Checker Framework integration (#126)

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+Thanks for taking the time to submit a contribution to Velocity! Your support
+is greatly appreciated.
+
+In this document, we'll give you some tips on making it more likely your
+contribution will be pulled.
+
+# Setting up a development environment
+
+This isn't as difficult as you may be led to believe. All you need to do is
+clone the Velocity repository in your favorite IDE and have your backend test
+servers set up to run behind Velocity.
+
+# Actually working on the code
+
+It is strongly recommended that you are familiar with the Minecraft protocol,
+proficient with using Java, and have familiarity with the libraries used in
+Velocity (particularly [Netty](https://netty.io), [Google Guava](https://github.com/google/guava),
+and the [Checker Framework annotations](https://checkerframework.org/)).
+While you can certainly work with the Velocity codebase without knowing any
+of this, it can be risky to proceed.
+
+Velocity does not currently obey any one general code style at the moment.
+Plans are [in the works](https://github.com/VelocityPowered/Velocity/issues/125)
+to define the code style the project will follow.
+
+# Notes on the build
+
+To reduce bugs and ensure code quality, we run the following tools on all commits
+and pull requests:
+
+* [Checker Framework](https://checkerframework.org/): an enhancement to Java's type
+  system that is designed to help catch bugs. Velocity runs the _Nullness Checker_
+  and the _Optional Checker_.
+* [Checkstyle](http://checkstyle.sourceforge.net/) (not currently in use): ensures
+  that your code is correctly formatted.

api/build.gradle

@@ -4,6 +4,8 @@ plugins {
     id 'maven-publish'
 }
 
+apply from: '../gradle/checkerframework.gradle'
+
 sourceSets {
     ap {
         compileClasspath += main.compileClasspath + main.output
@@ -17,7 +19,7 @@ dependencies {
     compile 'com.moandjiezana.toml:toml4j:0.7.2'
     compile "org.slf4j:slf4j-api:${slf4jVersion}"
     compile 'com.google.inject:guice:4.2.0'
-    compile 'org.checkerframework:checker-qual:2.5.4'
+    compile "org.checkerframework:checker-qual:${checkerFrameworkVersion}"
 
     testCompile "org.junit.jupiter:junit-jupiter-api:${junitVersion}"
     testCompile "org.junit.jupiter:junit-jupiter-engine:${junitVersion}"

api/src/ap/java/com/velocitypowered/api/plugin/ap/SerializedPluginDescription.java

@@ -2,6 +2,7 @@ package com.velocitypowered.api.plugin.ap;
 
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
+import com.google.common.collect.ImmutableList;
 import com.velocitypowered.api.plugin.Plugin;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
@@ -29,8 +30,8 @@ public class SerializedPluginDescription {
         this.version = Strings.emptyToNull(version);
         this.description = Strings.emptyToNull(description);
         this.url = Strings.emptyToNull(url);
-        this.authors = authors == null || authors.isEmpty() ? null : authors;
-        this.dependencies = dependencies == null || dependencies.isEmpty() ? null : dependencies;
+        this.authors = authors == null || authors.isEmpty() ? ImmutableList.of() : authors;
+        this.dependencies = dependencies == null || dependencies.isEmpty() ? ImmutableList.of() : dependencies;
         this.main = Preconditions.checkNotNull(main, "main");
     }
 
@@ -63,12 +64,12 @@ public class SerializedPluginDescription {
         return url;
     }
 
-    public @Nullable List<String> getAuthors() {
-        return authors;
+    public List<String> getAuthors() {
+        return authors == null ? ImmutableList.of() : authors;
     }
 
-    public @Nullable List<Dependency> getDependencies() {
-        return dependencies;
+    public List<Dependency> getDependencies() {
+        return dependencies == null ? ImmutableList.of() : dependencies;
     }
 
     public String getMain() {

api/src/main/java/com/velocitypowered/api/command/Command.java

@@ -15,7 +15,7 @@ public interface Command {
      * @param source the source of this command
      * @param args the arguments for this command
      */
-    void execute(@NonNull CommandSource source, @NonNull String[] args);
+    void execute(CommandSource source, String @NonNull [] args);
 
     /**
      * Provides tab complete suggestions for a command for a specified {@link CommandSource}.
@@ -23,7 +23,7 @@ public interface Command {
      * @param currentArgs the current, partial arguments for this command
      * @return tab complete suggestions
      */
-    default List<String> suggest(@NonNull CommandSource source, @NonNull String[] currentArgs) {
+    default List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
         return ImmutableList.of();
     }
 
@@ -38,7 +38,7 @@ public interface Command {
      * @param args the arguments for this command
      * @return whether the source has permission
      */
-    default boolean hasPermission(@NonNull CommandSource source, @NonNull String[] args) {
+    default boolean hasPermission(CommandSource source, String @NonNull [] args) {
         return true;
     }
 }

api/src/main/java/com/velocitypowered/api/command/CommandManager.java

@@ -1,7 +1,5 @@
 package com.velocitypowered.api.command;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 /**
  * Represents an interface to register a command executor with the proxy.
  */
@@ -11,13 +9,13 @@ public interface CommandManager {
      * @param command the command to register
      * @param aliases the alias to use
      */
-    void register(@NonNull Command command, String... aliases);
+    void register(Command command, String... aliases);
 
     /**
      * Unregisters a command.
      * @param alias the command alias to unregister
      */
-    void unregister(@NonNull String alias);
+    void unregister(String alias);
 
     /**
      * Attempts to execute a command from the specified {@code cmdLine}.
@@ -25,5 +23,5 @@ public interface CommandManager {
      * @param cmdLine the command to run
      * @return true if the command was found and executed, false if it was not
      */
-    boolean execute(@NonNull CommandSource source, @NonNull String cmdLine);
+    boolean execute(CommandSource source, String cmdLine);
 }

api/src/main/java/com/velocitypowered/api/command/CommandSource.java

@@ -2,7 +2,6 @@ package com.velocitypowered.api.command;
 
 import com.velocitypowered.api.permission.PermissionSubject;
 import net.kyori.text.Component;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
  * Represents something that can be used to run a {@link Command}.
@@ -12,5 +11,5 @@ public interface CommandSource extends PermissionSubject {
      * Sends the specified {@code component} to the invoker.
      * @param component the text component to send
      */
-    void sendMessage(@NonNull Component component);
+    void sendMessage(Component component);
 }

api/src/main/java/com/velocitypowered/api/event/EventHandler.java

@@ -1,12 +1,10 @@
 package com.velocitypowered.api.event;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 /**
  * Represents an interface to perform direct dispatch of an event. This makes integration easier to achieve with platforms
  * such as RxJava.
  */
 @FunctionalInterface
 public interface EventHandler<E> {
-    void execute(@NonNull E event);
+    void execute(E event);
 }

api/src/main/java/com/velocitypowered/api/event/EventManager.java

@@ -1,7 +1,5 @@
 package com.velocitypowered.api.event;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 import java.util.concurrent.CompletableFuture;
 
 /**
@@ -13,7 +11,7 @@ public interface EventManager {
      * @param plugin the plugin to associate with the listener
      * @param listener the listener to register
      */
-    void register(@NonNull Object plugin, @NonNull Object listener);
+    void register(Object plugin, Object listener);
 
     /**
      * Requests that the specified {@code handler} listen for events and associate it with the {@code plugin}.
@@ -22,7 +20,7 @@ public interface EventManager {
      * @param handler the handler to register
      * @param <E> the event type to handle
      */
-    default <E> void register(@NonNull Object plugin, @NonNull Class<E> eventClass, @NonNull EventHandler<E> handler) {
+    default <E> void register(Object plugin, Class<E> eventClass, EventHandler<E> handler) {
         register(plugin, eventClass, PostOrder.NORMAL, handler);
     }
 
@@ -34,7 +32,7 @@ public interface EventManager {
      * @param handler the handler to register
      * @param <E> the event type to handle
      */
-    <E> void register(@NonNull Object plugin, @NonNull Class<E> eventClass, @NonNull PostOrder postOrder, @NonNull EventHandler<E> handler);
+    <E> void register(Object plugin, Class<E> eventClass, PostOrder postOrder, EventHandler<E> handler);
 
     /**
      * Fires the specified event to the event bus asynchronously. This allows Velocity to continue servicing connections
@@ -42,13 +40,13 @@ public interface EventManager {
      * @param event the event to fire
      * @return a {@link CompletableFuture} representing the posted event
      */
-    @NonNull <E> CompletableFuture<E> fire(@NonNull E event);
+    <E> CompletableFuture<E> fire(E event);
 
     /**
      * Posts the specified event to the event bus and discards the result.
      * @param event the event to fire
      */
-    default void fireAndForget(@NonNull Object event) {
+    default void fireAndForget(Object event) {
         fire(event);
     }
 
@@ -56,14 +54,14 @@ public interface EventManager {
      * Unregisters all listeners for the specified {@code plugin}.
      * @param plugin the plugin to deregister listeners for
      */
-    void unregisterListeners(@NonNull Object plugin);
+    void unregisterListeners(Object plugin);
 
     /**
      * Unregisters a specific listener for a specific plugin.
      * @param plugin the plugin associated with the listener
      * @param listener the listener to deregister
      */
-    void unregisterListener(@NonNull Object plugin, @NonNull Object listener);
+    void unregisterListener(Object plugin, Object listener);
 
     /**
      * Unregisters a specific event handler for a specific plugin.
@@ -71,5 +69,5 @@ public interface EventManager {
      * @param handler the handler to register
      * @param <E> the event type to handle
      */
-    <E> void unregister(@NonNull Object plugin, @NonNull EventHandler<E> handler);
+    <E> void unregister(Object plugin, EventHandler<E> handler);
 }

api/src/main/java/com/velocitypowered/api/event/ResultedEvent.java

@@ -22,7 +22,7 @@ public interface ResultedEvent<R extends ResultedEvent.Result> {
      * Sets the result of this event. The result must be non-null.
      * @param result the new result
      */
-    void setResult(@NonNull R result);
+    void setResult(R result);
 
     /**
      * Represents a result for an event.
@@ -106,7 +106,7 @@ public interface ResultedEvent<R extends ResultedEvent.Result> {
             return ALLOWED;
         }
 
-        public static ComponentResult denied(@NonNull Component reason) {
+        public static ComponentResult denied(Component reason) {
             Preconditions.checkNotNull(reason, "reason");
             return new ComponentResult(false, reason);
         }

api/src/main/java/com/velocitypowered/api/event/connection/ConnectionHandshakeEvent.java

@@ -2,15 +2,14 @@ package com.velocitypowered.api.event.connection;
 
 import com.google.common.base.Preconditions;
 import com.velocitypowered.api.proxy.InboundConnection;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
  * This event is fired when a handshake is established between a client and Velocity.
  */
 public final class ConnectionHandshakeEvent {
-    private final @NonNull InboundConnection connection;
+    private final InboundConnection connection;
 
-    public ConnectionHandshakeEvent(@NonNull InboundConnection connection) {
+    public ConnectionHandshakeEvent(InboundConnection connection) {
         this.connection = Preconditions.checkNotNull(connection, "connection");
     }
 

api/src/main/java/com/velocitypowered/api/event/connection/DisconnectEvent.java

@@ -2,16 +2,15 @@ package com.velocitypowered.api.event.connection;
 
 import com.google.common.base.Preconditions;
 import com.velocitypowered.api.proxy.Player;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
  * This event is fired when a player disconnects from the proxy. Operations on the provided player, aside from basic
  * data retrieval operations, may behave in undefined ways.
  */
 public final class DisconnectEvent {
-    private @NonNull final Player player;
+    private final Player player;
 
-    public DisconnectEvent(@NonNull Player player) {
+    public DisconnectEvent(Player player) {
         this.player = Preconditions.checkNotNull(player, "player");
     }
 

api/src/main/java/com/velocitypowered/api/event/connection/LoginEvent.java

@@ -3,7 +3,6 @@ package com.velocitypowered.api.event.connection;
 import com.google.common.base.Preconditions;
 import com.velocitypowered.api.event.ResultedEvent;
 import com.velocitypowered.api.proxy.Player;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
  * This event is fired once the player has been authenticated but before they connect to a server on the proxy.
@@ -12,7 +11,7 @@ public final class LoginEvent implements ResultedEvent<ResultedEvent.ComponentRe
     private final Player player;
     private ComponentResult result;
 
-    public LoginEvent(@NonNull Player player) {
+    public LoginEvent(Player player) {
         this.player = Preconditions.checkNotNull(player, "player");
         this.result = ComponentResult.allowed();
     }
@@ -27,7 +26,7 @@ public final class LoginEvent implements ResultedEvent<ResultedEvent.ComponentRe
     }
 
     @Override
-    public void setResult(@NonNull ComponentResult result) {
+    public void setResult(ComponentResult result) {
         this.result = Preconditions.checkNotNull(result, "result");
     }
 

api/src/main/java/com/velocitypowered/api/event/connection/PluginMessageEvent.java

@@ -36,7 +36,7 @@ public final class PluginMessageEvent implements ResultedEvent<PluginMessageEven
     }
 
     @Override
-    public void setResult(@NonNull ForwardResult result) {
+    public void setResult(ForwardResult result) {
         this.result = Preconditions.checkNotNull(result, "result");
     }
 

api/src/main/java/com/velocitypowered/api/event/connection/PreLoginEvent.java

@@ -4,7 +4,6 @@ import com.google.common.base.Preconditions;
 import com.velocitypowered.api.event.ResultedEvent;
 import com.velocitypowered.api.proxy.InboundConnection;
 import net.kyori.text.Component;
-import net.kyori.text.serializer.ComponentSerializers;
 import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
@@ -62,11 +61,11 @@ public final class PreLoginEvent implements ResultedEvent<PreLoginEvent.PreLogin
         private static final PreLoginComponentResult FORCE_OFFLINEMODE = new PreLoginComponentResult(Result.FORCE_OFFLINE, null);
 
         private final Result result;
-        private final Optional<Component> reason;
+        private final @Nullable Component reason;
 
         private PreLoginComponentResult(Result result, @Nullable Component reason) {
             this.result = result;
-            this.reason = Optional.ofNullable(reason);
+            this.reason = reason;
         }
 
         @Override
@@ -75,7 +74,7 @@ public final class PreLoginEvent implements ResultedEvent<PreLoginEvent.PreLogin
         }
 
         public Optional<Component> getReason() {
-            return reason;
+            return Optional.ofNullable(reason);
         }
 
         public boolean isOnlineModeAllowed() {
@@ -88,19 +87,16 @@ public final class PreLoginEvent implements ResultedEvent<PreLoginEvent.PreLogin
 
         @Override
         public String toString() {
-            if (isForceOfflineMode()) {
-                return "allowed with force offline mode";
+            switch (result) {
+                case ALLOWED:
+                    return "allowed";
+                case FORCE_OFFLINE:
+                    return "allowed with force offline mode";
+                case FORCE_ONLINE:
+                    return "allowed with online mode";
+                default:
+                    return "denied";
             }
-            if (isOnlineModeAllowed()) {
-                return "allowed with online mode";
-            }
-            if (isAllowed()) {
-                return "allowed";
-            }
-            if (reason.isPresent()) {
-                return "denied: " + ComponentSerializers.PLAIN.serialize(reason.get());
-            }
-            return "denied";
         }
 
         /**

api/src/main/java/com/velocitypowered/api/event/permission/PermissionsSetupEvent.java

@@ -4,7 +4,6 @@ import com.google.common.base.Preconditions;
 import com.velocitypowered.api.permission.PermissionFunction;
 import com.velocitypowered.api.permission.PermissionProvider;
 import com.velocitypowered.api.permission.PermissionSubject;
-import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
@@ -22,7 +21,7 @@ public final class PermissionsSetupEvent {
         this.provider = this.defaultProvider = Preconditions.checkNotNull(provider, "provider");
     }
 
-    public @NonNull PermissionSubject getSubject() {
+    public PermissionSubject getSubject() {
         return this.subject;
     }
 
@@ -33,11 +32,11 @@ public final class PermissionsSetupEvent {
      * @param subject the subject
      * @return the obtained permission function
      */
-    public @NonNull PermissionFunction createFunction(PermissionSubject subject) {
+    public PermissionFunction createFunction(PermissionSubject subject) {
         return this.provider.createFunction(subject);
     }
 
-    public @NonNull PermissionProvider getProvider() {
+    public PermissionProvider getProvider() {
         return this.provider;
     }
 

api/src/main/java/com/velocitypowered/api/event/player/GameProfileRequestEvent.java

@@ -14,7 +14,7 @@ public final class GameProfileRequestEvent {
     private final InboundConnection connection;
     private final GameProfile originalProfile;
     private final boolean onlineMode;
-    private GameProfile gameProfile;
+    private @Nullable GameProfile gameProfile;
 
     public GameProfileRequestEvent(InboundConnection connection, GameProfile originalProfile, boolean onlineMode) {
         this.connection = Preconditions.checkNotNull(connection, "connection");

api/src/main/java/com/velocitypowered/api/event/player/PlayerChatEvent.java

@@ -36,7 +36,7 @@ public final class PlayerChatEvent implements ResultedEvent<PlayerChatEvent.Chat
     }
 
     @Override
-    public void setResult(@NonNull ChatResult result) {
+    public void setResult(ChatResult result) {
         this.result = Preconditions.checkNotNull(result, "result");
     }
 

api/src/main/java/com/velocitypowered/api/event/player/PlayerSettingsChangedEvent.java

@@ -17,7 +17,6 @@ public final class PlayerSettingsChangedEvent {
         return player;
     }
 
-    //New settings
     public PlayerSettings getPlayerSettings() {
         return playerSettings;
     }

api/src/main/java/com/velocitypowered/api/event/player/ServerPreConnectEvent.java

@@ -4,7 +4,6 @@ import com.google.common.base.Preconditions;
 import com.velocitypowered.api.event.ResultedEvent;
 import com.velocitypowered.api.proxy.Player;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
-import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Optional;
@@ -33,7 +32,7 @@ public final class ServerPreConnectEvent implements ResultedEvent<ServerPreConne
     }
 
     @Override
-    public void setResult(@NonNull ServerResult result) {
+    public void setResult(ServerResult result) {
         this.result = Preconditions.checkNotNull(result, "result");
     }
 
@@ -54,19 +53,17 @@ public final class ServerPreConnectEvent implements ResultedEvent<ServerPreConne
      * Represents the result of the {@link ServerPreConnectEvent}.
      */
     public static class ServerResult implements ResultedEvent.Result {
-        private static final ServerResult DENIED = new ServerResult(false, null);
+        private static final ServerResult DENIED = new ServerResult(null);
 
-        private final boolean allowed;
-        private final RegisteredServer server;
+        private final @Nullable RegisteredServer server;
 
-        private ServerResult(boolean allowed, @Nullable RegisteredServer server) {
-            this.allowed = allowed;
+        private ServerResult(@Nullable RegisteredServer server) {
             this.server = server;
         }
 
         @Override
         public boolean isAllowed() {
-            return allowed;
+            return server != null;
         }
 
         public Optional<RegisteredServer> getServer() {
@@ -75,10 +72,10 @@ public final class ServerPreConnectEvent implements ResultedEvent<ServerPreConne
 
         @Override
         public String toString() {
-            if (!allowed) {
-                return "denied";
+            if (server != null) {
+                return "allowed: connect to " + server.getServerInfo().getName();
             }
-            return "allowed: connect to " + server.getServerInfo().getName();
+            return "denied";
         }
 
         public static ServerResult denied() {
@@ -87,7 +84,7 @@ public final class ServerPreConnectEvent implements ResultedEvent<ServerPreConne
 
         public static ServerResult allowed(RegisteredServer server) {
             Preconditions.checkNotNull(server, "server");
-            return new ServerResult(true, server);
+            return new ServerResult(server);
         }
     }
 }

api/src/main/java/com/velocitypowered/api/permission/PermissionFunction.java

@@ -1,7 +1,5 @@
 package com.velocitypowered.api.permission;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 /**
  * Function that calculates the permission settings for a given
  * {@link PermissionSubject}.
@@ -29,5 +27,5 @@ public interface PermissionFunction {
      * @param permission the permission
      * @return the value the permission is set to
      */
-    @NonNull Tristate getPermissionValue(@NonNull String permission);
+    Tristate getPermissionValue(String permission);
 }

api/src/main/java/com/velocitypowered/api/permission/PermissionProvider.java

@@ -1,7 +1,5 @@
 package com.velocitypowered.api.permission;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 /**
  * Provides {@link PermissionFunction}s for {@link PermissionSubject}s.
  */
@@ -13,5 +11,5 @@ public interface PermissionProvider {
      * @param subject the subject
      * @return the function
      */
-    @NonNull PermissionFunction createFunction(@NonNull PermissionSubject subject);
+    PermissionFunction createFunction(PermissionSubject subject);
 }

api/src/main/java/com/velocitypowered/api/permission/PermissionSubject.java

@@ -1,7 +1,5 @@
 package com.velocitypowered.api.permission;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 /**
  * Represents a object that has a set of queryable permissions.
  */
@@ -12,7 +10,7 @@ public interface PermissionSubject {
      * @param permission the permission to check for
      * @return whether or not the subject has the permission
      */
-    default boolean hasPermission(@NonNull String permission) {
+    default boolean hasPermission(String permission) {
         return getPermissionValue(permission).asBoolean();
     }
 
