New command API (#330)

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

@@ -0,0 +1,48 @@
+package com.velocitypowered.api.command;
+
+import com.google.common.base.Preconditions;
+import com.mojang.brigadier.builder.LiteralArgumentBuilder;
+import com.mojang.brigadier.tree.LiteralCommandNode;
+
+/**
+ * A command that uses Brigadier for parsing the command and
+ * providing suggestions to the client.
+ */
+public final class BrigadierCommand implements Command {
+
+  /**
+   * Return code used by a {@link com.mojang.brigadier.Command} to indicate
+   * the command execution should be forwarded to the backend server.
+   */
+  public static final int FORWARD = 0xF6287429;
+
+  private final LiteralCommandNode<CommandSource> node;
+
+  /**
+   * Constructs a {@link BrigadierCommand} from the node returned by
+   * the given builder.
+   *
+   * @param builder the {@link LiteralCommandNode} builder
+   */
+  public BrigadierCommand(final LiteralArgumentBuilder<CommandSource> builder) {
+    this(Preconditions.checkNotNull(builder, "builder").build());
+  }
+
+  /**
+   * Constructs a {@link BrigadierCommand} from the given command node.
+   *
+   * @param node the command node
+   */
+  public BrigadierCommand(final LiteralCommandNode<CommandSource> node) {
+    this.node = Preconditions.checkNotNull(node, "node");
+  }
+
+  /**
+   * Returns the literal node for this command.
+   *
+   * @return the command node
+   */
+  public LiteralCommandNode<CommandSource> getNode() {
+    return node;
+  }
+}

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

@@ -1,59 +1,90 @@
 package com.velocitypowered.api.command;
 
 import com.google.common.collect.ImmutableList;
+import com.velocitypowered.api.proxy.Player;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
- * Represents a command that can be executed by a {@link CommandSource}, such as a {@link
- * com.velocitypowered.api.proxy.Player} or the console.
+ * Represents a command that can be executed by a {@link CommandSource}
+ * such as a {@link Player} or the console.
+ *
+ * <p>Velocity 1.1.0 introduces specialized command subinterfaces to separate
+ * command parsing concerns. These include, in order of preference:
+ *
+ * <ul>
+ * <li>{@link BrigadierCommand}, which supports parameterized arguments and
+ * specialized execution, tab complete suggestions and permission-checking logic.
+ *
+ * <li>{@link SimpleCommand}, modelled after the convention popularized by
+ * Bukkit and BungeeCord. Older classes directly implementing {@link Command}
+ * are suggested to migrate to this interface.
+ *
+ * <li>{@link RawCommand}, useful for bolting on external command frameworks
+ * to Velocity.
+ *
+ * </ul>
+ *
+ * <p>For this reason, the legacy {@code execute}, {@code suggest} and
+ * {@code hasPermission} methods are deprecated and will be removed
+ * in Velocity 2.0.0. We suggest implementing one of the more specific
+ * subinterfaces instead.
+ * The legacy methods are executed by a {@link CommandManager} if and only if
+ * the given command <b>directly</b> implements this interface.
  */
 public interface Command {
 
   /**
-   * Executes the command for the specified {@link CommandSource}.
+   * Executes the command for the specified source.
    *
-   * @param source the source of this command
-   * @param args the arguments for this command
+   * @param source the source to execute the command for
+   * @param args the arguments for the command
+   * @deprecated see {@link Command}
    */
-  void execute(CommandSource source, String @NonNull [] args);
+  @Deprecated
+  default void execute(final CommandSource source, final String @NonNull [] args) {
+    throw new UnsupportedOperationException();
+  }
 
   /**
-   * Provides tab complete suggestions for a command for a specified {@link CommandSource}.
+   * Provides tab complete suggestions for the specified source.
    *
-   * @param source the source to run the command for
-   * @param currentArgs the current, partial arguments for this command
-   * @return tab complete suggestions
+   * @param source the source to execute the command for
+   * @param currentArgs the partial arguments for the command
+   * @return the tab complete suggestions
+   * @deprecated see {@link Command}
    */
-  default List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
+  @Deprecated
+  default List<String> suggest(final CommandSource source, final String @NonNull [] currentArgs) {
     return ImmutableList.of();
   }
 
   /**
-   * Provides tab complete suggestions for a command for a specified {@link CommandSource}.
+   * Provides tab complete suggestions for the specified source.
    *
-   * @param source the source to run the command for
-   * @param currentArgs the current, partial arguments for this command
-   * @return tab complete suggestions
+   * @param source the source to execute the command for
+   * @param currentArgs the partial arguments for the command
+   * @return the tab complete suggestions
+   * @deprecated see {@link Command}
    */
-  default CompletableFuture<List<String>> suggestAsync(CommandSource source,
+  @Deprecated
+  default CompletableFuture<List<String>> suggestAsync(final CommandSource source,
                                                        String @NonNull [] currentArgs) {
     return CompletableFuture.completedFuture(suggest(source, currentArgs));
   }
 
   /**
-   * Tests to check if the {@code source} has permission to use this command with the provided
-   * {@code args}.
+   * Tests to check if the source has permission to perform the command with
+   * the provided arguments.
    *
-   * <p>If this method returns false, the handling will be forwarded onto
-   * the players current server.</p>
-   *
-   * @param source the source of the command
-   * @param args the arguments for this command
-   * @return whether the source has permission
+   * @param source the source to execute the command for
+   * @param args the arguments for the command
+   * @return {@code true} if the source has permission
+   * @deprecated see {@link Command}
    */
-  default boolean hasPermission(CommandSource source, String @NonNull [] args) {
+  @Deprecated
+  default boolean hasPermission(final CommandSource source, final String @NonNull [] args) {
     return true;
   }
 }

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

@@ -0,0 +1,23 @@
+package com.velocitypowered.api.command;
+
+/**
+ * Provides information related to the possible execution of a {@link Command}.
+ *
+ * @param <T> the type of the arguments
+ */
+public interface CommandInvocation<T> {
+
+  /**
+   * Returns the source to execute the command for.
+   *
+   * @return the command source
+   */
+  CommandSource source();
+
+  /**
+   * Returns the arguments after the command alias.
+   *
+   * @return the command arguments
+   */
+  T arguments();
+}

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

@@ -1,87 +1,125 @@
 package com.velocitypowered.api.command;
 
+import com.velocitypowered.api.event.command.CommandExecuteEvent;
 import java.util.concurrent.CompletableFuture;
 
 /**
- * Represents an interface to register a command executor with the proxy.
+ * Handles the registration and execution of commands.
  */
 public interface CommandManager {
 
   /**
-   * Registers the specified command with the manager with the specified aliases.
+   * Returns a builder to create a {@link CommandMeta} with
+   * the given alias.
+   *
+   * @param alias the first command alias
+   * @return a {@link CommandMeta} builder
+   */
+  CommandMeta.Builder metaBuilder(String alias);
+
+  /**
+   * Returns a builder to create a {@link CommandMeta} for
+   * the given Brigadier command.
+   *
+   * @param command the command
+   * @return a {@link CommandMeta} builder
+   */
+  CommandMeta.Builder metaBuilder(BrigadierCommand command);
+
+  /**
+   * Registers the specified command with the specified aliases.
    *
    * @param command the command to register
-   * @param aliases the alias to use
+   * @param aliases the command aliases
    *
+   * @throws IllegalArgumentException if one of the given aliases is already registered
    * @deprecated This method requires at least one alias, but this is only enforced at runtime.
-   *             Prefer {@link #register(String, Command, String...)} instead.
+   *             Prefer {@link #register(String, Command, String...)}
    */
   @Deprecated
   void register(Command command, String... aliases);
 
   /**
-   * Registers the specified command with the manager with the specified aliases.
+   * Registers the specified command with the specified aliases.
    *
-   * @param alias the first alias to register
+   * @param alias the first command alias
    * @param command the command to register
-   * @param otherAliases the other aliases to use
+   * @param otherAliases additional aliases
+   * @throws IllegalArgumentException if one of the given aliases is already registered
+   * @deprecated Prefer {@link #register(CommandMeta, Command)} instead.
    */
+  @Deprecated
   void register(String alias, Command command, String... otherAliases);
 
   /**
-   * Unregisters a command.
+   * Registers the specified Brigadier command.
+   *
+   * @param command the command to register
+   * @throws IllegalArgumentException if the node alias is already registered
+   */
+  void register(BrigadierCommand command);
+
+  /**
+   * Registers the specified command with the given metadata.
+   *
+   * @param meta the command metadata
+   * @param command the command to register
+   * @throws IllegalArgumentException if one of the given aliases is already registered
+   */
+  void register(CommandMeta meta, Command command);
+
+  /**
+   * Unregisters the specified command alias from the manager, if registered.
    *
    * @param alias the command alias to unregister
    */
   void unregister(String alias);
 
   /**
-   * Calls CommandExecuteEvent and attempts to execute a command using the specified {@code cmdLine}
-   * in a blocking fashion.
+   * Attempts to execute a command from the given {@code cmdLine} in
+   * a blocking fashion.
    *
-   * @param source the command's source
+   * @param source the source to execute the command for
    * @param cmdLine the command to run
-   * @return true if the command was found and executed, false if it was not
-   * 
-   * @deprecated This method will block current thread during event call and command execution.
-   *             Prefer {@link #executeAsync(CommandSource, String)} instead.
+   * @return {@code true} if the command was found and executed
+   * @deprecated this method blocks the current thread during the event call and
+   *             the command execution. Prefer {@link #executeAsync(CommandSource, String)}
+   *             instead.
    */
   @Deprecated
   boolean execute(CommandSource source, String cmdLine);
 
   /**
-   * Attempts to execute a command using the specified {@code cmdLine} in a blocking fashion without
-   * calling CommandExecuteEvent.
+   * Attempts to execute a command from the given {@code cmdLine} without
+   * firing a {@link CommandExecuteEvent} in a blocking fashion.
    *
-   * @param source the command's source
+   * @param source the source to execute the command for
    * @param cmdLine the command to run
-   * @return true if the command was found and executed, false if it was not
-   * 
-   * @deprecated This method will block current thread during event and command execution.
+   * @return {@code true} if the command was found and executed
+   * @deprecated this methods blocks the current thread during the command execution.
    *             Prefer {@link #executeImmediatelyAsync(CommandSource, String)} instead.
    */
   @Deprecated
   boolean executeImmediately(CommandSource source, String cmdLine);
 
   /**
-   * Calls CommandExecuteEvent and attempts to execute a command from the specified {@code cmdLine}
-   * async.
+   * Attempts to asynchronously execute a command from the given {@code cmdLine}.
    *
-   * @param source the command's source
+   * @param source the source to execute the command for
    * @param cmdLine the command to run
-   * @return A future that will be completed with the result of the command execution.
-   *         Can be completed exceptionally if exception was thrown during execution.
+   * @return a future that may be completed with the result of the command execution.
+   *         Can be completed exceptionally if an exception is thrown during execution.
    */
   CompletableFuture<Boolean> executeAsync(CommandSource source, String cmdLine);
 
   /**
-   * Attempts to execute a command from the specified {@code cmdLine} async
-   * without calling CommandExecuteEvent.
+   * Attempts to asynchronously execute a command from the given {@code cmdLine}
+   * without firing a {@link CommandExecuteEvent}.
    *
-   * @param source the command's source
+   * @param source the source to execute the command for
    * @param cmdLine the command to run
-   * @return A future that will be completed with the result of the command execution.
-   *         Can be completed exceptionally if exception was thrown during execution.
+   * @return a future that may be completed with the result of the command execution.
+   *         Can be completed exceptionally if an exception is thrown during execution.
    */
   CompletableFuture<Boolean> executeImmediatelyAsync(CommandSource source, String cmdLine);
 }

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

@@ -0,0 +1,57 @@
+package com.velocitypowered.api.command;
+
+import com.mojang.brigadier.tree.CommandNode;
+import java.util.Collection;
+
+/**
+ * Contains metadata for a {@link Command}.
+ */
+public interface CommandMeta {
+
+  /**
+   * Returns a non-empty collection containing the case-insensitive aliases
+   * used to execute the command.
+   *
+   * @return the command aliases
+   */
+  Collection<String> getAliases();
+
+  /**
+   * Returns a collection containing command nodes that provide additional
+   * argument metadata and tab-complete suggestions.
+   * Note some {@link Command} implementations may not support hinting.
+   *
+   * @return the hinting command nodes
+   */
+  Collection<CommandNode<CommandSource>> getHints();
+
+  /**
+   * Provides a fluent interface to create {@link CommandMeta}s.
+   */
+  interface Builder {
+
+    /**
+     * Specifies additional aliases that can be used to execute the command.
+     *
+     * @param aliases the command aliases
+     * @return this builder, for chaining
+     */
+    Builder aliases(String... aliases);
+
+    /**
+     * Specifies a command node providing additional argument metadata and
+     * tab-complete suggestions.
+     *
+     * @param node the command node
+     * @return this builder, for chaining
+     */
+    Builder hint(CommandNode<CommandSource> node);
+
+    /**
+     * Returns a newly-created {@link CommandMeta} based on the specified parameters.
+     *
+     * @return the built {@link CommandMeta}
+     */
+    CommandMeta build();
+  }
+}

