fix: adding a message handler while handling a message does not cause concurrency exceptions

Author: AlmasB

fxgl-io/src/main/java/com/almasb/fxgl/net/Connection.java

@@ -16,6 +16,7 @@
 import java.util.List;
 import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.CopyOnWriteArrayList;
 
 /**
  * @author Almas Baimagambetov ([email protected])
@@ -30,8 +31,8 @@ public abstract class Connection<T> {
 
     private PropertyMap localSessionData = new PropertyMap();
 
-    protected final List<MessageHandler<T>> messageHandlers = new ArrayList<>();
-    protected final List<MessageHandler<T>> messageHandlersFX = new ArrayList<>();
+    protected final List<MessageHandler<T>> messageHandlers = new CopyOnWriteArrayList<>();
+    protected final List<MessageHandler<T>> messageHandlersFX = new CopyOnWriteArrayList<>();
 
     protected BlockingQueue<T> messageQueue = new ArrayBlockingQueue<>(100);