@@ -22,5 +20,5 @@ public interface PermissionSubject {
      * @param permission the permission
      * @return the value the permission is set to
      */
-    @NonNull Tristate getPermissionValue(@NonNull String permission);
+    Tristate getPermissionValue(String permission);
 }

api/src/main/java/com/velocitypowered/api/permission/Tristate.java

@@ -37,7 +37,7 @@ public enum Tristate {
      * @param val the boolean value
      * @return {@link #TRUE} or {@link #FALSE}, if the value is <code>true</code> or <code>false</code>, respectively.
      */
-    public static @NonNull Tristate fromBoolean(boolean val) {
+    public static Tristate fromBoolean(boolean val) {
         return val ? TRUE : FALSE;
     }
 
@@ -51,7 +51,7 @@ public enum Tristate {
      * @return {@link #UNDEFINED}, {@link #TRUE} or {@link #FALSE}, if the value
      *         is <code>null</code>, <code>true</code> or <code>false</code>, respectively.
      */
-    public static @NonNull Tristate fromNullableBoolean(@Nullable Boolean val) {
+    public static Tristate fromNullableBoolean(@Nullable Boolean val) {
         return val == null ? UNDEFINED : val ? TRUE : FALSE;
     }
 

api/src/main/java/com/velocitypowered/api/plugin/PluginContainer.java

@@ -1,7 +1,5 @@
 package com.velocitypowered.api.plugin;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 import java.util.Optional;
 
 /**
@@ -13,7 +11,7 @@ public interface PluginContainer {
      *
      * @return the plugin's description
      */
-    @NonNull PluginDescription getDescription();
+    PluginDescription getDescription();
 
     /**
      * Returns the created plugin if it is available.

api/src/main/java/com/velocitypowered/api/plugin/PluginManager.java

@@ -1,16 +1,12 @@
 package com.velocitypowered.api.plugin;
 
-import org.checkerframework.checker.nullness.qual.NonNull;
-
 import java.nio.file.Path;
 import java.util.Collection;
 import java.util.Optional;
-import java.util.logging.Logger;
 
 /**
- * The class that manages plugins. This manager can retrieve
- * {@link PluginContainer}s from {@link Plugin} instances, getting
- * {@link Logger}s, etc.
+ * Manages plugins loaded on the proxy. This manager can retrieve {@link PluginContainer}s from plugin instances
+ * and inject arbitrary JAR files into the plugin classpath with {@link #addToClasspath(Object, Path)}.
  */
 public interface PluginManager {
     /**
@@ -19,7 +15,7 @@ public interface PluginManager {
      * @param instance the instance
      * @return the container
      */
-    @NonNull Optional<PluginContainer> fromInstance(@NonNull Object instance);
+    Optional<PluginContainer> fromInstance(Object instance);
 
     /**
      * Retrieves a {@link PluginContainer} based on its ID.
@@ -27,22 +23,22 @@ public interface PluginManager {
      * @param id the plugin ID
      * @return the plugin, if available
      */
-    @NonNull Optional<PluginContainer> getPlugin(@NonNull String id);
+    Optional<PluginContainer> getPlugin(String id);
 
     /**
      * Gets a {@link Collection} of all {@link PluginContainer}s.
      *
      * @return the plugins
      */
-    @NonNull Collection<PluginContainer> getPlugins();
+    Collection<PluginContainer> getPlugins();
 
     /**
      * Checks if a plugin is loaded based on its ID.
      *
-     * @param id the id of the {@link Plugin}
+     * @param id the id of the plugin
      * @return {@code true} if loaded
      */
-    boolean isLoaded(@NonNull String id);
+    boolean isLoaded(String id);
 
     /**
      * Adds the specified {@code path} to the plugin classpath.
@@ -51,5 +47,5 @@ public interface PluginManager {
      * @param path the path to the JAR you want to inject into the classpath
      * @throws UnsupportedOperationException if the operation is not applicable to this plugin
      */
-    void addToClasspath(@NonNull Object plugin, @NonNull Path path);
+    void addToClasspath(Object plugin, Path path);
 }

api/src/main/java/com/velocitypowered/api/plugin/meta/PluginDependency.java

@@ -1,6 +1,7 @@
 package com.velocitypowered.api.plugin.meta;
 
-import javax.annotation.Nullable;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
 import java.util.Objects;
 import java.util.Optional;
 
@@ -13,7 +14,8 @@ import static com.google.common.base.Strings.emptyToNull;
  */
 public final class PluginDependency {
     private final String id;
-    @Nullable private final String version;
+    @Nullable
+    private final String version;
 
     private final boolean optional;
 
@@ -53,7 +55,7 @@ public final class PluginDependency {
     }
 
     @Override
-    public boolean equals(Object o) {
+    public boolean equals(@Nullable Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
         PluginDependency that = (PluginDependency) o;

api/src/main/java/com/velocitypowered/api/proxy/Player.java

@@ -13,7 +13,6 @@ import com.velocitypowered.api.util.title.Title;
 import java.util.List;
 
 import net.kyori.text.Component;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 import java.util.Optional;
 import java.util.UUID;
@@ -62,7 +61,7 @@ public interface Player extends CommandSource, InboundConnection, ChannelMessage
      * Sends a chat message to the player's client.
      * @param component the chat message to send
      */
-    default void sendMessage(@NonNull Component component) {
+    default void sendMessage(Component component) {
         sendMessage(component, MessagePosition.CHAT);
     }
 
@@ -71,14 +70,14 @@ public interface Player extends CommandSource, InboundConnection, ChannelMessage
      * @param component the chat message to send
      * @param position the position for the message
      */
-    void sendMessage(@NonNull Component component, @NonNull MessagePosition position);
+    void sendMessage(Component component, MessagePosition position);
 
     /**
      * Creates a new connection request so that the player can connect to another server.
      * @param server the server to connect to
      * @return a new connection request
      */
-    ConnectionRequestBuilder createConnectionRequest(@NonNull RegisteredServer server);
+    ConnectionRequestBuilder createConnectionRequest(RegisteredServer server);
 
     /**
      * Gets the player's profile properties.

api/src/main/java/com/velocitypowered/api/proxy/ProxyServer.java

@@ -9,6 +9,7 @@ import com.velocitypowered.api.proxy.messages.ChannelRegistrar;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.proxy.server.ServerInfo;
 import com.velocitypowered.api.scheduler.Scheduler;
+import com.velocitypowered.api.util.ProxyVersion;
 import net.kyori.text.Component;
 
 import java.net.InetSocketAddress;
@@ -131,4 +132,10 @@ public interface ProxyServer {
      * @return the proxy config
      * */
     ProxyConfig getConfiguration();
+
+    /**
+     * Returns the version of the proxy.
+     * @return the proxy version
+     */
+    ProxyVersion getVersion();
 }

api/src/main/java/com/velocitypowered/api/proxy/messages/LegacyChannelIdentifier.java

@@ -2,6 +2,7 @@ package com.velocitypowered.api.proxy.messages;
 
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Objects;
 
@@ -27,7 +28,7 @@ public final class LegacyChannelIdentifier implements ChannelIdentifier {
     }
 
     @Override
-    public boolean equals(Object o) {
+    public boolean equals(@Nullable Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
         LegacyChannelIdentifier that = (LegacyChannelIdentifier) o;

api/src/main/java/com/velocitypowered/api/proxy/messages/MinecraftChannelIdentifier.java

@@ -2,6 +2,7 @@ package com.velocitypowered.api.proxy.messages;
 
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Objects;
 import java.util.regex.Pattern;
@@ -54,11 +55,11 @@ public final class MinecraftChannelIdentifier implements ChannelIdentifier {
 
     @Override
     public String toString() {
-        return getId() + " (modern)";
+        return namespace + ":" + name + " (modern)";
     }
 
     @Override
-    public boolean equals(Object o) {
+    public boolean equals(@Nullable Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
         MinecraftChannelIdentifier that = (MinecraftChannelIdentifier) o;

api/src/main/java/com/velocitypowered/api/proxy/player/TabList.java

@@ -3,6 +3,7 @@ package com.velocitypowered.api.proxy.player;
 import com.velocitypowered.api.proxy.Player;
 import com.velocitypowered.api.util.GameProfile;
 import net.kyori.text.Component;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Collection;
 import java.util.Optional;
@@ -47,5 +48,5 @@ public interface TabList {
 
     // Necessary because the TabListEntry implementation isn't in the api
     @Deprecated
-    TabListEntry buildEntry(GameProfile profile, Component displayName, int latency, int gameMode);
+    TabListEntry buildEntry(GameProfile profile, @Nullable Component displayName, int latency, int gameMode);
 }

api/src/main/java/com/velocitypowered/api/proxy/player/TabListEntry.java

@@ -1,9 +1,7 @@
 package com.velocitypowered.api.proxy.player;
 
-import com.google.common.base.Preconditions;
 import com.velocitypowered.api.util.GameProfile;
 import net.kyori.text.Component;
-import org.checkerframework.checker.nullness.qual.NonNull;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Optional;
@@ -16,7 +14,7 @@ public interface TabListEntry {
      * Returns the parent {@link TabList} of this {@code this} {@link TabListEntry}.
      * @return parent {@link TabList}
      */
-    @NonNull TabList getTabList();
+    TabList getTabList();
     
     /**
      * Returns the {@link GameProfile} of the entry, which uniquely identifies the entry
@@ -24,14 +22,14 @@ public interface TabListEntry {
      * as the player head in the tab list.
      * @return {@link GameProfile} of the entry
      */
-    @NonNull GameProfile getProfile();
+    GameProfile getProfile();
     
     /**
      * Returns {@link Optional} text {@link Component}, which if present is the text displayed for
      * {@code this} entry in the {@link TabList}, otherwise {@link GameProfile#getName()} is shown.
      * @return {@link Optional} text {@link Component} of name displayed in the tab list
      */
-    @NonNull Optional<Component> getDisplayName();
+    Optional<Component> getDisplayName();
     
     /**
      * Sets the text {@link Component} to be displayed for {@code this} {@link TabListEntry}.
@@ -39,7 +37,7 @@ public interface TabListEntry {
      * @param displayName to show in the {@link TabList} for {@code this} entry
      * @return {@code this}, for chaining
      */
-    @NonNull TabListEntry setDisplayName(@Nullable Component displayName);
+    TabListEntry setDisplayName(@Nullable Component displayName);
     
     /**
      * Returns the latency for {@code this} entry.
@@ -63,7 +61,7 @@ public interface TabListEntry {
      * @param latency to changed to
      * @return {@code this}, for chaining
      */
-    @NonNull TabListEntry setLatency(int latency);
+    TabListEntry setLatency(int latency);
     
     /**
      * Gets the game mode {@code this} entry has been set to.
@@ -99,9 +97,9 @@ public interface TabListEntry {
      * @see TabListEntry
      */
     class Builder {
-        private TabList tabList;
-        private GameProfile profile;
-        private Component displayName;
+        private @Nullable TabList tabList;
+        private @Nullable GameProfile profile;
+        private @Nullable Component displayName;
         private int latency = 0;
         private int gameMode = 0;
 
@@ -167,9 +165,12 @@ public interface TabListEntry {
          * @return the constructed {@link TabListEntry}
          */
         public TabListEntry build() {
-            Preconditions.checkState(tabList != null, "The Tablist must be set when building a TabListEntry");
-            Preconditions.checkState(profile != null, "The GameProfile must be set when building a TabListEntry");
-
+            if (tabList == null) {
+                throw new IllegalStateException("The Tablist must be set when building a TabListEntry");
+            }
+            if (profile == null) {
+                throw new IllegalStateException("The GameProfile must be set when building a TabListEntry");
+            }
             return tabList.buildEntry(profile, displayName, latency, gameMode);
         }
     }

api/src/main/java/com/velocitypowered/api/proxy/server/ServerInfo.java

@@ -1,7 +1,7 @@
 package com.velocitypowered.api.proxy.server;
 
 import com.google.common.base.Preconditions;
-import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.net.InetSocketAddress;
 import java.util.Objects;
@@ -10,24 +10,24 @@ import java.util.Objects;
  * ServerInfo represents a server that a player can connect to. This object is immutable and safe for concurrent access.
  */
 public final class ServerInfo {
-    private final @NonNull String name;
-    private final @NonNull InetSocketAddress address;
+    private final String name;
+    private final InetSocketAddress address;
 
     /**
      * Creates a new ServerInfo object.
      * @param name the name for the server
      * @param address the address of the server to connect to
      */
-    public ServerInfo(@NonNull String name, @NonNull InetSocketAddress address) {
+    public ServerInfo(String name, InetSocketAddress address) {
         this.name = Preconditions.checkNotNull(name, "name");
         this.address = Preconditions.checkNotNull(address, "address");
     }
 
-    public final @NonNull String getName() {
+    public final String getName() {
         return name;
     }
 
-    public final @NonNull InetSocketAddress getAddress() {
+    public final InetSocketAddress getAddress() {
         return address;
     }
 
@@ -40,7 +40,7 @@ public final class ServerInfo {
     }
 
     @Override
-    public final boolean equals(Object o) {
+    public final boolean equals(@Nullable Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
         ServerInfo that = (ServerInfo) o;

api/src/main/java/com/velocitypowered/api/proxy/server/ServerPing.java

@@ -14,7 +14,7 @@ import java.util.*;
  */
 public final class ServerPing {
     private final Version version;
-    private final Players players;
+    private final @Nullable Players players;
     private final Component description;
     private final @Nullable Favicon favicon;
     private final @Nullable ModInfo modinfo;
@@ -89,14 +89,14 @@ public final class ServerPing {
      * A builder for {@link ServerPing} objects.
      */
     public static final class Builder {
-        private Version version;
+        private Version version = new Version(0, "Unknown");
         private int onlinePlayers;
         private int maximumPlayers;
         private final List<SamplePlayer> samplePlayers = new ArrayList<>();
-        private String modType;
+        private String modType = "FML";
         private final List<ModInfo.Mod> mods = new ArrayList<>();
-        private Component description;
-        private Favicon favicon;
+        private @Nullable Component description;
+        private @Nullable Favicon favicon;
         private boolean nullOutPlayers;
         private boolean nullOutModinfo;
 
@@ -165,6 +165,12 @@ public final class ServerPing {
         }
 
         public ServerPing build() {
+            if (this.version == null) {
+                throw new IllegalStateException("version not specified");
+            }
+            if (this.description == null) {
+                throw new IllegalStateException("no server description supplied");
+            }
             return new ServerPing(version, nullOutPlayers ? null : new Players(onlinePlayers, maximumPlayers, samplePlayers),
                     description, favicon, nullOutModinfo ? null : new ModInfo(modType, mods));
         }
@@ -185,12 +191,12 @@ public final class ServerPing {
             return samplePlayers;
         }
 
-        public Component getDescription() {
-            return description;
+        public Optional<Component> getDescription() {
+            return Optional.ofNullable(description);
         }
 
-        public Favicon getFavicon() {
-            return favicon;
+        public Optional<Favicon> getFavicon() {
+            return Optional.ofNullable(favicon);
         }
 
         public String getModType() {

api/src/main/java/com/velocitypowered/api/scheduler/Scheduler.java

@@ -1,5 +1,8 @@
 package com.velocitypowered.api.scheduler;
 
+import org.checkerframework.common.value.qual.IntRange;
+import org.checkerframework.common.value.qual.IntRangeFromNonNegative;
+
 import java.util.concurrent.TimeUnit;
 
 /**
@@ -24,7 +27,7 @@ public interface Scheduler {
          * @param unit the unit of time for {@code time}
          * @return this builder, for chaining
          */
-        TaskBuilder delay(long time, TimeUnit unit);
+        TaskBuilder delay(@IntRange(from = 0) long time, TimeUnit unit);
 
         /**
          * Specifies that the task should continue running after waiting for the specified amount, until it is cancelled.
@@ -32,7 +35,7 @@ public interface Scheduler {
          * @param unit the unit of time for {@code time}
          * @return this builder, for chaining
          */
-        TaskBuilder repeat(long time, TimeUnit unit);
+        TaskBuilder repeat(@IntRange(from = 0) long time, TimeUnit unit);
 
         /**
          * Clears the delay on this task.

api/src/main/java/com/velocitypowered/api/util/Favicon.java

@@ -1,7 +1,7 @@
 package com.velocitypowered.api.util;
 
 import com.google.common.base.Preconditions;
-import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import javax.imageio.ImageIO;
 import java.awt.image.BufferedImage;
@@ -25,7 +25,7 @@ public final class Favicon {
      * of functions.
      * @param base64Url the url for use with this favicon
      */
-    public Favicon(@NonNull String base64Url) {
+    public Favicon(String base64Url) {
         this.base64Url = Preconditions.checkNotNull(base64Url, "base64Url");
     }
 
@@ -38,7 +38,7 @@ public final class Favicon {
     }
 
     @Override
-    public boolean equals(Object o) {
+    public boolean equals(@Nullable Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
         Favicon favicon = (Favicon) o;
@@ -62,7 +62,7 @@ public final class Favicon {
      * @param image the image to use for the favicon
      * @return the created {@link Favicon} instance
      */
-    public static Favicon create(@NonNull BufferedImage image) {
+    public static Favicon create(BufferedImage image) {
         Preconditions.checkNotNull(image, "image");
         Preconditions.checkArgument(image.getWidth() == 64 && image.getHeight() == 64, "Image does not have" +
                 " 64x64 dimensions (found %sx%s)", image.getWidth(), image.getHeight());
@@ -81,7 +81,7 @@ public final class Favicon {
      * @return the created {@link Favicon} instance
      * @throws IOException if the file could not be read from the path
      */
-    public static Favicon create(@NonNull Path path) throws IOException {
+    public static Favicon create(Path path) throws IOException {
         try (InputStream stream = Files.newInputStream(path)) {
             return create(ImageIO.read(stream));
         }

api/src/main/java/com/velocitypowered/api/util/GameProfile.java

@@ -2,7 +2,6 @@ package com.velocitypowered.api.util;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 import java.util.List;
 import java.util.UUID;
@@ -15,7 +14,7 @@ public final class GameProfile {
     private final String name;
     private final List<Property> properties;
 
-    public GameProfile(@NonNull String id, @NonNull String name, @NonNull List<Property> properties) {
+    public GameProfile(String id, String name, List<Property> properties) {
         this.id = Preconditions.checkNotNull(id, "id");
         this.name = Preconditions.checkNotNull(name, "name");
         this.properties = ImmutableList.copyOf(properties);
@@ -42,7 +41,7 @@ public final class GameProfile {
      * @param username the username to use
      * @return the new offline-mode game profile
      */
-    public static GameProfile forOfflinePlayer(@NonNull String username) {
+    public static GameProfile forOfflinePlayer(String username) {
         Preconditions.checkNotNull(username, "username");
         String id = UuidUtils.toUndashed(UuidUtils.generateOfflinePlayerUuid(username));
         return new GameProfile(id, username, ImmutableList.of());
@@ -62,7 +61,7 @@ public final class GameProfile {
         private final String value;
         private final String signature;
 
-        public Property(@NonNull String name, @NonNull String value, @NonNull String signature) {
+        public Property(String name, String value, String signature) {
             this.name = Preconditions.checkNotNull(name, "name");
             this.value = Preconditions.checkNotNull(value, "value");
             this.signature = Preconditions.checkNotNull(signature, "signature");

api/src/main/java/com/velocitypowered/api/util/ProxyVersion.java

@@ -0,0 +1,57 @@
+package com.velocitypowered.api.util;
+
+import com.google.common.base.Preconditions;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+import java.util.Objects;
+
+/**
+ * Provides a version object for the proxy.
+ */
+public final class ProxyVersion {
+    private final String name;
+    private final String vendor;
+    private final String version;
+
+    public ProxyVersion(String name, String vendor, String version) {
+        this.name = Preconditions.checkNotNull(name, "name");
+        this.vendor = Preconditions.checkNotNull(vendor, "vendor");
+        this.version = Preconditions.checkNotNull(version, "version");
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getVendor() {
+        return vendor;
+    }
+
+    public String getVersion() {
+        return version;
+    }
+
+    @Override
+    public boolean equals(@Nullable Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        ProxyVersion that = (ProxyVersion) o;
+        return Objects.equals(name, that.name) &&
+                Objects.equals(vendor, that.vendor) &&
+                Objects.equals(version, that.version);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(name, vendor, version);
+    }
+
+    @Override
+    public String toString() {
+        return "ProxyVersion{" +
+                "name='" + name + '\'' +
+                ", vendor='" + vendor + '\'' +
+                ", version='" + version + '\'' +
+                '}';
+    }
+}

api/src/main/java/com/velocitypowered/api/util/UuidUtils.java

@@ -2,7 +2,6 @@ package com.velocitypowered.api.util;
 
 import com.google.common.base.Preconditions;
 import com.google.common.base.Strings;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
 import java.nio.charset.StandardCharsets;
 import java.util.Objects;
@@ -21,7 +20,7 @@ public final class UuidUtils {
      * @param string the string to convert
      * @return the UUID object
      */
-    public static @NonNull UUID fromUndashed(final @NonNull String string) {
+    public static UUID fromUndashed(final String string) {
         Objects.requireNonNull(string, "string");
         Preconditions.checkArgument(string.length() == 32, "Length is incorrect");
         return new UUID(
@@ -35,7 +34,7 @@ public final class UuidUtils {
      * @param uuid the UUID to convert
      * @return the undashed UUID
      */
-    public static @NonNull String toUndashed(final @NonNull UUID uuid) {
+    public static String toUndashed(final UUID uuid) {
         Preconditions.checkNotNull(uuid, "uuid");
         return Strings.padStart(Long.toHexString(uuid.getMostSignificantBits()), 16, '0') +
                 Strings.padStart(Long.toHexString(uuid.getLeastSignificantBits()), 16, '0');
@@ -46,7 +45,7 @@ public final class UuidUtils {
      * @param username the username to use
      * @return the offline mode UUID
      */
-    public static @NonNull UUID generateOfflinePlayerUuid(@NonNull String username) {
+    public static UUID generateOfflinePlayerUuid(String username) {
         return UUID.nameUUIDFromBytes(("OfflinePlayer:" + username).getBytes(StandardCharsets.UTF_8));
     }
 }

api/src/main/java/com/velocitypowered/api/util/title/TextTitle.java

@@ -11,8 +11,8 @@ import java.util.Optional;
  * Represents a "full" title, including all components. This class is immutable.
  */
 public final class TextTitle implements Title {
-    private final Component title;
-    private final Component subtitle;
+    private final @Nullable Component title;
+    private final @Nullable Component subtitle;
     private final int stay;
     private final int fadeIn;
     private final int fadeOut;
@@ -94,7 +94,7 @@ public final class TextTitle implements Title {
     }
 