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

@@ -0,0 +1,54 @@
+package com.velocitypowered.api.command;
+
+import com.google.common.collect.ImmutableList;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * A command that can be executed with arbitrary arguments.
+ *
+ * @param <I> the type of the command invocation object
+ */
+public interface InvocableCommand<I extends CommandInvocation<?>> extends Command {
+
+  /**
+   * Executes the command for the specified invocation.
+   *
+   * @param invocation the invocation context
+   */
+  void execute(I invocation);
+
+  /**
+   * Provides tab complete suggestions for the specified invocation.
+   *
+   * @param invocation the invocation context
+   * @return the tab complete suggestions
+   */
+  default List<String> suggest(final I invocation) {
+    return ImmutableList.of();
+  }
+
+  /**
+   * Provides tab complete suggestions for the specified invocation.
+   *
+   * @param invocation the invocation context
+   * @return the tab complete suggestions
+   * @implSpec defaults to wrapping the value returned by {@link #suggest(CommandInvocation)}
+   */
+  default CompletableFuture<List<String>> suggestAsync(final I invocation) {
+    return CompletableFuture.completedFuture(suggest(invocation));
+  }
+
+  /**
+   * Tests to check if the source has permission to perform the specified invocation.
+   *
+   * <p>If the method returns {@code false}, the handling is forwarded onto
+   * the players current server.
+   *
+   * @param invocation the invocation context
+   * @return {@code true} if the source has permission
+   */
+  default boolean hasPermission(final I invocation) {
+    return true;
+  }
+}

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

@@ -1,67 +1,97 @@
 package com.velocitypowered.api.command;
 
-import com.google.common.collect.ImmutableList;
 import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import org.checkerframework.checker.nullness.qual.NonNull;
 
 /**
- * A specialized sub-interface of {@code Command} which indicates that the proxy should pass a
- * raw command to the command. This is useful for bolting on external command frameworks to
- * Velocity.
+ * A specialized sub-interface of {@code Command} which indicates the proxy should pass
+ * the command and its arguments directly without further processing.
+ * This is useful for bolting on external command frameworks to Velocity.
  */
-public interface RawCommand extends Command {
-  /**
-   * Executes the command for the specified {@link CommandSource}.
-   *
-   * @param source the source of this command
-   * @param commandLine the full command line after the command name
-   */
-  void execute(CommandSource source, String commandLine);
+public interface RawCommand extends InvocableCommand<RawCommand.Invocation> {
 
-  default void execute(CommandSource source, String @NonNull [] args) {
+  /**
+   * Executes the command for the specified source.
+   *
+   * @param source the source to execute the command for
+   * @param cmdLine the arguments for the command
+   * @deprecated see {@link Command}
+   */
+  @Deprecated
+  default void execute(final CommandSource source, final String cmdLine) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Deprecated
+  @Override
+  default void execute(final CommandSource source, final String @NonNull [] args) {
     execute(source, String.join(" ", args));
   }
 
-  /**
-   * Provides tab complete suggestions for a command for a specified {@link CommandSource}.
-   *
-   * @param source the source to run the command for
-   * @param currentLine the current, partial command line for this command
-   * @return tab complete suggestions
-   */
-  default CompletableFuture<List<String>> suggest(CommandSource source, String currentLine) {
-    return CompletableFuture.completedFuture(ImmutableList.of());
+  @Override
+  default void execute(Invocation invocation) {
+    // Guarantees ABI compatibility
   }
 
+  /**
+   * Provides tab complete suggestions for the specified source.
+   *
+   * @param source the source to execute the command for
+   * @param currentArgs the partial arguments for the command
+   * @return the tab complete suggestions
+   * @deprecated see {@link Command}
+   */
+  @Deprecated
+  default CompletableFuture<List<String>> suggest(final CommandSource source,
+                                                  final String currentArgs) {
+    // This method even has an inconsistent return type
+    throw new UnsupportedOperationException();
+  }
+
+  @Deprecated
   @Override
-  default List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
+  default List<String> suggest(final CommandSource source, final String @NonNull [] currentArgs) {
     return suggestAsync(source, currentArgs).join();
   }
 
+  @Deprecated
   @Override
-  default CompletableFuture<List<String>> suggestAsync(CommandSource source,
-      String @NonNull [] currentArgs) {
+  default CompletableFuture<List<String>> suggestAsync(final CommandSource source,
+                                                       final String @NonNull [] currentArgs) {
     return suggest(source, String.join(" ", currentArgs));
   }
 
+  /**
+   * Tests to check if the source has permission to perform the command with
+   * the provided arguments.
+   *
+   * @param source the source to execute the command for
+   * @param cmdLine the arguments for the command
+   * @return {@code true} if the source has permission
+   * @deprecated see {@link Command}
+   */
+  @Deprecated
+  default boolean hasPermission(final CommandSource source, final String cmdLine) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Deprecated
   @Override
-  default boolean hasPermission(CommandSource source, String @NonNull [] args) {
+  default boolean hasPermission(final CommandSource source, final String @NonNull [] args) {
     return hasPermission(source, String.join(" ", args));
   }
 
   /**
-   * Tests to check if the {@code source} has permission to use this command with the provided
-   * {@code args}.
-   *
-   * <p>If this method returns false, the handling will be forwarded onto
-   * the players current server.</p>
-   *
-   * @param source the source of the command
-   * @param commandLine the arguments for this command
-   * @return whether the source has permission
+   * Contains the invocation data for a raw command.
    */
-  default boolean hasPermission(CommandSource source, String commandLine) {
-    return true;
+  interface Invocation extends CommandInvocation<String> {
+
+    /**
+     * Returns the used alias to execute the command.
+     *
+     * @return the used command alias
+     */
+    String alias();
   }
 }

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

@@ -0,0 +1,20 @@
+package com.velocitypowered.api.command;
+
+import org.checkerframework.checker.nullness.qual.NonNull;
+
+/**
+ * A simple command, modelled after the convention popularized by
+ * Bukkit and BungeeCord.
+ *
+ * <p>Prefer using {@link BrigadierCommand}, which is also
+ * backwards-compatible with older clients.
+ */
+public interface SimpleCommand extends InvocableCommand<SimpleCommand.Invocation> {
+
+  /**
+   * Contains the invocation data for a simple command.
+   */
+  interface Invocation extends CommandInvocation<String @NonNull []> {
+
+  }
+}

api/src/main/java/com/velocitypowered/api/command/package-info.java

@@ -1,4 +1,4 @@
 /**
- * Provides a simple command framework.
+ * Provides a command framework.
  */
 package com.velocitypowered.api.command;

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

@@ -21,11 +21,11 @@ import com.velocitypowered.api.util.ProxyVersion;
 import com.velocitypowered.api.util.bossbar.BossBar;
 import com.velocitypowered.api.util.bossbar.BossBarColor;
 import com.velocitypowered.api.util.bossbar.BossBarOverlay;
-import com.velocitypowered.proxy.command.GlistCommand;
-import com.velocitypowered.proxy.command.ServerCommand;
-import com.velocitypowered.proxy.command.ShutdownCommand;
-import com.velocitypowered.proxy.command.VelocityCommand;
 import com.velocitypowered.proxy.command.VelocityCommandManager;
+import com.velocitypowered.proxy.command.builtin.GlistCommand;
+import com.velocitypowered.proxy.command.builtin.ServerCommand;
+import com.velocitypowered.proxy.command.builtin.ShutdownCommand;
+import com.velocitypowered.proxy.command.builtin.VelocityCommand;
 import com.velocitypowered.proxy.config.VelocityConfiguration;
 import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
 import com.velocitypowered.proxy.console.VelocityConsole;
@@ -194,7 +194,7 @@ public class VelocityServer implements ProxyServer, ForwardingAudience {
     commandManager.register("velocity", new VelocityCommand(this));
     commandManager.register("server", new ServerCommand(this));
     commandManager.register("shutdown", new ShutdownCommand(this),"end");
-    commandManager.register("glist", new GlistCommand(this));
+    new GlistCommand(this).register();
 
     try {
       Path configPath = Paths.get("velocity.toml");

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

@@ -0,0 +1,31 @@
+package com.velocitypowered.proxy.command;
+
+import com.google.common.base.Preconditions;
+import com.velocitypowered.api.command.CommandInvocation;
+import com.velocitypowered.api.command.CommandSource;
+
+/**
+ * Abstract base class for {@link CommandInvocation} implementations.
+ *
+ * @param <T> the type of the arguments
+ */
+abstract class AbstractCommandInvocation<T> implements CommandInvocation<T> {
+
+  private final CommandSource source;
+  private final T arguments;
+
+  protected AbstractCommandInvocation(final CommandSource source, final T arguments) {
+    this.source = Preconditions.checkNotNull(source, "source");
+    this.arguments = Preconditions.checkNotNull(arguments, "arguments");
+  }
+
+  @Override
+  public CommandSource source() {
+    return source;
+  }
+
+  @Override
+  public T arguments() {
+    return arguments;
+  }
+}

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

@@ -0,0 +1,23 @@
+package com.velocitypowered.proxy.command;
+
+import com.mojang.brigadier.context.CommandContext;
+import com.velocitypowered.api.command.CommandInvocation;
+import com.velocitypowered.api.command.CommandSource;
+
+/**
+ * Creates command invocation contexts for the given {@link CommandSource}
+ * and command line arguments.
+ *
+ * @param <I> the type of the built invocation
+ */
+@FunctionalInterface
+public interface CommandInvocationFactory<I extends CommandInvocation<?>> {
+
+  /**
+   * Returns an invocation context for the given Brigadier context.
+   *
+   * @param context the command context
+   * @return the built invocation context
+   */
+  I create(final CommandContext<CommandSource> context);
+}

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

@@ -0,0 +1,97 @@
+package com.velocitypowered.proxy.command;
+
+import com.mojang.brigadier.context.CommandContext;
+import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.velocitypowered.api.command.BrigadierCommand;
+import com.velocitypowered.api.command.Command;
+import com.velocitypowered.api.command.CommandInvocation;
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.command.InvocableCommand;
+import com.velocitypowered.api.command.RawCommand;
+import com.velocitypowered.api.command.SimpleCommand;
+import com.velocitypowered.proxy.util.BrigadierUtils;
+
+@FunctionalInterface
+public interface CommandNodeFactory<T extends Command> {
+
+  InvocableCommandNodeFactory<SimpleCommand.Invocation> SIMPLE =
+      new InvocableCommandNodeFactory<>() {
+        @Override
+        protected SimpleCommand.Invocation createInvocation(
+                final CommandContext<CommandSource> context) {
+          return VelocitySimpleCommandInvocation.FACTORY.create(context);
+        }
+      };
+
+  InvocableCommandNodeFactory<RawCommand.Invocation> RAW =
+      new InvocableCommandNodeFactory<>() {
+        @Override
+        protected RawCommand.Invocation createInvocation(
+                final CommandContext<CommandSource> context) {
+          return VelocityRawCommandInvocation.FACTORY.create(context);
+        }
+      };
+
+  CommandNodeFactory<Command> FALLBACK = (alias, command) ->
+      BrigadierUtils.buildRawArgumentsLiteral(alias,
+        context -> {
+          CommandSource source = context.getSource();
+          String[] args = BrigadierUtils.getSplitArguments(context);
+
+          if (!command.hasPermission(source, args)) {
+            return BrigadierCommand.FORWARD;
+          }
+          command.execute(source, args);
+          return 1;
+        },
+        (context, builder) -> {
+          String[] args = BrigadierUtils.getSplitArguments(context);
+          return command.suggestAsync(context.getSource(), args).thenApply(values -> {
+            for (String value : values) {
+              builder.suggest(value);
+            }
+
+            return builder.build();
+          });
+        });
+
+  /**
+   * Returns a Brigadier node for the execution of the given command.
+   *
+   * @param alias the command alias
+   * @param command the command to execute
+   * @return the command node
+   */
+  LiteralCommandNode<CommandSource> create(String alias, T command);
+
+  abstract class InvocableCommandNodeFactory<I extends CommandInvocation<?>>
+          implements CommandNodeFactory<InvocableCommand<I>> {
+
+    @Override
+    public LiteralCommandNode<CommandSource> create(
+            final String alias, final InvocableCommand<I> command) {
+      return BrigadierUtils.buildRawArgumentsLiteral(alias,
+          context -> {
+            I invocation = createInvocation(context);
+            if (!command.hasPermission(invocation)) {
+              return BrigadierCommand.FORWARD;
+            }
+            command.execute(invocation);
+            return 1;
+          },
+          (context, builder) -> {
+            I invocation = createInvocation(context);
+
+            return command.suggestAsync(invocation).thenApply(values -> {
+              for (String value : values) {
+                builder.suggest(value);
+              }
+
+              return builder.build();
+            });
+          });
+    }
+
+    protected abstract I createInvocation(final CommandContext<CommandSource> context);
+  }
+}

