Netty

public class ConnectHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelActive(ChannelHandlerContext ctx)   // channelActive是回调函数
            throws Exception {
        System.out.println(
                "Client " + ctx.channel().remoteAddress() + " connected");
    }
}

    public static void connect() {
        Channel channel = new NioSocketChannel();
        // Does not block
        ChannelFuture future = channel.connect(new InetSocketAddress("192.168.0.1", 25));
        future.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) {
                if (future.isSuccess()) {
                    ByteBuf buffer = Unpooled.copiedBuffer(
                            "Hello", Charset.defaultCharset());
                    ChannelFuture wf = future.channel()
                            .writeAndFlush(buffer);
                    // ...
                } else {
                    Throwable cause = future.cause();
                    cause.printStackTrace();
                }
            }
        });
    }

@Sharable
public class EchoServerHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        ByteBuf in = (ByteBuf) msg;
        System.out.println("Server received: " + in.toString(CharsetUtil.UTF_8));
        ctx.write(in);    // 将接收到的消息写给发送者，而不冲刷出站消息
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx)
            throws Exception {
        ctx.writeAndFlush(Unpooled.EMPTY_BUFFER)   // 将未决消息冲刷到远程节点，并且关闭该Channel
           .addListener(ChannelFutureListener.CLOSE);
    }

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

public class EchoServer {
    private final int port;

    public EchoServer(int port) {
        this.port = port;
    }

    public static void main(String[] args)
        throws Exception {
        if (args.length != 1) {
            System.err.println("Usage: " + EchoServer.class.getSimpleName() + " <port>");
            return;
        }
        int port = Integer.parseInt(args[0]);
        new EchoServer(port).start();
    }

    public void start() throws Exception {
        final EchoServerHandler serverHandler = new EchoServerHandler();
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(group)
                .channel(NioServerSocketChannel.class)     // 指定所使用的NIO传输Channel
                .localAddress(new InetSocketAddress(port)) // 使用指定的端口设置套接字地址
                .childHandler(new ChannelInitializer<SocketChannel>() { // 添加Handler到Channel的ChannelPipeline
                    @Override
                    public void initChannel(SocketChannel ch) throws Exception {
                        ch.pipeline().addLast(serverHandler);
                    }
                });

            ChannelFuture f = b.bind().sync();   // 异步地绑定服务器；调用sync()方法阻塞等待直到绑定完成
            System.out.println(EchoServer.class.getName() +
                " started and listening for connections on " + f.channel().localAddress());
            f.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully().sync();
        }
    }
}

@Sharable
public class EchoClientHandler extends SimpleChannelInboundHandler<ByteBuf> {
    @Override
    public void channelActive(ChannelHandlerContext ctx) {  // 在当服务器的连接已经建立之后将被调用
        ctx.writeAndFlush(Unpooled.copiedBuffer("Netty rocks!",
                CharsetUtil.UTF_8));
    }

    @Override
    public void channelRead0(ChannelHandlerContext ctx, ByteBuf in) { // 当从服务器接收消息时被调用
        System.out.println("Client received: " + in.toString(CharsetUtil.UTF_8));
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx,  // 在处理过程中引发异常时被调用
        Throwable cause) {
        cause.printStackTrace();
        ctx.close();
    }
}

public class EchoClient {
    private final String host;
    private final int port;

    public EchoClient(String host, int port) {
        this.host = host;
        this.port = port;
    }

    public void start() throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(group)
                .channel(NioSocketChannel.class)
                .remoteAddress(new InetSocketAddress(host, port))  // 设置服务器的InetSocketAddress
                .handler(new ChannelInitializer<SocketChannel>() { // 向ChannelPipeline中添加一个EchoClientHandler实例
                    @Override
                    public void initChannel(SocketChannel ch)
                        throws Exception {
                        ch.pipeline().addLast(
                             new EchoClientHandler());
                    }
                });
            ChannelFuture f = b.connect().sync(); // 连接到远程节点，阻塞等待直到连接完成
            f.channel().closeFuture().sync();     // 阻塞，直到Channel关闭
        } finally {
            group.shutdownGracefully().sync();
        }
    }

    public static void main(String[] args) throws Exception {
        if (args.length != 2) {
            System.err.println("Usage: " + EchoClient.class.getSimpleName() + " <host> <port>" );
            return;
        }

        final String host = args[0];
        final int port = Integer.parseInt(args[1]);
        new EchoClient(host, port).start();
    }
}