Java Netty客戶端無法將消息發送到服務器，但可以通過telnet切斷服務器

Question

我只是在學習Java Netty，以通過套接字7000使客戶端服務器變得非常簡單。服務器正在運行，它將回顯從客戶端收到的消息。 如果我使用telnet localhost 7000並將消息發送到服務器並接收回顯消息，則它可以工作。

但是，對於Java Netty客戶端，它不起作用。 服務器什么都沒收到，客戶端什么也沒發送？ 當我嘗試向控制台添加一些文字時。

這是此示例的類：

客戶

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);
    }
}

服務器

    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，當我運行EchoServer時，然后運行EchoClient，EchoClient的輸出為

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

從EchoServer的輸出是

Sending message: message from server.

Answer 1

在您的代碼中， DelimiterBasedFrameDecoder處理程序位於解碼器/編碼器之前。 因此，如果傳輸的消息沒有預期的定界符（例如，在這種情況下為新行），則兩個客戶端/服務器將繼續等待並認為該消息仍在傳輸。 因此，有兩種方法可以解決您的問題。

1. 在客戶端和服務器上均刪除new DelimiterBasedFrameDecoder(Integer.MAX_VALUE, Delimiters.lineDelimiter())
2. 每次通過通道發送消息時，請添加新的行分隔符。 否則，該消息將無法從另一端解碼。 請在下面的示例代碼中找到

EchoClientHandler類

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類

  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());
    }
}

服務器輸出

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

客戶輸出

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

Answer 2

比較您的客戶端和telnet之間的協議（使用LoggingHandler或Wireshark），如果這樣做，您會注意到這2個協議之間有1個主要區別。

不同之處在於，您發送的消息末尾沒有換行符，而telnet卻沒有。

通過在消息末尾添加新的換行符，或編寫一個自動為您執行此操作的新處理程序，即可輕松解決此問題。

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

您可以進行的另一個改進是調用writeAndFlush而不是先寫然后刷新，這還可以防止您在編碼時被中斷而錯過對另一個方法的調用。