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

@@ -1,110 +0,0 @@
-package com.velocitypowered.proxy.command;
-
-import com.google.common.collect.ImmutableList;
-import com.velocitypowered.api.command.Command;
-import com.velocitypowered.api.command.CommandSource;
-import com.velocitypowered.api.permission.Tristate;
-import com.velocitypowered.api.proxy.Player;
-import com.velocitypowered.api.proxy.ProxyServer;
-import com.velocitypowered.api.proxy.server.RegisteredServer;
-import java.util.List;
-import java.util.Optional;
-import net.kyori.adventure.text.TextComponent;
-import net.kyori.adventure.text.format.NamedTextColor;
-import org.checkerframework.checker.nullness.qual.NonNull;
-
-public class GlistCommand implements Command {
-
-  private final ProxyServer server;
-
-  public GlistCommand(ProxyServer server) {
-    this.server = server;
-  }
-
-  @Override
-  public void execute(CommandSource source, String @NonNull [] args) {
-    if (args.length == 0) {
-      sendTotalProxyCount(source);
-      source.sendMessage(
-          TextComponent.builder("To view all players on servers, use ", NamedTextColor.YELLOW)
-              .append("/glist all", NamedTextColor.DARK_AQUA)
-              .append(".", NamedTextColor.YELLOW)
-              .build());
-    } else if (args.length == 1) {
-      String arg = args[0];
-      if (arg.equalsIgnoreCase("all")) {
-        for (RegisteredServer server : BuiltinCommandUtil.sortedServerList(server)) {
-          sendServerPlayers(source, server, true);
-        }
-        sendTotalProxyCount(source);
-      } else {
-        Optional<RegisteredServer> registeredServer = server.getServer(arg);
-        if (!registeredServer.isPresent()) {
-          source.sendMessage(
-              TextComponent.of("Server " + arg + " doesn't exist.", NamedTextColor.RED));
-          return;
-        }
-        sendServerPlayers(source, registeredServer.get(), false);
-      }
-    } else {
-      source.sendMessage(TextComponent.of("Too many arguments.", NamedTextColor.RED));
-    }
-  }
-
-  private void sendTotalProxyCount(CommandSource target) {
-    target.sendMessage(TextComponent.builder("There are ", NamedTextColor.YELLOW)
-        .append(Integer.toString(server.getAllPlayers().size()), NamedTextColor.GREEN)
-        .append(" player(s) online.", NamedTextColor.YELLOW)
-        .build());
-  }
-
-  private void sendServerPlayers(CommandSource target, RegisteredServer server, boolean fromAll) {
-    List<Player> onServer = ImmutableList.copyOf(server.getPlayersConnected());
-    if (onServer.isEmpty() && fromAll) {
-      return;
-    }
-
-    TextComponent.Builder builder = TextComponent.builder()
-        .append(TextComponent.of("[" + server.getServerInfo().getName() + "] ",
-            NamedTextColor.DARK_AQUA))
-        .append("(" + onServer.size() + ")", NamedTextColor.GRAY)
-        .append(": ")
-        .resetStyle();
-
-    for (int i = 0; i < onServer.size(); i++) {
-      Player player = onServer.get(i);
-      builder.append(player.getUsername());
-
-      if (i + 1 < onServer.size()) {
-        builder.append(", ");
-      }
-    }
-
-    target.sendMessage(builder.build());
-  }
-
-  @Override
-  public List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
-    ImmutableList.Builder<String> options = ImmutableList.builder();
-    for (RegisteredServer server : server.getAllServers()) {
-      options.add(server.getServerInfo().getName());
-    }
-    options.add("all");
-
-    switch (currentArgs.length) {
-      case 0:
-        return options.build();
-      case 1:
-        return options.build().stream()
-            .filter(o -> o.regionMatches(true, 0, currentArgs[0], 0, currentArgs[0].length()))
-            .collect(ImmutableList.toImmutableList());
-      default:
-        return ImmutableList.of();
-    }
-  }
-
-  @Override
-  public boolean hasPermission(CommandSource source, String @NonNull [] args) {
-    return source.getPermissionValue("velocity.command.glist") == Tristate.TRUE;
-  }
-}

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

@@ -1,31 +0,0 @@
-package com.velocitypowered.proxy.command;
-
-import com.velocitypowered.api.command.Command;
-import com.velocitypowered.api.command.CommandSource;
-import com.velocitypowered.proxy.VelocityServer;
-import net.kyori.adventure.text.serializer.legacy.LegacyComponentSerializer;
-import org.checkerframework.checker.nullness.qual.NonNull;
-
-public class ShutdownCommand implements Command {
-
-  private final VelocityServer server;
-
-  public ShutdownCommand(VelocityServer server) {
-    this.server = server;
-  }
-
-  @Override
-  public void execute(CommandSource source, String @NonNull [] args) {
-    if (args.length == 0) {
-      server.shutdown(true);
-    } else {
-      String reason = String.join(" ", args);
-      server.shutdown(true, LegacyComponentSerializer.legacy('&').deserialize(reason));
-    }
-  }
-
-  @Override
-  public boolean hasPermission(CommandSource source, String @NonNull [] args) {
-    return source == server.getConsoleCommandSource();
-  }
-}

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

@@ -1,273 +1,229 @@
 package com.velocitypowered.proxy.command;
 
 import com.google.common.base.Preconditions;
-import com.google.common.collect.ImmutableList;
-import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Lists;
+import com.mojang.brigadier.CommandDispatcher;
+import com.mojang.brigadier.ParseResults;
+import com.mojang.brigadier.exceptions.CommandSyntaxException;
+import com.mojang.brigadier.suggestion.Suggestion;
+import com.mojang.brigadier.tree.CommandNode;
+import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.velocitypowered.api.command.BrigadierCommand;
 import com.velocitypowered.api.command.Command;
 import com.velocitypowered.api.command.CommandManager;
+import com.velocitypowered.api.command.CommandMeta;
 import com.velocitypowered.api.command.CommandSource;
 import com.velocitypowered.api.command.RawCommand;
+import com.velocitypowered.api.command.SimpleCommand;
 import com.velocitypowered.api.event.command.CommandExecuteEvent;
 import com.velocitypowered.api.event.command.CommandExecuteEvent.CommandResult;
 import com.velocitypowered.proxy.plugin.VelocityEventManager;
+import com.velocitypowered.proxy.util.BrigadierUtils;
 import java.util.Arrays;
-import java.util.HashMap;
+import java.util.Iterator;
 import java.util.List;
 import java.util.Locale;
-import java.util.Map;
-import java.util.Set;
 import java.util.concurrent.CompletableFuture;
+import net.kyori.adventure.text.TextComponent;
+import net.kyori.adventure.text.format.NamedTextColor;
 
 public class VelocityCommandManager implements CommandManager {
 
-  private final Map<String, RawCommand> commands = new HashMap<>();
+  private final CommandDispatcher<CommandSource> dispatcher;
   private final VelocityEventManager eventManager;
 
-  public VelocityCommandManager(VelocityEventManager eventManager) {
-    this.eventManager = eventManager;
+  public VelocityCommandManager(final VelocityEventManager eventManager) {
+    this.eventManager = Preconditions.checkNotNull(eventManager);
+    this.dispatcher = new CommandDispatcher<>();
+  }
+
+  @Override
+  public CommandMeta.Builder metaBuilder(final String alias) {
+    Preconditions.checkNotNull(alias, "alias");
+    return new VelocityCommandMeta.Builder(alias);
+  }
+
+  @Override
+  public CommandMeta.Builder metaBuilder(final BrigadierCommand command) {
+    Preconditions.checkNotNull(command, "command");
+    return new VelocityCommandMeta.Builder(command.getNode().getName());
   }
 
   @Override
-  @Deprecated
   public void register(final Command command, final String... aliases) {
     Preconditions.checkArgument(aliases.length > 0, "no aliases provided");
     register(aliases[0], command, Arrays.copyOfRange(aliases, 1, aliases.length));
   }
 
   @Override
-  public void register(String alias, Command command, String... otherAliases) {
+  public void register(final String alias, final Command command, final String... otherAliases) {
     Preconditions.checkNotNull(alias, "alias");
+    Preconditions.checkNotNull(command, "command");
     Preconditions.checkNotNull(otherAliases, "otherAliases");
-    Preconditions.checkNotNull(command, "executor");
+    Preconditions.checkArgument(!hasCommand(alias), "alias already registered");
+    register(metaBuilder(alias).aliases(otherAliases).build(), command);
+  }
 
-    RawCommand rawCmd = RegularCommandWrapper.wrap(command);
-    this.commands.put(alias.toLowerCase(Locale.ENGLISH), rawCmd);
+  @Override
+  public void register(final BrigadierCommand command) {
+    Preconditions.checkNotNull(command, "command");
+    register(metaBuilder(command).build(), command);
+  }
 
-    for (int i = 0, length = otherAliases.length; i < length; i++) {
-      final String alias1 = otherAliases[i];
-      Preconditions.checkNotNull(alias1, "alias at index %s", i + 1);
-      this.commands.put(alias1.toLowerCase(Locale.ENGLISH), rawCmd);
+  @Override
+  public void register(final CommandMeta meta, final Command command) {
+    Preconditions.checkNotNull(meta, "meta");
+    Preconditions.checkNotNull(command, "command");
+
+    Iterator<String> aliasIterator = meta.getAliases().iterator();
+    String alias = aliasIterator.next();
+
+    LiteralCommandNode<CommandSource> node = null;
+    if (command instanceof BrigadierCommand) {
+      node = ((BrigadierCommand) command).getNode();
+    } else if (command instanceof SimpleCommand) {
+      node = CommandNodeFactory.SIMPLE.create(alias, (SimpleCommand) command);
+    } else if (command instanceof RawCommand) {
+      // This ugly hack will be removed in Velocity 2.0. Most if not all plugins
+      // have side-effect free #suggest methods. We rely on the newer RawCommand
+      // throwing UOE.
+      RawCommand asRaw = (RawCommand) command;
+      try {
+        asRaw.suggest(null, new String[0]);
+      } catch (final UnsupportedOperationException e) {
+        node = CommandNodeFactory.RAW.create(alias, asRaw);
+      } catch (final Exception ignored) {
+        // The implementation probably relies on a non-null source
+      }
+    }
+    if (node == null) {
+      node = CommandNodeFactory.FALLBACK.create(alias, command);
+    }
+
+    if (!(command instanceof BrigadierCommand)) {
+      for (CommandNode<CommandSource> hint : meta.getHints()) {
+        node.addChild(BrigadierUtils.wrapForHinting(hint, node.getCommand()));
+      }
+    }
+
+    dispatcher.getRoot().addChild(node);
+    while (aliasIterator.hasNext()) {
+      String otherAlias = aliasIterator.next();
+      Preconditions.checkArgument(!hasCommand(otherAlias),
+              "alias %s is already registered", otherAlias);
+      dispatcher.getRoot().addChild(BrigadierUtils.buildRedirect(otherAlias, node));
     }
   }
 
   @Override
   public void unregister(final String alias) {
-    Preconditions.checkNotNull(alias, "name");
-    this.commands.remove(alias.toLowerCase(Locale.ENGLISH));
+    Preconditions.checkNotNull(alias, "alias");
+    CommandNode<CommandSource> node =
+            dispatcher.getRoot().getChild(alias.toLowerCase(Locale.ENGLISH));
+    if (node != null) {
+      dispatcher.getRoot().getChildren().remove(node);
+    }
   }
 
   /**
-   * Calls CommandExecuteEvent.
-   * @param source the command's source
-   * @param cmd the command
-   * @return CompletableFuture of event
+   * Fires a {@link CommandExecuteEvent}.
+   *
+   * @param source the source to execute the command for
+   * @param cmdLine the command to execute
+   * @return the {@link CompletableFuture} of the event
    */
-  public CompletableFuture<CommandExecuteEvent> callCommandEvent(CommandSource source, String cmd) {
+  public CompletableFuture<CommandExecuteEvent> callCommandEvent(final CommandSource source,
+                                                                 final String cmdLine) {
     Preconditions.checkNotNull(source, "source");
-    Preconditions.checkNotNull(cmd, "cmd");
-    return eventManager.fire(new CommandExecuteEvent(source, cmd));
+    Preconditions.checkNotNull(cmdLine, "cmdLine");
+    return eventManager.fire(new CommandExecuteEvent(source, cmdLine));
   }
 
   @Override
-  public boolean execute(CommandSource source, String cmdLine) {
-    Preconditions.checkNotNull(source, "source");
-    Preconditions.checkNotNull(cmdLine, "cmdLine");
-
-    CommandExecuteEvent event = callCommandEvent(source, cmdLine).join();
-    CommandResult commandResult = event.getResult();
-    if (commandResult.isForwardToServer() || !commandResult.isAllowed()) {
-      return false;
-    }
-    cmdLine = commandResult.getCommand().orElse(event.getCommand());
-
-    return executeImmediately(source, cmdLine);
+  public boolean execute(final CommandSource source, final String cmdLine) {
+    return executeAsync(source, cmdLine).join();
   }
 
   @Override
-  public boolean executeImmediately(CommandSource source, String cmdLine) {
+  public boolean executeImmediately(final CommandSource source, final String cmdLine) {
     Preconditions.checkNotNull(source, "source");
     Preconditions.checkNotNull(cmdLine, "cmdLine");
 
-    String alias = cmdLine;
-    String args = "";
-    int firstSpace = cmdLine.indexOf(' ');
-    if (firstSpace != -1) {
-      alias = cmdLine.substring(0, firstSpace);
-      args = cmdLine.substring(firstSpace);
-    }
-    RawCommand command = commands.get(alias.toLowerCase(Locale.ENGLISH));
-    if (command == null) {
-      return false;
-    }
-
+    ParseResults<CommandSource> results = parse(cmdLine, source, true);
     try {
-      if (!command.hasPermission(source, args)) {
-        return false;
+      return dispatcher.execute(results) != BrigadierCommand.FORWARD;
+    } catch (final CommandSyntaxException e) {
+      boolean isSyntaxError = !e.getType().equals(
+              CommandSyntaxException.BUILT_IN_EXCEPTIONS.dispatcherUnknownCommand());
+      if (isSyntaxError) {
+        source.sendMessage(TextComponent.of(e.getMessage(), NamedTextColor.RED));
       }
-      command.execute(source, args);
-      return true;
-    } catch (Exception e) {
+      return false;
+    } catch (final Exception e) {
       throw new RuntimeException("Unable to invoke command " + cmdLine + " for " + source, e);
     }
   }
 
