Copy invocation handler list in Java client (#43402)

Author: BrennanConroy

src/SignalR/clients/java/signalr/core/src/main/java/com/microsoft/signalr/CallbackMap.java

@@ -31,7 +31,12 @@ public InvocationHandler put(String target, Object action, Type... types) {
                     }
                 }
             }
+            methodHandlers = new ArrayList<>(methodHandlers);
             methodHandlers.add(handler);
+
+            // replace List in handlers map
+            handlers.remove(target);
+            handlers.put(target, methodHandlers);
             return handler;
         } finally {
             lock.unlock();
@@ -41,7 +46,7 @@ public InvocationHandler put(String target, Object action, Type... types) {
     public List<InvocationHandler> get(String key) {
         try {
             lock.lock();
-            return handlers.get(key);
+            return this.handlers.get(key);
         } finally {
             lock.unlock();
         }
@@ -55,4 +60,21 @@ public void remove(String key) {
             lock.unlock();
         }
     }
+
+    public void remove(String key, InvocationHandler handler) {
+        try {
+            lock.lock();
+            List<InvocationHandler> handlers = this.handlers.get(key);
+            if (handlers != null) {
+                handlers = new ArrayList<>(handlers);
+                handlers.remove(handler);
+
+                // replace List in handlers map
+                this.handlers.remove(key);
+                this.handlers.put(key, handlers);
+            }
+        } finally {
+            lock.unlock();
+        }
+    }
 }

src/SignalR/clients/java/signalr/core/src/main/java/com/microsoft/signalr/Subscription.java

@@ -23,9 +23,6 @@ public class Subscription {
      * Removes the client method handler represented by this subscription.
      */
     public void unsubscribe() {
-        List<InvocationHandler> handler = this.handlers.get(target);
-        if (handler != null) {
-            handler.remove(this.handler);
-        }
+        this.handlers.remove(this.target, this.handler);
     }
 }