netty断线重连原理 断线重接最好用的方法）

心跳机制

何为心跳

所谓心跳, 即在 TCP 长连接中, 客户端和服务器之间定期发送的一种特殊的数据包, 通知对方自己还在线, 以确保 TCP 连接的有效性.

注：心跳包还有另一个作用，经常被忽略，即：一个连接如果长时间不用，防火墙或者路由器就会断开该连接。

如何实现

核心Handler —— IdleStateHandler

在 Netty 中, 实现心跳机制的关键是 IdleStateHandler, 那么这个 Handler 如何使用呢? 先看下它的构造器：

public IdleStateHandler(int readerIdleTimeSeconds, int writerIdleTimeSeconds, int allIdleTimeSeconds) {
 this((long)readerIdleTimeSeconds, (long)writerIdleTimeSeconds, (long)allIdleTimeSeconds, TimeUnit.SECONDS);
}

这里解释下三个参数的含义：

• readerIdleTimeSeconds: 读超时. 即当在指定的时间间隔内没有从 Channel 读取到数据时, 会触发一个 READER_IDLE 的 IdleStateEvent 事件.
• writerIdleTimeSeconds: 写超时. 即当在指定的时间间隔内没有数据写入到 Channel 时, 会触发一个 WRITER_IDLE 的 IdleStateEvent 事件.
• allIdleTimeSeconds: 读/写超时. 即当在指定的时间间隔内没有读或写操作时, 会触发一个ALL_IDLE 的 IdleStateEvent 事件.

注：这三个参数默认的时间单位是秒。若需要指定其他时间单位，可以使用另一个构造方法：IdleStateHandler(boolean observeOutput, long readerIdleTime, long writerIdleTime, long allIdleTime, TimeUnit unit)

在看下面的实现之前，建议先了解一下IdleStateHandler的实现原理。

下面直接上代码，需要注意的地方，会在代码中通过注释进行说明。

使用IdleStateHandler实现心跳

下面将使用IdleStateHandler来实现心跳，Client端连接到Server端后，会循环执行一个任务：随机等待几秒，然后ping一下Server端，即发送一个心跳包。当等待的时间超过规定时间，将会发送失败，以为Server端在此之前已经主动断开连接了。代码如下：

Client端

ClientIdleStateTrigger —— 心跳触发器

类ClientIdleStateTrigger也是一个Handler，只是重写了userEventTriggered方法，用于捕获IdleState.WRITER_IDLE事件（未在指定时间内向服务器发送数据），然后向Server端发送一个心跳包。

public class ClientIdleStateTrigger extends ChannelInboundHandlerAdapter {
 public static final String HEART_BEAT = "heart beat!";
 @Override
 public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
 if (evt instanceof IdleStateEvent) {
 IdleState state = ((IdleStateEvent) evt).state();
 if (state == IdleState.WRITER_IDLE) {
 // write heartbeat to server
 ctx.writeAndFlush(HEART_BEAT);
 }
 } else {
 super.userEventTriggered(ctx, evt);
 }
 }
}

Pinger —— 心跳发射器

public class Pinger extends ChannelInboundHandlerAdapter {
 private Random random = new Random();
 private int baseRandom = 8;
 private Channel channel;
 @Override
 public void channelActive(ChannelHandlerContext ctx) throws Exception {
 super.channelActive(ctx);
 this.channel = ctx.channel();
 ping(ctx.channel());
 }
 private void ping(Channel channel) {
 int second = Math.max(1, random.nextInt(baseRandom));
 System.out.println("next heart beat will send after " + second + "s.");
 ScheduledFuture<?> future = channel.eventLoop().schedule(new Runnable() {
 @Override
 public void run() {
 if (channel.isActive()) {
 System.out.println("sending heart beat to the server...");
 channel.writeAndFlush(ClientIdleStateTrigger.HEART_BEAT);
 } else {
 System.err.println("The connection had broken, cancel the task that will send a heart beat.");
 channel.closeFuture();
 throw new RuntimeException();
 }
 }
 }, second, TimeUnit.SECONDS);
 future.addListener(new GenericFutureListener() {
 @Override
 public void operationComplete(Future future) throws Exception {
 if (future.isSuccess()) {
 ping(channel);
 }
 }
 });
 }
 @Override
 public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
 // 当Channel已经断开的情况下, 仍然发送数据, 会抛异常, 该方法会被调用.
 cause.printStackTrace();
 ctx.close();
 }
}

ClientHandlersInitializer —— 客户端处理器集合的初始化类

public class ClientHandlersInitializer extends ChannelInitializer<SocketChannel> {
 private ReconnectHandler reconnectHandler;
 private EchoHandler echoHandler;
 public ClientHandlersInitializer(TcpClient tcpClient) {
 Assert.notNull(tcpClient, "TcpClient can not be null.");
 this.reconnectHandler = new ReconnectHandler(tcpClient);
 this.echoHandler = new EchoHandler();
 }
 @Override
 protected void initChannel(SocketChannel ch) throws Exception {
 ChannelPipeline pipeline = ch.pipeline();
 pipeline.addLast(new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 4, 0, 4));
 pipeline.addLast(new LengthFieldPrepender(4));
 pipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
 pipeline.addLast(new StringEncoder(CharsetUtil.UTF_8));
 pipeline.addLast(new Pinger());
 }
}