     @Override
-    public boolean equals(Object o) {
+    public boolean equals(@Nullable Object o) {
         if (this == o) return true;
         if (o == null || getClass() != o.getClass()) return false;
         TextTitle textTitle = (TextTitle) o;
@@ -193,12 +193,12 @@ public final class TextTitle implements Title {
             return this;
         }
 
-        public Component getTitle() {
-            return title;
+        public Optional<Component> getTitle() {
+            return Optional.ofNullable(title);
         }
 
-        public Component getSubtitle() {
-            return subtitle;
+        public Optional<Component> getSubtitle() {
+            return Optional.ofNullable(subtitle);
         }
 
         public int getStay() {

build.gradle

@@ -16,6 +16,7 @@ allprojects {
         log4jVersion = '2.11.1'
         nettyVersion = '4.1.30.Final'
         guavaVersion = '25.1-jre'
+        checkerFrameworkVersion = '2.5.6'
 
         getCurrentBranchName = {
             new ByteArrayOutputStream().withStream { os ->

gradle/checkerframework.gradle

@@ -0,0 +1,70 @@
+///////////////////////////////////////////////////////////////////////////
+/// Checker Framework pluggable type-checking
+///
+
+repositories {
+    mavenCentral()
+}
+
+configurations {
+    checkerFrameworkCheckerJar {
+        description = 'the Checker Framework, including the Type Annotations compiler'
+    }
+
+    checkerFrameworkAnnotatedJDK {
+        description = 'a copy of JDK classes with Checker Framework type qualifers inserted'
+    }
+}
+
+// By default, use Checker Framework from Maven Central.
+// Pass -PcfLocal to use a locally-built version of the Checker Framework.
+dependencies {
+    if (!rootProject.hasProperty('cfLocal')) {
+        checkerFrameworkAnnotatedJDK "org.checkerframework:jdk8:${checkerFrameworkVersion}"
+        checkerFrameworkCheckerJar "org.checkerframework:checker:${checkerFrameworkVersion}"
+        implementation "org.checkerframework:checker-qual:${checkerFrameworkVersion}"
+    } else if (System.getenv("CHECKERFRAMEWORK") == null) {
+        throw new GradleException("Environment variable CHECKERFRAMEWORK is not set")
+    } else if (! file(System.getenv("CHECKERFRAMEWORK")).exists()) {
+        throw new GradleException("Environment variable CHECKERFRAMEWORK is set to non-existent directory " + System.getenv("CHECKERFRAMEWORK"));
+    } else {
+        ext.checkerframeworkdist = "$System.env.CHECKERFRAMEWORK/checker/dist"
+        checkerFrameworkAnnotatedJDK fileTree(dir: "${ext.checkerframeworkdist}", include: "jdk8.jar")
+        checkerFrameworkCheckerJar fileTree(dir: "${ext.checkerframeworkdist}", include: 'checker.jar')
+        implementation fileTree(dir: "${ext.checkerframeworkdist}", include: 'checker-qual.jar')
+    }
+}
+
+// // To type-check all projects.
+// allprojects {
+//   tasks.withType(JavaCompile).all { JavaCompile compile ->
+//     compile.doFirst {
+//       compile.options.compilerArgs = [
+//         '-processor', 'org.checkerframework.checker.formatter.FormatterChecker,org.checkerframework.checker.index.IndexChecker,org.checkerframework.checker.lock.LockChecker,org.checkerframework.checker.nullness.NullnessChecker,org.checkerframework.checker.signature.SignatureChecker',
+//         '-Xmaxerrs', '10000',
+//         '-Awarns',    // -Awarns turns Checker Framework errors into warnings
+//         '-AcheckPurityAnnotations',
+//         '-processorpath', "${configurations.checkerFrameworkCheckerJar.asPath}",
+//         "-Xbootclasspath/p:${configurations.checkerFrameworkAnnotatedJDK.asPath}",
+//         "-Astubs=$System.env.CHECKERFRAMEWORK/checker/resources/javadoc.astub" // TODO: does not work when downloading from Maven Central
+//       ]
+//     }
+//   }
+// }
+
+// To typecheck only the current project's main source set (in a multi-project
+// build), use this instead:
+compileJava {
+    doFirst {
+        options.compilerArgs = [
+                '-processor', 'org.checkerframework.checker.nullness.NullnessChecker',
+                '-processor', 'org.checkerframework.checker.optional.OptionalChecker',
+                '-Xmaxerrs', '10000',
+                '-Xmaxwarns', '10000',
+                // '-Awarns',      // -Awarns turns Checker Framework errors into warnings
+                //'-AcheckPurityAnnotations', // Disabled for Velocity, wish we could do better
+                '-processorpath', "${configurations.checkerFrameworkCheckerJar.asPath}",
+                "-Xbootclasspath/p:${configurations.checkerFrameworkAnnotatedJDK.asPath}"
+        ]
+    }
+}

native/build.gradle

@@ -2,9 +2,13 @@ plugins {
     id 'java'
 }
 
+apply from: '../gradle/checkerframework.gradle'
+
 dependencies {
     compile "com.google.guava:guava:${guavaVersion}"
     compile "io.netty:netty-buffer:${nettyVersion}"
+    compile "org.checkerframework:checker-qual:${checkerFrameworkVersion}"
+
     testCompile "org.junit.jupiter:junit-jupiter-api:${junitVersion}"
     testCompile "org.junit.jupiter:junit-jupiter-engine:${junitVersion}"
 }

native/src/main/java/com/velocitypowered/natives/util/NativeCodeLoader.java

@@ -1,88 +1,64 @@
 package com.velocitypowered.natives.util;
 
-import com.google.common.collect.ImmutableList;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.List;
 import java.util.function.BooleanSupplier;
 import java.util.function.Supplier;
 
-public class NativeCodeLoader<T> implements Supplier<T> {
-    private final List<Variant<T>> variants;
-    private volatile Variant<T> selected;
+public final class NativeCodeLoader<T> implements Supplier<T> {
+    private final Variant<T> selected;
 
-    public NativeCodeLoader(List<Variant<T>> variants) {
-        this.variants = ImmutableList.copyOf(variants);
+    NativeCodeLoader(List<Variant<T>> variants) {
+        this.selected = getVariant(variants);
     }
 
     @Override
     public T get() {
-        return tryLoad().object;
+        return selected.object;
     }
 
-    private Variant<T> tryLoad() {
-        if (selected != null) {
-            return selected;
-        }
-
-        synchronized (this) {
-            if (selected != null) {
-                return selected;
+    private static <T> Variant<T> getVariant(List<Variant<T>> variants) {
+        for (Variant<T> variant : variants) {
+            T got = variant.get();
+            if (got == null) {
+                continue;
             }
-
-            for (Variant<T> variant : variants) {
-                T got = variant.get();
-                if (got == null) {
-                    continue;
-                }
-                selected = variant;
-                return selected;
-            }
-            throw new IllegalArgumentException("Can't find any suitable variants");
+            return variant;
         }
+        throw new IllegalArgumentException("Can't find any suitable variants");
     }
 
     public String getLoadedVariant() {
-        return tryLoad().name;
+        return selected.name;
     }
 
     static class Variant<T> {
-        private volatile boolean available;
+        private Status status;
         private final Runnable setup;
         private final String name;
         private final T object;
-        private volatile boolean hasBeenSetup = false;
 
-        Variant(BooleanSupplier available, Runnable setup, String name, T object) {
-            this.available = available.getAsBoolean();
+        Variant(BooleanSupplier possiblyAvailable, Runnable setup, String name, T object) {
+            this.status = possiblyAvailable.getAsBoolean() ? Status.POSSIBLY_AVAILABLE : Status.NOT_AVAILABLE;
             this.setup = setup;
             this.name = name;
             this.object = object;
         }
 
-        public T get() {
-            if (!available) {
+        public @Nullable T get() {
+            if (status == Status.NOT_AVAILABLE || status == Status.SETUP_FAILURE) {
                 return null;
             }
 
             // Make sure setup happens only once
-            if (!hasBeenSetup) {
-                synchronized (this) {
-                    // We change availability if need be below, may as well check it again here for sanity.
-                    if (!available) {
-                        return null;
-                    }
-
-                    // Okay, now try the setup if we haven't done so yet.
-                    if (!hasBeenSetup) {
-                        try {
-                            setup.run();
-                            hasBeenSetup = true;
-                            return object;
-                        } catch (Exception e) {
-                            available = false;
-                            return null;
-                        }
-                    }
+            if (status == Status.POSSIBLY_AVAILABLE) {
+                try {
+                    setup.run();
+                    status = Status.SETUP;
+                } catch (Exception e) {
+                    status = Status.SETUP_FAILURE;
+                    return null;
                 }
             }
 
@@ -90,9 +66,16 @@ public class NativeCodeLoader<T> implements Supplier<T> {
         }
     }
 
-    static final BooleanSupplier MACOS = () -> System.getProperty("os.name").equalsIgnoreCase("Mac OS X") &&
+    private enum Status {
+        NOT_AVAILABLE,
+        POSSIBLY_AVAILABLE,
+        SETUP,
+        SETUP_FAILURE
+    }
+
+    static final BooleanSupplier MACOS = () -> System.getProperty("os.name", "").equalsIgnoreCase("Mac OS X") &&
             System.getProperty("os.arch").equals("x86_64");
-    static final BooleanSupplier LINUX = () -> System.getProperties().getProperty("os.name").equalsIgnoreCase("Linux") &&
+    static final BooleanSupplier LINUX = () -> System.getProperties().getProperty("os.name", "").equalsIgnoreCase("Linux") &&
             System.getProperty("os.arch").equals("amd64");
     static final BooleanSupplier ALWAYS = () -> true;
 }

native/src/main/java/com/velocitypowered/natives/util/Natives.java

@@ -8,6 +8,7 @@ import com.velocitypowered.natives.encryption.JavaVelocityCipher;
 import com.velocitypowered.natives.encryption.VelocityCipherFactory;
 
 import java.io.IOException;
+import java.io.InputStream;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
@@ -21,7 +22,12 @@ public class Natives {
         return () -> {
             try {
                 Path tempFile = Files.createTempFile("native-", path.substring(path.lastIndexOf('.')));
-                Files.copy(Natives.class.getResourceAsStream(path), tempFile, StandardCopyOption.REPLACE_EXISTING);
+                InputStream nativeLib = Natives.class.getResourceAsStream(path);
+                if (nativeLib == null) {
+                    throw new IllegalStateException("Native library " + path + " not found.");
+                }
+
+                Files.copy(nativeLib, tempFile, StandardCopyOption.REPLACE_EXISTING);
                 Runtime.getRuntime().addShutdownHook(new Thread(() -> {
                     try {
                         Files.deleteIfExists(tempFile);

proxy/build.gradle

@@ -4,6 +4,8 @@ plugins {
     id 'de.sebastianboegl.shadow.transformer.log4j' version '2.1.1'
 }
 
+apply from: '../gradle/checkerframework.gradle'
+
 compileJava {
     options.compilerArgs += ['-proc:none']
 }

proxy/src/main/java/com/velocitypowered/proxy/Velocity.java

@@ -8,8 +8,6 @@ import java.text.DecimalFormat;
 public class Velocity {
     private static final Logger logger = LogManager.getLogger(Velocity.class);
 
-    private static long startTime;
-
     static {
         // We use BufferedImage for favicons, and on macOS this puts the Java application in the dock. How inconvenient.
         // Force AWT to work with its head chopped off.
@@ -17,8 +15,7 @@ public class Velocity {
     }
 
     public static void main(String... args) {
-        startTime = System.currentTimeMillis();
-        logger.info("Booting up Velocity {}...", Velocity.class.getPackage().getImplementationVersion());
+        long startTime = System.currentTimeMillis();
 
         VelocityServer server = new VelocityServer();
         server.start();

proxy/src/main/java/com/velocitypowered/proxy/VelocityServer.java

@@ -1,5 +1,6 @@
 package com.velocitypowered.proxy;
 
+import com.google.common.base.MoreObjects;
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.gson.Gson;
@@ -7,12 +8,14 @@ import com.google.gson.GsonBuilder;
 import com.velocitypowered.api.event.EventManager;
 import com.velocitypowered.api.event.proxy.ProxyInitializeEvent;
 import com.velocitypowered.api.event.proxy.ProxyShutdownEvent;
+import com.velocitypowered.api.plugin.PluginContainer;
 import com.velocitypowered.api.plugin.PluginManager;
 import com.velocitypowered.api.proxy.Player;
 import com.velocitypowered.api.proxy.ProxyServer;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.proxy.server.ServerInfo;
 import com.velocitypowered.api.util.Favicon;
+import com.velocitypowered.api.util.ProxyVersion;
 import com.velocitypowered.proxy.command.ServerCommand;
 import com.velocitypowered.proxy.command.ShutdownCommand;
 import com.velocitypowered.proxy.command.VelocityCommand;
@@ -39,6 +42,7 @@ import net.kyori.text.TextComponent;
 import net.kyori.text.serializer.GsonComponentSerializer;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.*;
 
 import java.net.InetSocketAddress;
 import java.nio.file.Files;
@@ -58,36 +62,54 @@ public class VelocityServer implements ProxyServer {
             .registerTypeHierarchyAdapter(Favicon.class, new FaviconSerializer())
             .create();
 
-    private final ConnectionManager cm = new ConnectionManager(this);
-    private VelocityConfiguration configuration;
-    private NettyHttpClient httpClient;
-    private KeyPair serverKeyPair;
-    private final ServerMap servers = new ServerMap(this);
+    private @MonotonicNonNull ConnectionManager cm;
+    private @MonotonicNonNull VelocityConfiguration configuration;
+    private @MonotonicNonNull NettyHttpClient httpClient;
+    private @MonotonicNonNull KeyPair serverKeyPair;
+    private @MonotonicNonNull ServerMap servers;
     private final VelocityCommandManager commandManager = new VelocityCommandManager();
     private final AtomicBoolean shutdownInProgress = new AtomicBoolean(false);
     private boolean shutdown = false;
-    private final VelocityPluginManager pluginManager = new VelocityPluginManager(this);
+    private @MonotonicNonNull VelocityPluginManager pluginManager;
 
     private final Map<UUID, ConnectedPlayer> connectionsByUuid = new ConcurrentHashMap<>();
     private final Map<String, ConnectedPlayer> connectionsByName = new ConcurrentHashMap<>();
-    private final VelocityConsole console = new VelocityConsole(this);
-    private Ratelimiter ipAttemptLimiter;
-    private VelocityEventManager eventManager;
-    private VelocityScheduler scheduler;
-    private VelocityChannelRegistrar channelRegistrar;
-
-    VelocityServer() {
-        commandManager.register(new VelocityCommand(), "velocity");
-        commandManager.register(new ServerCommand(this), "server");
-        commandManager.register(new ShutdownCommand(this), "shutdown", "end");
-    }
+    private @MonotonicNonNull VelocityConsole console;
+    private @MonotonicNonNull Ratelimiter ipAttemptLimiter;
+    private @MonotonicNonNull VelocityEventManager eventManager;
+    private @MonotonicNonNull VelocityScheduler scheduler;
+    private final VelocityChannelRegistrar channelRegistrar = new VelocityChannelRegistrar();
 
     public KeyPair getServerKeyPair() {
+        if (serverKeyPair == null) {
+            throw new AssertionError();
+        }
         return serverKeyPair;
     }
 
     public VelocityConfiguration getConfiguration() {
-        return configuration;
+        VelocityConfiguration cfg = this.configuration;
+        if (cfg == null) {
+            throw new IllegalStateException("Configuration not initialized!");
+        }
+        return cfg;
+    }
+
+    @Override
+    public ProxyVersion getVersion() {
+        Package pkg = VelocityServer.class.getPackage();
+        String implName, implVersion, implVendor;
+        if (pkg != null) {
+            implName = MoreObjects.firstNonNull(pkg.getImplementationTitle(), "Velocity");
+            implVersion = MoreObjects.firstNonNull(pkg.getImplementationVersion(), "<unknown>");
+            implVendor = MoreObjects.firstNonNull(pkg.getImplementationVendor(), "Velocity Contributors");
+        } else {
+            implName = "Velocity";
+            implVersion = "<unknown>";
+            implVendor = "Velocity Contributors";
+        }
+
+        return new ProxyVersion(implName, implVendor, implVersion);
     }
 
     @Override
@@ -95,7 +117,25 @@ public class VelocityServer implements ProxyServer {
         return commandManager;
     }
 
+    @EnsuresNonNull({"serverKeyPair", "servers", "pluginManager", "eventManager", "scheduler", "console", "cm", "configuration"})
     public void start() {
+        logger.info("Booting up {} {}...", getVersion().getName(), getVersion().getVersion());
+
+        serverKeyPair = EncryptionUtils.createRsaKeyPair(1024);
+        pluginManager = new VelocityPluginManager(this);
+        eventManager = new VelocityEventManager(pluginManager);
+        scheduler = new VelocityScheduler(pluginManager);
+        console = new VelocityConsole(this);
+        cm = new ConnectionManager(this);
+        servers = new ServerMap(this);
+
+        cm.logChannelInformation();
+
+        // Initialize commands first
+        commandManager.register(new VelocityCommand(this), "velocity");
+        commandManager.register(new ServerCommand(this), "server");
+        commandManager.register(new ShutdownCommand(this), "shutdown", "end");
+
         try {
             Path configPath = Paths.get("velocity.toml");
             configuration = VelocityConfiguration.read(configPath);
@@ -118,22 +158,13 @@ public class VelocityServer implements ProxyServer {
             servers.register(new ServerInfo(entry.getKey(), AddressUtil.parseAddress(entry.getValue())));
         }
 
-        serverKeyPair = EncryptionUtils.createRsaKeyPair(1024);
         ipAttemptLimiter = new Ratelimiter(configuration.getLoginRatelimit());
         httpClient = new NettyHttpClient(this);
-        eventManager = new VelocityEventManager(pluginManager);
-        scheduler = new VelocityScheduler(pluginManager);
-        channelRegistrar = new VelocityChannelRegistrar();
         loadPlugins();
 
-        try {
-            // Go ahead and fire the proxy initialization event. We block since plugins should have a chance
-            // to fully initialize before we accept any connections to the server.
-            eventManager.fire(new ProxyInitializeEvent()).get();
-        } catch (InterruptedException | ExecutionException e) {
-            // Ignore, we don't care. InterruptedException is unlikely to happen (and if it does, you've got bigger
-            // issues) and there is almost no chance ExecutionException will be thrown.
-        }
+        // Go ahead and fire the proxy initialization event. We block since plugins should have a chance
+        // to fully initialize before we accept any connections to the server.
+        eventManager.fire(new ProxyInitializeEvent()).join();
 
         // init console permissions after plugins are loaded
         console.setupPermissions();
@@ -145,6 +176,7 @@ public class VelocityServer implements ProxyServer {
         }
     }
 
+    @RequiresNonNull({"pluginManager", "eventManager"})
     private void loadPlugins() {
         logger.info("Loading plugins...");
 
@@ -166,18 +198,20 @@ public class VelocityServer implements ProxyServer {
         }
 
         // Register the plugin main classes so that we may proceed with firing the proxy initialize event
-        pluginManager.getPlugins().forEach(container -> {
-            container.getInstance().ifPresent(plugin -> eventManager.register(plugin, plugin));
-        });
+        for (PluginContainer plugin : pluginManager.getPlugins()) {
+            Optional<?> instance = plugin.getInstance();
+            if (instance.isPresent()) {
+                eventManager.register(instance.get(), instance.get());
+            }
+        }
 
         logger.info("Loaded {} plugins", pluginManager.getPlugins().size());
     }
 
-    public ServerMap getServers() {
-        return servers;
-    }
-
     public Bootstrap initializeGenericBootstrap() {
+        if (cm == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return this.cm.createWorker();
     }
 
