Netty断线重连

一、无法接收消息的问题

在使用Netty作为客户端与服务器通信的框架时，难免会遇到断线重连的问题。其中一个可能会出现的问题是：客户端在断线重连之后无法接收到消息。

出现这个问题的原因可能是断线后客户端并没有依靠Netty提供的重连机制重新建立起连接。

解决这个问题的方法是，建议在客户端与服务器通信时使用Netty提供的ChannelFutureListener，在连接建立成功后向服务器发送一条连接建立成功的消息。

一个简单的示例代码如下：

public class Client {
    public static void main(String[] args) {
        Bootstrap bootstrap = new Bootstrap()
            .group(new NioEventLoopGroup())
            .channel(NioSocketChannel.class)
            .handler(new SimpleChannelInboundHandler() {
                @Override
                protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
                    System.out.println(msg);
                }
            });
 
        ChannelFuture channelFuture = bootstrap.connect("localhost", 8888);
        channelFuture.addListener((ChannelFutureListener) future -> {
            if (future.isSuccess()) {
                Channel channel = future.channel();
                channel.writeAndFlush("hello world");
            }
        });
    }
}

二、Netty客户端断线重连

Netty内置了断线重连的机制，可以通过ChannelFutureListener实现。

在客户端与服务器连接过程中，我们可以在ChannelFutureListener的operationComplete方法中添加断线重连的逻辑，确保当连接断开时能够自动重连。

示例代码如下:

public class Client {
    private static final int MAX_RETRY = 3; // 最大重试次数
    private static final int PORT = 8888;
    private static final String HOST = "localhost";
 
    public static void main(String[] args) {
        Bootstrap bootstrap = new Bootstrap()
            .group(new NioEventLoopGroup())
            .channel(NioSocketChannel.class)
            .handler(new SimpleChannelInboundHandler() {
                @Override
                protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
                    System.out.println(msg);
                }
            });
 
        connect(bootstrap, HOST, PORT, MAX_RETRY);
    }
 
    private static void connect(Bootstrap bootstrap, String host, int port, int retry) {
        bootstrap.connect(host, port).addListener((ChannelFutureListener) futrue -> {
            if (futrue.isSuccess()) {
                System.out.println("连接成功");
            } else if (retry == 0) {
                System.out.println("重试次数已用完，放弃连接！");
            } else {
                // 第几次重连
                int order = (MAX_RETRY - retry) + 1;
                // 本次重连的间隔
                int delay = 1 < connect(bootstrap, host, port, retry - 1), delay, TimeUnit
                        .SECONDS);
            }
        });
    }
}

三、TCPClient断线重连

Netty提供了TCPClient，内置了断线重连的功能，可以通过实现ReconnectStrategy接口来控制重连策略。

示例代码如下:

public class Client {
    public static void main(String[] args) {
        // 重连策略
        ReconnectStrategy strategy = new FixedIntervalReconnectStrategy(5, TimeUnit.SECONDS);
        // TCP客户端
        TCPClient client = TCPClient.newClient(new InetSocketAddress("localhost", 8888))
                .reconnectStrategy(strategy);
        client.handle((in, out) -> {
            in.receive().asString().subscribe(System.out::println);
            return Flux.never();
        });
        client.start().block();
    }
}

四、Netty断开重连

Netty提供了ChannelOption.CONNECT_TIMEOUT_MILLIS来控制连接超时时间。如果赋值为0，则表示一直重试。

示例代码如下：

public class Client {
    private static final int PORT = 8888;
    private static final String HOST = "localhost";
 
    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup();
 
        Bootstrap bootstrap = new Bootstrap()
            .group(group)
            .channel(NioSocketChannel.class)
            .option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 0)
            .handler(new SimpleChannelInboundHandler() {
                @Override
                protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
                    System.out.println(msg);
                }
            });
 
        ChannelFuture channelFuture = bootstrap.connect(HOST, PORT);
        channelFuture.addListener(new ChannelFutureListener() {
            public void operationComplete(ChannelFuture future) throws Exception {
                if (future.isSuccess()) {
                    System.out.println("连接成功");
                } else {
                    System.out.println("连接失败！");
                    future.channel().eventLoop().schedule(() -> bootstrap.connect(HOST, PORT), 3, TimeUnit.SECONDS);
                }
            }
        });
    }
}

五、Netty自动重连

Netty通过实现ReconnectStrategy接口可以自动重连，并且可以自定义重连策略，示例代码如下：

public class Client {
    public static void main(String[] args) {
        Bootstrap bootstrap = new Bootstrap()
            .group(new NioEventLoopGroup())
            .channel(NioSocketChannel.class)
            .handler(new SimpleChannelInboundHandler() {
                @Override
                protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
                    System.out.println(msg);
                }
            });
 
