Netty斷線重連

一、無法接收消息的問題

在使用Netty作為客戶端與伺服器通信的框架時，難免會遇到斷線重連的問題。其中一個可能會出現的問題是：客戶端在斷線重連之後無法接收到消息。

出現這個問題的原因可能是斷線後客戶端並沒有依靠Netty提供的重連機制重新建立起連接。

解決這個問題的方法是，建議在客戶端與伺服器通信時使用Netty提供的ChannelFutureListener，在連接建立成功後向伺服器發送一條連接建立成功的消息。

一個簡單的示例代碼如下：

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客戶端斷線重連

Netty內置了斷線重連的機制，可以通過ChannelFutureListener實現。

在客戶端與伺服器連接過程中，我們可以在ChannelFutureListener的operationComplete方法中添加斷線重連的邏輯，確保當連接斷開時能夠自動重連。

示例代碼如下:

public class Client {
    private static final int MAX_RETRY = 3; // 最大重試次數
    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("連接成功");
            } else if (retry == 0) {
                System.out.println("重試次數已用完，放棄連接！");
            } else {
                // 第幾次重連
                int order = (MAX_RETRY - retry) + 1;
                // 本次重連的間隔
                int delay = 1 < connect(bootstrap, host, port, retry - 1), delay, TimeUnit
                        .SECONDS);
            }
        });
    }
}

三、TCPClient斷線重連

Netty提供了TCPClient，內置了斷線重連的功能，可以通過實現ReconnectStrategy介面來控制重連策略。

示例代碼如下:

public class Client {
    public static void main(String[] args) {
        // 重連策略
        ReconnectStrategy strategy = new FixedIntervalReconnectStrategy(5, TimeUnit.SECONDS);
        // TCP客戶端
        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斷開重連

Netty提供了ChannelOption.CONNECT_TIMEOUT_MILLIS來控制連接超時時間。如果賦值為0，則表示一直重試。

示例代碼如下：

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("連接成功");
                } else {
                    System.out.println("連接失敗！");
                    future.channel().eventLoop().schedule(() -> bootstrap.connect(HOST, PORT), 3, TimeUnit.SECONDS);
                }
            }
        });
    }
}

五、Netty自動重連

Netty通過實現ReconnectStrategy介面可以自動重連，並且可以自定義重連策略，示例代碼如下：

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連續無限斷線重連

在實際開發中，NettyClient很可能會出現連續無限斷線重連的問題。

解決這個問題的方法是，在連接建立成功後發送一條心跳消息，並設置一個固定的時間間隔發送心跳消息。這樣一來，伺服器與客戶端之間就會持續保持連接。

示例代碼如下：

public class Client {
    private static final String HOST = "localhost";
    private static final int PORT = 8888;
    private static final int RE_CONNECT_TIME = 5000; // 斷線重連間隔
 
    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("連接斷開，正在重試連接...");
                            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("連接成功！");
                            // 連接成功後，定時發送心跳消息
                            heartbeatFuture = ctx.channel().eventLoop().scheduleAtFixedRate(() -> {
                                String heartbeatMsg = "ping";
                                System.out.println("發送心跳消息：" + 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("伺服器拒絕連接！");
                            } else {
                                System.out.println("連接異常斷開，正在重試連接...");
                                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連接被對方重設

在使用Netty進行客戶端和服務端之間通信時，可能會出現連接被對方重設的情況。這種情況下，客戶端需要自動重連。

示例代碼如下：

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("連接成功");
            } else {
                System.out.println("連接失敗！");
                future.channel().eventLoop().schedule(() -> bootstrap.connect(HOST, PORT), 3, TimeUnit.SECONDS);
            }
        });
 
        // 檢測連接是否斷開
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
        executor.scheduleAtFixedRate(() -> {
            if (channelFuture.isDone() && !channelFuture.isSuccess()) {
                System.out.println("檢測到連接已經斷開，開始重連...");
                bootstrap.connect(HOST, PORT);
            }
        }, 0, 2, TimeUnit.SECONDS);
    }
}

本文主要介紹了Netty斷線重連的相關知識，包括Netty斷線重連後無法接收消息的問題、Netty客戶端斷線重連、TCPClient斷線重連、Netty斷開重連、Netty自動重連、NettyClient連續無限斷線重連、Netty連接被對方重設等問題，並提供詳細的示例代碼。

原創文章，作者：小藍，如若轉載，請註明出處：https://www.506064.com/zh-tw/n/200507.html

Netty斷線重連

一、無法接收消息的問題

二、Netty客戶端斷線重連

三、TCPClient斷線重連

四、Netty斷開重連

五、Netty自動重連

六、NettyClient連續無限斷線重連

七、Netty連接被對方重設

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