[ISSUE #3109]Return a no-support value when not match any request code

broker/src/main/java/org/apache/rocketmq/broker/processor/AdminBrokerProcessor.java

@@ -116,6 +116,7 @@ import org.apache.rocketmq.remoting.netty.NettyRequestProcessor;
 import org.apache.rocketmq.remoting.protocol.LanguageCode;
 import org.apache.rocketmq.remoting.protocol.RemotingCommand;
 import org.apache.rocketmq.remoting.protocol.RemotingSerializable;
+import org.apache.rocketmq.remoting.protocol.RemotingSysResponseCode;
 import org.apache.rocketmq.store.ConsumeQueue;
 import org.apache.rocketmq.store.ConsumeQueueExt;
 import org.apache.rocketmq.store.DefaultMessageStore;
@@ -235,10 +236,8 @@ public class AdminBrokerProcessor extends AsyncNettyRequestProcessor implements
             case RequestCode.GET_BROKER_CLUSTER_ACL_CONFIG:
                 return getBrokerClusterAclConfig(ctx, request);
             default:
-                break;
+                return getUnknownCmdResponse(ctx, request);
         }
-
-        return null;
     }
 
     @Override
@@ -459,6 +458,13 @@ public class AdminBrokerProcessor extends AsyncNettyRequestProcessor implements
         return null;
     }
 
+    private RemotingCommand getUnknownCmdResponse(ChannelHandlerContext ctx, RemotingCommand request) {
+        String error = " request type " + request.getCode() + " not supported";
+        final RemotingCommand response =
+                RemotingCommand.createResponseCommand(RemotingSysResponseCode.REQUEST_CODE_NOT_SUPPORTED, error);
+        return response;
+    }
+
     private RemotingCommand getAllTopicConfig(ChannelHandlerContext ctx, RemotingCommand request) {
         final RemotingCommand response = RemotingCommand.createResponseCommand(GetAllTopicConfigResponseHeader.class);
         // final GetAllTopicConfigResponseHeader responseHeader =