@@ -186,6 +220,10 @@ public class VelocityServer implements ProxyServer {
     }
 
     public void shutdown() {
+        if (eventManager == null || pluginManager == null || cm == null || scheduler == null) {
+            throw new AssertionError();
+        }
+
         if (!shutdownInProgress.compareAndSet(false, true)) {
             return;
         }
@@ -210,10 +248,16 @@ public class VelocityServer implements ProxyServer {
     }
 
     public NettyHttpClient getHttpClient() {
+        if (httpClient == null) {
+            throw new IllegalStateException("HTTP client not initialized");
+        }
         return httpClient;
     }
 
     public Ratelimiter getIpAttemptLimiter() {
+        if (ipAttemptLimiter == null) {
+            throw new IllegalStateException("Ratelimiter not initialized");
+        }
         return ipAttemptLimiter;
     }
 
@@ -268,41 +312,65 @@ public class VelocityServer implements ProxyServer {
     @Override
     public Optional<RegisteredServer> getServer(String name) {
         Preconditions.checkNotNull(name, "name");
+        if (servers == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return servers.getServer(name);
     }
 
     @Override
     public Collection<RegisteredServer> getAllServers() {
+        if (servers == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return servers.getAllServers();
     }
 
     @Override
     public RegisteredServer registerServer(ServerInfo server) {
+        if (servers == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return servers.register(server);
     }
 
     @Override
     public void unregisterServer(ServerInfo server) {
+        if (servers == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         servers.unregister(server);
     }
 
     @Override
     public VelocityConsole getConsoleCommandSource() {
+        if (console == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return console;
     }
 
     @Override
     public PluginManager getPluginManager() {
+        if (pluginManager == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return pluginManager;
     }
 
     @Override
     public EventManager getEventManager() {
+        if (eventManager == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return eventManager;
     }
 
     @Override
     public VelocityScheduler getScheduler() {
+        if (scheduler == null) {
+            throw new IllegalStateException("Server did not initialize properly.");
+        }
         return scheduler;
     }
 
@@ -313,6 +381,9 @@ public class VelocityServer implements ProxyServer {
 
     @Override
     public InetSocketAddress getBoundAddress() {
+        if (configuration == null) {
+            throw new IllegalStateException("No configuration"); // even though you'll never get the chance... heh, heh
+        }
         return configuration.getBind();
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/command/ServerCommand.java

@@ -27,7 +27,7 @@ public class ServerCommand implements Command {
     }
 
     @Override
-    public void execute(CommandSource source, String[] args) {
+    public void execute(CommandSource source, String @NonNull [] args) {
         if (!(source instanceof Player)) {
             source.sendMessage(TextComponent.of("Only players may run this command.", TextColor.RED));
             return;
@@ -76,7 +76,7 @@ public class ServerCommand implements Command {
     }
 
     @Override
-    public List<String> suggest(CommandSource source, String[] currentArgs) {
+    public List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
         if (currentArgs.length == 0) {
             return server.getAllServers().stream()
                     .map(rs -> rs.getServerInfo().getName())
@@ -92,7 +92,7 @@ public class ServerCommand implements Command {
     }
 
     @Override
-    public boolean hasPermission(@NonNull CommandSource source, @NonNull String[] args) {
+    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
         return source.getPermissionValue("velocity.command.server") != Tristate.FALSE;
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/command/ShutdownCommand.java

@@ -15,7 +15,7 @@ public class ShutdownCommand implements Command {
     }
 
     @Override
-    public void execute(CommandSource source, String[] args) {
+    public void execute(CommandSource source, String @NonNull [] args) {
         if (source != server.getConsoleCommandSource()) {
             source.sendMessage(TextComponent.of("You are not allowed to use this command.", TextColor.RED));
             return;
@@ -24,7 +24,7 @@ public class ShutdownCommand implements Command {
     }
 
     @Override
-    public boolean hasPermission(@NonNull CommandSource source, @NonNull String[] args) {
+    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
         return source == server.getConsoleCommandSource();
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommand.java

@@ -1,12 +1,12 @@
 package com.velocitypowered.proxy.command;
 
-import com.google.common.base.MoreObjects;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.velocitypowered.api.command.Command;
 import com.velocitypowered.api.command.CommandSource;
 import com.velocitypowered.api.permission.Tristate;
-import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.api.proxy.ProxyServer;
+import com.velocitypowered.api.util.ProxyVersion;
 import net.kyori.text.TextComponent;
 import net.kyori.text.event.ClickEvent;
 import net.kyori.text.format.TextColor;
@@ -20,9 +20,13 @@ import java.util.Map;
 import java.util.stream.Collectors;
 
 public class VelocityCommand implements Command {
-    private final Map<String, Command> subcommands = ImmutableMap.<String, Command>builder()
-            .put("version", Info.INSTANCE)
-            .build();
+    private final Map<String, Command> subcommands;
+
+    public VelocityCommand(ProxyServer server) {
+        this.subcommands = ImmutableMap.<String, Command>builder()
+                .put("version", new Info(server))
+                .build();
+    }
 
     private void usage(CommandSource source) {
         String commandText = "/velocity <" + String.join("|", subcommands.keySet()) + ">";
@@ -30,7 +34,7 @@ public class VelocityCommand implements Command {
     }
 
     @Override
-    public void execute(CommandSource source, String[] args) {
+    public void execute(CommandSource source, String @NonNull [] args) {
         if (args.length == 0) {
             usage(source);
             return;
@@ -41,11 +45,13 @@ public class VelocityCommand implements Command {
             usage(source);
             return;
         }
-        command.execute(source, Arrays.copyOfRange(args, 1, args.length));
+        @SuppressWarnings("nullness")
+        String[] actualArgs = Arrays.copyOfRange(args, 1, args.length);
+        command.execute(source, actualArgs);
     }
 
     @Override
-    public List<String> suggest(@NonNull CommandSource source, @NonNull String[] currentArgs) {
+    public List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
         if (currentArgs.length == 0) {
             return ImmutableList.copyOf(subcommands.keySet());
         }
@@ -60,11 +66,13 @@ public class VelocityCommand implements Command {
         if (command == null) {
             return ImmutableList.of();
         }
-        return command.suggest(source, Arrays.copyOfRange(currentArgs, 1, currentArgs.length));
+        @SuppressWarnings("nullness")
+        String[] actualArgs = Arrays.copyOfRange(currentArgs, 1, currentArgs.length);
+        return command.suggest(source, actualArgs);
     }
 
     @Override
-    public boolean hasPermission(@NonNull CommandSource source, @NonNull String[] args) {
+    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
         if (args.length == 0) {
             return true;
         }
@@ -72,29 +80,33 @@ public class VelocityCommand implements Command {
         if (command == null) {
             return true;
         }
-        return command.hasPermission(source, Arrays.copyOfRange(args, 1, args.length));
+        @SuppressWarnings("nullness")
+        String[] actualArgs = Arrays.copyOfRange(args, 1, args.length);
+        return command.hasPermission(source, actualArgs);
     }
 
     private static class Info implements Command {
-        static final Info INSTANCE = new Info();
-        private Info() {}
+        private final ProxyServer server;
+
+        private Info(ProxyServer server) {
+            this.server = server;
+        }
 
         @Override
-        public void execute(@NonNull CommandSource source, @NonNull String[] args) {
-            String implName = MoreObjects.firstNonNull(VelocityServer.class.getPackage().getImplementationTitle(), "Velocity");
-            String implVersion = MoreObjects.firstNonNull(VelocityServer.class.getPackage().getImplementationVersion(), "<unknown>");
-            String implVendor = MoreObjects.firstNonNull(VelocityServer.class.getPackage().getImplementationVendor(), "Velocity Contributors");
-            TextComponent velocity = TextComponent.builder(implName + " ")
+        public void execute(CommandSource source, String @NonNull [] args) {
+            ProxyVersion version = server.getVersion();
+
+            TextComponent velocity = TextComponent.builder(version.getName() + " ")
                     .decoration(TextDecoration.BOLD, true)
                     .color(TextColor.DARK_AQUA)
-                    .append(TextComponent.of(implVersion).decoration(TextDecoration.BOLD, false))
+                    .append(TextComponent.of(version.getVersion()).decoration(TextDecoration.BOLD, false))
                     .build();
-            TextComponent copyright = TextComponent.of("Copyright 2018 " + implVendor + ". " + implName + " is freely licensed under the terms of the " +
+            TextComponent copyright = TextComponent.of("Copyright 2018 " + version.getVendor() + ". " + version.getName() + " is freely licensed under the terms of the " +
                     "MIT License.");
             source.sendMessage(velocity);
             source.sendMessage(copyright);
 
-            if (implName.equals("Velocity")) {
+            if (version.getName().equals("Velocity")) {
                 TextComponent velocityWebsite = TextComponent.builder()
                         .content("Visit the ")
                         .append(TextComponent.builder("Velocity website")
@@ -112,7 +124,7 @@ public class VelocityCommand implements Command {
         }
 
         @Override
-        public boolean hasPermission(@NonNull CommandSource source, @NonNull String[] args) {
+        public boolean hasPermission(CommandSource source, String @NonNull [] args) {
             return source.getPermissionValue("velocity.command.info") != Tristate.FALSE;
         }
     }

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommandManager.java

@@ -1,9 +1,11 @@
 package com.velocitypowered.proxy.command;
 
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
 import com.velocitypowered.api.command.Command;
 import com.velocitypowered.api.command.CommandManager;
 import com.velocitypowered.api.command.CommandSource;
+import org.checkerframework.checker.nullness.qual.NonNull;
 
 import java.util.*;
 
@@ -11,7 +13,7 @@ public class VelocityCommandManager implements CommandManager {
     private final Map<String, Command> commands = new HashMap<>();
 
     @Override
-    public void register(final Command command, final String... aliases) {
+    public void register(@NonNull final Command command, final String... aliases) {
         Preconditions.checkNotNull(aliases, "aliases");
         Preconditions.checkNotNull(command, "executor");
         for (int i = 0, length = aliases.length; i < length; i++) {
@@ -22,13 +24,13 @@ public class VelocityCommandManager implements CommandManager {
     }
 
     @Override
-    public void unregister(final String alias) {
+    public void unregister(@NonNull final String alias) {
         Preconditions.checkNotNull(alias, "name");
         this.commands.remove(alias.toLowerCase(Locale.ENGLISH));
     }
 
     @Override
-    public boolean execute(CommandSource source, String cmdLine) {
+    public boolean execute(@NonNull CommandSource source, @NonNull String cmdLine) {
         Preconditions.checkNotNull(source, "invoker");
         Preconditions.checkNotNull(cmdLine, "cmdLine");
 
@@ -38,6 +40,7 @@ public class VelocityCommandManager implements CommandManager {
         }
 
         String alias = split[0];
+        @SuppressWarnings("nullness")
         String[] actualArgs = Arrays.copyOfRange(split, 1, split.length);
         Command command = commands.get(alias.toLowerCase(Locale.ENGLISH));
         if (command == null) {
@@ -60,13 +63,13 @@ public class VelocityCommandManager implements CommandManager {
         return commands.containsKey(command);
     }
 
-    public Optional<List<String>> offerSuggestions(CommandSource source, String cmdLine) {
+    public List<String> offerSuggestions(CommandSource source, String cmdLine) {
         Preconditions.checkNotNull(source, "source");
         Preconditions.checkNotNull(cmdLine, "cmdLine");
 
         String[] split = cmdLine.split(" ", -1);
         if (split.length == 0) {
-            return Optional.empty();
+            return ImmutableList.of();
         }
 
         String alias = split[0];
@@ -78,21 +81,22 @@ public class VelocityCommandManager implements CommandManager {
                     availableCommands.add("/" + entry.getKey());
                 }
             }
-            return Optional.of(availableCommands);
+            return availableCommands;
         }
 
+        @SuppressWarnings("nullness")
         String[] actualArgs = Arrays.copyOfRange(split, 1, split.length);
         Command command = commands.get(alias.toLowerCase(Locale.ENGLISH));
         if (command == null) {
-            return Optional.empty();
+            return ImmutableList.of();
         }
 
         try {
             if (!command.hasPermission(source, actualArgs)) {
-                return Optional.empty();
+                return ImmutableList.of();
             }
 
-            return Optional.of(command.suggest(source, actualArgs));
+            return command.suggest(source, actualArgs);
         } catch (Exception e) {
             throw new RuntimeException("Unable to invoke suggestions for command " + alias + " for " + source, e);
         }

proxy/src/main/java/com/velocitypowered/proxy/config/AnnotatedConfig.java

@@ -116,7 +116,7 @@ public abstract class AnnotatedConfig {
 
                 // Get a key name for config
                 ConfigKey key = field.getAnnotation(ConfigKey.class);
-                String name = key == null ? field.getName() : key.value(); // Use field name if @ConfigKey annotation is not present
+                String name = safeKey(key == null ? field.getName() : key.value()); // Use field name if @ConfigKey annotation is not present
 
                 // Check if field is table.
                 Table table = field.getAnnotation(Table.class);
@@ -130,7 +130,7 @@ public abstract class AnnotatedConfig {
                     @SuppressWarnings("unchecked")
                     Map<String, ?> map = (Map<String, ?>) field.get(dumpable);
                     for (Entry<String, ?> entry : map.entrySet()) {
-                        lines.add(entry.getKey() + " = " + serialize(entry.getValue())); // Save map data
+                        lines.add(safeKey(entry.getKey()) + " = " + serialize(entry.getValue())); // Save map data
                     }
                     lines.add(""); // Add empty line
                     continue;
@@ -193,7 +193,7 @@ public abstract class AnnotatedConfig {
         return value != null ? value.toString() : "null";
     }
 
-    private String safeKey(String key) {
+    private static String safeKey(String key) {
         if(key.contains(".") && !(key.indexOf('"') == 0 && key.lastIndexOf('"') == (key.length() - 1))) {
             return '"' + key + '"';
         }

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -10,6 +10,8 @@ import com.velocitypowered.proxy.util.AddressUtil;
 import net.kyori.text.Component;
 import net.kyori.text.serializer.ComponentSerializers;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.io.IOException;
 import java.io.Reader;
@@ -78,10 +80,10 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
     private final Query query;
 
     @Ignore
-    private Component motdAsComponent;
+    private @MonotonicNonNull Component motdAsComponent;
 
     @Ignore
-    private Favicon favicon;
+    private @Nullable Favicon favicon;
 
     public VelocityConfiguration(Servers servers, ForcedHosts forcedHosts, Advanced advanced, Query query) {
         this.servers = servers;

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -16,6 +16,7 @@ import io.netty.handler.codec.haproxy.HAProxyMessage;
 import io.netty.util.ReferenceCountUtil;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import javax.crypto.SecretKey;
 import javax.crypto.spec.SecretKeySpec;
@@ -35,10 +36,10 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
     private final Channel channel;
     private SocketAddress remoteAddress;
     private StateRegistry state;
-    private MinecraftSessionHandler sessionHandler;
+    private @Nullable MinecraftSessionHandler sessionHandler;
     private int protocolVersion;
     private int nextProtocolVersion;
-    private MinecraftConnectionAssociation association;
+    private @Nullable MinecraftConnectionAssociation association;
     private boolean isLegacyForge;
     private final VelocityServer server;
     private boolean canSendLegacyFMLResetPacket = false;
@@ -74,6 +75,12 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
 
     @Override
     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
+        if (sessionHandler == null) {
+            // No session handler available, do nothing
+            ReferenceCountUtil.release(msg);
+            return;
+        }
+
         if (msg instanceof MinecraftPacket) {
             if (sessionHandler.beforeHandle()) {
                 return;
@@ -197,6 +204,7 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
         }
     }
 
+    @Nullable
     public MinecraftSessionHandler getSessionHandler() {
         return sessionHandler;
     }
@@ -243,6 +251,7 @@ public class MinecraftConnection extends ChannelInboundHandlerAdapter {
         channel.pipeline().addBefore(FRAME_ENCODER, CIPHER_ENCODER, new MinecraftCipherEncoder(encryptionCipher));
     }
 
+    @Nullable
     public MinecraftConnectionAssociation getAssociation() {
         return association;
     }

proxy/src/main/java/com/velocitypowered/proxy/connection/VelocityConstants.java

@@ -7,4 +7,6 @@ public class VelocityConstants {
 
     public static final String VELOCITY_IP_FORWARDING_CHANNEL = "velocity:player_info";
     public static final int FORWARDING_VERSION = 1;
+
+    public static final byte[] EMPTY_BYTE_ARRAY = new byte[0];
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/BackendPlaySessionHandler.java

@@ -4,6 +4,7 @@ import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.event.player.ServerConnectedEvent;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.connection.forge.ForgeConstants;
@@ -22,7 +23,12 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
     BackendPlaySessionHandler(VelocityServer server, VelocityServerConnection serverConn) {
         this.server = server;
         this.serverConn = serverConn;
-        this.playerSessionHandler = (ClientPlaySessionHandler) serverConn.getPlayer().getConnection().getSessionHandler();
+
+        MinecraftSessionHandler psh = serverConn.getPlayer().getConnection().getSessionHandler();
+        if (!(psh instanceof ClientPlaySessionHandler)) {
+            throw new IllegalStateException("Initializing BackendPlaySessionHandler with no backing client play session handler!");
+        }
+        this.playerSessionHandler = (ClientPlaySessionHandler) psh;
     }
 
     @Override
@@ -74,6 +80,11 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
     @Override
     public boolean handle(PluginMessage packet) {
+        MinecraftConnection smc = serverConn.getConnection();
+        if (smc == null) {
+            return true;
+        }
+
         if (!canForwardPluginMessage(packet)) {
             return true;
         }
@@ -105,9 +116,9 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
         server.getEventManager().fire(event)
                 .thenAcceptAsync(pme -> {
                     if (pme.getResult().isAllowed()) {
-                        serverConn.getPlayer().getConnection().write(packet);
+                        smc.write(packet);
                     }
-                }, serverConn.getConnection().eventLoop());
+                }, smc.eventLoop());
         return true;
     }
 
@@ -152,16 +163,18 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
     }
 
     private boolean canForwardPluginMessage(PluginMessage message) {
-        ClientPlaySessionHandler playerHandler =
-                (ClientPlaySessionHandler) serverConn.getPlayer().getConnection().getSessionHandler();
+        MinecraftConnection mc = serverConn.getConnection();
+        if (mc == null) {
+            return false;
+        }
         boolean isMCOrFMLMessage;
-        if (serverConn.getConnection().getProtocolVersion() <= ProtocolConstants.MINECRAFT_1_12_2) {
+        if (mc.getProtocolVersion() <= ProtocolConstants.MINECRAFT_1_12_2) {
             String channel = message.getChannel();
             isMCOrFMLMessage = channel.startsWith("MC|") || channel.startsWith(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL);
         } else {
             isMCOrFMLMessage = message.getChannel().startsWith("minecraft:");
         }
-        return isMCOrFMLMessage || playerHandler.getClientPluginMsgChannels().contains(message.getChannel()) ||
+        return isMCOrFMLMessage || playerSessionHandler.getClientPluginMsgChannels().contains(message.getChannel()) ||
                 server.getChannelRegistrar().registered(message.getChannel());
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/LoginSessionHandler.java

@@ -6,6 +6,7 @@ import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.config.VelocityConfiguration;
+import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
@@ -38,6 +39,14 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         this.resultFuture = resultFuture;
     }
 
+    private MinecraftConnection ensureMinecraftConnection() {
+        MinecraftConnection mc = serverConn.getConnection();
+        if (mc == null) {
+            throw new IllegalStateException("Not connected to backend server!");
+        }
+        return mc;
+    }
+
     @Override
     public boolean handle(EncryptionRequest packet) {
         throw new IllegalStateException("Backend server is online-mode!");
@@ -45,6 +54,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
     @Override
     public boolean handle(LoginPluginMessage packet) {
+        MinecraftConnection mc = ensureMinecraftConnection();
         VelocityConfiguration configuration = server.getConfiguration();
         if (configuration.getPlayerInfoForwardingMode() == PlayerInfoForwarding.MODERN && packet.getChannel()
                 .equals(VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL)) {
@@ -54,7 +64,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
             response.setData(createForwardingData(configuration.getForwardingSecret(),
                     serverConn.getPlayer().getRemoteAddress().getHostString(),
                     serverConn.getPlayer().getProfile()));
-            serverConn.getConnection().write(response);
+            mc.write(response);
             informationForwarded = true;
         } else {
             // Don't understand
@@ -62,7 +72,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
             response.setSuccess(false);
             response.setId(packet.getId());
             response.setData(Unpooled.EMPTY_BUFFER);
-            serverConn.getConnection().write(response);
+            mc.write(response);
         }
         return true;
     }
@@ -76,7 +86,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
     @Override
     public boolean handle(SetCompression packet) {
-        serverConn.getConnection().setCompressionThreshold(packet.getThreshold());
+        ensureMinecraftConnection().setCompressionThreshold(packet.getThreshold());
         return true;
     }
 
