How to get server response with netty client

Question

I want to write a netty based client. It should have method public String send(String msg); which should return response from the server or some future - doesen\'t

Answer 1

Returning a Future<String> for the method is simple, we are going to implement the following method signature:

public Futute<String> sendMessage(String msg) {

The is relatively easy to do when you are known with the async programming structures. To solve the design problem, we are going to do the following steps:

1. When a message is written, add a Promise<String> to a ArrayBlockingQueue<Promise>

   This will serve as a list of what messages have recently been send, and allows us to change our Future<String> objects return result.

2. When a message arrives back into the handler, resolve it against the head of the Queue

   This allows us to get the correct future to change.

3. Update the state of the Promise<String>

   We call promise.setSuccess() to finally set the state on the object, this will propagate back to the future object.

Example code

public class ClientHandler extends SimpleChannelInboundHandler<String> {
    private ChannelHandlerContext ctx;
    private BlockingQueue<Promise<String>> messageList = new ArrayBlockingQueue<>(16);

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        super.channelActive(ctx);
        this.ctx = ctx;
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) {
        super.channelInactive(ctx);
        synchronized(this){
            Promise<String> prom;
            while((prom = messageList.poll()) != null) 
                prom.setFailure(new IOException("Connection lost"));
            messageList = null;
        }
    }

    public Future<String> sendMessage(String message) {
        if(ctx == null) 
            throw new IllegalStateException();
        return sendMessage(message, ctx.executor().newPromise());
    }

    public Future<String> sendMessage(String message, Promise<String> prom) {
        synchronized(this){
            if(messageList == null) {
                // Connection closed
                prom.setFailure(new IllegalStateException());
            } else if(messageList.offer(prom)) { 
                // Connection open and message accepted
                ctx.writeAndFlush(message).addListener();
            } else { 
                // Connection open and message rejected
                prom.setFailure(new BufferOverflowException());
            }
            return prom;
        }
    }
    @Override
    protected void messageReceived(ChannelHandlerContext ctx, String msg) {
        synchronized(this){
            if(messageList != null) {
                 messageList.poll().setSuccess(msg);
            }
        }
    }
}

Documentation breakdown

• private ChannelHandlerContext ctx;

  Used to store our reference to the ChannelHandlerContext, we use this so we can create promises

• private BlockingQueue<Promise<String>> messageList = new ArrayBlockingQueue<>();

  We keep the past messages in this list so we can change the result of the future

• public void channelActive(ChannelHandlerContext ctx)

  Called by netty when the connection becomes active. Init our variables here.

• public void channelInactive(ChannelHandlerContext ctx)

  Called by netty when the connection becomes inactive, either due to error or normal connection close.

• protected void messageReceived(ChannelHandlerContext ctx, String msg)

  Called by netty when a new message arrives, here pick out the head of the queue, and then we call setsuccess on it.

Warning advise

When using futures, there is 1 thing you need to lookout for, do not call get() from 1 of the netty threads if the future isn't done yet, failure to follow this simple rule will either result in a deadlock or a BlockingOperationException.

Answer 2

Calling channel.writeAndFlush(msg); already returns a ChannelFuture. To handle the result of this method call, you could add a listener to the future like this:

future.addListener(new ChannelFutureListener() {
    public void operationComplete(ChannelFuture future) {
        // Perform post-closure operation
        // ...
    }
}); 

(this is taken from the Netty documentation see: Netty doc)

Answer 3

You can find the sample in netty project. We can save the result into the last handler's custom fields. In the following code, it is handler.getFactorial() that is what we want.

refer to http://www.lookatsrc.com/source/io/netty/example/factorial/FactorialClient.java?a=io.netty:netty-all

FactorialClient.java

public final class FactorialClient {

    static final boolean SSL = System.getProperty("ssl") != null;
    static final String HOST = System.getProperty("host", "127.0.0.1");
    static final int PORT = Integer.parseInt(System.getProperty("port", "8322"));
    static final int COUNT = Integer.parseInt(System.getProperty("count", "1000"));

    public static void main(String[] args) throws Exception {
        // Configure SSL.
        final SslContext sslCtx;
        if (SSL) {
            sslCtx = SslContextBuilder.forClient()
                .trustManager(InsecureTrustManagerFactory.INSTANCE).build();
        } else {
            sslCtx = null;
        }

        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group)
             .channel(NioSocketChannel.class)
             .handler(new FactorialClientInitializer(sslCtx));

            // Make a new connection.
            ChannelFuture f = b.connect(HOST, PORT).sync();

            // Get the handler instance to retrieve the answer.
            FactorialClientHandler handler =
                (FactorialClientHandler) f.channel().pipeline().last();

            // Print out the answer.
            System.err.format("Factorial of %,d is: %,d", COUNT, handler.getFactorial());
        } finally {
            group.shutdownGracefully();
        }
    }
}

public class FactorialClientHandler extends SimpleChannelInboundHandler<BigInteger> {

    private ChannelHandlerContext ctx;
    private int receivedMessages;
    private int next = 1;
    final BlockingQueue<BigInteger> answer = new LinkedBlockingQueue<BigInteger>();

    public BigInteger getFactorial() {
        boolean interrupted = false;
        try {
            for (;;) {
                try {
                    return answer.take();
                } catch (InterruptedException ignore) {
                    interrupted = true;
                }
            }
        } finally {
            if (interrupted) {
                Thread.currentThread().interrupt();
            }
        }
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        this.ctx = ctx;
        sendNumbers();
    }

    @Override
    public void channelRead0(ChannelHandlerContext ctx, final BigInteger msg) {
        receivedMessages ++;
        if (receivedMessages == FactorialClient.COUNT) {
            // Offer the answer after closing the connection.
            ctx.channel().close().addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) {
                    boolean offered = answer.offer(msg);
                    assert offered;
                }
            });
        }
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        cause.printStackTrace();
        ctx.close();
    }

    private void sendNumbers() {
        // Do not send more than 4096 numbers.
        ChannelFuture future = null;
        for (int i = 0; i < 4096 && next <= FactorialClient.COUNT; i++) {
            future = ctx.write(Integer.valueOf(next));
            next++;
        }
        if (next <= FactorialClient.COUNT) {
            assert future != null;
            future.addListener(numberSender);
        }
        ctx.flush();
    }

    private final ChannelFutureListener numberSender = new ChannelFutureListener() {
        @Override
        public void operationComplete(ChannelFuture future) throws Exception {
            if (future.isSuccess()) {
                sendNumbers();
            } else {
                future.cause().printStackTrace();
                future.channel().close();
            }
        }
    };
}