Netty.io messageReceived覆盖不带instanceof

Question

In netty, MessageEvent (wrapper for messages) has a method Object getMessage() to get the real carried message from the network. Reading the source I noticed they heavily use the instanceof operator to switch among methods.

However, having a wide variety of message types I would like to avoid a method like this:

public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
   if (e.getMessage() instanceof MessageType1) {
      ...
   } else if (e.getMessage() instanceof MessageType2) {
      ...
   } ... {
      ...
   } else if (e.getMessage() instanceof MessageTypeN) {
      ...
   } else {
      ctx.sendUpstream(e);
   }
}

Writing different methods taking advantage of polymorphism would be much better, like:

public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
   // MessageType implements Message
   if (e.getMessage() instanceof Message) {
      handleMessage(ctx, (Message) e.getMessage());
   } else {
      ctx.sendUpstream(e);
   }
}

void handleMessage(ChannelHandlerContext ctx, MessageType1 m) {
    ...
}

...

void handleMessage(ChannelHandlerContext ctx, MessageTypeN m) {
    ...
}

But i cannot due to downcasting limitations. Is there a clean way to do this or am I tied to instanceof cascade? I could bring the logic out the Handler using .doSomething() methods inside Message sub-types but I'd like to keep the business logic inside the netty pipeline.

Answer 1

Solved applying Visitor Pattern:

public interface Handler {
    void handleMessage(ChannelHandlerContext ctx, MessageType1 m);
    void handleMessage(ChannelHandlerContext ctx, MessageType2 m);
    ...
    void handleMessage(ChannelHandlerContext ctx, MessageTypeN m);
}

then:

@Sharable
public class MessageHandler extends SimpleChannelHandler implements Handler {
   ...
   @Override
   public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
     if (e.getMessage() instanceof Message) {
        Message m = (Message) e.getMessage();
        m.handleTo(ctx, this);
    } else {
        ctx.sendUpstream(e);
    }
}

@Override
public void handleMessage(ChannelHandlerContext ctx, MessageType1 m) {
    ...
}

and

public interface Message {
   /*
    * Will be {
    *    handler.handleMessage(ctx, this);
    * }
    * everywhere.
    */
   void handleTo(ChannelHandlerContext ctx, Handler handler);
}