-
   @Override
-  public CompletableFuture<Boolean> executeAsync(CommandSource source, String cmdLine) {
-    CompletableFuture<Boolean> result = new CompletableFuture<>();
-    callCommandEvent(source, cmdLine).thenAccept(event -> {
+  public CompletableFuture<Boolean> executeAsync(final CommandSource source, final String cmdLine) {
+    Preconditions.checkNotNull(source, "source");
+    Preconditions.checkNotNull(cmdLine, "cmdLine");
+
+    return callCommandEvent(source, cmdLine).thenApply(event -> {
       CommandResult commandResult = event.getResult();
       if (commandResult.isForwardToServer() || !commandResult.isAllowed()) {
-        result.complete(false);
-      }
-      String command = commandResult.getCommand().orElse(event.getCommand());
-      try {
-        result.complete(executeImmediately(source, command));
-      } catch (Exception e) {
-        result.completeExceptionally(e);
-      }
-    });
-    return result;
-  }
-
-  @Override
-  public CompletableFuture<Boolean> executeImmediatelyAsync(CommandSource source, String cmdLine) {
-    Preconditions.checkNotNull(source, "source");
-    Preconditions.checkNotNull(cmdLine, "cmdLine");
-    CompletableFuture<Boolean> result = new CompletableFuture<>();
-    eventManager.getService().execute(() -> {
-      try {
-        result.complete(executeImmediately(source, cmdLine));
-      } catch (Exception e) {
-        result.completeExceptionally(e);
-      }
-    });
-    return result;
-  }
-
-  public boolean hasCommand(String command) {
-    return commands.containsKey(command);
-  }
-
-  public Set<String> getAllRegisteredCommands() {
-    return ImmutableSet.copyOf(commands.keySet());
-  }
-
-  /**
-   * Offer suggestions to fill in the command.
-   * @param source the source for the command
-   * @param cmdLine the partially completed command
-   * @return a {@link CompletableFuture} eventually completed with a {@link List}, possibly empty
-   */
-  public CompletableFuture<List<String>> offerSuggestions(CommandSource source, String cmdLine) {
-    Preconditions.checkNotNull(source, "source");
-    Preconditions.checkNotNull(cmdLine, "cmdLine");
-
-    int firstSpace = cmdLine.indexOf(' ');
-    if (firstSpace == -1) {
-      // Offer to fill in commands.
-      ImmutableList.Builder<String> availableCommands = ImmutableList.builder();
-      for (Map.Entry<String, RawCommand> entry : commands.entrySet()) {
-        if (entry.getKey().regionMatches(true, 0, cmdLine, 0, cmdLine.length())
-            && entry.getValue().hasPermission(source, new String[0])) {
-          availableCommands.add("/" + entry.getKey());
-        }
-      }
-      return CompletableFuture.completedFuture(availableCommands.build());
-    }
-
-    String alias = cmdLine.substring(0, firstSpace);
-    String args = cmdLine.substring(firstSpace);
-    RawCommand command = commands.get(alias.toLowerCase(Locale.ENGLISH));
-    if (command == null) {
-      // No such command, so we can't offer any tab complete suggestions.
-      return CompletableFuture.completedFuture(ImmutableList.of());
-    }
-
-    try {
-      if (!command.hasPermission(source, args)) {
-        return CompletableFuture.completedFuture(ImmutableList.of());
-      }
-      return command.suggest(source, args)
-          .thenApply(ImmutableList::copyOf);
-    } catch (Exception e) {
-      throw new RuntimeException(
-          "Unable to invoke suggestions for command " + cmdLine + " for " + source, e);
-    }
-  }
-
-  /**
-   * Determines if the {@code source} has permission to run the {@code cmdLine}.
-   * @param source the source to check against
-   * @param cmdLine the command to run
-   * @return {@code true} if the command can be run, otherwise {@code false}
-   */
-  public boolean hasPermission(CommandSource source, String cmdLine) {
-    Preconditions.checkNotNull(source, "source");
-    Preconditions.checkNotNull(cmdLine, "cmdLine");
-
-    String alias = cmdLine;
-    String args = "";
-    int firstSpace = cmdLine.indexOf(' ');
-    if (firstSpace != -1) {
-      alias = cmdLine.substring(0, firstSpace);
-      args = cmdLine.substring(firstSpace).trim();
-    }
-    RawCommand command = commands.get(alias.toLowerCase(Locale.ENGLISH));
-    if (command == null) {
         return false;
       }
-
-    try {
-      return command.hasPermission(source, args);
-    } catch (Exception e) {
-      throw new RuntimeException(
-          "Unable to invoke suggestions for command " + alias + " for " + source, e);
-    }
-  }
-
-  private static class RegularCommandWrapper implements RawCommand {
-
-    private final Command delegate;
-
-    private RegularCommandWrapper(Command delegate) {
-      this.delegate = delegate;
-    }
-
-    private static String[] split(String line) {
-      if (line.isEmpty()) {
-        return new String[0];
-      }
-
-      String[] trimmed = line.trim().split(" ", -1);
-      if (line.endsWith(" ") && !line.trim().isEmpty()) {
-        // To work around a 1.13+ specific bug we have to inject a space at the end of the arguments
-        trimmed = Arrays.copyOf(trimmed, trimmed.length + 1);
-        trimmed[trimmed.length - 1] = "";
-      }
-      return trimmed;
+      return executeImmediately(source, commandResult.getCommand().orElse(event.getCommand()));
+    });
   }
 
   @Override
-    public void execute(CommandSource source, String commandLine) {
-      delegate.execute(source, split(commandLine));
+  public CompletableFuture<Boolean> executeImmediatelyAsync(
+          final CommandSource source, final String cmdLine) {
+    Preconditions.checkNotNull(source, "source");
+    Preconditions.checkNotNull(cmdLine, "cmdLine");
+
+    return CompletableFuture.supplyAsync(
+        () -> executeImmediately(source, cmdLine), eventManager.getService());
   }
 
-    @Override
-    public CompletableFuture<List<String>> suggest(CommandSource source, String currentLine) {
-      return delegate.suggestAsync(source, split(currentLine));
+  /**
+   * Returns suggestions to fill in the given command.
+   *
+   * @param source the source to execute the command for
+   * @param cmdLine the partially completed command
+   * @return a {@link CompletableFuture} eventually completed with a {@link List},
+   *         possibly empty
+   */
+  public CompletableFuture<List<String>> offerSuggestions(final CommandSource source,
+                                                          final String cmdLine) {
+    Preconditions.checkNotNull(source, "source");
+    Preconditions.checkNotNull(cmdLine, "cmdLine");
+
+    ParseResults<CommandSource> parse = parse(cmdLine, source, false);
+    return dispatcher.getCompletionSuggestions(parse)
+            .thenApply(suggestions -> Lists.transform(suggestions.getList(), Suggestion::getText));
   }
 
-    @Override
-    public boolean hasPermission(CommandSource source, String commandLine) {
-      return delegate.hasPermission(source, split(commandLine));
+  private ParseResults<CommandSource> parse(final String cmdLine, final CommandSource source,
+                                            final boolean trim) {
+    String normalized = BrigadierUtils.normalizeInput(cmdLine, trim);
+    return dispatcher.parse(normalized, source);
   }
 
-    static RawCommand wrap(Command command) {
-      if (command instanceof RawCommand) {
-        return (RawCommand) command;
-      }
-      return new RegularCommandWrapper(command);
+  /**
+   * Returns whether the given alias is registered on this manager.
+   *
+   * @param alias the command alias to check
+   * @return {@code true} if the alias is registered
+   */
+  public boolean hasCommand(final String alias) {
+    Preconditions.checkNotNull(alias, "alias");
+    return dispatcher.getRoot().getChild(alias.toLowerCase(Locale.ENGLISH)) != null;
   }
+
+  public CommandDispatcher<CommandSource> getDispatcher() {
+    return dispatcher;
   }
 }

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

@@ -0,0 +1,70 @@
+package com.velocitypowered.proxy.command;
+
+import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.mojang.brigadier.tree.CommandNode;
+import com.velocitypowered.api.command.CommandMeta;
+import com.velocitypowered.api.command.CommandSource;
+import java.util.Collection;
+import java.util.List;
+import java.util.Locale;
+import java.util.Set;
+
+final class VelocityCommandMeta implements CommandMeta {
+
+  static final class Builder implements CommandMeta.Builder {
+
+    private final ImmutableSet.Builder<String> aliases;
+    private final ImmutableList.Builder<CommandNode<CommandSource>> hints;
+
+    public Builder(final String alias) {
+      Preconditions.checkNotNull(alias, "alias");
+      this.aliases = ImmutableSet.<String>builder()
+              .add(alias.toLowerCase(Locale.ENGLISH));
+      this.hints = ImmutableList.builder();
+    }
+
+    @Override
+    public CommandMeta.Builder aliases(final String... aliases) {
+      Preconditions.checkNotNull(aliases, "aliases");
+      for (int i = 0, length = aliases.length; i < length; i++) {
+        final String alias1 = aliases[i];
+        Preconditions.checkNotNull(alias1, "alias at index %s", i);
+        this.aliases.add(alias1.toLowerCase(Locale.ENGLISH));
+      }
+      return this;
+    }
+
+    @Override
+    public CommandMeta.Builder hint(final CommandNode<CommandSource> node) {
+      Preconditions.checkNotNull(node, "node");
+      hints.add(node);
+      return this;
+    }
+
+    @Override
+    public CommandMeta build() {
+      return new VelocityCommandMeta(aliases.build(), hints.build());
+    }
+  }
+
+  private final Set<String> aliases;
+  private final List<CommandNode<CommandSource>> hints;
+
+  private VelocityCommandMeta(
+          final Set<String> aliases, final List<CommandNode<CommandSource>> hints) {
+    this.aliases = aliases;
+    this.hints = hints;
+  }
+
+  @Override
+  public Collection<String> getAliases() {
+    return aliases;
+  }
+
+  @Override
+  public Collection<CommandNode<CommandSource>> getHints() {
+    return hints;
+  }
+}

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

@@ -0,0 +1,37 @@
+package com.velocitypowered.proxy.command;
+
+import com.google.common.base.Preconditions;
+import com.mojang.brigadier.context.CommandContext;
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.command.RawCommand;
+import com.velocitypowered.proxy.util.BrigadierUtils;
+
+final class VelocityRawCommandInvocation extends AbstractCommandInvocation<String>
+        implements RawCommand.Invocation {
+
+  static final Factory FACTORY = new Factory();
+
+  static class Factory implements CommandInvocationFactory<RawCommand.Invocation> {
+
+    @Override
+    public RawCommand.Invocation create(final CommandContext<CommandSource> context) {
+      return new VelocityRawCommandInvocation(
+              context.getSource(),
+              BrigadierUtils.getAlias(context),
+              BrigadierUtils.getRawArguments(context));
+    }
+  }
+
+  private final String alias;
+
+  private VelocityRawCommandInvocation(final CommandSource source,
+                               final String alias, final String arguments) {
+    super(source, arguments);
+    this.alias = Preconditions.checkNotNull(alias);
+  }
+
+  @Override
+  public String alias() {
+    return alias;
+  }
+}

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

@@ -0,0 +1,25 @@
+package com.velocitypowered.proxy.command;
+
+import com.mojang.brigadier.context.CommandContext;
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.command.SimpleCommand;
+import com.velocitypowered.proxy.util.BrigadierUtils;
+
+final class VelocitySimpleCommandInvocation extends AbstractCommandInvocation<String[]>
+        implements SimpleCommand.Invocation {
+
+  static final Factory FACTORY = new Factory();
+
+  static class Factory implements CommandInvocationFactory<SimpleCommand.Invocation> {
+
+    @Override
+    public SimpleCommand.Invocation create(final CommandContext<CommandSource> context) {
+      final String[] arguments = BrigadierUtils.getSplitArguments(context);
+      return new VelocitySimpleCommandInvocation(context.getSource(), arguments);
+    }
+  }
+
+  VelocitySimpleCommandInvocation(final CommandSource source, final String[] arguments) {
+    super(source, arguments);
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/command/builtin/BuiltinCommandUtil.java

@@ -1,4 +1,4 @@
-package com.velocitypowered.proxy.command;
+package com.velocitypowered.proxy.command.builtin;
 
 import com.velocitypowered.api.proxy.ProxyServer;
 import com.velocitypowered.api.proxy.server.RegisteredServer;

