Change our data structures to use concurrent equivalents, netty is multi-threaded.

src/main/java/us/myles/ViaVersion/api/protocol/ProtocolPipeline.java

@@ -11,12 +11,12 @@ import us.myles.ViaVersion.protocols.base.ProtocolInfo;
 
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.LinkedList;
 import java.util.List;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.logging.Level;
 
 public class ProtocolPipeline extends Protocol {
-    LinkedList<Protocol> protocolList;
+    List<Protocol> protocolList;
     private UserConnection userConnection;
 
     public ProtocolPipeline(UserConnection userConnection) {
@@ -26,9 +26,9 @@ public class ProtocolPipeline extends Protocol {
 
     @Override
     protected void registerPackets() {
-        protocolList = new LinkedList<>();
+        protocolList = new CopyOnWriteArrayList<>();
         // This is a pipeline so we register basic pipes
-        protocolList.addLast(ProtocolRegistry.BASE_PROTOCOL);
+        protocolList.add(ProtocolRegistry.BASE_PROTOCOL);
     }
 
     @Override
@@ -54,7 +54,7 @@ public class ProtocolPipeline extends Protocol {
      */
     public void add(Protocol protocol) {
         if (protocolList != null) {
-            protocolList.addLast(protocol);
+            protocolList.add(protocol);
             protocol.init(userConnection);
         } else {
             throw new NullPointerException("Tried to add protocol to early");

src/main/java/us/myles/ViaVersion/protocols/protocol1_9to1_8/storage/EntityTracker.java

@@ -2,6 +2,7 @@ package us.myles.ViaVersion.protocols.protocol1_9to1_8.storage;
 
 import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
+import com.google.common.collect.Sets;
 import io.netty.buffer.ByteBuf;
 import lombok.Getter;
 import lombok.Setter;
@@ -26,16 +27,17 @@ import us.myles.ViaVersion.protocols.protocol1_9to1_8.chat.GameMode;
 import us.myles.ViaVersion.protocols.protocol1_9to1_8.metadata.NewType;
 
 import java.util.*;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
 
 @Getter
 public class EntityTracker extends StoredObject {
-    private final Map<Integer, UUID> uuidMap = new HashMap<>();
-    private final Map<Integer, EntityType> clientEntityTypes = new HashMap<>();
-    private final Map<Integer, Integer> vehicleMap = new HashMap<>();
-    private final Map<Integer, BossBar> bossBarMap = new HashMap<>();
-    private final Set<Integer> validBlocking = new HashSet<>();
-    private final Set<Integer> knownHolograms = new HashSet<>();
+    private final Map<Integer, UUID> uuidMap = new ConcurrentHashMap<>();
+    private final Map<Integer, EntityType> clientEntityTypes = new ConcurrentHashMap<>();
+    private final Map<Integer, Integer> vehicleMap = new ConcurrentHashMap<>();
+    private final Map<Integer, BossBar> bossBarMap = new ConcurrentHashMap<>();
+    private final Set<Integer> validBlocking = Sets.newConcurrentHashSet();
+    private final Set<Integer> knownHolograms = Sets.newConcurrentHashSet();
     private final Cache<Position, Material> blockInteractions = CacheBuilder.newBuilder().maximumSize(10).expireAfterAccess(250, TimeUnit.MILLISECONDS).build();
     @Setter
     private boolean blocking = false;