@@ -90,7 +100,8 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
         }
 
         // The player has been logged on to the backend server.
-        serverConn.getConnection().setState(StateRegistry.PLAY);
+        MinecraftConnection smc = ensureMinecraftConnection();
+        smc.setState(StateRegistry.PLAY);
         VelocityServerConnection existingConnection = serverConn.getPlayer().getConnectedServer();
         if (existingConnection == null) {
             // Strap on the play session handler
@@ -104,14 +115,14 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
             existingConnection.disconnect();
         }
 
-        serverConn.getConnection().getChannel().config().setAutoRead(false);
+        smc.getChannel().config().setAutoRead(false);
         server.getEventManager().fire(new ServerConnectedEvent(serverConn.getPlayer(), serverConn.getServer()))
                 .whenCompleteAsync((x, error) -> {
                     resultFuture.complete(ConnectionRequestResults.SUCCESSFUL);
-                    serverConn.getConnection().setSessionHandler(new BackendPlaySessionHandler(server, serverConn));
+                    smc.setSessionHandler(new BackendPlaySessionHandler(server, serverConn));
                     serverConn.getPlayer().setConnectedServer(serverConn);
-                    serverConn.getConnection().getChannel().config().setAutoRead(true);
-                }, serverConn.getConnection().eventLoop());
+                    smc.getChannel().config().setAutoRead(true);
+                }, smc.eventLoop());
         return true;
     }
 

proxy/src/main/java/com/velocitypowered/proxy/connection/backend/VelocityServerConnection.java

@@ -1,6 +1,8 @@
 package com.velocitypowered.proxy.connection.backend;
 
 import com.google.common.base.Preconditions;
+import com.google.common.base.Verify;
+import com.google.common.base.VerifyException;
 import com.velocitypowered.api.proxy.ConnectionRequestBuilder;
 import com.velocitypowered.api.proxy.ServerConnection;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
@@ -24,10 +26,12 @@ import io.netty.channel.Channel;
 import io.netty.channel.ChannelFutureListener;
 import io.netty.channel.ChannelInitializer;
 import io.netty.handler.timeout.ReadTimeoutHandler;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.TimeUnit;
 
+import static com.google.common.base.Verify.verify;
 import static com.velocitypowered.proxy.VelocityServer.GSON;
 import static com.velocitypowered.proxy.network.Connections.*;
 
@@ -35,7 +39,7 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     private final VelocityRegisteredServer registeredServer;
     private final ConnectedPlayer proxyPlayer;
     private final VelocityServer server;
-    private MinecraftConnection connection;
+    private @Nullable MinecraftConnection connection;
     private boolean legacyForge = false;
     private boolean hasCompletedJoin = false;
     private boolean gracefulDisconnect = false;
@@ -72,6 +76,11 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
                     if (future.isSuccess()) {
                         connection = future.channel().pipeline().get(MinecraftConnection.class);
 
+                        // This is guaranteed not to be null, but Checker Framework is whining about it anyway
+                        if (connection == null) {
+                            throw new VerifyException("MinecraftConnection not injected into pipeline");
+                        }
+
                         // Kick off the connection process
                         connection.setSessionHandler(new LoginSessionHandler(server, VelocityServerConnection.this, result));
                         startHandshake();
@@ -93,6 +102,11 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     }
 
     private void startHandshake() {
+        MinecraftConnection mc = connection;
+        if (mc == null) {
+            throw new IllegalStateException("No connection established!");
+        }
+
         PlayerInfoForwarding forwardingMode = server.getConfiguration().getPlayerInfoForwardingMode();
 
         // Initiate a handshake.
@@ -107,17 +121,15 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
             handshake.setServerAddress(registeredServer.getServerInfo().getAddress().getHostString());
         }
         handshake.setPort(registeredServer.getServerInfo().getAddress().getPort());
-        connection.write(handshake);
+        mc.write(handshake);
 
         int protocolVersion = proxyPlayer.getConnection().getNextProtocolVersion();
-        connection.setProtocolVersion(protocolVersion);
-        connection.setState(StateRegistry.LOGIN);
-
-        ServerLogin login = new ServerLogin();
-        login.setUsername(proxyPlayer.getUsername());
-        connection.write(login);
+        mc.setProtocolVersion(protocolVersion);
+        mc.setState(StateRegistry.LOGIN);
+        mc.write(new ServerLogin(proxyPlayer.getUsername()));
     }
 
+    @Nullable
     public MinecraftConnection getConnection() {
         return connection;
     }
@@ -154,10 +166,16 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     public boolean sendPluginMessage(ChannelIdentifier identifier, byte[] data) {
         Preconditions.checkNotNull(identifier, "identifier");
         Preconditions.checkNotNull(data, "data");
+
+        MinecraftConnection mc = connection;
+        if (mc == null) {
+            throw new IllegalStateException("Not connected to a server!");
+        }
+
         PluginMessage message = new PluginMessage();
         message.setChannel(identifier.getId());
         message.setData(data);
-        connection.write(message);
+        mc.write(message);
         return true;
     }
 

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientPlaySessionHandler.java

@@ -1,10 +1,12 @@
 package com.velocitypowered.proxy.connection.client;
 
+import com.google.common.base.Preconditions;
 import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.event.player.PlayerChatEvent;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.api.util.ModInfo;
 import com.velocitypowered.proxy.VelocityServer;
+import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.connection.backend.VelocityServerConnection;
 import com.velocitypowered.proxy.connection.forge.ForgeConstants;
@@ -19,6 +21,7 @@ import net.kyori.text.TextComponent;
 import net.kyori.text.format.TextColor;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.*;
 
@@ -36,7 +39,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     private final Set<String> clientPluginMsgChannels = new HashSet<>();
     private final Queue<PluginMessage> loginPluginMessages = new ArrayDeque<>();
     private final VelocityServer server;
-    private TabCompleteRequest outstandingTabComplete;
+    private @Nullable TabCompleteRequest outstandingTabComplete;
 
     public ClientPlaySessionHandler(VelocityServer server, ConnectedPlayer player) {
         this.player = player;
@@ -53,8 +56,13 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     public boolean handle(KeepAlive packet) {
         VelocityServerConnection serverConnection = player.getConnectedServer();
         if (serverConnection != null && packet.getRandomId() == serverConnection.getLastPingId()) {
+            MinecraftConnection smc = serverConnection.getConnection();
+            if (smc == null) {
+                // eat the packet
+                return true;
+            }
             player.setPing(System.currentTimeMillis() - serverConnection.getLastPingSent());
-            serverConnection.getConnection().write(packet);
+            smc.write(packet);
             serverConnection.resetLastPingId();
         }
         return true;
@@ -84,15 +92,23 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             if (serverConnection == null) {
                 return true;
             }
+            MinecraftConnection smc = serverConnection.getConnection();
+            if (smc == null) {
+                return true;
+            }
             PlayerChatEvent event = new PlayerChatEvent(player, msg);
             server.getEventManager().fire(event)
                     .thenAcceptAsync(pme -> {
-                        if (pme.getResult().equals(PlayerChatEvent.ChatResult.allowed())){
-                            serverConnection.getConnection().write(packet);
-                        } else if (pme.getResult().isAllowed() && pme.getResult().getMessage().isPresent()){
-                            serverConnection.getConnection().write(Chat.createServerbound(pme.getResult().getMessage().get()));
+                        PlayerChatEvent.ChatResult chatResult = pme.getResult();
+                        if (chatResult.isAllowed()) {
+                            Optional<String> eventMsg = pme.getResult().getMessage();
+                            if (eventMsg.isPresent()) {
+                                smc.write(Chat.createServerbound(eventMsg.get()));
+                            } else {
+                                smc.write(packet);
+                            }
                         }
-                    }, serverConnection.getConnection().eventLoop());
+                    }, smc.eventLoop());
         }
         return true;
     }
@@ -105,10 +121,10 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             if (spacePos > 0) {
                 String cmd = packet.getCommand().substring(1, spacePos);
                 if (server.getCommandManager().hasCommand(cmd)) {
-                    Optional<List<String>> suggestions = server.getCommandManager().offerSuggestions(player, packet.getCommand().substring(1));
-                    if (suggestions.isPresent()) {
+                    List<String> suggestions = server.getCommandManager().offerSuggestions(player, packet.getCommand().substring(1));
+                    if (suggestions.size() > 0) {
                         TabCompleteResponse resp = new TabCompleteResponse();
-                        resp.getOffers().addAll(suggestions.get());
+                        resp.getOffers().addAll(suggestions);
                         player.getConnection().write(resp);
                         return true;
                     }
@@ -121,58 +137,68 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
 
     @Override
     public boolean handle(PluginMessage packet) {
-        if (PluginMessageUtil.isMCRegister(packet)) {
-            List<String> actuallyRegistered = new ArrayList<>();
-            List<String> channels = PluginMessageUtil.getChannels(packet);
-            for (String channel : channels) {
-                if (clientPluginMsgChannels.size() >= MAX_PLUGIN_CHANNELS &&
-                        !clientPluginMsgChannels.contains(channel)) {
-                    throw new IllegalStateException("Too many plugin message channels registered");
+        VelocityServerConnection serverConn = player.getConnectedServer();
+        MinecraftConnection backendConn = serverConn != null ? serverConn.getConnection() : null;
+        if (serverConn != null && backendConn != null) {
+            if (PluginMessageUtil.isMCRegister(packet)) {
+                List<String> actuallyRegistered = new ArrayList<>();
+                List<String> channels = PluginMessageUtil.getChannels(packet);
+                for (String channel : channels) {
+                    if (clientPluginMsgChannels.size() >= MAX_PLUGIN_CHANNELS &&
+                            !clientPluginMsgChannels.contains(channel)) {
+                        throw new IllegalStateException("Too many plugin message channels registered");
+                    }
+                    if (clientPluginMsgChannels.add(channel)) {
+                        actuallyRegistered.add(channel);
+                    }
                 }
-                if (clientPluginMsgChannels.add(channel)) {
-                    actuallyRegistered.add(channel);
-                }
-            }
 
-            if (actuallyRegistered.size() > 0) {
-                PluginMessage newRegisterPacket = PluginMessageUtil.constructChannelsPacket(player.getConnectedServer()
-                        .getConnection().getProtocolVersion(), actuallyRegistered);
-                player.getConnectedServer().getConnection().write(newRegisterPacket);
-            }
-        } else if (PluginMessageUtil.isMCUnregister(packet)) {
-            List<String> channels = PluginMessageUtil.getChannels(packet);
-            clientPluginMsgChannels.removeAll(channels);
-            player.getConnectedServer().getConnection().write(packet);
-        } else if (PluginMessageUtil.isMCBrand(packet)) {
-            player.getConnectedServer().getConnection().write(PluginMessageUtil.rewriteMCBrand(packet));
-        } else if (player.getConnectedServer().isLegacyForge() && !player.getConnectedServer().hasCompletedJoin()) {
-            if (packet.getChannel().equals(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
-                if (!player.getModInfo().isPresent()) {
-                    ForgeUtil.readModList(packet).ifPresent(mods -> player.setModInfo(new ModInfo("FML", mods)));
+                if (actuallyRegistered.size() > 0) {
+                    PluginMessage newRegisterPacket = PluginMessageUtil.constructChannelsPacket(backendConn
+                            .getProtocolVersion(), actuallyRegistered);
+                    backendConn.write(newRegisterPacket);
                 }
-                
-                // Always forward the FML handshake to the remote server.
-                player.getConnectedServer().getConnection().write(packet);
+                return true;
+            } else if (PluginMessageUtil.isMCUnregister(packet)) {
+                List<String> channels = PluginMessageUtil.getChannels(packet);
+                clientPluginMsgChannels.removeAll(channels);
+                backendConn.write(packet);
+                return true;
+            } else if (PluginMessageUtil.isMCBrand(packet)) {
+                backendConn.write(PluginMessageUtil.rewriteMCBrand(packet));
+                return true;
+            } else if (backendConn.isLegacyForge() && !serverConn.hasCompletedJoin()) {
+                if (packet.getChannel().equals(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL)) {
+                    if (!player.getModInfo().isPresent()) {
+                        List<ModInfo.Mod> mods = ForgeUtil.readModList(packet);
+                        if (!mods.isEmpty()) {
+                            player.setModInfo(new ModInfo("FML", mods));
+                        }
+                    }
+
+                    // Always forward the FML handshake to the remote server.
+                    backendConn.write(packet);
+                } else {
+                    // The client is trying to send messages too early. This is primarily caused by mods, but it's further
+                    // aggravated by Velocity. To work around these issues, we will queue any non-FML handshake messages to
+                    // be sent once the JoinGame packet has been received by the proxy.
+                    loginPluginMessages.add(packet);
+                }
+                return true;
             } else {
-                // The client is trying to send messages too early. This is primarily caused by mods, but it's further
-                // aggravated by Velocity. To work around these issues, we will queue any non-FML handshake messages to
-                // be sent once the JoinGame packet has been received by the proxy.
-                loginPluginMessages.add(packet);
-            }
-        } else {
-            ChannelIdentifier id = server.getChannelRegistrar().getFromId(packet.getChannel());
-            if (id == null) {
-                player.getConnectedServer().getConnection().write(packet);
-            } else {
-                PluginMessageEvent event = new PluginMessageEvent(player, player.getConnectedServer(), id, packet.getData());
-                server.getEventManager().fire(event)
-                        .thenAcceptAsync(pme -> {
-                            if (pme.getResult().isAllowed()) {
-                                player.getConnectedServer().getConnection().write(packet);
-                            }
-                        }, player.getConnectedServer().getConnection().eventLoop());
+                ChannelIdentifier id = server.getChannelRegistrar().getFromId(packet.getChannel());
+                if (id == null) {
+                    backendConn.write(packet);
+                } else {
+                    PluginMessageEvent event = new PluginMessageEvent(player, serverConn, id, packet.getData());
+                    server.getEventManager().fire(event)
+                            .thenAcceptAsync(pme -> {
+                                backendConn.write(packet);
+                            }, backendConn.eventLoop());
+                }
             }
         }
+
         return true;
     }
 
@@ -184,9 +210,9 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             return;
         }
 
-        // If we don't want to handle this packet, just forward it on.
-        if (serverConnection.hasCompletedJoin()) {
-            serverConnection.getConnection().write(packet);
+        MinecraftConnection smc = serverConnection.getConnection();
+        if (smc != null && serverConnection.hasCompletedJoin()) {
+            smc.write(packet);
         }
     }
 
@@ -198,8 +224,9 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             return;
         }
 
-        if (serverConnection.hasCompletedJoin()) {
-            serverConnection.getConnection().write(buf.retain());
+        MinecraftConnection smc = serverConnection.getConnection();
+        if (smc != null && serverConnection.hasCompletedJoin()) {
+            smc.write(buf.retain());
         }
     }
 
@@ -222,11 +249,23 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         VelocityServerConnection server = player.getConnectedServer();
         if (server != null) {
             boolean writable = player.getConnection().getChannel().isWritable();
-            server.getConnection().getChannel().config().setAutoRead(writable);
+            MinecraftConnection smc = server.getConnection();
+            if (smc != null) {
+                smc.getChannel().config().setAutoRead(writable);
+            }
         }
     }
 
     public void handleBackendJoinGame(JoinGame joinGame) {
+        VelocityServerConnection serverConn = player.getConnectedServer();
+        if (serverConn == null) {
+            throw new IllegalStateException("No server connection for " + player + ", but JoinGame packet received");
+        }
+        MinecraftConnection serverMc = serverConn.getConnection();
+        if (serverMc == null) {
+            throw new IllegalStateException("Server connection for " + player + " is disconnected, but JoinGame packet received");
+        }
+
         if (!spawned) {
             // Nothing special to do with regards to spawning the player
             spawned = true;
@@ -272,7 +311,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
         serverBossBars.clear();
 
         // Tell the server about this client's plugin message channels.
-        int serverVersion = player.getConnectedServer().getConnection().getProtocolVersion();
+        int serverVersion = serverMc.getProtocolVersion();
         Collection<String> toRegister = new HashSet<>(clientPluginMsgChannels);
         if (serverVersion >= ProtocolConstants.MINECRAFT_1_13) {
             toRegister.addAll(server.getChannelRegistrar().getModernChannelIds());
@@ -280,14 +319,13 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             toRegister.addAll(server.getChannelRegistrar().getIdsForLegacyConnections());
         }
         if (!toRegister.isEmpty()) {
-            player.getConnectedServer().getConnection().delayedWrite(PluginMessageUtil.constructChannelsPacket(
-                    serverVersion, toRegister));
+            serverMc.delayedWrite(PluginMessageUtil.constructChannelsPacket(serverVersion, toRegister));
         }
 
         // If we had plugin messages queued during login/FML handshake, send them now.
         PluginMessage pm;
         while ((pm = loginPluginMessages.poll()) != null) {
-            player.getConnectedServer().getConnection().delayedWrite(pm);
+            serverMc.delayedWrite(pm);
         }
 
         // Clear any title from the previous server.
@@ -295,9 +333,9 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
 
         // Flush everything
         player.getConnection().flush();
-        player.getConnectedServer().getConnection().flush();
-        player.getConnectedServer().setHasCompletedJoin(true);
-        if (player.getConnectedServer().isLegacyForge()) {
+        serverMc.flush();
+        serverConn.setHasCompletedJoin(true);
+        if (serverConn.isLegacyForge()) {
             // We only need to indicate we can send a reset packet if we complete a handshake, that is,
             // logged onto a Forge server.
             //
@@ -326,8 +364,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
             if (!outstandingTabComplete.isAssumeCommand()) {
                 String command = outstandingTabComplete.getCommand().substring(1);
                 try {
-                    Optional<List<String>> offers = server.getCommandManager().offerSuggestions(player, command);
-                    offers.ifPresent(strings -> response.getOffers().addAll(strings));
+                    response.getOffers().addAll(server.getCommandManager().offerSuggestions(player, command));
                 } catch (Exception e) {
                     logger.error("Unable to provide tab list completions for {} for command '{}'", player.getUsername(),
                             command, e);

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ClientSettingsWrapper.java

@@ -3,6 +3,7 @@ package com.velocitypowered.proxy.connection.client;
 import com.velocitypowered.api.proxy.player.PlayerSettings;
 import com.velocitypowered.api.proxy.player.SkinParts;
 import com.velocitypowered.proxy.protocol.packet.ClientSettings;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Locale;
 
@@ -11,7 +12,7 @@ public class ClientSettingsWrapper implements PlayerSettings {
     
     private final ClientSettings settings;
     private final SkinParts parts;
-    private Locale locale = null;
+    private @Nullable Locale locale;
 
     ClientSettingsWrapper(ClientSettings settings) {
         this.settings = settings;

proxy/src/main/java/com/velocitypowered/proxy/connection/client/ConnectedPlayer.java

@@ -63,27 +63,28 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     private static final Logger logger = LogManager.getLogger(ConnectedPlayer.class);
 
     private final MinecraftConnection connection;
-    private final InetSocketAddress virtualHost;
+    private @Nullable final InetSocketAddress virtualHost;
     private GameProfile profile;
-    private PermissionFunction permissionFunction = null;
+    private PermissionFunction permissionFunction;
     private int tryIndex = 0;
     private long ping = -1;
-    private VelocityServerConnection connectedServer;
-    private VelocityServerConnection connectionInFlight;
-    private PlayerSettings settings;
-    private ModInfo modInfo;
+    private @Nullable VelocityServerConnection connectedServer;
+    private @Nullable VelocityServerConnection connectionInFlight;
+    private @Nullable PlayerSettings settings;
+    private @Nullable ModInfo modInfo;
     private final VelocityTabList tabList;
     private final VelocityServer server;
 
     @MonotonicNonNull
     private List<String> serversToTry = null;
-    
-    ConnectedPlayer(VelocityServer server, GameProfile profile, MinecraftConnection connection, InetSocketAddress virtualHost) {
+
+    ConnectedPlayer(VelocityServer server, GameProfile profile, MinecraftConnection connection, @Nullable InetSocketAddress virtualHost) {
         this.server = server;
         this.tabList = new VelocityTabList(connection);
         this.profile = profile;
         this.connection = connection;
         this.virtualHost = virtualHost;
+        this.permissionFunction = (permission) -> Tristate.UNDEFINED;
     }
 
     @Override
@@ -123,8 +124,9 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     }
 
     void setPlayerSettings(ClientSettings settings) {
-        this.settings = new ClientSettingsWrapper(settings);
-        server.getEventManager().fireAndForget(new PlayerSettingsChangedEvent(this, this.settings));
+        ClientSettingsWrapper cs = new ClientSettingsWrapper(settings);
+        this.settings = cs;
+        server.getEventManager().fireAndForget(new PlayerSettingsChangedEvent(this, cs));
     }
     
     public Optional<ModInfo> getModInfo() {
@@ -133,7 +135,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     
     void setModInfo(ModInfo modInfo) {
         this.modInfo = modInfo;
-        server.getEventManager().fireAndForget(new PlayerModInfoEvent(this, this.modInfo));
+        server.getEventManager().fireAndForget(new PlayerModInfoEvent(this, modInfo));
     }
     