proxy/src/main/java/com/velocitypowered/proxy/command/builtin/GlistCommand.java

@@ -0,0 +1,120 @@
+package com.velocitypowered.proxy.command.builtin;
+
+import static com.mojang.brigadier.arguments.StringArgumentType.getString;
+
+import com.google.common.collect.ImmutableList;
+import com.mojang.brigadier.arguments.StringArgumentType;
+import com.mojang.brigadier.builder.LiteralArgumentBuilder;
+import com.mojang.brigadier.builder.RequiredArgumentBuilder;
+import com.mojang.brigadier.context.CommandContext;
+import com.mojang.brigadier.tree.ArgumentCommandNode;
+import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.velocitypowered.api.command.BrigadierCommand;
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.permission.Tristate;
+import com.velocitypowered.api.proxy.Player;
+import com.velocitypowered.api.proxy.ProxyServer;
+import com.velocitypowered.api.proxy.server.RegisteredServer;
+import java.util.List;
+import java.util.Optional;
+import net.kyori.adventure.text.TextComponent;
+import net.kyori.adventure.text.format.NamedTextColor;
+
+public class GlistCommand {
+
+  private static final String SERVER_ARG = "server";
+
+  private final ProxyServer server;
+
+  public GlistCommand(ProxyServer server) {
+    this.server = server;
+  }
+
+  /**
+   * Registers this command.
+   */
+  public void register() {
+    LiteralCommandNode<CommandSource> totalNode = LiteralArgumentBuilder
+            .<CommandSource>literal("glist")
+            .requires(source ->
+                    source.getPermissionValue("velocity.command.glist") == Tristate.TRUE)
+            .executes(this::totalCount)
+            .build();
+    ArgumentCommandNode<CommandSource, String> serverNode = RequiredArgumentBuilder
+            .<CommandSource, String>argument(SERVER_ARG, StringArgumentType.string())
+            .suggests((context, builder) -> {
+              for (RegisteredServer server : server.getAllServers()) {
+                builder.suggest(server.getServerInfo().getName());
+              }
+              builder.suggest("all");
+              return builder.buildFuture();
+            })
+            .executes(this::serverCount)
+            .build();
+    totalNode.addChild(serverNode);
+    server.getCommandManager().register(new BrigadierCommand(totalNode));
+  }
+
+  private int totalCount(final CommandContext<CommandSource> context) {
+    final CommandSource source = context.getSource();
+    sendTotalProxyCount(source);
+    source.sendMessage(
+        TextComponent.builder("To view all players on servers, use ", NamedTextColor.YELLOW)
+            .append("/glist all", NamedTextColor.DARK_AQUA)
+            .append(".", NamedTextColor.YELLOW)
+            .build());
+    return 1;
+  }
+
+  private int serverCount(final CommandContext<CommandSource> context) {
+    final CommandSource source = context.getSource();
+    final String serverName = getString(context, SERVER_ARG);
+    if (serverName.equalsIgnoreCase("all")) {
+      for (RegisteredServer server : BuiltinCommandUtil.sortedServerList(server)) {
+        sendServerPlayers(source, server, true);
+      }
+      sendTotalProxyCount(source);
+    } else {
+      Optional<RegisteredServer> registeredServer = server.getServer(serverName);
+      if (!registeredServer.isPresent()) {
+        source.sendMessage(
+            TextComponent.of("Server " + serverName + " doesn't exist.", NamedTextColor.RED));
+        return -1;
+      }
+      sendServerPlayers(source, registeredServer.get(), false);
+    }
+    return 1;
+  }
+
+  private void sendTotalProxyCount(CommandSource target) {
+    target.sendMessage(TextComponent.builder("There are ", NamedTextColor.YELLOW)
+        .append(Integer.toString(server.getAllPlayers().size()), NamedTextColor.GREEN)
+        .append(" player(s) online.", NamedTextColor.YELLOW)
+        .build());
+  }
+
+  private void sendServerPlayers(CommandSource target, RegisteredServer server, boolean fromAll) {
+    List<Player> onServer = ImmutableList.copyOf(server.getPlayersConnected());
+    if (onServer.isEmpty() && fromAll) {
+      return;
+    }
+
+    TextComponent.Builder builder = TextComponent.builder()
+        .append(TextComponent.of("[" + server.getServerInfo().getName() + "] ",
+            NamedTextColor.DARK_AQUA))
+        .append("(" + onServer.size() + ")", NamedTextColor.GRAY)
+        .append(": ")
+        .resetStyle();
+
+    for (int i = 0; i < onServer.size(); i++) {
+      Player player = onServer.get(i);
+      builder.append(player.getUsername());
+
+      if (i + 1 < onServer.size()) {
+        builder.append(", ");
+      }
+    }
+
+    target.sendMessage(builder.build());
+  }
+}

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

@@ -1,10 +1,10 @@
-package com.velocitypowered.proxy.command;
+package com.velocitypowered.proxy.command.builtin;
 
 import static net.kyori.adventure.text.event.HoverEvent.showText;
 
 import com.google.common.collect.ImmutableList;
-import com.velocitypowered.api.command.Command;
 import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.command.SimpleCommand;
 import com.velocitypowered.api.permission.Tristate;
 import com.velocitypowered.api.proxy.Player;
 import com.velocitypowered.api.proxy.ProxyServer;
@@ -18,9 +18,8 @@ import java.util.stream.Stream;
 import net.kyori.adventure.text.TextComponent;
 import net.kyori.adventure.text.event.ClickEvent;
 import net.kyori.adventure.text.format.NamedTextColor;
-import org.checkerframework.checker.nullness.qual.NonNull;
 
-public class ServerCommand implements Command {
+public class ServerCommand implements SimpleCommand {
 
   public static final int MAX_SERVERS_TO_LIST = 50;
   private final ProxyServer server;
@@ -30,7 +29,10 @@ public class ServerCommand implements Command {
   }
 
   @Override
-  public void execute(CommandSource source, String @NonNull [] args) {
+  public void execute(final SimpleCommand.Invocation invocation) {
+    final CommandSource source = invocation.source();
+    final String[] args = invocation.arguments();
+
     if (!(source instanceof Player)) {
       source.sendMessage(TextComponent.of("Only players may run this command.",
           NamedTextColor.RED));
@@ -102,9 +104,11 @@ public class ServerCommand implements Command {
   }
 
   @Override
-  public List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
-    Stream<String> possibilities = Stream.concat(Stream.of("all"), server.getAllServers()
-        .stream().map(rs -> rs.getServerInfo().getName()));
+  public List<String> suggest(final SimpleCommand.Invocation invocation) {
+    final String[] currentArgs = invocation.arguments();
+    Stream<String> possibilities = server.getAllServers().stream()
+            .map(rs -> rs.getServerInfo().getName());
+
     if (currentArgs.length == 0) {
       return possibilities.collect(Collectors.toList());
     } else if (currentArgs.length == 1) {
@@ -117,7 +121,7 @@ public class ServerCommand implements Command {
   }
 
   @Override
-  public boolean hasPermission(CommandSource source, String @NonNull [] args) {
-    return source.getPermissionValue("velocity.command.server") != Tristate.FALSE;
+  public boolean hasPermission(final SimpleCommand.Invocation invocation) {
+    return invocation.source().getPermissionValue("velocity.command.server") != Tristate.FALSE;
   }
 }

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

@@ -0,0 +1,25 @@
+package com.velocitypowered.proxy.command.builtin;
+
+import com.velocitypowered.api.command.RawCommand;
+import com.velocitypowered.proxy.VelocityServer;
+import net.kyori.adventure.text.serializer.legacy.LegacyComponentSerializer;
+
+public class ShutdownCommand implements RawCommand {
+
+  private final VelocityServer server;
+
+  public ShutdownCommand(VelocityServer server) {
+    this.server = server;
+  }
+
+  @Override
+  public void execute(final Invocation invocation) {
+    String reason = invocation.arguments();
+    server.shutdown(true, LegacyComponentSerializer.legacy('&').deserialize(reason));
+  }
+
+  @Override
+  public boolean hasPermission(final Invocation invocation) {
+    return invocation.source() == server.getConsoleCommandSource();
+  }
+}

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

@@ -1,10 +1,10 @@
-package com.velocitypowered.proxy.command;
+package com.velocitypowered.proxy.command.builtin;
 
 import com.google.common.base.Joiner;
 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.command.SimpleCommand;
 import com.velocitypowered.api.permission.Tristate;
 import com.velocitypowered.api.plugin.PluginContainer;
 import com.velocitypowered.api.plugin.PluginDescription;
@@ -25,16 +25,28 @@ import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.checkerframework.checker.nullness.qual.NonNull;
 
-public class VelocityCommand implements Command {
+public class VelocityCommand implements SimpleCommand {
 
-  private final Map<String, Command> subcommands;
+  private interface SubCommand {
+
+    void execute(final CommandSource source, final String @NonNull [] args);
+
+    default List<String> suggest(final CommandSource source, final String @NonNull [] currentArgs) {
+      return ImmutableList.of();
+    }
+
+    boolean hasPermission(final CommandSource source, final String @NonNull [] args);
+  }
+
+  private final Map<String, SubCommand> commands;
 
   /**
    * Initializes the command object for /velocity.
+   *
    * @param server the Velocity server
    */
   public VelocityCommand(VelocityServer server) {
-    this.subcommands = ImmutableMap.<String, Command>builder()
+    this.commands = ImmutableMap.<String, SubCommand>builder()
         .put("version", new Info(server))
         .put("plugins", new Plugins(server))
         .put("reload", new Reload(server))
@@ -42,7 +54,7 @@ public class VelocityCommand implements Command {
   }
 
   private void usage(CommandSource source) {
-    String availableCommands = subcommands.entrySet().stream()
+    String availableCommands = commands.entrySet().stream()
         .filter(e -> e.getValue().hasPermission(source, new String[0]))
         .map(Map.Entry::getKey)
         .collect(Collectors.joining("|"));
@@ -51,13 +63,16 @@ public class VelocityCommand implements Command {
   }
 
   @Override
-  public void execute(CommandSource source, String @NonNull [] args) {
+  public void execute(final SimpleCommand.Invocation invocation) {
+    final CommandSource source = invocation.source();
+    final String[] args = invocation.arguments();
+
     if (args.length == 0) {
       usage(source);
       return;
     }
 
-    Command command = subcommands.get(args[0].toLowerCase(Locale.US));
+    SubCommand command = commands.get(args[0].toLowerCase(Locale.US));
     if (command == null) {
       usage(source);
       return;
@@ -68,16 +83,19 @@ public class VelocityCommand implements Command {
   }
 
   @Override
-  public List<String> suggest(CommandSource source, String @NonNull [] currentArgs) {
+  public List<String> suggest(final SimpleCommand.Invocation invocation) {
+    final CommandSource source = invocation.source();
+    final String[] currentArgs = invocation.arguments();
+
     if (currentArgs.length == 0) {
-      return subcommands.entrySet().stream()
+      return commands.entrySet().stream()
               .filter(e -> e.getValue().hasPermission(source, new String[0]))
               .map(Map.Entry::getKey)
               .collect(ImmutableList.toImmutableList());
     }
 
     if (currentArgs.length == 1) {
-      return subcommands.entrySet().stream()
+      return commands.entrySet().stream()
           .filter(e -> e.getKey().regionMatches(true, 0, currentArgs[0], 0,
               currentArgs[0].length()))
           .filter(e -> e.getValue().hasPermission(source, new String[0]))
@@ -85,7 +103,7 @@ public class VelocityCommand implements Command {
           .collect(ImmutableList.toImmutableList());
     }
 
-    Command command = subcommands.get(currentArgs[0].toLowerCase(Locale.US));
+    SubCommand command = commands.get(currentArgs[0].toLowerCase(Locale.US));
     if (command == null) {
       return ImmutableList.of();
     }
@@ -95,11 +113,14 @@ public class VelocityCommand implements Command {
   }
 
   @Override
-  public boolean hasPermission(CommandSource source, String @NonNull [] args) {
+  public boolean hasPermission(final SimpleCommand.Invocation invocation) {
+    final CommandSource source = invocation.source();
+    final String[] args = invocation.arguments();
+
     if (args.length == 0) {
-      return subcommands.values().stream().anyMatch(e -> e.hasPermission(source, args));
+      return commands.values().stream().anyMatch(e -> e.hasPermission(source, args));
     }
-    Command command = subcommands.get(args[0].toLowerCase(Locale.US));
+    SubCommand command = commands.get(args[0].toLowerCase(Locale.US));
     if (command == null) {
       return true;
     }
@@ -108,7 +129,7 @@ public class VelocityCommand implements Command {
     return command.hasPermission(source, actualArgs);
   }
 
-  private static class Reload implements Command {
+  private static class Reload implements SubCommand {
 
     private static final Logger logger = LogManager.getLogger(Reload.class);
     private final VelocityServer server;
@@ -136,12 +157,12 @@ public class VelocityCommand implements Command {
     }
 
     @Override
-    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
+    public boolean hasPermission(final CommandSource source, final String @NonNull [] args) {
       return source.getPermissionValue("velocity.command.reload") == Tristate.TRUE;
     }
   }
 
-  private static class Info implements Command {
+  private static class Info implements SubCommand {
 
     private final ProxyServer server;
 
@@ -189,12 +210,12 @@ public class VelocityCommand implements Command {
     }
 
     @Override
-    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
+    public boolean hasPermission(final CommandSource source, final String @NonNull [] args) {
       return source.getPermissionValue("velocity.command.info") != Tristate.FALSE;
     }
   }
 
-  private static class Plugins implements Command {
+  private static class Plugins implements SubCommand {
 
     private final ProxyServer server;
 
@@ -260,7 +281,7 @@ public class VelocityCommand implements Command {
     }
 
     @Override
-    public boolean hasPermission(CommandSource source, String @NonNull [] args) {
+    public boolean hasPermission(final CommandSource source, final String @NonNull [] args) {
       return source.getPermissionValue("velocity.command.plugins") == Tristate.TRUE;
     }
   }

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

@@ -346,6 +346,10 @@ public class VelocityConfiguration implements ProxyConfig {
     return advanced.isFailoverOnUnexpectedServerDisconnect();
   }
 
+  public boolean isAnnounceProxyCommands() {
+    return advanced.isAnnounceProxyCommands();
+  }
+
   public boolean isLogCommandExecutions() {
     return advanced.isLogCommandExecutions();
   }
@@ -586,6 +590,7 @@ public class VelocityConfiguration implements ProxyConfig {
     private boolean bungeePluginMessageChannel = true;
     private boolean showPingRequests = false;
     private boolean failoverOnUnexpectedServerDisconnect = true;
+    private boolean announceProxyCommands = true;
     private boolean logCommandExecutions = false;
 
     private Advanced() {
@@ -604,6 +609,7 @@ public class VelocityConfiguration implements ProxyConfig {
         this.showPingRequests = config.getOrElse("show-ping-requests", false);
         this.failoverOnUnexpectedServerDisconnect = config
             .getOrElse("failover-on-unexpected-server-disconnect", true);
+        this.announceProxyCommands = config.getOrElse("announce-proxy-commands", true);
         this.logCommandExecutions = config.getOrElse("log-command-executions", false);
       }
     }
@@ -648,6 +654,10 @@ public class VelocityConfiguration implements ProxyConfig {
       return failoverOnUnexpectedServerDisconnect;
     }
 
+    public boolean isAnnounceProxyCommands() {
+      return announceProxyCommands;
+    }
+
     public boolean isLogCommandExecutions() {
       return logCommandExecutions;
     }
@@ -665,6 +675,7 @@ public class VelocityConfiguration implements ProxyConfig {
           + ", bungeePluginMessageChannel=" + bungeePluginMessageChannel
           + ", showPingRequests=" + showPingRequests
           + ", failoverOnUnexpectedServerDisconnect=" + failoverOnUnexpectedServerDisconnect
+          + ", announceProxyCommands=" + announceProxyCommands
           + ", logCommandExecutions=" + logCommandExecutions
           + '}';
     }

