Java Netty客户端无法将消息发送到服务器，但可以通过telnet切断服务器

Question

我只是在学习Java Netty，以通过套接字7000使客户端服务器变得非常简单。服务器正在运行，它将回显从客户端收到的消息。 如果我使用telnet localhost 7000并将消息发送到服务器并接收回显消息，则它可以工作。

但是，对于Java Netty客户端，它不起作用。 服务器什么都没收到，客户端什么也没发送？ 当我尝试向控制台添加一些文字时。

这是此示例的类：

客户

public final class EchoClient {

    public static void main(String[] args) throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            Bootstrap bootstrap = new Bootstrap();
            bootstrap.group(group)
                    .channel(NioSocketChannel.class)
                    .option(ChannelOption.TCP_NODELAY, true)
                    .handler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(
                                    new LoggingHandler(LogLevel.TRACE),
                                    new DelimiterBasedFrameDecoder(Integer.MAX_VALUE, Delimiters.lineDelimiter()),
                                    new StringEncoder(),
                                    new StringDecoder(),
                                    new EchoClientHandler());
                        }
                    });

            // Start the connection attempt.
            bootstrap.connect("localhost", 7000).sync().channel().closeFuture().sync();
            System.out.println("Message sent successfully.");
        } finally {
            group.shutdownGracefully();
        }
    }
}

public class EchoClientHandler extends SimpleChannelInboundHandler<String> {


    /**
     * Constructor for the class
     *
     * @param message the message you want to transmit
     */
    public EchoClientHandler() {

    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        String message = "message from client.";
        System.out.println("Sending message: " + message);
        ctx.write(message);
        ctx.flush();
        ctx.close();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        System.out.println("Error caught in the communication service: " + cause);
        ctx.close();
    }


    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
        System.out.println("Received message: " + msg);
    }
}

服务器

    public final class EchoServer {

    static final boolean SSL = System.getProperty("ssl") != null;
    static final int PORT = Integer.parseInt(System.getProperty("port", "7000"));

    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap serverBootstrap = new ServerBootstrap();
            serverBootstrap.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .handler(new LoggingHandler(LogLevel.TRACE))
                    .childHandler(new ChannelInitializer<SocketChannel>() {
                        @Override
                        public void initChannel(SocketChannel ch) throws Exception {
                            ch.pipeline().addLast(
                                    new LoggingHandler(LogLevel.TRACE),
                                    new DelimiterBasedFrameDecoder(Integer.MAX_VALUE, Delimiters.lineDelimiter()),
                                    new StringEncoder(),
                                    new StringDecoder(),
                                    new EchoServerHandler());
                        }
                    });

            System.out.println("Server is listening on port 7000.");

            // Start the server.
            ChannelFuture channelFuture = serverBootstrap.bind("localhost", 7000).sync();

            // Wait until the server socket is closed.
            channelFuture.channel().closeFuture().sync();


        } finally {
            // Shut down all event loops to terminate all threads.
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}



public class EchoServerHandler extends SimpleChannelInboundHandler<String> {

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        String message = "message from server.";
        System.out.println("Sending message: " + message);
        ctx.write(message);
        ctx.flush();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        // Close the connection when an exception is raised.
        System.out.println("Error in receiving message.");
        cause.printStackTrace();
        ctx.close();
    }

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String message) throws Exception {
        System.out.println("Received message: " + message);
        ctx.write(message);
        ctx.flush();
        //ctx.close();
    }
}

因此，当我运行EchoServer时，然后运行EchoClient，EchoClient的输出为

Sending message: message from client.
Message sent successfully.
Then application stopped.

从EchoServer的输出是

Sending message: message from server.

Answer 1

在您的代码中， DelimiterBasedFrameDecoder处理程序位于解码器/编码器之前。 因此，如果传输的消息没有预期的定界符（例如，在这种情况下为新行），则两个客户端/服务器将继续等待并认为该消息仍在传输。 因此，有两种方法可以解决您的问题。

1. 在客户端和服务器上均删除new DelimiterBasedFrameDecoder(Integer.MAX_VALUE, Delimiters.lineDelimiter())
2. 每次通过通道发送消息时，请添加新的行分隔符。 否则，该消息将无法从另一端解码。 请在下面的示例代码中找到

EchoClientHandler类

public class EchoClientHandler extends SimpleChannelInboundHandler<String> {

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        String message = "message from client.";
        System.out.println("Sending message: " + message);
        ctx.writeAndFlush(message + System.lineSeparator());
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        System.out.println("Error caught in the communication service: " + cause);
        ctx.close();
    }

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
        System.out.println("Received message: " + msg);
    }
}

EchoServerHandler类

  public class EchoServerHandler extends SimpleChannelInboundHandler<String> {

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        String message = "message from server.";
        System.out.println("Sending message: " + message);
        ctx.writeAndFlush(message + System.lineSeparator());
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        System.out.println("Error in receiving message.");
        cause.printStackTrace();
        ctx.close();
    }

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, String message) throws Exception {
        System.out.println("Received message: " + message);
        ctx.writeAndFlush(message + System.lineSeparator());
    }
}

服务器输出

Server is listening on port 7000.
Sending message: message from server.
Received message: message from client.

客户输出

Sending message: message from client.
Received message: message from server.
Received message: message from client.

Answer 2

比较您的客户端和telnet之间的协议（使用LoggingHandler或Wireshark），如果这样做，您会注意到这2个协议之间有1个主要区别。

不同之处在于，您发送的消息末尾没有换行符，而telnet却没有。

通过在消息末尾添加新的换行符，或编写一个自动为您执行此操作的新处理程序，即可轻松解决此问题。

ctx.write(message + "\n");

您可以进行的另一个改进是调用writeAndFlush而不是先写然后刷新，这还可以防止您在编码时被中断而错过对另一个方法的调用。