     @Override
@@ -161,7 +163,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     }
 
     @Override
-    public void sendMessage(@NonNull Component component, @NonNull MessagePosition position) {
+    public void sendMessage(Component component, MessagePosition position) {
         Preconditions.checkNotNull(component, "component");
         Preconditions.checkNotNull(position, "position");
 
@@ -193,7 +195,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     }
 
     @Override
-    public ConnectionRequestBuilder createConnectionRequest(@NonNull RegisteredServer server) {
+    public ConnectionRequestBuilder createConnectionRequest(RegisteredServer server) {
         return new ConnectionRequestBuilderImpl(server);
     }
     
@@ -243,16 +245,19 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
                 connection.delayedWrite(TitlePacket.resetForProtocolVersion(connection.getProtocolVersion()));
             }
 
-            if (tt.getTitle().isPresent()) {
+            Optional<Component> titleText = tt.getTitle();
+            if (titleText.isPresent()) {
                 TitlePacket titlePkt = new TitlePacket();
                 titlePkt.setAction(TitlePacket.SET_TITLE);
-                titlePkt.setComponent(ComponentSerializers.JSON.serialize(tt.getTitle().get()));
+                titlePkt.setComponent(ComponentSerializers.JSON.serialize(titleText.get()));
                 connection.delayedWrite(titlePkt);
             }
-            if (tt.getSubtitle().isPresent()) {
+
+            Optional<Component> subtitleText = tt.getSubtitle();
+            if (subtitleText.isPresent()) {
                 TitlePacket titlePkt = new TitlePacket();
                 titlePkt.setAction(TitlePacket.SET_SUBTITLE);
-                titlePkt.setComponent(ComponentSerializers.JSON.serialize(tt.getSubtitle().get()));
+                titlePkt.setComponent(ComponentSerializers.JSON.serialize(subtitleText.get()));
                 connection.delayedWrite(titlePkt);
             }
 
@@ -270,16 +275,23 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
 
     }
 
+    @Nullable
     public VelocityServerConnection getConnectedServer() {
         return connectedServer;
     }
 
     public void handleConnectionException(RegisteredServer server, Throwable throwable) {
-        Throwable wrapped = throwable;
-        if (throwable instanceof CompletionException) {
-            wrapped = throwable.getCause();
+        if (throwable == null) {
+            throw new NullPointerException("throwable");
         }
 
+        Throwable wrapped = throwable;
+        if (throwable instanceof CompletionException) {
+            Throwable cause = throwable.getCause();
+            if (cause != null) {
+                wrapped = cause;
+            }
+        }
         String error = ThrowableUtils.briefDescription(wrapped);
         String userMessage;
         if (connectedServer != null && connectedServer.getServerInfo().equals(server.getServerInfo())) {
@@ -366,7 +378,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
 
         String toTryName = serversToTry.get(tryIndex);
         tryIndex++;
-        return server.getServers().getServer(toTryName);
+        return server.getServer(toTryName);
     }
 
     private Optional<ConnectionRequestBuilder.Status> checkServer(RegisteredServer server) {
@@ -390,13 +402,14 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         ServerPreConnectEvent event = new ServerPreConnectEvent(this, request.getServer());
         return server.getEventManager().fire(event)
                 .thenCompose((newEvent) -> {
-                    if (!newEvent.getResult().isAllowed()) {
+                    Optional<RegisteredServer> connectTo = newEvent.getResult().getServer();
+                    if (!connectTo.isPresent()) {
                         return CompletableFuture.completedFuture(
                                 ConnectionRequestResults.plainResult(ConnectionRequestBuilder.Status.CONNECTION_CANCELLED)
                         );
                     }
 
-                    RegisteredServer rs = newEvent.getResult().getServer().get();
+                    RegisteredServer rs = connectTo.get();
                     Optional<ConnectionRequestBuilder.Status> lastCheck = checkServer(rs);
                     if (lastCheck.isPresent()) {
                         return CompletableFuture.completedFuture(ConnectionRequestResults.plainResult(lastCheck.get()));
@@ -406,9 +419,13 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     }
 
     public void setConnectedServer(VelocityServerConnection serverConnection) {
-        if (this.connectedServer != null && !serverConnection.getServerInfo().equals(connectedServer.getServerInfo())) {
+        VelocityServerConnection oldConnection = this.connectedServer;
+        if (oldConnection != null && !serverConnection.getServerInfo().equals(oldConnection.getServerInfo())) {
             this.tryIndex = 0;
         }
+        if (serverConnection == connectionInFlight) {
+            connectionInFlight = null;
+        }
         this.connectedServer = serverConnection;
     }
 
@@ -426,6 +443,20 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
         connection.closeWith(Disconnect.create(reason));
     }
 
+    private MinecraftConnection ensureBackendConnection() {
+        VelocityServerConnection sc = this.connectedServer;
+        if (sc == null) {
+            throw new IllegalStateException("No backend connection");
+        }
+
+        MinecraftConnection mc = sc.getConnection();
+        if (mc == null) {
+            throw new IllegalStateException("Backend connection is not connected to a server");
+        }
+
+        return mc;
+    }
+
     void teardown() {
         if (connectionInFlight != null) {
             connectionInFlight.disconnect();
@@ -439,11 +470,11 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
 
     @Override
     public String toString() {
-        return "[connected player] " + getProfile().getName() + " (" + getRemoteAddress() + ")";
+        return "[connected player] " + profile.getName() + " (" + getRemoteAddress() + ")";
     }
 
     @Override
-    public @NonNull Tristate getPermissionValue(@NonNull String permission) {
+    public Tristate getPermissionValue(String permission) {
         return permissionFunction.getPermissionValue(permission);
     }
 
@@ -461,7 +492,7 @@ public class ConnectedPlayer implements MinecraftConnectionAssociation, Player {
     @Override
     public void spoofChatInput(String input) {
         Preconditions.checkArgument(input.length() <= Chat.MAX_SERVERBOUND_MESSAGE_LENGTH, "input cannot be greater than " + Chat.MAX_SERVERBOUND_MESSAGE_LENGTH + " characters in length");
-        connectedServer.getConnection().write(Chat.createServerbound(input));
+        ensureBackendConnection().write(Chat.createServerbound(input));
     }
 
     private class ConnectionRequestBuilderImpl implements ConnectionRequestBuilder {

proxy/src/main/java/com/velocitypowered/proxy/connection/client/LoginSessionHandler.java

@@ -22,10 +22,12 @@ import com.velocitypowered.proxy.util.EncryptionUtils;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufUtil;
 import io.netty.buffer.Unpooled;
+import net.kyori.text.Component;
 import net.kyori.text.TextComponent;
 import net.kyori.text.format.TextColor;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
 import java.net.InetSocketAddress;
 import java.net.MalformedURLException;
@@ -38,6 +40,8 @@ import java.util.List;
 import java.util.Optional;
 import java.util.concurrent.ThreadLocalRandom;
 
+import static com.velocitypowered.proxy.connection.VelocityConstants.EMPTY_BYTE_ARRAY;
+
 public class LoginSessionHandler implements MinecraftSessionHandler {
 
     private static final Logger logger = LogManager.getLogger(LoginSessionHandler.class);
@@ -47,10 +51,10 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     private final VelocityServer server;
     private final MinecraftConnection inbound;
     private final InboundConnection apiInbound;
-    private ServerLogin login;
-    private byte[] verify;
+    private @MonotonicNonNull ServerLogin login;
+    private byte[] verify = EMPTY_BYTE_ARRAY;
     private int playerInfoId;
-    private ConnectedPlayer connectedPlayer;
+    private @MonotonicNonNull ConnectedPlayer connectedPlayer;
 
     public LoginSessionHandler(VelocityServer server, MinecraftConnection inbound, InboundConnection apiInbound) {
         this.server = Preconditions.checkNotNull(server, "server");
@@ -62,12 +66,9 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     public boolean handle(ServerLogin packet) {
         this.login = packet;
         if (inbound.getProtocolVersion() >= ProtocolConstants.MINECRAFT_1_13) {
-            LoginPluginMessage message = new LoginPluginMessage();
             playerInfoId = ThreadLocalRandom.current().nextInt();
-            message.setId(playerInfoId);
-            message.setChannel(VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL);
-            message.setData(Unpooled.EMPTY_BUFFER);
-            inbound.write(message);
+            inbound.write(new LoginPluginMessage(playerInfoId, VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL,
+                    Unpooled.EMPTY_BUFFER));
         } else {
             beginPreLogin();
         }
@@ -92,6 +93,15 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
     @Override
     public boolean handle(EncryptionResponse packet) {
+        ServerLogin login = this.login;
+        if (login == null) {
+            throw new IllegalStateException("No ServerLogin packet received yet.");
+        }
+
+        if (verify.length == 0) {
+            throw new IllegalStateException("No EncryptionRequest packet sent yet.");
+        }
+
         try {
             KeyPair serverKeyPair = server.getServerKeyPair();
             byte[] decryptedVerifyToken = EncryptionUtils.decryptRsa(serverKeyPair, packet.getVerifyToken());
@@ -149,6 +159,10 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     }
 
     private void beginPreLogin() {
+        ServerLogin login = this.login;
+        if (login == null) {
+            throw new IllegalStateException("No ServerLogin packet received yet.");
+        }
         PreLoginEvent event = new PreLoginEvent(apiInbound, login.getUsername());
         server.getEventManager().fire(event)
                 .thenRunAsync(() -> {
@@ -157,9 +171,10 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
                         return;
                     }
                     PreLoginComponentResult result = event.getResult();
-                    if (!result.isAllowed()) {
+                    Optional<Component> disconnectReason = result.getReason();
+                    if (disconnectReason.isPresent()) {
                         // The component is guaranteed to be provided if the connection was denied.
-                        inbound.closeWith(Disconnect.create(event.getResult().getReason().get()));
+                        inbound.closeWith(Disconnect.create(disconnectReason.get()));
                         return;
                     }
 
@@ -212,13 +227,13 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
                                 // The player was disconnected
                                 return;
                             }
-                            if (!event.getResult().isAllowed()) {
-                                // The component is guaranteed to be provided if the connection was denied.
-                                player.disconnect(event.getResult().getReason().get());
-                                return;
-                            }
 
-                            handleProxyLogin(player);
+                            Optional<Component> reason = event.getResult().getReason();
+                            if (reason.isPresent()) {
+                                player.disconnect(reason.get());
+                            } else {
+                                handleProxyLogin(player);
+                            }
                         }, inbound.eventLoop());
         });
 

proxy/src/main/java/com/velocitypowered/proxy/connection/client/StatusSessionHandler.java

@@ -49,9 +49,7 @@ public class StatusSessionHandler implements MinecraftSessionHandler {
         ProxyPingEvent event = new ProxyPingEvent(inboundWrapper, initialPing);
         server.getEventManager().fire(event)
                 .thenRunAsync(() -> {
-                    StatusResponse response = new StatusResponse();
-                    response.setStatus(VelocityServer.GSON.toJson(event.getPing()));
-                    connection.write(response);
+                    connection.write(new StatusResponse(VelocityServer.GSON.toJson(event.getPing())));
                 }, connection.eventLoop());
         return true;
     }

proxy/src/main/java/com/velocitypowered/proxy/connection/forge/ForgeUtil.java

@@ -9,15 +9,13 @@ import io.netty.buffer.ByteBuf;
 import io.netty.buffer.Unpooled;
 
 import java.util.List;
-import java.util.Optional;
 
 public class ForgeUtil {
-
     private ForgeUtil() {
         throw new AssertionError();
     }
 
-    public static Optional<List<ModInfo.Mod>> readModList(PluginMessage message) {
+    public static List<ModInfo.Mod> readModList(PluginMessage message) {
         Preconditions.checkNotNull(message, "message");
         Preconditions.checkArgument(message.getChannel().equals(ForgeConstants.FORGE_LEGACY_HANDSHAKE_CHANNEL),
                 "message is not a FML HS plugin message");
@@ -36,10 +34,10 @@ public class ForgeUtil {
                     mods.add(new ModInfo.Mod(id, version));
                 }
 
-                return Optional.of(mods.build());
+                return mods.build();
             }
 
-            return Optional.empty();
+            return ImmutableList.of();
         } finally {
             byteBuf.release();
         }

proxy/src/main/java/com/velocitypowered/proxy/console/VelocityConsole.java

@@ -53,16 +53,14 @@ public final class VelocityConsole extends SimpleTerminalConsole implements Comm
                 .completer((reader, parsedLine, list) -> {
                     try {
                         boolean isCommand = parsedLine.line().indexOf(' ') == -1;
-                        Optional<List<String>> o = this.server.getCommandManager().offerSuggestions(this, parsedLine.line());
-                        o.ifPresent(offers -> {
-                            for (String offer : offers) {
-                                if (isCommand) {
-                                    list.add(new Candidate(offer.substring(1)));
-                                } else {
-                                    list.add(new Candidate(offer));
-                                }
+                        List<String> offers = this.server.getCommandManager().offerSuggestions(this, parsedLine.line());
+                        for (String offer : offers) {
+                            if (isCommand) {
+                                list.add(new Candidate(offer.substring(1)));
+                            } else {
+                                list.add(new Candidate(offer));
                             }
-                        });
+                        }
                     } catch (Exception e) {
                         logger.error("An error occurred while trying to perform tab completion.", e);
                     }

proxy/src/main/java/com/velocitypowered/proxy/network/ConnectionManager.java

@@ -12,6 +12,7 @@ import io.netty.channel.EventLoopGroup;
 import io.netty.channel.WriteBufferWaterMark;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.initialization.qual.UnderInitialization;
 
 import java.net.InetSocketAddress;
 import java.util.HashSet;
@@ -27,16 +28,15 @@ public final class ConnectionManager {
     private final VelocityServer server;
     public final ServerChannelInitializerHolder serverChannelInitializer;
 
-    public ConnectionManager(final VelocityServer server) {
+    public ConnectionManager(VelocityServer server) {
         this.server = server;
         this.transportType = TransportType.bestType();
         this.bossGroup = this.transportType.createEventLoopGroup(TransportType.Type.BOSS);
         this.workerGroup = this.transportType.createEventLoopGroup(TransportType.Type.WORKER);
         this.serverChannelInitializer = new ServerChannelInitializerHolder(new ServerChannelInitializer(this.server));
-        this.logChannelInformation();
     }
 
-    private void logChannelInformation() {
+    public void logChannelInformation() {
         LOGGER.info("Connections will use {} channels, {} compression, {} ciphers", this.transportType, Natives.compressor.getLoadedVariant(), Natives.cipher.getLoadedVariant());
     }
 

proxy/src/main/java/com/velocitypowered/proxy/network/http/NettyHttpClient.java

@@ -1,5 +1,6 @@
 package com.velocitypowered.proxy.network.http;
 
+import com.google.common.base.VerifyException;
 import com.velocitypowered.proxy.VelocityServer;
 import io.netty.bootstrap.Bootstrap;
 import io.netty.channel.Channel;
@@ -61,6 +62,9 @@ public class NettyHttpClient {
                 .addListener(future -> {
                     if (future.isSuccess()) {
                         Channel channel = (Channel) future.getNow();
+                        if (channel == null) {
+                            throw new VerifyException("Null channel retrieved from pool!");
+                        }
                         channel.pipeline().addLast("collector", new SimpleHttpResponseCollector(reply));
 
                         DefaultFullHttpRequest request = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, url.getPath() + "?" + url.getQuery());

proxy/src/main/java/com/velocitypowered/proxy/plugin/PluginClassLoader.java

@@ -16,10 +16,13 @@ public class PluginClassLoader extends URLClassLoader {
 
     public PluginClassLoader(URL[] urls) {
         super(urls);
+    }
+
+    public void addToClassloaders() {
         loaders.add(this);
     }
 
-    public void addPath(Path path) {
+    void addPath(Path path) {
         try {
             addURL(path.toUri().toURL());
         } catch (MalformedURLException e) {

proxy/src/main/java/com/velocitypowered/proxy/plugin/VelocityEventManager.java

@@ -12,7 +12,6 @@ import com.velocitypowered.api.plugin.PluginManager;
 import net.kyori.event.EventSubscriber;
 import net.kyori.event.PostResult;
 import net.kyori.event.SimpleEventBus;
-import net.kyori.event.method.EventExecutor;
 import net.kyori.event.method.MethodScanner;
 import net.kyori.event.method.MethodSubscriptionAdapter;
 import net.kyori.event.method.SimpleMethodSubscriptionAdapter;
@@ -20,12 +19,14 @@ import net.kyori.event.method.asm.ASMEventExecutorFactory;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.checkerframework.checker.nullness.qual.NonNull;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.lang.reflect.Method;
 import java.net.URL;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.IdentityHashMap;
+import java.util.Objects;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -38,20 +39,24 @@ public class VelocityEventManager implements EventManager {
             .synchronizedListMultimap(Multimaps.newListMultimap(new IdentityHashMap<>(), ArrayList::new));
     private final ListMultimap<Object, EventHandler<?>> registeredHandlersByPlugin = Multimaps
             .synchronizedListMultimap(Multimaps.newListMultimap(new IdentityHashMap<>(), ArrayList::new));
-    private final VelocityEventBus bus = new VelocityEventBus(
-            new ASMEventExecutorFactory<>(new PluginClassLoader(new URL[0])),
-            new VelocityMethodScanner());
+    private final SimpleEventBus<Object> bus;
+    private final MethodSubscriptionAdapter<Object> methodAdapter;
     private final ExecutorService service;
     private final PluginManager pluginManager;
 
     public VelocityEventManager(PluginManager pluginManager) {
+        PluginClassLoader cl = new PluginClassLoader(new URL[0]);
+        cl.addToClassloaders();
+        this.bus = new SimpleEventBus<>(Object.class);
+        this.methodAdapter = new SimpleMethodSubscriptionAdapter<>(bus, new ASMEventExecutorFactory<>(cl),
+                new VelocityMethodScanner());
         this.pluginManager = pluginManager;
         this.service = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors(), new ThreadFactoryBuilder()
                 .setNameFormat("Velocity Event Executor - #%d").setDaemon(true).build());
     }
 
     @Override
-    public void register(@NonNull Object plugin, @NonNull Object listener) {
+    public void register(Object plugin, Object listener) {
         Preconditions.checkNotNull(plugin, "plugin");
         Preconditions.checkNotNull(listener, "listener");
         Preconditions.checkArgument(pluginManager.fromInstance(plugin).isPresent(), "Specified plugin is not loaded");
@@ -59,11 +64,12 @@ public class VelocityEventManager implements EventManager {
             throw new IllegalArgumentException("Trying to register the plugin main instance. Velocity already takes care of this for you.");
         }
         registeredListenersByPlugin.put(plugin, listener);
-        bus.register(listener);
+        methodAdapter.register(listener);
     }
 
     @Override
-    public <E> void register(@NonNull Object plugin, @NonNull Class<E> eventClass, @NonNull PostOrder postOrder, @NonNull EventHandler<E> handler) {
+    @SuppressWarnings("type.argument.type.incompatible")
+    public <E> void register(Object plugin, Class<E> eventClass, PostOrder postOrder, EventHandler<E> handler) {
         Preconditions.checkNotNull(plugin, "plugin");
         Preconditions.checkNotNull(eventClass, "eventClass");
         Preconditions.checkNotNull(postOrder, "postOrder");
@@ -72,8 +78,10 @@ public class VelocityEventManager implements EventManager {
     }
 
     @Override
-    public <E> @NonNull CompletableFuture<E> fire(@NonNull E event) {
-        Preconditions.checkNotNull(event, "event");
+    public <E> CompletableFuture<E> fire(E event) {
+        if (event == null) {
+            throw new NullPointerException("event");
+        }
         if (!bus.hasSubscribers(event.getClass())) {
             // Optimization: nobody's listening.
             return CompletableFuture.completedFuture(event);
@@ -99,30 +107,30 @@ public class VelocityEventManager implements EventManager {
     }
 
     @Override
-    public void unregisterListeners(@NonNull Object plugin) {
+    public void unregisterListeners(Object plugin) {
         Preconditions.checkNotNull(plugin, "plugin");
         Preconditions.checkArgument(pluginManager.fromInstance(plugin).isPresent(), "Specified plugin is not loaded");
         Collection<Object> listeners = registeredListenersByPlugin.removeAll(plugin);
-        listeners.forEach(bus::unregister);
+        listeners.forEach(methodAdapter::unregister);
         Collection<EventHandler<?>> handlers = registeredHandlersByPlugin.removeAll(plugin);
-        handlers.forEach(bus::unregister);
+        handlers.forEach(handler -> bus.unregister(new KyoriToVelocityHandler<>(handler, PostOrder.LAST)));
     }
 
     @Override
-    public void unregisterListener(@NonNull Object plugin, @NonNull Object listener) {
+    public void unregisterListener(Object plugin, Object listener) {
         Preconditions.checkNotNull(plugin, "plugin");
         Preconditions.checkNotNull(listener, "listener");
         Preconditions.checkArgument(pluginManager.fromInstance(plugin).isPresent(), "Specified plugin is not loaded");
         registeredListenersByPlugin.remove(plugin, listener);
-        bus.unregister(listener);
+        methodAdapter.unregister(listener);
     }
 