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

@@ -3,10 +3,9 @@ package com.velocitypowered.proxy.connection.backend;
 import static com.velocitypowered.proxy.connection.backend.BungeeCordMessageResponder.getBungeeCordChannel;
 
 import com.google.common.collect.ImmutableList;
-import com.mojang.brigadier.arguments.StringArgumentType;
-import com.mojang.brigadier.builder.LiteralArgumentBuilder;
-import com.mojang.brigadier.builder.RequiredArgumentBuilder;
-import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.mojang.brigadier.tree.CommandNode;
+import com.mojang.brigadier.tree.RootCommandNode;
+import com.velocitypowered.api.command.CommandSource;
 import com.velocitypowered.api.event.command.PlayerAvailableCommandsEvent;
 import com.velocitypowered.api.event.connection.PluginMessageEvent;
 import com.velocitypowered.api.network.ProtocolVersion;
@@ -18,7 +17,6 @@ import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.connection.util.ConnectionMessages;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.packet.AvailableCommands;
-import com.velocitypowered.proxy.protocol.packet.AvailableCommands.ProtocolSuggestionProvider;
 import com.velocitypowered.proxy.protocol.packet.BossBar;
 import com.velocitypowered.proxy.protocol.packet.Disconnect;
 import com.velocitypowered.proxy.protocol.packet.KeepAlive;
@@ -30,6 +28,7 @@ import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufUtil;
 import io.netty.buffer.Unpooled;
 import io.netty.handler.timeout.ReadTimeoutException;
+import java.util.Collection;
 
 public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
@@ -164,23 +163,18 @@ public class BackendPlaySessionHandler implements MinecraftSessionHandler {
 
   @Override
   public boolean handle(AvailableCommands commands) {
+    RootCommandNode<CommandSource> rootNode = commands.getRootNode();
+    if (server.getConfiguration().isAnnounceProxyCommands()) {
       // Inject commands from the proxy.
-    for (String command : server.getCommandManager().getAllRegisteredCommands()) {
-      if (!server.getCommandManager().hasPermission(serverConn.getPlayer(), command)) {
-        continue;
+      Collection<CommandNode<CommandSource>> proxyNodes = server.getCommandManager().getDispatcher()
+              .getRoot().getChildren();
+      for (CommandNode<CommandSource> node : proxyNodes) {
+        rootNode.addChild(node);
       }
-
-      LiteralCommandNode<Object> root = LiteralArgumentBuilder.literal(command)
-          .then(RequiredArgumentBuilder.argument("args", StringArgumentType.greedyString())
-              .suggests(new ProtocolSuggestionProvider("minecraft:ask_server"))
-              .build())
-          .executes((ctx) -> 0)
-          .build();
-      commands.getRootNode().addChild(root);
     }
 
     server.getEventManager().fire(
-        new PlayerAvailableCommandsEvent(serverConn.getPlayer(), commands.getRootNode()))
+        new PlayerAvailableCommandsEvent(serverConn.getPlayer(), rootNode))
         .thenAcceptAsync(event -> playerConnection.write(commands), playerConnection.eventLoop());
     return true;
   }

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

@@ -130,7 +130,7 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
       server.getCommandManager().callCommandEvent(player, msg.substring(1))
           .thenComposeAsync(event -> processCommandExecuteResult(originalCommand,
               event.getResult()))
-          .whenCompleteAsync((ignored, throwable) -> {
+          .whenComplete((ignored, throwable) -> {
             if (server.getConfiguration().isLogCommandExecutions()) {
               logger.info("{} -> executed command /{}", player, originalCommand);
             }
@@ -414,7 +414,6 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
           for (String suggestion : suggestions) {
             offers.add(new Offer(suggestion));
           }
-
           int startPos = packet.getCommand().lastIndexOf(' ') + 1;
           if (startPos > 0) {
             TabCompleteResponse resp = new TabCompleteResponse();
@@ -460,9 +459,10 @@ public class ClientPlaySessionHandler implements MinecraftSessionHandler {
     String command = request.getCommand().substring(1);
     server.getCommandManager().offerSuggestions(player, command)
         .thenAcceptAsync(offers -> {
+          boolean needsSlash = player.getProtocolVersion().compareTo(MINECRAFT_1_13) < 0;
           try {
             for (String offer : offers) {
-              response.getOffers().add(new Offer(offer, null));
+              response.getOffers().add(new Offer(needsSlash ? "/" + offer : offer, null));
             }
             response.getOffers().sort(null);
             player.getConnection().write(response);

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

@@ -71,17 +71,12 @@ public final class VelocityConsole extends SimpleTerminalConsole implements Cons
         .appName("Velocity")
         .completer((reader, parsedLine, list) -> {
           try {
-            boolean isCommand = parsedLine.line().indexOf(' ') == -1;
             List<String> offers = this.server.getCommandManager()
                 .offerSuggestions(this, parsedLine.line())
                 .join(); // Console doesn't get harmed much by this...
             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/protocol/packet/AvailableCommands.java

@@ -16,6 +16,7 @@ import com.mojang.brigadier.tree.ArgumentCommandNode;
 import com.mojang.brigadier.tree.CommandNode;
 import com.mojang.brigadier.tree.LiteralCommandNode;
 import com.mojang.brigadier.tree.RootCommandNode;
+import com.velocitypowered.api.command.CommandSource;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
@@ -44,14 +45,13 @@ public class AvailableCommands implements MinecraftPacket {
   private static final byte FLAG_IS_REDIRECT = 0x08;
   private static final byte FLAG_HAS_SUGGESTIONS = 0x10;
 
-  // Note: Velocity doesn't use Brigadier for command handling. This may change in Velocity 2.0.0.
-  private @MonotonicNonNull RootCommandNode<Object> rootNode;
+  private @MonotonicNonNull RootCommandNode<CommandSource> rootNode;
 
   /**
    * Returns the root node.
    * @return the root node
    */
-  public RootCommandNode<Object> getRootNode() {
+  public RootCommandNode<CommandSource> getRootNode() {
     if (rootNode == null) {
       throw new IllegalStateException("Packet not yet deserialized");
     }
@@ -87,16 +87,16 @@ public class AvailableCommands implements MinecraftPacket {
     }
 
     int rootIdx = ProtocolUtils.readVarInt(buf);
-    rootNode = (RootCommandNode<Object>) wireNodes[rootIdx].built;
+    rootNode = (RootCommandNode<CommandSource>) wireNodes[rootIdx].built;
   }
 
   @Override
   public void encode(ByteBuf buf, Direction direction, ProtocolVersion protocolVersion) {
     // Assign all the children an index.
-    Deque<CommandNode<Object>> childrenQueue = new ArrayDeque<>(ImmutableList.of(rootNode));
-    Object2IntMap<CommandNode<Object>> idMappings = new Object2IntLinkedOpenHashMap<>();
+    Deque<CommandNode<CommandSource>> childrenQueue = new ArrayDeque<>(ImmutableList.of(rootNode));
+    Object2IntMap<CommandNode<CommandSource>> idMappings = new Object2IntLinkedOpenHashMap<>();
     while (!childrenQueue.isEmpty()) {
-      CommandNode<Object> child = childrenQueue.poll();
+      CommandNode<CommandSource> child = childrenQueue.poll();
       if (!idMappings.containsKey(child)) {
         idMappings.put(child, idMappings.size());
         childrenQueue.addAll(child.getChildren());
@@ -105,14 +105,14 @@ public class AvailableCommands implements MinecraftPacket {
 
     // Now serialize the children.
     ProtocolUtils.writeVarInt(buf, idMappings.size());
-    for (CommandNode<Object> child : idMappings.keySet()) {
+    for (CommandNode<CommandSource> child : idMappings.keySet()) {
       serializeNode(child, buf, idMappings);
     }
     ProtocolUtils.writeVarInt(buf, idMappings.getInt(rootNode));
   }
 
-  private static void serializeNode(CommandNode<Object> node, ByteBuf buf,
-      Object2IntMap<CommandNode<Object>> idMappings) {
+  private static void serializeNode(CommandNode<CommandSource> node, ByteBuf buf,
+      Object2IntMap<CommandNode<CommandSource>> idMappings) {
     byte flags = 0;
     if (node.getRedirect() != null) {
       flags |= FLAG_IS_REDIRECT;
@@ -127,7 +127,7 @@ public class AvailableCommands implements MinecraftPacket {
       flags |= NODE_TYPE_LITERAL;
     } else if (node instanceof ArgumentCommandNode<?, ?>) {
       flags |= NODE_TYPE_ARGUMENT;
-      if (((ArgumentCommandNode) node).getCustomSuggestions() != null) {
+      if (((ArgumentCommandNode<CommandSource, ?>) node).getCustomSuggestions() != null) {
         flags |= FLAG_HAS_SUGGESTIONS;
       }
     } else {
@@ -136,7 +136,7 @@ public class AvailableCommands implements MinecraftPacket {
 
     buf.writeByte(flags);
     ProtocolUtils.writeVarInt(buf, node.getChildren().size());
-    for (CommandNode<Object> child : node.getChildren()) {
+    for (CommandNode<CommandSource> child : node.getChildren()) {
       ProtocolUtils.writeVarInt(buf, idMappings.getInt(child));
     }
     if (node.getRedirect() != null) {
@@ -145,20 +145,17 @@ public class AvailableCommands implements MinecraftPacket {
 
     if (node instanceof ArgumentCommandNode<?, ?>) {
       ProtocolUtils.writeString(buf, node.getName());
-      ArgumentPropertyRegistry.serialize(buf, ((ArgumentCommandNode) node).getType());
+      ArgumentPropertyRegistry.serialize(buf,
+              ((ArgumentCommandNode<CommandSource, ?>) node).getType());
 
-      if (((ArgumentCommandNode) node).getCustomSuggestions() != null) {
-        // The unchecked cast is required, but it is not particularly relevant because we check for
-        // a more specific type later. (Even then, we only pull out one field.)
-        @SuppressWarnings("unchecked")
-        SuggestionProvider<Object> provider = ((ArgumentCommandNode) node).getCustomSuggestions();
-
-        if (!(provider instanceof ProtocolSuggestionProvider)) {
-          throw new IllegalArgumentException("Suggestion provider " + provider.getClass().getName()
-            + " is not valid.");
+      if (((ArgumentCommandNode<CommandSource, ?>) node).getCustomSuggestions() != null) {
+        SuggestionProvider<CommandSource> provider = ((ArgumentCommandNode<CommandSource, ?>) node)
+                .getCustomSuggestions();
+        String name = "minecraft:ask_server";
+        if (provider instanceof ProtocolSuggestionProvider) {
+          name = ((ProtocolSuggestionProvider) provider).name;
         }
-
-        ProtocolUtils.writeString(buf, ((ProtocolSuggestionProvider) provider).name);
+        ProtocolUtils.writeString(buf, name);
       }
     } else if (node instanceof LiteralCommandNode<?>) {
       ProtocolUtils.writeString(buf, node.getName());
@@ -188,7 +185,7 @@ public class AvailableCommands implements MinecraftPacket {
         String name = ProtocolUtils.readString(buf);
         ArgumentType<?> argumentType = ArgumentPropertyRegistry.deserialize(buf);
 
-        RequiredArgumentBuilder<Object, ?> argumentBuilder = RequiredArgumentBuilder
+        RequiredArgumentBuilder<CommandSource, ?> argumentBuilder = RequiredArgumentBuilder
             .argument(name, argumentType);
         if ((flags & FLAG_HAS_SUGGESTIONS) != 0) {
           argumentBuilder.suggests(new ProtocolSuggestionProvider(ProtocolUtils.readString(buf)));
@@ -205,11 +202,11 @@ public class AvailableCommands implements MinecraftPacket {
     private final byte flags;
     private final int[] children;
     private final int redirectTo;
-    private final @Nullable ArgumentBuilder<Object, ?> args;
-    private @MonotonicNonNull CommandNode<Object> built;
+    private final @Nullable ArgumentBuilder<CommandSource, ?> args;
+    private @MonotonicNonNull CommandNode<CommandSource> built;
 
     private WireNode(int idx, byte flags, int[] children, int redirectTo,
-        @Nullable ArgumentBuilder<Object, ?> args) {
+        @Nullable ArgumentBuilder<CommandSource, ?> args) {
       this.idx = idx;
       this.flags = flags;
       this.children = children;
@@ -251,7 +248,7 @@ public class AvailableCommands implements MinecraftPacket {
 
           // If executable, add a dummy command
           if ((flags & FLAG_EXECUTABLE) != 0) {
-            args.executes((Command<Object>) context -> 0);
+            args.executes((Command<CommandSource>) context -> 0);
           }
 
           this.built = args.build();
@@ -267,7 +264,7 @@ public class AvailableCommands implements MinecraftPacket {
 
       // Associate children with nodes
       for (int child : children) {
-        CommandNode<Object> childNode = wireNodes[child].built;
+        CommandNode<CommandSource> childNode = wireNodes[child].built;
         if (!(childNode instanceof RootCommandNode)) {
           built.addChild(childNode);
         }
@@ -286,9 +283,11 @@ public class AvailableCommands implements MinecraftPacket {
 
       if (args != null) {
         if (args instanceof LiteralArgumentBuilder) {
-          helper.add("argsLabel", ((LiteralArgumentBuilder) args).getLiteral());
+          helper.add("argsLabel",
+                  ((LiteralArgumentBuilder<CommandSource>) args).getLiteral());
         } else if (args instanceof RequiredArgumentBuilder) {
-          helper.add("argsName", ((RequiredArgumentBuilder) args).getName());
+          helper.add("argsName",
+                  ((RequiredArgumentBuilder<CommandSource, ?>) args).getName());
         }
       }
 
