UnsupportedOperationException when sending messages through ServerBootstrap ChannelPipeline in Netty

Question

I am using Netty 5.0.

I have a complementary client bootstrap for which I took the SecureChatClient.java example from netty github. Wenn I send messages from the client bootstrap to the server it works perfectly fine. When I try to send messages from the server bootstrap to the client (after successfully initiating a connection/channel through the client first) I get a java.lang.UnsupportedOperationException without any further information on it. Sending messages from server to client is done via code above.

Is a serverbootstrap for receiving only?

Is a serverbootstrap not meant to be able to write messages back to the client as shown above? By that I mean, messages can enter a ChannelPipeline from a socket up through the ChannelHandlers, but only the ChannelHandlers are supposed to be writing responses back down the ChannelPipeline and out the socket. So in a ServerBootstrap a user is not meant to be able to send messages down the ChannelPipeline from outside the Pipeline. (Hope that makes sense)

Or am I simply missing something?

My code follows:

    // Ports.
    int serverPort = 8080;

    EventLoopGroup bossGroup    = new NioEventLoopGroup();
    EventLoopGroup workerGroup  = new NioEventLoopGroup();
    try {
        ServerBootstrap b = new ServerBootstrap();
        b.group(bossGroup, workerGroup)
         .channel(NioServerSocketChannel.class)
         .childHandler(new ChannelInitializer<SocketChannel>() {
             @Override
             public void initChannel(SocketChannel ch) throws Exception {
                 ch.pipeline().addLast("MyMessageHandler", new MyMessageHandler());
             }
         })
         .option(ChannelOption.SO_BACKLOG, 128)
         .childOption(ChannelOption.SO_KEEPALIVE, true);

        // Bind and start to accept incoming connections.
        ChannelFuture f = b.bind(serverPort).sync();
        Channel ch = f.channel();

        System.out.println("Server: Running!");

      // Read commands from the stdin.
      ChannelFuture lastWriteFuture = null;
      BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
      while(true)
      {
          String line = in.readLine();
          if (line == null) break;

          ByteBuf getOut = buffer(64);
          getOut.writeBytes(line.getBytes());

          // Sends the received line to the server.
          lastWriteFuture = ch.writeAndFlush(getOut);

          lastWriteFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture cf) throws Exception {
                    if(cf.isSuccess()) {
                        System.out.println("CFListener: SUCCESS! YEAH! HELL! YEAH!");
                    } else {
                        System.out.println("CFListener: failure! FAILure! FAILURE!");
                        System.out.println(cf.cause());
                    }
                }
            });

      }

               // Wait until all messages are flushed before closing the channel.
      if (lastWriteFuture != null) {
          lastWriteFuture.sync();
      }


        // Wait until the server socket is closed.
        // In this example, this does not happen, but you can do that to gracefully
        // shut down your server.
        f.channel().closeFuture().sync();
    } catch (InterruptedException | UnsupportedOperationException e) {
        e.printStackTrace();
    } finally {
        workerGroup.shutdownGracefully();
        bossGroup.shutdownGracefully();
    }

I started using the following example: https://github.com/netty/netty/tree/4.1/example/src/main/java/io/netty/example/securechat

My problem is that I get the following exception when calling ch.writeAndFlush:

java.lang.UnsupportedOperationException
at io.netty.channel.socket.nio.NioServerSocketChannel.filterOutboundMessage(NioServerSocketChannel.java:184)
at io.netty.channel.AbstractChannel$AbstractUnsafe.write(AbstractChannel.java:784)
at io.netty.channel.DefaultChannelPipeline$HeadContext.write(DefaultChannelPipeline.java:1278)
at io.netty.channel.ChannelHandlerInvokerUtil.invokeWriteNow(ChannelHandlerInvokerUtil.java:158)
at io.netty.channel.DefaultChannelHandlerInvoker$WriteTask.run(DefaultChannelHandlerInvoker.java:440)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:328)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:354)
at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:116)
at io.netty.util.internal.chmv8.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1412)
at io.netty.util.internal.chmv8.ForkJoinTask.doExec(ForkJoinTask.java:280)
at io.netty.util.internal.chmv8.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:877)
at io.netty.util.internal.chmv8.ForkJoinPool.scan(ForkJoinPool.java:1706)
at io.netty.util.internal.chmv8.ForkJoinPool.runWorker(ForkJoinPool.java:1661)
at io.netty.util.internal.chmv8.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:126)

