Java netty 客户端无法向服务器发送消息，但 telnet 可以服务器

Question

我正在学习 Java Netty 通过套接字 7000 制作非常简单的客户端-服务器。服务器正在运行ning 并且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，它就会工作。

但是，对于 Java Netty 客户端，它不起作用。服务器什么也没收到，客户端什么也没发送？当我试图向控制台添加一些内容时。

这里是这个例子的类：

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();
    }
}

所以当我运行 EchoServer，然后我运行 EchoClient，EchoClient 的输出是

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

EchoServer 的输出是

Sending message: message from server.

Answer 1

比较客户端和 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，您发送的邮件末尾没有换行符，而 telnet 有。

解决这个问题真的很简单，只需在消息末尾添加一个新的换行符，或者编写一个新的处理程序来自动为您执行此操作。

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

您可以做的另一个改进是调用 writeAndFlush 而不是先写入，然后刷新，这也可以防止您在编码时被打断时错过对其他方法的调用。

Answer 2

在您的代码中，DelimiterBasedFrameDecoder 处理程序出现在 decoder/encoder 之前。因此，如果传输的消息没有预期的分隔符（例如本例中的新行），client/server 将继续等待并认为消息仍在传输。因此，有两种可能的方法可以解决您的问题。

在客户端和服务器中删除 new DelimiterBasedFrameDecoder(Integer.MAX_VALUE, Delimiters.lineDelimiter())
每次通过频道发送消息时，请添加新的行分隔符。否则，无法从另一端解码消息。请在下面找到示例代码

EchoClientHandlerclass

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);
    }
}

EchoServerHandlerclass

  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());
    }
}

来自服务器的输出

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

来自客户端的输出

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

Java netty 客户端无法向服务器发送消息，但 telnet 可以服务器

Java netty client cannot send message to server, but telnet to sever ok

java

sockets

telnet

netty