@@ -300,7 +299,7 @@ public class AvailableCommands implements MinecraftPacket {
    * A placeholder {@link SuggestionProvider} used internally to preserve the suggestion provider
    * name.
    */
-  public static class ProtocolSuggestionProvider implements SuggestionProvider<Object> {
+  public static class ProtocolSuggestionProvider implements SuggestionProvider<CommandSource> {
 
     private final String name;
 
@@ -309,7 +308,7 @@ public class AvailableCommands implements MinecraftPacket {
     }
 
     @Override
-    public CompletableFuture<Suggestions> getSuggestions(CommandContext<Object> context,
+    public CompletableFuture<Suggestions> getSuggestions(CommandContext<CommandSource> context,
         SuggestionsBuilder builder) throws CommandSyntaxException {
       return builder.buildFuture();
     }

proxy/src/main/java/com/velocitypowered/proxy/util/BrigadierUtils.java

@@ -0,0 +1,149 @@
+package com.velocitypowered.proxy.util;
+
+import com.google.common.base.Preconditions;
+import com.mojang.brigadier.Command;
+import com.mojang.brigadier.arguments.StringArgumentType;
+import com.mojang.brigadier.builder.ArgumentBuilder;
+import com.mojang.brigadier.builder.LiteralArgumentBuilder;
+import com.mojang.brigadier.builder.RequiredArgumentBuilder;
+import com.mojang.brigadier.context.CommandContext;
+import com.mojang.brigadier.suggestion.SuggestionProvider;
+import com.mojang.brigadier.tree.CommandNode;
+import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.velocitypowered.api.command.CommandSource;
+import java.util.Locale;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+/**
+ * Provides utilities for working with Brigadier commands.
+ */
+public final class BrigadierUtils {
+
+  /**
+   * Returns a literal node that redirects its execution to
+   * the given destination node.
+   *
+   * @param alias the command alias
+   * @param destination the destination node
+   * @return the built node
+   */
+  public static LiteralCommandNode<CommandSource> buildRedirect(
+          final String alias, final LiteralCommandNode<CommandSource> destination) {
+    // Redirects only work for nodes with children, but break the top argument-less command.
+    // Manually adding the root command after setting the redirect doesn't fix it.
+    // See https://github.com/Mojang/brigadier/issues/46). Manually clone the node instead.
+    LiteralArgumentBuilder<CommandSource> builder = LiteralArgumentBuilder
+            .<CommandSource>literal(alias.toLowerCase(Locale.ENGLISH))
+            .requires(destination.getRequirement())
+            .forward(
+                destination.getRedirect(), destination.getRedirectModifier(), destination.isFork())
+            .executes(destination.getCommand());
+    for (CommandNode<CommandSource> child : destination.getChildren()) {
+      builder.then(child);
+    }
+    return builder.build();
+  }
+
+  /**
+   * Returns a literal node that optionally accepts arguments
+   * as a raw {@link String}.
+   *
+   * @param alias the literal alias
+   * @param brigadierCommand the command to execute
+   * @param suggestionProvider the suggestion provider
+   * @return the built node
+   */
+  public static LiteralCommandNode<CommandSource> buildRawArgumentsLiteral(
+          final String alias, final Command<CommandSource> brigadierCommand,
+          SuggestionProvider<CommandSource> suggestionProvider) {
+    return LiteralArgumentBuilder
+            .<CommandSource>literal(alias.toLowerCase(Locale.ENGLISH))
+            .then(RequiredArgumentBuilder
+                .<CommandSource, String>argument("arguments", StringArgumentType.greedyString())
+                .suggests(suggestionProvider)
+                .executes(brigadierCommand))
+            .executes(brigadierCommand)
+            .build();
+  }
+
+  /**
+   * Returns the used command alias.
+   *
+   * @param context the command context
+   * @return the parsed command alias
+   */
+  public static String getAlias(final CommandContext<CommandSource> context) {
+    return context.getNodes().get(0).getNode().getName();
+  }
+
+  /**
+   * Returns the raw {@link String} arguments of a command execution.
+   *
+   * @param context the command context
+   * @return the parsed arguments
+   */
+  public static String getRawArguments(final CommandContext<CommandSource> context) {
+    String cmdLine = context.getInput();
+    int firstSpace = cmdLine.indexOf(' ');
+    if (firstSpace == -1) {
+      return "";
+    }
+    return cmdLine.substring(firstSpace + 1);
+  }
+
+  /**
+   * Returns the splitted arguments of a command node built with
+   * {@link #buildRawArgumentsLiteral(String, Command, SuggestionProvider)}.
+   *
+   * @param context the command context
+   * @return the parsed arguments
+   */
+  public static String[] getSplitArguments(final CommandContext<CommandSource> context) {
+    String line = getRawArguments(context);
+    if (line.isEmpty()) {
+      return new String[0];
+    }
+    return line.trim().split(" ", -1);
+  }
+
+  /**
+   * Returns the normalized representation of the given command input.
+   *
+   * @param cmdLine the command input
+   * @param trim whether to trim argument-less inputs
+   * @return the normalized command
+   */
+  public static String normalizeInput(final String cmdLine, final boolean trim) {
+    // Command aliases are case insensitive, but Brigadier isn't
+    String command = trim ? cmdLine.trim() : cmdLine;
+    int firstSpace = command.indexOf(' ');
+    if (firstSpace != -1) {
+      return command.substring(0, firstSpace).toLowerCase(Locale.ENGLISH)
+              + command.substring(firstSpace);
+    }
+    return command.toLowerCase(Locale.ENGLISH);
+  }
+
+  /**
+   * Prepares the given command node prior for hinting metadata to
+   * a {@link com.velocitypowered.api.command.Command}.
+   *
+   * @param node the command node to be wrapped
+   * @param command the command to execute
+   * @return the wrapped command node
+   */
+  public static CommandNode<CommandSource> wrapForHinting(
+          final CommandNode<CommandSource> node, final @Nullable Command<CommandSource> command) {
+    Preconditions.checkNotNull(node, "node");
+    ArgumentBuilder<CommandSource, ?> builder = node.createBuilder();
+    builder.executes(command);
+    for (CommandNode<CommandSource> child : node.getChildren()) {
+      builder.then(wrapForHinting(child, command));
+    }
+    return builder.build();
+  }
+
+  private BrigadierUtils() {
+    throw new AssertionError();
+  }
+}

proxy/src/main/resources/default-velocity.toml

@@ -123,6 +123,9 @@ show-ping-requests = false
 # can disable this setting to use the BungeeCord behavior.
 failover-on-unexpected-server-disconnect = true
 
+# Declares the proxy commands to 1.13+ clients.
+announce-proxy-commands = true
+
 # Enables the logging of commands
 log-command-executions = false
 