        ReconnectHandler reconnectHandler = ReconnectHandler.builder()
            .reconnectInterval(1L, TimeUnit.SECONDS)
            .reconnectAttempts(Integer.MAX_VALUE)
            .addReconnectListener(() -> System.out.println("Start Reconnect"))
            .addListener(future -> {
                    if (future.isSuccess()) {
                        System.out.println("Reconnect Succeeded");
                    } else {
                        System.out.println("Reconnect Failed");
                    }
                })
            .build();
 
        bootstrap.handler(reconnectHandler);
        bootstrap.connect(new InetSocketAddress("localhost", 8888)).awaitUninterruptibly();
    }
}

六、NettyClient连续无限断线重连

在实际开发中，NettyClient很可能会出现连续无限断线重连的问题。

解决这个问题的方法是，在连接建立成功后发送一条心跳消息，并设置一个固定的时间间隔发送心跳消息。这样一来，服务器与客户端之间就会持续保持连接。

示例代码如下：

public class Client {
    private static final String HOST = "localhost";
    private static final int PORT = 8888;
    private static final int RE_CONNECT_TIME = 5000; // 断线重连间隔
 
    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup();
 
        Bootstrap bootstrap = new Bootstrap()
            .group(group)
            .channel(NioSocketChannel.class)
            .handler(new ChannelInitializer() {
                protected void initChannel(SocketChannel ch) throws Exception {
                    ch.pipeline().addLast(new StringDecoder());
                    ch.pipeline().addLast(new StringEncoder());
                    ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
                        private ScheduledFuture reconnectFuture;
                        private ScheduledFuture heartbeatFuture;
 
                        @Override
                        public void channelInactive(ChannelHandlerContext ctx) {
                            System.out.println("连接断开，正在重试连接...");
                            reconnectFuture = ctx.channel().eventLoop().schedule(() -> connect(bootstrap, HOST, PORT), 
                            RE_CONNECT_TIME, TimeUnit.MILLISECONDS);
                            super.channelInactive(ctx);
                        }
 
                        @Override
                        public void channelActive(ChannelHandlerContext ctx) throws Exception {
                            System.out.println("连接成功！");
                            // 连接成功后，定时发送心跳消息
                            heartbeatFuture = ctx.channel().eventLoop().scheduleAtFixedRate(() -> {
                                String heartbeatMsg = "ping";
                                System.out.println("发送心跳消息：" + heartbeatMsg);
                                ctx.writeAndFlush(heartbeatMsg);
                            }, 0, 3000, TimeUnit.MILLISECONDS);
                            super.channelActive(ctx);
                        }
 
                        @Override
                        public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
                            if (cause instanceof ConnectException) {
                                System.out.println("服务器拒绝连接！");
                            } else {
                                System.out.println("连接异常断开，正在重试连接...");
                                reconnectFuture = ctx.channel().eventLoop().schedule(() -> connect(bootstrap, HOST, PORT),
                                RE_CONNECT_TIME, TimeUnit.MILLISECONDS);
                            }
                        }
                    });
                }
            });
 
        connect(bootstrap, HOST, PORT);
    }
 
    private static void connect(Bootstrap bootstrap, String host, int port) {
        try {
            ChannelFuture channelFuture = bootstrap.connect(host, port).sync();
            channelFuture.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

七、Netty连接被对方重设

在使用Netty进行客户端和服务端之间通信时，可能会出现连接被对方重设的情况。这种情况下，客户端需要自动重连。

示例代码如下：

public class Client {
    private static final int PORT = 8888;
    private static final String HOST = "localhost";
 
    public static void main(String[] args) {
        EventLoopGroup group = new NioEventLoopGroup();
 
        Bootstrap bootstrap = new Bootstrap()
            .group(group)
            .channel(NioSocketChannel.class)
            .handler(new SimpleChannelInboundHandler() {
                @Override
                protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception {
                    System.out.println(msg);
                }
            });
 
        ChannelFuture channelFuture = bootstrap.connect(HOST, PORT);
        channelFuture.addListener((ChannelFutureListener) future -> {
            if (future.isSuccess()) {
                System.out.println("连接成功");
            } else {
                System.out.println("连接失败！");
                future.channel().eventLoop().schedule(() -> bootstrap.connect(HOST, PORT), 3, TimeUnit.SECONDS);
            }
        });
 
        // 检测连接是否断开
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
        executor.scheduleAtFixedRate(() -> {
            if (channelFuture.isDone() && !channelFuture.isSuccess()) {
                System.out.println("检测到连接已经断开，开始重连...");
                bootstrap.connect(HOST, PORT);
            }
        }, 0, 2, TimeUnit.SECONDS);
    }
}

本文主要介绍了Netty断线重连的相关知识，包括Netty断线重连后无法接收消息的问题、Netty客户端断线重连、TCPClient断线重连、Netty断开重连、Netty自动重连、NettyClient连续无限断线重连、Netty连接被对方重设等问题，并提供详细的示例代码。

原创文章，作者：小蓝，如若转载，请注明出处：https://www.506064.com/n/200507.html

Netty断线重连

一、无法接收消息的问题

二、Netty客户端断线重连

三、TCPClient断线重连

四、Netty断开重连

五、Netty自动重连

六、NettyClient连续无限断线重连

七、Netty连接被对方重设

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