     @Override
-    public <E> void unregister(@NonNull Object plugin, @NonNull EventHandler<E> handler) {
+    public <E> void unregister(Object plugin, EventHandler<E> handler) {
         Preconditions.checkNotNull(plugin, "plugin");
         Preconditions.checkNotNull(handler, "listener");
         registeredHandlersByPlugin.remove(plugin, handler);
-        bus.unregister(handler);
+        bus.unregister(new KyoriToVelocityHandler<>(handler, PostOrder.LAST));
     }
 
     public boolean shutdown() throws InterruptedException {
@@ -130,27 +138,6 @@ public class VelocityEventManager implements EventManager {
         return service.awaitTermination(10, TimeUnit.SECONDS);
     }
 
-    private static class VelocityEventBus extends SimpleEventBus<Object> {
-        private final MethodSubscriptionAdapter<Object> methodAdapter;
-
-        VelocityEventBus(EventExecutor.@NonNull Factory<Object, Object> factory, @NonNull MethodScanner<Object> methodScanner) {
-            super(Object.class);
-            this.methodAdapter = new SimpleMethodSubscriptionAdapter<>(this, factory, methodScanner);
-        }
-
-        void register(Object listener) {
-            this.methodAdapter.register(listener);
-        }
-
-        void unregister(Object listener) {
-            this.methodAdapter.unregister(listener);
-        }
-
-        void unregister(EventHandler<?> handler) {
-            this.unregister(s -> s instanceof KyoriToVelocityHandler && ((KyoriToVelocityHandler<?>) s).getHandler().equals(handler));
-        }
-    }
-
     private static class VelocityMethodScanner implements MethodScanner<Object> {
         @Override
         public boolean shouldRegister(@NonNull Object listener, @NonNull Method method) {
@@ -159,7 +146,11 @@ public class VelocityEventManager implements EventManager {
 
         @Override
         public int postOrder(@NonNull Object listener, @NonNull Method method) {
-            return method.getAnnotation(Subscribe.class).order().ordinal();
+            Subscribe annotation = method.getAnnotation(Subscribe.class);
+            if (annotation == null) {
+                throw new IllegalStateException("Trying to determine post order for listener without @Subscribe annotation");
+            }
+            return annotation.order().ordinal();
         }
 
         @Override
@@ -190,5 +181,18 @@ public class VelocityEventManager implements EventManager {
         public EventHandler<E> getHandler() {
             return handler;
         }
+
+        @Override
+        public boolean equals(@Nullable Object o) {
+            if (this == o) return true;
+            if (o == null || getClass() != o.getClass()) return false;
+            KyoriToVelocityHandler<?> that = (KyoriToVelocityHandler<?>) o;
+            return Objects.equals(handler, that.handler);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(handler);
+        }
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/plugin/VelocityPluginManager.java

@@ -31,12 +31,15 @@ public class VelocityPluginManager implements PluginManager {
         this.server = checkNotNull(server, "server");
     }
 
-    private void registerPlugin(@NonNull PluginContainer plugin) {
+    private void registerPlugin(PluginContainer plugin) {
         plugins.put(plugin.getDescription().getId(), plugin);
-        plugin.getInstance().ifPresent(instance -> pluginInstances.put(instance, plugin));
+        Optional<?> instance = plugin.getInstance();
+        if (instance.isPresent()) {
+            pluginInstances.put(instance.get(), plugin);
+        }
     }
 
-    public void loadPlugins(@NonNull Path directory) throws IOException {
+    public void loadPlugins(Path directory) throws IOException {
         checkNotNull(directory, "directory");
         checkArgument(Files.isDirectory(directory), "provided path isn't a directory");
 
@@ -86,7 +89,7 @@ public class VelocityPluginManager implements PluginManager {
     }
 
     @Override
-    public @NonNull Optional<PluginContainer> fromInstance(@NonNull Object instance) {
+    public Optional<PluginContainer> fromInstance(Object instance) {
         checkNotNull(instance, "instance");
 
         if (instance instanceof PluginContainer) {
@@ -97,23 +100,23 @@ public class VelocityPluginManager implements PluginManager {
     }
 
     @Override
-    public @NonNull Optional<PluginContainer> getPlugin(@NonNull String id) {
+    public Optional<PluginContainer> getPlugin(String id) {
         checkNotNull(id, "id");
         return Optional.ofNullable(plugins.get(id));
     }
 
     @Override
-    public @NonNull Collection<PluginContainer> getPlugins() {
+    public Collection<PluginContainer> getPlugins() {
         return Collections.unmodifiableCollection(plugins.values());
     }
 
     @Override
-    public boolean isLoaded(@NonNull String id) {
+    public boolean isLoaded(String id) {
         return plugins.containsKey(id);
     }
 
     @Override
-    public void addToClasspath(@NonNull Object plugin, @NonNull Path path) {
+    public void addToClasspath(Object plugin, Path path) {
         checkNotNull(plugin, "instance");
         checkNotNull(path, "path");
         checkArgument(pluginInstances.containsKey(plugin), "plugin is not loaded");

proxy/src/main/java/com/velocitypowered/proxy/plugin/loader/JavaPluginLoader.java

@@ -12,6 +12,7 @@ import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.plugin.PluginClassLoader;
 import com.velocitypowered.proxy.plugin.loader.java.JavaVelocityPluginDescription;
 import com.velocitypowered.proxy.plugin.loader.java.VelocityPluginModule;
+import org.checkerframework.checker.nullness.qual.NonNull;
 
 import java.io.BufferedInputStream;
 import java.io.InputStreamReader;
@@ -51,6 +52,7 @@ public class JavaPluginLoader implements PluginLoader {
         PluginClassLoader loader = new PluginClassLoader(
                 new URL[] {source.toUri().toURL() }
         );
+        loader.addToClassloaders();
 
         Class mainClass = loader.loadClass(pd.getMain());
         return createDescription(pd, source, mainClass);
@@ -72,6 +74,10 @@ public class JavaPluginLoader implements PluginLoader {
         Injector injector = Guice.createInjector(new VelocityPluginModule(server, javaDescription, baseDirectory));
         Object instance = injector.getInstance(javaDescription.getMainClass());
 
+        if (instance == null) {
+            throw new IllegalStateException("Got nothing from injector for plugin " + javaDescription.getId());
+        }
+
         return new VelocityPluginContainer(description, instance);
     }
 
@@ -93,10 +99,8 @@ public class JavaPluginLoader implements PluginLoader {
     private VelocityPluginDescription createDescription(SerializedPluginDescription description, Path source, Class mainClass) {
         Set<PluginDependency> dependencies = new HashSet<>();
 
-        if (description.getDependencies() != null) {
-            for (SerializedPluginDescription.Dependency dependency : description.getDependencies()) {
-                dependencies.add(toDependencyMeta(dependency));
-            }
+        for (SerializedPluginDescription.Dependency dependency : description.getDependencies()) {
+            dependencies.add(toDependencyMeta(dependency));
         }
 
         return new JavaVelocityPluginDescription(

proxy/src/main/java/com/velocitypowered/proxy/plugin/loader/VelocityPluginDescription.java

@@ -33,7 +33,7 @@ public class VelocityPluginDescription implements PluginDescription {
         this.description = Strings.emptyToNull(description);
         this.url = Strings.emptyToNull(url);
         this.authors = authors == null ? ImmutableList.of() : ImmutableList.copyOf(authors);
-        this.dependencies = Maps.uniqueIndex(dependencies, PluginDependency::getId);
+        this.dependencies = Maps.uniqueIndex(dependencies, d -> d == null ? null : d.getId());
         this.source = source;
     }
 

proxy/src/main/java/com/velocitypowered/proxy/plugin/util/PluginDependencyUtils.java

@@ -8,14 +8,15 @@ import com.google.common.graph.GraphBuilder;
 import com.google.common.graph.MutableGraph;
 import com.velocitypowered.api.plugin.PluginDescription;
 import com.velocitypowered.api.plugin.meta.PluginDependency;
+import org.checkerframework.checker.nullness.qual.NonNull;
 
 import java.util.*;
 
 public class PluginDependencyUtils {
-    public static List<PluginDescription> sortCandidates(List<PluginDescription> candidates) {
+    public static List<PluginDescription> sortCandidates(List<@NonNull PluginDescription> candidates) {
         // Create our graph, we're going to be using this for Kahn's algorithm.
         MutableGraph<PluginDescription> graph = GraphBuilder.directed().allowsSelfLoops(false).build();
-        Map<String, PluginDescription> candidateMap = Maps.uniqueIndex(candidates, PluginDescription::getId);
+        Map<String, PluginDescription> candidateMap = Maps.uniqueIndex(candidates, d -> d == null ? null : d.getId());
 
         // Add edges
         for (PluginDescription description : candidates) {
@@ -36,7 +37,7 @@ public class PluginDependencyUtils {
         // Actually run Kahn's algorithm
         List<PluginDescription> sorted = new ArrayList<>();
         while (!noEdges.isEmpty()) {
-            PluginDescription candidate = noEdges.poll();
+            PluginDescription candidate = noEdges.remove();
             sorted.add(candidate);
 
             for (PluginDescription node : ImmutableSet.copyOf(graph.adjacentNodes(candidate))) {

proxy/src/main/java/com/velocitypowered/proxy/protocol/ProtocolConstants.java

@@ -48,6 +48,10 @@ public enum ProtocolConstants { ;
 
     public enum Direction {
         SERVERBOUND,
-        CLIENTBOUND
+        CLIENTBOUND;
+
+        public StateRegistry.PacketRegistry.ProtocolVersion getProtocol(StateRegistry state, int protocolVersion) {
+            return (this == SERVERBOUND ? state.SERVERBOUND : state.CLIENTBOUND).getVersion(protocolVersion);
+        }
     }
 }

proxy/src/main/java/com/velocitypowered/proxy/protocol/StateRegistry.java

@@ -1,12 +1,12 @@
 package com.velocitypowered.proxy.protocol;
 
-import com.google.common.base.Strings;
 import com.google.common.primitives.ImmutableIntArray;
 import com.velocitypowered.proxy.protocol.packet.*;
 import io.netty.util.collection.IntObjectHashMap;
 import io.netty.util.collection.IntObjectMap;
 import it.unimi.dsi.fastutil.objects.Object2IntMap;
 import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Objects;
 import java.util.function.Supplier;
@@ -36,6 +36,9 @@ public enum StateRegistry {
     },
     PLAY {
         {
+            SERVERBOUND.fallback = false;
+            CLIENTBOUND.fallback = false;
+
             SERVERBOUND.register(TabCompleteRequest.class, TabCompleteRequest::new,
                     map(0x14, MINECRAFT_1_8, false),
                     map(0x01, MINECRAFT_1_9, false),
@@ -150,8 +153,8 @@ public enum StateRegistry {
 
     public static final int STATUS_ID = 1;
     public static final int LOGIN_ID = 2;
-    public final PacketRegistry CLIENTBOUND = new PacketRegistry(ProtocolConstants.Direction.CLIENTBOUND, this);
-    public final PacketRegistry SERVERBOUND = new PacketRegistry(ProtocolConstants.Direction.SERVERBOUND, this);
+    public final PacketRegistry CLIENTBOUND = new PacketRegistry(ProtocolConstants.Direction.CLIENTBOUND);
+    public final PacketRegistry SERVERBOUND = new PacketRegistry(ProtocolConstants.Direction.SERVERBOUND);
 
     public static class PacketRegistry {
         private static final IntObjectMap<ImmutableIntArray> LINKED_PROTOCOL_VERSIONS = new IntObjectHashMap<>();
@@ -165,19 +168,18 @@ public enum StateRegistry {
         }
 
         private final ProtocolConstants.Direction direction;
-        private final StateRegistry state;
         private final IntObjectMap<ProtocolVersion> versions = new IntObjectHashMap<>(16);
+        private boolean fallback = true;
 
-        public PacketRegistry(Direction direction, StateRegistry state) {
+        public PacketRegistry(Direction direction) {
             this.direction = direction;
-            this.state = state;
             ProtocolConstants.SUPPORTED_VERSIONS.forEach(version -> versions.put(version, new ProtocolVersion(version)));
         }
 
         public ProtocolVersion getVersion(final int version) {
             ProtocolVersion result = versions.get(version);
             if (result == null) {
-                if (state != PLAY) {
+                if (fallback) {
                     return getVersion(MINIMUM_GENERIC_VERSION);
                 }
                 throw new IllegalArgumentException("Could not find data for protocol version " + version);
@@ -224,7 +226,7 @@ public enum StateRegistry {
                 this.packetClassToId.defaultReturnValue(Integer.MIN_VALUE);
             }
 
-            public MinecraftPacket createPacket(final int id) {
+            public @Nullable MinecraftPacket createPacket(final int id) {
                 final Supplier<? extends MinecraftPacket> supplier = this.packetIdToSupplier.get(id);
                 if (supplier == null) {
                     return null;
@@ -242,23 +244,6 @@ public enum StateRegistry {
                 }
                 return id;
             }
-
-            @Override
-            public String toString() {
-                StringBuilder mappingAsString = new StringBuilder("{");
-                for (Object2IntMap.Entry<Class<? extends MinecraftPacket>> entry : packetClassToId.object2IntEntrySet()) {
-                    mappingAsString.append(entry.getKey().getSimpleName()).append(" -> ")
-                            .append("0x")
-                            .append(Strings.padStart(Integer.toHexString(entry.getIntValue()), 2, '0'))
-                            .append(", ");
-                }
-                mappingAsString.setLength(mappingAsString.length() - 2);
-                mappingAsString.append("}");
-                return "ProtocolVersion{" +
-                        "id=" + id +
-                        ", packetClassToId=" + mappingAsString.toString() +
-                        '}';
-            }
         }
     }
 
@@ -283,7 +268,7 @@ public enum StateRegistry {
         }
 
         @Override
-        public boolean equals(Object o) {
+        public boolean equals(@Nullable Object o) {
             if (this == o) return true;
             if (o == null || getClass() != o.getClass()) return false;
             PacketMapping that = (PacketMapping) o;

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftDecoder.java

@@ -13,14 +13,14 @@ import io.netty.handler.codec.MessageToMessageDecoder;
 import java.util.List;
 
 public class MinecraftDecoder extends MessageToMessageDecoder<ByteBuf> {
-    private StateRegistry state;
     private final ProtocolConstants.Direction direction;
+    private StateRegistry state;
     private StateRegistry.PacketRegistry.ProtocolVersion protocolVersion;
 
     public MinecraftDecoder(ProtocolConstants.Direction direction) {
-        this.state = StateRegistry.HANDSHAKE;
         this.direction = Preconditions.checkNotNull(direction, "direction");
-        this.setProtocolVersion(ProtocolConstants.MINIMUM_GENERIC_VERSION);
+        this.protocolVersion = direction.getProtocol(StateRegistry.HANDSHAKE, ProtocolConstants.MINIMUM_GENERIC_VERSION);
+        this.state = StateRegistry.HANDSHAKE;
     }
 
     @Override
@@ -51,24 +51,12 @@ public class MinecraftDecoder extends MessageToMessageDecoder<ByteBuf> {
         }
     }
 
-    public StateRegistry.PacketRegistry.ProtocolVersion getProtocolVersion() {
-        return protocolVersion;
-    }
-
     public void setProtocolVersion(int protocolVersion) {
-        this.protocolVersion = (this.direction == ProtocolConstants.Direction.CLIENTBOUND ? this.state.CLIENTBOUND : this.state.SERVERBOUND).getVersion(protocolVersion);
-    }
-
-    public StateRegistry getState() {
-        return state;
+        this.protocolVersion = direction.getProtocol(state, protocolVersion);
     }
 
     public void setState(StateRegistry state) {
         this.state = state;
         this.setProtocolVersion(protocolVersion.id);
     }
-
-    public ProtocolConstants.Direction getDirection() {
-        return direction;
-    }
 }

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftEncoder.java

@@ -10,14 +10,14 @@ import io.netty.channel.ChannelHandlerContext;
 import io.netty.handler.codec.MessageToByteEncoder;
 
 public class MinecraftEncoder extends MessageToByteEncoder<MinecraftPacket> {
-    private StateRegistry state;
     private final ProtocolConstants.Direction direction;
+    private StateRegistry state;
     private StateRegistry.PacketRegistry.ProtocolVersion protocolVersion;
 
     public MinecraftEncoder(ProtocolConstants.Direction direction) {
-        this.state = StateRegistry.HANDSHAKE;
         this.direction = Preconditions.checkNotNull(direction, "direction");
-        this.setProtocolVersion(ProtocolConstants.MINIMUM_GENERIC_VERSION);
+        this.protocolVersion = direction.getProtocol(StateRegistry.HANDSHAKE, ProtocolConstants.MINIMUM_GENERIC_VERSION);
+        this.state = StateRegistry.HANDSHAKE;
     }
 
     @Override
@@ -27,24 +27,12 @@ public class MinecraftEncoder extends MessageToByteEncoder<MinecraftPacket> {
         msg.encode(out, direction, protocolVersion.id);
     }
 
-    public StateRegistry.PacketRegistry.ProtocolVersion getProtocolVersion() {
-        return protocolVersion;
-    }
-
     public void setProtocolVersion(final int protocolVersion) {
-        this.protocolVersion = (this.direction == ProtocolConstants.Direction.CLIENTBOUND ? this.state.CLIENTBOUND : this.state.SERVERBOUND).getVersion(protocolVersion);
-    }
-
-    public StateRegistry getState() {
-        return state;
+        this.protocolVersion = direction.getProtocol(state, protocolVersion);
     }
 
     public void setState(StateRegistry state) {
         this.state = state;
         this.setProtocolVersion(protocolVersion.id);
     }
-
-    public ProtocolConstants.Direction getDirection() {
-        return direction;
-    }
 }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/BossBar.java

@@ -5,6 +5,7 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.UUID;
 
@@ -15,15 +16,18 @@ public class BossBar implements MinecraftPacket {
     public static final int UPDATE_NAME = 3;
     public static final int UPDATE_STYLE = 4;
     public static final int UPDATE_PROPERTIES = 5;
-    private UUID uuid;
+    private @Nullable UUID uuid;
     private int action;
-    private String name;
+    private @Nullable String name;
     private float percent;
     private int color;
     private int overlay;
     private short flags;
 
     public UUID getUuid() {
+        if (uuid == null) {
+            throw new IllegalStateException("No boss bar UUID specified");
+        }
         return uuid;
     }
 
@@ -39,7 +43,7 @@ public class BossBar implements MinecraftPacket {
         this.action = action;
     }
 
-    public String getName() {
+    public @Nullable String getName() {
         return name;
     }
 
@@ -124,10 +128,16 @@ public class BossBar implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (uuid == null) {
+            throw new IllegalStateException("No boss bar UUID specified");
+        }
         ProtocolUtils.writeUuid(buf, uuid);
         ProtocolUtils.writeVarInt(buf, action);
         switch (action) {
             case ADD:
+                if (name == null) {
+                    throw new IllegalStateException("No name specified!");
+                }
                 ProtocolUtils.writeString(buf, name);
                 buf.writeFloat(percent);
                 ProtocolUtils.writeVarInt(buf, color);
@@ -140,6 +150,9 @@ public class BossBar implements MinecraftPacket {
                 buf.writeFloat(percent);
                 break;
             case UPDATE_NAME:
+                if (name == null) {
+                    throw new IllegalStateException("No name specified!");
+                }
                 ProtocolUtils.writeString(buf, name);
                 break;
             case UPDATE_STYLE:

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Chat.java

@@ -8,12 +8,13 @@ import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
 import net.kyori.text.Component;
 import net.kyori.text.serializer.ComponentSerializers;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class Chat implements MinecraftPacket {
     public static final byte CHAT = (byte) 0;
     public static final int MAX_SERVERBOUND_MESSAGE_LENGTH = 256;
 
-    private String message;
+    private @Nullable String message;
     private byte type;
 
     public Chat() {
@@ -25,6 +26,9 @@ public class Chat implements MinecraftPacket {
     }
 
     public String getMessage() {
+        if (message == null) {
+            throw new IllegalStateException("Message is not specified");
+        }
         return message;
     }
 
@@ -58,6 +62,9 @@ public class Chat implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (message == null) {
+            throw new IllegalStateException("Message is not specified");
+        }
         ProtocolUtils.writeString(buf, message);
         if (direction == ProtocolConstants.Direction.CLIENTBOUND) {
             buf.writeByte(type);

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/ClientSettings.java

@@ -5,10 +5,11 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class ClientSettings implements MinecraftPacket {
 
-    private String locale;
+    private @Nullable String locale;
     private byte viewDistance;
     private int chatVisibility;
     private boolean chatColors;
@@ -28,6 +29,9 @@ public class ClientSettings implements MinecraftPacket {
     }
 
     public String getLocale() {
+        if (locale == null) {
+            throw new IllegalStateException("No locale specified");
+        }
         return locale;
     }
 
@@ -102,6 +106,9 @@ public class ClientSettings implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (locale == null) {
+            throw new IllegalStateException("No locale specified");
+        }
         ProtocolUtils.writeString(buf, locale);
         buf.writeByte(viewDistance);
         ProtocolUtils.writeVarInt(buf, chatVisibility);