proxy/src/test/java/com/velocitypowered/proxy/command/CommandManagerTests.java

@@ -0,0 +1,523 @@
+package com.velocitypowered.proxy.command;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.junit.jupiter.api.Assertions.fail;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
+import com.mojang.brigadier.arguments.IntegerArgumentType;
+import com.mojang.brigadier.arguments.StringArgumentType;
+import com.mojang.brigadier.builder.LiteralArgumentBuilder;
+import com.mojang.brigadier.builder.RequiredArgumentBuilder;
+import com.mojang.brigadier.tree.ArgumentCommandNode;
+import com.mojang.brigadier.tree.CommandNode;
+import com.mojang.brigadier.tree.LiteralCommandNode;
+import com.velocitypowered.api.command.BrigadierCommand;
+import com.velocitypowered.api.command.Command;
+import com.velocitypowered.api.command.CommandMeta;
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.command.RawCommand;
+import com.velocitypowered.api.command.SimpleCommand;
+import com.velocitypowered.proxy.plugin.MockEventManager;
+import com.velocitypowered.proxy.plugin.VelocityEventManager;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.junit.jupiter.api.Test;
+
+public class CommandManagerTests {
+
+  private static final VelocityEventManager EVENT_MANAGER = new MockEventManager();
+
+  static {
+    Runtime.getRuntime().addShutdownHook(new Thread(() -> {
+      try {
+        EVENT_MANAGER.shutdown();
+      } catch (InterruptedException e) {
+        Thread.currentThread().interrupt();
+      }
+    }));
+  }
+
+  static VelocityCommandManager createManager() {
+    return new VelocityCommandManager(EVENT_MANAGER);
+  }
+
+  @Test
+  void testConstruction() {
+    VelocityCommandManager manager = createManager();
+    assertFalse(manager.hasCommand("foo"));
+    assertTrue(manager.getDispatcher().getRoot().getChildren().isEmpty());
+    assertFalse(manager.execute(MockCommandSource.INSTANCE, "foo"));
+    assertFalse(manager.executeImmediately(MockCommandSource.INSTANCE, "bar"));
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "").join().isEmpty());
+  }
+
+  @Test
+  void testBrigadierRegister() {
+    VelocityCommandManager manager = createManager();
+    LiteralCommandNode<CommandSource> node = LiteralArgumentBuilder
+            .<CommandSource>literal("foo")
+            .build();
+    BrigadierCommand command = new BrigadierCommand(node);
+    manager.register(command);
+
+    assertEquals(node, command.getNode());
+    assertTrue(manager.hasCommand("fOo"));
+
+    LiteralCommandNode<CommandSource> barNode = LiteralArgumentBuilder
+            .<CommandSource>literal("bar")
+            .build();
+    BrigadierCommand aliasesCommand = new BrigadierCommand(barNode);
+    CommandMeta meta = manager.metaBuilder(aliasesCommand)
+            .aliases("baZ")
+            .build();
+
+    assertEquals(ImmutableSet.of("bar", "baz"), meta.getAliases());
+    assertTrue(meta.getHints().isEmpty());
+    manager.register(meta, aliasesCommand);
+    assertTrue(manager.hasCommand("bAr"));
+    assertTrue(manager.hasCommand("Baz"));
+  }
+
+  @Test
+  void testSimpleRegister() {
+    VelocityCommandManager manager = createManager();
+    SimpleCommand command = new NoopSimpleCommand();
+
+    manager.register("Foo", command);
+    assertTrue(manager.hasCommand("foO"));
+    manager.unregister("fOo");
+    assertFalse(manager.hasCommand("foo"));
+
+    manager.register("foo", command, "bAr", "BAZ");
+    assertTrue(manager.hasCommand("bar"));
+    assertTrue(manager.hasCommand("bAz"));
+  }
+
+  @Test
+  void testRawRegister() {
+    VelocityCommandManager manager = createManager();
+    RawCommand command = new NoopRawCommand();
+
+    assertThrows(IllegalArgumentException.class, () -> manager.register(command),
+            "no aliases throws");
+    manager.register(command, "foO", "BAR");
+    assertTrue(manager.hasCommand("fOo"));
+    assertTrue(manager.hasCommand("bar"));
+  }
+
+  @Test
+  void testDeprecatedRegister() {
+    VelocityCommandManager manager = createManager();
+    Command command = new NoopDeprecatedCommand();
+
+    manager.register("foo", command);
+    assertTrue(manager.hasCommand("foO"));
+  }
+
+  @Test
+  void testAlreadyRegisteredThrows() {
+    VelocityCommandManager manager = createManager();
+    manager.register("bar", new NoopDeprecatedCommand());
+    assertThrows(IllegalArgumentException.class, () ->
+            manager.register("BAR", new NoopSimpleCommand()));
+    assertThrows(IllegalArgumentException.class, () -> {
+      CommandMeta meta = manager.metaBuilder("baz")
+              .aliases("BAr")
+              .build();
+      manager.register(meta, new NoopRawCommand());
+    });
+  }
+
+  @Test
+  void testBrigadierExecute() {
+    VelocityCommandManager manager = createManager();
+    AtomicBoolean executed = new AtomicBoolean(false);
+    AtomicBoolean checkedRequires = new AtomicBoolean(false);
+    LiteralCommandNode<CommandSource> node = LiteralArgumentBuilder
+            .<CommandSource>literal("buy")
+            .executes(context -> {
+              assertEquals(MockCommandSource.INSTANCE, context.getSource());
+              assertEquals("buy", context.getInput());
+              executed.set(true);
+              return 1;
+            })
+            .build();
+    CommandNode<CommandSource> quantityNode = RequiredArgumentBuilder
+            .<CommandSource, Integer>argument("quantity", IntegerArgumentType.integer(12, 16))
+            .requires(source -> {
+              assertEquals(MockCommandSource.INSTANCE, source);
+              checkedRequires.set(true);
+              return true;
+            })
+            .executes(context -> {
+              int argument = IntegerArgumentType.getInteger(context, "quantity");
+              assertEquals(14, argument);
+              executed.set(true);
+              return 1;
+            })
+            .build();
+    CommandNode<CommandSource> productNode = RequiredArgumentBuilder
+            .<CommandSource, String>argument("product", StringArgumentType.string())
+            .requires(source -> {
+              checkedRequires.set(true);
+              return false;
+            })
+            .executes(context -> fail("was executed"))
+            .build();
+    quantityNode.addChild(productNode);
+    node.addChild(quantityNode);
+    manager.register(new BrigadierCommand(node));
+
+    assertTrue(manager.executeAsync(MockCommandSource.INSTANCE, "buy ").join());
+    assertTrue(executed.compareAndSet(true, false), "was executed");
+    assertTrue(manager.executeImmediatelyAsync(MockCommandSource.INSTANCE, "buy 14").join());
+    assertTrue(checkedRequires.compareAndSet(true, false));
+    assertTrue(executed.get());
+    assertFalse(manager.execute(MockCommandSource.INSTANCE, "buy 9"),
+            "Invalid arg returns false");
+    assertFalse(manager.executeImmediately(MockCommandSource.INSTANCE, "buy 12 bananas"));
+    assertTrue(checkedRequires.get());
+  }
+
+  @Test
+  void testSimpleExecute() {
+    VelocityCommandManager manager = createManager();
+    AtomicBoolean executed = new AtomicBoolean(false);
+    SimpleCommand command = invocation -> {
+      assertEquals(MockCommandSource.INSTANCE, invocation.source());
+      assertArrayEquals(new String[] {"bar", "254"}, invocation.arguments());
+      executed.set(true);
+    };
+    manager.register("foo", command);
+
+    assertTrue(manager.executeAsync(MockCommandSource.INSTANCE, "foo bar 254").join());
+    assertTrue(executed.get());
+
+    SimpleCommand noPermsCommand = new SimpleCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        fail("was executed");
+      }
+
+      @Override
+      public boolean hasPermission(final Invocation invocation) {
+        return false;
+      }
+    };
+
+    manager.register("dangerous", noPermsCommand, "veryDangerous");
+    assertFalse(manager.execute(MockCommandSource.INSTANCE, "dangerous"));
+    assertFalse(manager.executeImmediately(MockCommandSource.INSTANCE, "verydangerous 123"));
+  }
+
+  @Test
+  void testRawExecute() {
+    VelocityCommandManager manager = createManager();
+    AtomicBoolean executed = new AtomicBoolean(false);
+    RawCommand command = new RawCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        assertEquals(MockCommandSource.INSTANCE, invocation.source());
+        assertEquals("lobby 23", invocation.arguments());
+        executed.set(true);
+      }
+    };
+    manager.register("sendMe", command);
+
+    assertTrue(manager.executeImmediately(MockCommandSource.INSTANCE, "sendMe lobby 23"));
+    assertTrue(executed.compareAndSet(true, false));
+
+    RawCommand noArgsCommand = new RawCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        assertEquals("", invocation.arguments());
+        executed.set(true);
+      }
+    };
+    manager.register("noargs", noArgsCommand);
+
+    assertTrue(manager.executeImmediately(MockCommandSource.INSTANCE, "noargs"));
+    assertTrue(executed.get());
+    assertTrue(manager.executeImmediately(MockCommandSource.INSTANCE, "noargs "));
+
+    RawCommand noPermsCommand = new RawCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        fail("was executed");
+      }
+
+      @Override
+      public boolean hasPermission(final Invocation invocation) {
+        return false;
+      }
+    };
+
+    manager.register("sendThem", noPermsCommand);
+    assertFalse(manager.executeImmediately(MockCommandSource.INSTANCE, "sendThem foo"));
+  }
+
+  @Test
+  void testDeprecatedExecute() {
+    VelocityCommandManager manager = createManager();
+    AtomicBoolean executed = new AtomicBoolean(false);
+    Command command = new Command() {
+      @Override
+      public void execute(final CommandSource source, final String @NonNull [] args) {
+        assertEquals(MockCommandSource.INSTANCE, source);
+        assertArrayEquals(new String[] { "boo", "123" }, args);
+        executed.set(true);
+      }
+    };
+    manager.register("foo", command);
+
+    assertTrue(manager.execute(MockCommandSource.INSTANCE, "foo boo 123"));
+    assertTrue(executed.get());
+
+    Command noPermsCommand = new Command() {
+      @Override
+      public boolean hasPermission(final CommandSource source, final String @NonNull [] args) {
+        return false;
+      }
+    };
+
+    manager.register("oof", noPermsCommand, "veryOof");
+    assertFalse(manager.execute(MockCommandSource.INSTANCE, "veryOOF"));
+    assertFalse(manager.executeImmediately(MockCommandSource.INSTANCE, "ooF boo 54321"));
+  }
+
+  @Test
+  void testSuggestions() {
+    VelocityCommandManager manager = createManager();
+
+    LiteralCommandNode<CommandSource> brigadierNode = LiteralArgumentBuilder
+            .<CommandSource>literal("brigadier")
+            .build();
+    CommandNode<CommandSource> nameNode = RequiredArgumentBuilder
+            .<CommandSource, String>argument("name", StringArgumentType.string())
+            .build();
+    CommandNode<CommandSource> numberNode = RequiredArgumentBuilder
+            .<CommandSource, Integer>argument("quantity", IntegerArgumentType.integer())
+            .suggests((context, builder) -> builder.suggest(2).suggest(3).buildFuture())
+            .build();
+    nameNode.addChild(numberNode);
+    brigadierNode.addChild(nameNode);
+    manager.register(new BrigadierCommand(brigadierNode));
+
+    SimpleCommand simpleCommand = new SimpleCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        fail();
+      }
+
+      @Override
+      public List<String> suggest(final Invocation invocation) {
+        switch (invocation.arguments().length) {
+          case 0:
+            return ImmutableList.of("foo", "bar");
+          case 1:
+            return ImmutableList.of("123");
+          default:
+            return ImmutableList.of();
+        }
+      }
+    };
+    manager.register("simple", simpleCommand);
+
+    RawCommand rawCommand = new RawCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        fail();
+      }
+
+      @Override
+      public List<String> suggest(final Invocation invocation) {
+        switch (invocation.arguments()) {
+          case "":
+            return ImmutableList.of("foo", "baz");
+          case "foo ":
+            return ImmutableList.of("2", "3", "5", "7");
+          case "bar ":
+            return ImmutableList.of("11", "13", "17");
+          default:
+            return ImmutableList.of();
+        }
+      }
+    };
+    manager.register("raw", rawCommand);
+
+    Command deprecatedCommand = new Command() {
+      @Override
+      public List<String> suggest(
+              final CommandSource source, final String @NonNull [] currentArgs) {
+        switch (currentArgs.length) {
+          case 0:
+            return ImmutableList.of("boo", "scary");
+          case 1:
+            return ImmutableList.of("123", "456");
+          default:
+            return ImmutableList.of();
+        }
+      }
+    };
+    manager.register("deprecated", deprecatedCommand);
+
+    assertEquals(
+            ImmutableList.of("brigadier", "deprecated", "raw", "simple"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "").join(),
+            "literals are in alphabetical order");
+    assertEquals(
+            ImmutableList.of("brigadier"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "briga").join());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "brigadier")
+            .join().isEmpty());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "brigadier ")
+            .join().isEmpty());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "brigadier foo")
+            .join().isEmpty());
+    assertEquals(
+            ImmutableList.of("2", "3"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "brigadier foo ").join());
+    assertEquals(
+            ImmutableList.of("bar", "foo"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "simple ").join());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "simple")
+            .join().isEmpty());
+    assertEquals(
+            ImmutableList.of("123"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "simPle foo ").join());
+    assertEquals(
+            ImmutableList.of("baz", "foo"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "raw ").join());
+    assertEquals(
+            ImmutableList.of("2", "3", "5", "7"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "raw foo ").join());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "raw foo")
+            .join().isEmpty());
+    assertEquals(
+            ImmutableList.of("11", "13", "17"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "rAW bar ").join());
+    assertEquals(
+            ImmutableList.of("boo", "scary"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "deprecated ").join());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "deprecated")
+            .join().isEmpty());
+    assertEquals(
+            ImmutableList.of("123", "456"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "deprEcated foo ").join());
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "deprecated foo 789 ")
+            .join().isEmpty());
+  }
+
+  @Test
+  void testBrigadierSuggestionPermissions() {
+    VelocityCommandManager manager = createManager();
+    LiteralCommandNode<CommandSource> manageNode = LiteralArgumentBuilder
+            .<CommandSource>literal("manage")
+            .requires(source -> false)
+            .build();
+    CommandNode<CommandSource> idNode = RequiredArgumentBuilder
+            .<CommandSource, Integer>argument("id", IntegerArgumentType.integer(0))
+            .suggests((context, builder) -> fail("called suggestion builder"))
+            .build();
+    manageNode.addChild(idNode);
+    manager.register(new BrigadierCommand(manageNode));
+
+    // Brigadier doesn't call the children predicate when requesting suggestions.
+    // However, it won't query children if the source doesn't pass the parent
+    // #requires predicate.
+    assertTrue(manager.offerSuggestions(MockCommandSource.INSTANCE, "manage ")
+            .join().isEmpty());
+  }
+
+  @Test
+  void testHinting() {
+    VelocityCommandManager manager = createManager();
+    AtomicBoolean executed = new AtomicBoolean(false);
+    AtomicBoolean calledSuggestionProvider = new AtomicBoolean(false);
+    AtomicReference<String> expectedArgs = new AtomicReference<>();
+    RawCommand command = new RawCommand() {
+      @Override
+      public void execute(final Invocation invocation) {
+        assertEquals(expectedArgs.get(), invocation.arguments());
+        executed.set(true);
+      }
+
+      @Override
+      public List<String> suggest(final Invocation invocation) {
+        return ImmutableList.of("raw");
+      }
+    };
+
+    CommandNode<CommandSource> barHint = LiteralArgumentBuilder
+            .<CommandSource>literal("bar")
+            .executes(context -> fail("hints don't get executed"))
+            .build();
+    ArgumentCommandNode<CommandSource, Integer> numberArg = RequiredArgumentBuilder
+            .<CommandSource, Integer>argument("number", IntegerArgumentType.integer())
+            .suggests((context, builder) -> {
+              calledSuggestionProvider.set(true);
+              return builder.suggest("456").buildFuture();
+            })
+            .build();
+    barHint.addChild(numberArg);
+    CommandNode<CommandSource> bazHint = LiteralArgumentBuilder
+            .<CommandSource>literal("baz")
+            .build();
+    CommandMeta meta = manager.metaBuilder("foo")
+            .aliases("foo2")
+            .hint(barHint)
+            .hint(bazHint)
+            .build();
+    manager.register(meta, command);
+
+    expectedArgs.set("notBarOrBaz");
+    assertTrue(manager.execute(MockCommandSource.INSTANCE, "foo notBarOrBaz"));
+    assertTrue(executed.compareAndSet(true, false));
+    expectedArgs.set("anotherArg 123");
+    assertTrue(manager.execute(MockCommandSource.INSTANCE, "Foo2 anotherArg 123"));
+    assertTrue(executed.compareAndSet(true, false));
+    expectedArgs.set("bar");
+    assertTrue(manager.execute(MockCommandSource.INSTANCE, "foo bar"));
+    assertTrue(executed.compareAndSet(true, false));
+    expectedArgs.set("bar 123");
+    assertTrue(manager.execute(MockCommandSource.INSTANCE, "foo2 bar 123"));
+    assertTrue(executed.compareAndSet(true, false));
+
+    assertEquals(ImmutableList.of("bar", "baz", "raw"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "foo ").join());
+    assertFalse(calledSuggestionProvider.get());
+    assertEquals(ImmutableList.of("456"),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "foo bar ").join());
+    assertTrue(calledSuggestionProvider.compareAndSet(true, false));
+    assertEquals(ImmutableList.of(),
+            manager.offerSuggestions(MockCommandSource.INSTANCE, "foo2 baz ").join());
+  }
+
+  static class NoopSimpleCommand implements SimpleCommand {
+    @Override
+    public void execute(final Invocation invocation) {
+
+    }
+  }
+
+  static class NoopRawCommand implements RawCommand {
+    @Override
+    public void execute(final Invocation invocation) {
+
+    }
+  }
+
+  static class NoopDeprecatedCommand implements Command {
+    @Override
+    public void execute(final CommandSource source, final String @NonNull [] args) {
+
+    }
+  }
+}

proxy/src/test/java/com/velocitypowered/proxy/command/MockCommandSource.java

@@ -0,0 +1,20 @@
+package com.velocitypowered.proxy.command;
+
+import com.velocitypowered.api.command.CommandSource;
+import com.velocitypowered.api.permission.Tristate;
+import net.kyori.text.Component;
+
+public class MockCommandSource implements CommandSource {
+
+  public static CommandSource INSTANCE = new MockCommandSource();
+
+  @Override
+  public void sendMessage(final Component component) {
+
+  }
+
+  @Override
+  public Tristate getPermissionValue(final String permission) {
+    return Tristate.UNDEFINED;
+  }
+}

proxy/src/test/java/com/velocitypowered/proxy/plugin/MockEventManager.java

@@ -0,0 +1,11 @@
+package com.velocitypowered.proxy.plugin;
+
+/**
+ * A mock {@link VelocityEventManager}. Must be shutdown after use!
+ */
+public class MockEventManager extends VelocityEventManager {
+
+  public MockEventManager() {
+    super(MockPluginManager.INSTANCE);
+  }
+}

proxy/src/test/java/com/velocitypowered/proxy/plugin/MockPluginManager.java

@@ -0,0 +1,38 @@
+package com.velocitypowered.proxy.plugin;
+
+import com.google.common.collect.ImmutableList;
+import com.velocitypowered.api.plugin.PluginContainer;
+import com.velocitypowered.api.plugin.PluginManager;
+import java.nio.file.Path;
+import java.util.Collection;
+import java.util.Optional;
+
+public class MockPluginManager implements PluginManager {
+
+  public static PluginManager INSTANCE = new MockPluginManager();
+
+  @Override
+  public Optional<PluginContainer> fromInstance(final Object instance) {
+    return Optional.empty();
+  }
+
+  @Override
+  public Optional<PluginContainer> getPlugin(final String id) {
+    return Optional.empty();
+  }
+
+  @Override
+  public Collection<PluginContainer> getPlugins() {
+    return ImmutableList.of();
+  }
+
+  @Override
+  public boolean isLoaded(final String id) {
+    return false;
+  }
+
+  @Override
+  public void addToClasspath(final Object plugin, final Path path) {
+
+  }
+}