In response to your new ChannelGroupException error: Can you cast f.cause() to a ChannelGroupException, and then print every value inside its iterator() — Ferrybig
I put the whole thing into my question. I hope I did the iterator-thing right. I am opened for improvements or corrections. — Spyro
@Ferrybig Solved the problem due to your debugging skills. Thank you so much! Put the answer up with some more details in it. If there was a way to give people credit, let me know. Would like to give kudos to you somehow. — Spyro
I am unsure how you solved the latter part, but at stackoverflow we usually keep only 1 question inside a question, so we can have a accepted answer, at the moment your question is based on 2 seperate parts, where the first part is your orginal problem I help you debugging with, and the latter problem you posted an answer with, I think its better to remove the second part and your "half" answer and then mark mine as accepted, as it solved your initial question, and prevent people voting to close your question as "Too broad" — Ferrybig

Ferrybig Ferrybig · Accepted Answer · 2016-01-11T08:39:54

You cannot write to a ServerChannel, you can only connect to normal channels. Your call to writeAndFlush is failing for this reason.

To send a message to every client, you should store the channel of every client inside a ChannelGroup and invoke writeAndFlush() on that.

A quick way to do this is adding another handler to your ServerBootstrap that puts the incoming connections inside the ChannelGroup, a quick implementation of this would be this:

// In your main:
ChannelGroup allChannels =
         new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);

// In your ChannelInitializer<SocketChannel>
ch.pipeline().addLast("grouper", new GlobalSendHandler());

// New class:
public class MyHandler extends ChannelInboundHandlerAdapter {
     @Override
     public void channelActive(ChannelHandlerContext ctx) {
         allChannels.add(ctx.channel());
         super.channelActive(ctx);
     }
 }

Then we can call the following to send a message to every connection, this returns a ChannelGroupFuture instead of a normal ChannelFuture:

allChannels.writeAndFlush(getOut);

Your total code would look like this with the fixes from above:

// Ports.
int serverPort = 8080;

ChannelGroup allChannels =
         new DefaultChannelGroup(GlobalEventExecutor.INSTANCE);

EventLoopGroup bossGroup    = new NioEventLoopGroup();
EventLoopGroup workerGroup  = new NioEventLoopGroup();
try {
    ServerBootstrap b = new ServerBootstrap();
    b.group(bossGroup, workerGroup)
     .channel(NioServerSocketChannel.class)
     .childHandler(new ChannelInitializer<SocketChannel>() {
         @Override
         public void initChannel(SocketChannel ch) throws Exception {
             ch.pipeline().addLast("MyMessageHandler", new MyMessageHandler());
             ch.pipeline().addLast("grouper", new GlobalSendHandler());
         }
     })
     .option(ChannelOption.SO_BACKLOG, 128)
     .childOption(ChannelOption.SO_KEEPALIVE, true);

    // Bind and start to accept incoming connections.
    ChannelFuture f = b.bind(serverPort).sync();
    Channel ch = f.channel();

    System.out.println("Server: Running!");

  // Read commands from the stdin.
  ChannelGroupFuture lastWriteFuture = null;
  BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
  while(true)
  {
      String line = in.readLine();
      if (line == null) break;

      ByteBuf getOut = buffer(64);
      getOut.writeBytes(line.getBytes());

      // Sends the received line to the server.
      lastWriteFuture = allChannels.writeAndFlush(getOut);

      lastWriteFuture.addListener(new ChannelGroupFutureListener() {
            @Override
            public void operationComplete(ChannelGroupFuture cf) throws Exception {
                if(cf.isSuccess()) {
                    System.out.println("CFListener: SUCCESS! YEAH! HELL! YEAH!");
                } else {
                    System.out.println("CFListener: failure! FAILure! FAILURE!");
                    System.out.println(cf.cause());
                }
            }
        });

  }

           // Wait until all messages are flushed before closing the channel.
  if (lastWriteFuture != null) {
      lastWriteFuture.sync();
  }


    // Wait until the server socket is closed.
    // In this example, this does not happen, but you can do that to gracefully
    // shut down your server.
    f.channel().closeFuture().sync();
} catch (InterruptedException | UnsupportedOperationException e) {
    e.printStackTrace();
} finally {
    workerGroup.shutdownGracefully();
    bossGroup.shutdownGracefully();
}

UnsupportedOperationException when sending messages through ServerBootstrap ChannelPipeline in Netty

2 Answers