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Disconnect.java

@@ -8,22 +8,26 @@ import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
 import net.kyori.text.Component;
 import net.kyori.text.serializer.ComponentSerializers;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class Disconnect implements MinecraftPacket {
-    private String reason;
+    private @Nullable String reason;
 
     public Disconnect() {
     }
 
     public Disconnect(String reason) {
-        this.reason = reason;
+        this.reason = Preconditions.checkNotNull(reason, "reason");
     }
 
     public String getReason() {
+        if (reason == null) {
+            throw new IllegalStateException("No reason specified");
+        }
         return reason;
     }
 
-    public void setReason(String reason) {
+    public void setReason(@Nullable String reason) {
         this.reason = reason;
     }
 
@@ -41,6 +45,9 @@ public class Disconnect implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (reason == null) {
+            throw new IllegalStateException("No reason specified.");
+        }
         ProtocolUtils.writeString(buf, reason);
     }
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/EncryptionRequest.java

@@ -8,10 +8,12 @@ import io.netty.buffer.ByteBuf;
 
 import java.util.Arrays;
 
+import static com.velocitypowered.proxy.connection.VelocityConstants.*;
+
 public class EncryptionRequest implements MinecraftPacket {
     private String serverId = "";
-    private byte[] publicKey;
-    private byte[] verifyToken;
+    private byte[] publicKey = EMPTY_BYTE_ARRAY;
+    private byte[] verifyToken = EMPTY_BYTE_ARRAY;
 
     public byte[] getPublicKey() {
         return publicKey;

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/EncryptionResponse.java

@@ -8,9 +8,11 @@ import io.netty.buffer.ByteBuf;
 
 import java.util.Arrays;
 
+import static com.velocitypowered.proxy.connection.VelocityConstants.*;
+
 public class EncryptionResponse implements MinecraftPacket {
-    private byte[] sharedSecret;
-    private byte[] verifyToken;
+    private byte[] sharedSecret = EMPTY_BYTE_ARRAY;
+    private byte[] verifyToken = EMPTY_BYTE_ARRAY;
 
     public byte[] getSharedSecret() {
         return sharedSecret;

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Handshake.java

@@ -8,7 +8,7 @@ import io.netty.buffer.ByteBuf;
 
 public class Handshake implements MinecraftPacket {
     private int protocolVersion;
-    private String serverAddress;
+    private String serverAddress = "";
     private int port;
     private int nextStatus;
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/HeaderAndFooter.java

@@ -1,5 +1,6 @@
 package com.velocitypowered.proxy.protocol.packet;
 
+import com.google.common.base.Preconditions;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants.Direction;
@@ -11,36 +12,29 @@ import net.kyori.text.serializer.ComponentSerializers;
 import static com.velocitypowered.proxy.protocol.ProtocolUtils.writeString;
 
 public class HeaderAndFooter implements MinecraftPacket {
-
-    private static final HeaderAndFooter RESET = new HeaderAndFooter("{\"translate\":\"\"}", "{\"translate\":\"\"}");
+    private static final String EMPTY_COMPONENT = "{\"translate\":\"\"}";
+    private static final HeaderAndFooter RESET = new HeaderAndFooter();
     
     private String header;
     private String footer;
 
     public HeaderAndFooter() {
+        this(EMPTY_COMPONENT, EMPTY_COMPONENT);
     }
 
     public HeaderAndFooter(String header, String footer) {
-        this.header = header;
-        this.footer = footer;
+        this.header = Preconditions.checkNotNull(header, "header");
+        this.footer = Preconditions.checkNotNull(footer, "footer");
     }
 
     public String getHeader() {
         return header;
     }
 
-    public void setHeader(String header) {
-        this.header = header;
-    }
-
     public String getFooter() {
         return footer;
     }
 
-    public void setFooter(String footer) {
-        this.footer = footer;
-    }
-
     @Override
     public void decode(ByteBuf buf, Direction direction, int protocolVersion) {
         throw new UnsupportedOperationException("Decode is not implemented");

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGame.java

@@ -5,6 +5,7 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class JoinGame implements MinecraftPacket {
     private int entityId;
@@ -12,7 +13,7 @@ public class JoinGame implements MinecraftPacket {
     private int dimension;
     private short difficulty;
     private short maxPlayers;
-    private String levelType;
+    private @Nullable String levelType;
     private boolean reducedDebugInfo;
 
     public int getEntityId() {
@@ -56,6 +57,9 @@ public class JoinGame implements MinecraftPacket {
     }
 
     public String getLevelType() {
+        if (levelType == null) {
+            throw new IllegalStateException("No level type specified.");
+        }
         return levelType;
     }
 
@@ -110,6 +114,9 @@ public class JoinGame implements MinecraftPacket {
         }
         buf.writeByte(difficulty);
         buf.writeByte(maxPlayers);
+        if (levelType == null) {
+            throw new IllegalStateException("No level type specified.");
+        }
         ProtocolUtils.writeString(buf, levelType);
         buf.writeBoolean(reducedDebugInfo);
     }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/LegacyPingResponse.java

@@ -6,14 +6,11 @@ import net.kyori.text.serializer.ComponentSerializers;
 
 public class LegacyPingResponse {
     private static final ServerPing.Players FAKE_PLAYERS = new ServerPing.Players(0, 0, ImmutableList.of());
-    private int protocolVersion;
-    private String serverVersion;
-    private String motd;
-    private int playersOnline;
-    private int playersMax;
-
-    public LegacyPingResponse() {
-    }
+    private final int protocolVersion;
+    private final String serverVersion;
+    private final String motd;
+    private final int playersOnline;
+    private final int playersMax;
 
     public LegacyPingResponse(int protocolVersion, String serverVersion, String motd, int playersOnline, int playersMax) {
         this.protocolVersion = protocolVersion;
@@ -27,42 +24,22 @@ public class LegacyPingResponse {
         return protocolVersion;
     }
 
-    public void setProtocolVersion(int protocolVersion) {
-        this.protocolVersion = protocolVersion;
-    }
-
     public String getServerVersion() {
         return serverVersion;
     }
 
-    public void setServerVersion(String serverVersion) {
-        this.serverVersion = serverVersion;
-    }
-
     public String getMotd() {
         return motd;
     }
 
-    public void setMotd(String motd) {
-        this.motd = motd;
-    }
-
     public int getPlayersOnline() {
         return playersOnline;
     }
 
-    public void setPlayersOnline(int playersOnline) {
-        this.playersOnline = playersOnline;
-    }
-
     public int getPlayersMax() {
         return playersMax;
     }
 
-    public void setPlayersMax(int playersMax) {
-        this.playersMax = playersMax;
-    }
-
     @Override
     public String toString() {
         return "LegacyPingResponse{" +

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/LoginPluginMessage.java

@@ -6,36 +6,38 @@ import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.Unpooled;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class LoginPluginMessage implements MinecraftPacket {
     private int id;
-    private String channel;
-    private ByteBuf data;
+    private @Nullable String channel;
+    private ByteBuf data = Unpooled.EMPTY_BUFFER;
+
+    public LoginPluginMessage() {
+
+    }
+
+    public LoginPluginMessage(int id, @Nullable String channel, ByteBuf data) {
+        this.id = id;
+        this.channel = channel;
+        this.data = data;
+    }
 
     public int getId() {
         return id;
     }
 
-    public void setId(int id) {
-        this.id = id;
-    }
-
     public String getChannel() {
+        if (channel == null) {
+            throw new IllegalStateException("Channel is not specified!");
+        }
         return channel;
     }
 
-    public void setChannel(String channel) {
-        this.channel = channel;
-    }
-
     public ByteBuf getData() {
         return data;
     }
 
-    public void setData(ByteBuf data) {
-        this.data = data;
-    }
-
     @Override
     public String toString() {
         return "LoginPluginMessage{" +
@@ -59,6 +61,9 @@ public class LoginPluginMessage implements MinecraftPacket {
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
         ProtocolUtils.writeVarInt(buf, id);
+        if (channel == null) {
+            throw new IllegalStateException("Channel is not specified!");
+        }
         ProtocolUtils.writeString(buf, channel);
         buf.writeBytes(data);
     }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/LoginPluginResponse.java

@@ -10,7 +10,7 @@ import io.netty.buffer.Unpooled;
 public class LoginPluginResponse implements MinecraftPacket {
     private int id;
     private boolean success;
-    private ByteBuf data;
+    private ByteBuf data = Unpooled.EMPTY_BUFFER;
 
     public int getId() {
         return id;

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/PlayerListItem.java

@@ -1,5 +1,6 @@
 package com.velocitypowered.proxy.protocol.packet;
 
+import com.google.common.collect.ImmutableList;
 import com.velocitypowered.api.proxy.player.TabListEntry;
 import com.velocitypowered.api.util.GameProfile;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
@@ -9,6 +10,7 @@ import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
 import net.kyori.text.Component;
 import net.kyori.text.serializer.ComponentSerializers;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -21,11 +23,11 @@ public class PlayerListItem implements MinecraftPacket {
     public static final int UPDATE_DISPLAY_NAME = 3;
     public static final int REMOVE_PLAYER = 4;
     private int action;
-    private List<Item> items;
+    private final List<Item> items = new ArrayList<>();
 
     public PlayerListItem(int action, List<Item> items) {
         this.action = action;
-        this.items = items;
+        this.items.addAll(items);
     }
 
     public PlayerListItem() {}
@@ -41,7 +43,6 @@ public class PlayerListItem implements MinecraftPacket {
     @Override
     public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
         action = ProtocolUtils.readVarInt(buf);
-        items = new ArrayList<>();
         int length = ProtocolUtils.readVarInt(buf);
 
         for (int i = 0; i < length; i++) {
@@ -57,7 +58,8 @@ public class PlayerListItem implements MinecraftPacket {
                     if (hasDisplayName) {
                         item.setDisplayName(ComponentSerializers.JSON.deserialize(ProtocolUtils.readString(buf)));
                     }
-                } break;
+                    break;
+                }
                 case UPDATE_GAMEMODE:
                     item.setGameMode(ProtocolUtils.readVarInt(buf));
                     break;
@@ -81,7 +83,7 @@ public class PlayerListItem implements MinecraftPacket {
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
         ProtocolUtils.writeVarInt(buf, action);
         ProtocolUtils.writeVarInt(buf, items.size());
-        for (Item item: items) {
+        for (Item item : items) {
             ProtocolUtils.writeUuid(buf, item.getUuid());
             switch (action) {
                 case ADD_PLAYER:
@@ -113,7 +115,7 @@ public class PlayerListItem implements MinecraftPacket {
         return handler.handle(this);
     }
 
-    private void writeDisplayName(ByteBuf buf, Component displayName) {
+    private void writeDisplayName(ByteBuf buf, @Nullable Component displayName) {
         buf.writeBoolean(displayName != null);
         if (displayName != null) {
             ProtocolUtils.writeString(buf, ComponentSerializers.JSON.serialize(displayName));
@@ -122,11 +124,11 @@ public class PlayerListItem implements MinecraftPacket {
 
     public static class Item {
         private final UUID uuid;
-        private String name;
-        private List<GameProfile.Property> properties;
+        private String name = "";
+        private List<GameProfile.Property> properties = ImmutableList.of();
         private int gameMode;
         private int latency;
-        private Component displayName;
+        private @Nullable Component displayName;
 
         public Item(UUID uuid) {
             this.uuid = uuid;
@@ -181,11 +183,11 @@ public class PlayerListItem implements MinecraftPacket {
             return this;
         }
 
-        public Component getDisplayName() {
+        public @Nullable Component getDisplayName() {
             return displayName;
         }
 
-        public Item setDisplayName(Component displayName) {
+        public Item setDisplayName(@Nullable Component displayName) {
             this.displayName = displayName;
             return this;
         }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/PluginMessage.java

@@ -6,12 +6,18 @@ import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufUtil;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+import static com.velocitypowered.proxy.connection.VelocityConstants.*;
 
 public class PluginMessage implements MinecraftPacket {
-    private String channel;
-    private byte[] data;
+    private @Nullable String channel;
+    private byte[] data = EMPTY_BYTE_ARRAY;
 
     public String getChannel() {
+        if (channel == null) {
+            throw new IllegalStateException("Channel is not specified.");
+        }
         return channel;
     }
 
@@ -44,6 +50,9 @@ public class PluginMessage implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (channel == null) {
+            throw new IllegalStateException("Channel is not specified.");
+        }
         ProtocolUtils.writeString(buf, channel);
         buf.writeBytes(data);
     }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/Respawn.java

@@ -10,7 +10,7 @@ public class Respawn implements MinecraftPacket {
     private int dimension;
     private short difficulty;
     private short gamemode;
-    private String levelType;
+    private String levelType = "";
 
     public Respawn() {
     }

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/ServerLogin.java

@@ -1,20 +1,27 @@
 package com.velocitypowered.proxy.protocol.packet;
 
+import com.google.common.base.Preconditions;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class ServerLogin implements MinecraftPacket {
-    private String username;
+    private @Nullable String username;
 
-    public String getUsername() {
-        return username;
+    public ServerLogin() {}
+
+    public ServerLogin(String username) {
+        this.username = Preconditions.checkNotNull(username, "username");
     }
 
-    public void setUsername(String username) {
-        this.username = username;
+    public String getUsername() {
+        if (username == null) {
+            throw new IllegalStateException("No username found!");
+        }
+        return username;
     }
 
     @Override
@@ -31,6 +38,9 @@ public class ServerLogin implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (username == null) {
+            throw new IllegalStateException("No username found!");
+        }
         ProtocolUtils.writeString(buf, username);
     }
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/ServerLoginSuccess.java

@@ -5,14 +5,18 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.UUID;
 
 public class ServerLoginSuccess implements MinecraftPacket {
-    private UUID uuid;
-    private String username;
+    private @Nullable UUID uuid;
+    private @Nullable String username;
 
     public UUID getUuid() {
+        if (uuid == null) {
+            throw new IllegalStateException("No UUID specified!");
+        }
         return uuid;
     }
 
@@ -21,6 +25,9 @@ public class ServerLoginSuccess implements MinecraftPacket {
     }
 
     public String getUsername() {
+        if (username == null) {
+            throw new IllegalStateException("No username specified!");
+        }
         return username;
     }
 
@@ -44,7 +51,13 @@ public class ServerLoginSuccess implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (uuid == null) {
+            throw new IllegalStateException("No UUID specified!");
+        }
         ProtocolUtils.writeString(buf, uuid.toString());
+        if (username == null) {
+            throw new IllegalStateException("No username specified!");
+        }
         ProtocolUtils.writeString(buf, username);
     }
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/StatusResponse.java

@@ -5,16 +5,22 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class StatusResponse implements MinecraftPacket {
-    private String status;
+    private @Nullable String status;
 
-    public String getStatus() {
-        return status;
+    public StatusResponse() {}
+
+    public StatusResponse(String status) {
+        this.status = status;
     }
 
-    public void setStatus(String status) {
-        this.status = status;
+    public String getStatus() {
+        if (status == null) {
+            throw new IllegalStateException("Status is not specified");
+        }
+        return status;
     }
 
     @Override
@@ -31,6 +37,9 @@ public class StatusResponse implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (status == null) {
+            throw new IllegalStateException("Status is not specified");
+        }
         ProtocolUtils.writeString(buf, status);
     }
 

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/TabCompleteRequest.java

@@ -5,16 +5,20 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import static com.velocitypowered.proxy.protocol.ProtocolConstants.MINECRAFT_1_9;
 
 public class TabCompleteRequest implements MinecraftPacket {
-    private String command;
+    private @Nullable String command;
     private boolean assumeCommand;
     private boolean hasPosition;
     private long position;
 
     public String getCommand() {
+        if (command == null) {
+            throw new IllegalStateException("Command is not specified");
+        }
         return command;
     }
 
@@ -70,6 +74,9 @@ public class TabCompleteRequest implements MinecraftPacket {
 
     @Override
     public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        if (command == null) {
+            throw new IllegalStateException("Command is not specified");
+        }
         ProtocolUtils.writeString(buf, command);
         if (protocolVersion >= MINECRAFT_1_9) {
             buf.writeBoolean(assumeCommand);

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/TitlePacket.java

@@ -5,6 +5,7 @@ import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class TitlePacket implements MinecraftPacket {
     public static final int SET_TITLE = 0;
@@ -18,7 +19,7 @@ public class TitlePacket implements MinecraftPacket {
     public static final int RESET_OLD = 4;
 
     private int action;
-    private String component;
+    private @Nullable String component;
     private int fadeIn;
     private int stay;
     private int fadeOut;
@@ -37,6 +38,9 @@ public class TitlePacket implements MinecraftPacket {
                 case SET_TITLE:
                 case SET_SUBTITLE:
                 case SET_ACTION_BAR:
+                    if (component == null) {
+                        throw new IllegalStateException("No component found for " + action);
+                    }
                     ProtocolUtils.writeString(buf, component);
                     break;
                 case SET_TIMES:
@@ -52,6 +56,9 @@ public class TitlePacket implements MinecraftPacket {
             switch (action) {
                 case SET_TITLE:
                 case SET_SUBTITLE:
+                    if (component == null) {
+                        throw new IllegalStateException("No component found for " + action);
+                    }
                     ProtocolUtils.writeString(buf, component);
                     break;
                 case SET_TIMES_OLD:
@@ -74,11 +81,11 @@ public class TitlePacket implements MinecraftPacket {
         this.action = action;
     }
 
-    public String getComponent() {
+    public @Nullable String getComponent() {
         return component;
     }
 
-    public void setComponent(String component) {
+    public void setComponent(@Nullable String component) {
         this.component = component;
     }
 

proxy/src/main/java/com/velocitypowered/proxy/scheduler/VelocityScheduler.java

@@ -11,6 +11,7 @@ import com.velocitypowered.api.scheduler.Scheduler;
 import com.velocitypowered.api.scheduler.TaskStatus;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Collection;
 import java.util.HashSet;
@@ -92,6 +93,7 @@ public class VelocityScheduler implements Scheduler {
         public ScheduledTask schedule() {
             VelocityTask task = new VelocityTask(plugin, runnable, delay, repeat);
             tasksByPlugin.put(plugin, task);
+            task.schedule();
             return task;
         }
     }
@@ -99,12 +101,19 @@ public class VelocityScheduler implements Scheduler {
     private class VelocityTask implements Runnable, ScheduledTask {
         private final Object plugin;
         private final Runnable runnable;
-        private ScheduledFuture<?> future;
-        private volatile Thread currentTaskThread;
+        private final long delay;
+        private final long repeat;
+        private @Nullable ScheduledFuture<?> future;
+        private volatile @Nullable Thread currentTaskThread;
 
         private VelocityTask(Object plugin, Runnable runnable, long delay, long repeat) {
             this.plugin = plugin;
             this.runnable = runnable;
+            this.delay = delay;
+            this.repeat = repeat;
+        }
+
+        public void schedule() {
             if (repeat == 0) {
                 this.future = timerExecutionService.schedule(this, delay, TimeUnit.MILLISECONDS);
             } else {
@@ -155,14 +164,10 @@ public class VelocityScheduler implements Scheduler {
                 try {
                     runnable.run();
                 } catch (Exception e) {
-                    // Since we can't catch InterruptedException separately...
-                    if (e instanceof InterruptedException) {
-                        onFinish();
-                    } else {
-                        Log.logger.error("Exception in task {} by plugin {}", runnable, plugin);
-                    }
+                    Log.logger.error("Exception in task {} by plugin {}", runnable, plugin);
+                } finally {
+                    currentTaskThread = null;
                 }
-                currentTaskThread = null;
             });
         }
 

proxy/src/main/java/com/velocitypowered/proxy/server/ServerMap.java

@@ -5,6 +5,7 @@ import com.google.common.collect.ImmutableList;
 import com.velocitypowered.api.proxy.server.RegisteredServer;
 import com.velocitypowered.api.proxy.server.ServerInfo;
 import com.velocitypowered.proxy.VelocityServer;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 import java.util.Collection;
 import java.util.Locale;
@@ -13,10 +14,10 @@ import java.util.Optional;
 import java.util.concurrent.ConcurrentHashMap;
 
 public class ServerMap {
-    private final VelocityServer server;
+    private final @Nullable VelocityServer server;
     private final Map<String, RegisteredServer> servers = new ConcurrentHashMap<>();
 
-    public ServerMap(VelocityServer server) {
+    public ServerMap(@Nullable VelocityServer server) {
         this.server = server;
     }
 
@@ -49,7 +50,9 @@ public class ServerMap {
         Preconditions.checkNotNull(serverInfo, "serverInfo");
         String lowerName = serverInfo.getName().toLowerCase(Locale.US);
         RegisteredServer rs = servers.get(lowerName);
-        Preconditions.checkArgument(rs != null, "Server with name %s is not registered!", serverInfo.getName());
+        if (rs == null) {
+            throw new IllegalArgumentException("Server with name " + serverInfo.getName() + " is not registered!");
+        }
         Preconditions.checkArgument(rs.getServerInfo().equals(serverInfo), "Trying to remove server %s with differing information", serverInfo.getName());
         Preconditions.checkState(servers.remove(lowerName, rs), "Server with name %s replaced whilst unregistering", serverInfo.getName());
     }