注： 上面的Handler集合，除了Pinger，其他都是编解码器和解决粘包，可以忽略。

TcpClient —— TCP连接的客户端

public class TcpClient {
 private String host;
 private int port;
 private Bootstrap bootstrap;
 
 private Channel channel;
 public TcpClient(String host, int port) {
 this(host, port, new ExponentialBackOffRetry(1000, Integer.MAX_VALUE, 60 * 1000));
 }
 public TcpClient(String host, int port, RetryPolicy retryPolicy) {
 this.host = host;
 this.port = port;
 init();
 }
 
 public void connect() {
 synchronized (bootstrap) {
 ChannelFuture future = bootstrap.connect(host, port);
 this.channel = future.channel();
 }
 }
 private void init() {
 EventLoopGroup group = new NioEventLoopGroup();
 // bootstrap 可重用, 只需在TcpClient实例化的时候初始化即可.
 bootstrap = new Bootstrap();
 bootstrap.group(group)
 .channel(NioSocketChannel.class)
 .handler(new ClientHandlersInitializer(TcpClient.this));
 }
 public static void main(String[] args) {
 TcpClient tcpClient = new TcpClient("localhost", 2222);
 tcpClient.connect();
 }
}

Server端

ServerIdleStateTrigger —— 断连触发器

public class ServerIdleStateTrigger extends ChannelInboundHandlerAdapter {
 @Override
 public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
 if (evt instanceof IdleStateEvent) {
 IdleState state = ((IdleStateEvent) evt).state();
 if (state == IdleState.READER_IDLE) {
 // 在规定时间内没有收到客户端的上行数据, 主动断开连接
 ctx.disconnect();
 }
 } else {
 super.userEventTriggered(ctx, evt);
 }
 }
}

ServerBizHandler —— 服务器端的业务处理器

@ChannelHandler.Sharable
public class ServerBizHandler extends SimpleChannelInboundHandler<String> {
 private final String REC_HEART_BEAT = "I had received the heart beat!";
 @Override
 protected void channelRead0(ChannelHandlerContext ctx, String data) throws Exception {
 try {
 System.out.println("receive data: " + data);
// ctx.writeAndFlush(REC_HEART_BEAT);
 } catch (Exception e) {
 e.printStackTrace();
 }
 }
 @Override
 public void channelActive(ChannelHandlerContext ctx) throws Exception {
 System.out.println("Established connection with the remote client.");
 // do something
 ctx.fireChannelActive();
 }
 @Override
 public void channelInactive(ChannelHandlerContext ctx) throws Exception {
 System.out.println("Disconnected with the remote client.");
 // do something
 ctx.fireChannelInactive();
 }
 @Override
 public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
 cause.printStackTrace();
 ctx.close();
 }
}

ServerHandlerInitializer —— 服务器端处理器集合的初始化类

public class ServerHandlerInitializer extends ChannelInitializer<SocketChannel> {
 protected void initChannel(SocketChannel ch) throws Exception {
 ch.pipeline().addLast("idleStateHandler", new IdleStateHandler(5, 0, 0));
 ch.pipeline().addLast("idleStateTrigger", new ServerIdleStateTrigger());
 ch.pipeline().addLast("frameDecoder", new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 4, 0, 4));
 ch.pipeline().addLast("frameEncoder", new LengthFieldPrepender(4));
 ch.pipeline().addLast("decoder", new StringDecoder());
 ch.pipeline().addLast("encoder", new StringEncoder());
 ch.pipeline().addLast("bizHandler", new ServerBizHandler());
 }
}

注：new IdleStateHandler(5, 0, 0)该handler代表如果在5秒内没有收到来自客户端的任何数据包（包括但不限于心跳包），将会主动断开与该客户端的连接。

TcpServer —— 服务器端

public class TcpServer {
 private int port;
 private ServerHandlerInitializer serverHandlerInitializer;
 public TcpServer(int port) {
 this.port = port;
 this.serverHandlerInitializer = new ServerHandlerInitializer();
 }
 public void start() {
 EventLoopGroup bossGroup = new NioEventLoopGroup(1);
 EventLoopGroup workerGroup = new NioEventLoopGroup();
 try {
 ServerBootstrap bootstrap = new ServerBootstrap();
 bootstrap.group(bossGroup, workerGroup)
 .channel(NioServerSocketChannel.class)
 .childHandler(this.serverHandlerInitializer);
 // 绑定端口，开始接收进来的连接
 ChannelFuture future = bootstrap.bind(port).sync();
 System.out.println("Server start listen at " + port);
 future.channel().closeFuture().sync();
 } catch (Exception e) {
 bossGroup.shutdownGracefully();
 workerGroup.shutdownGracefully();
 e.printStackTrace();
 }
 }
 public static void main(String[] args) throws Exception {
 int port = 2222;
 new TcpServer(port).start();
 }
}

至此，所有代码已经编写完毕。

测试

首先启动客户端，再启动服务器端。启动完成后，在客户端的控制台上，可以看到打印如下类似日志：

客户端控制台输出的日志

在服务器端可以看到控制台输出了类似如下的日志：

服务器端控制台输出的日志

可以看到，客户端在发送4个心跳包后，第5个包因为等待时间较长，等到真正发送的时候，发现连接已断开了；而服务器端收到客户端的4个心跳数据包后，迟迟等不到下一个数据包，所以果断断开该连接。

异常情况

在测试过程中，有可能会出现如下情况：

异常情况

出现这种情况的原因是：在连接已断开的情况下，仍然向服务器端发送心跳包。虽然在发送心跳包之前会使用channel.isActive()判断连接是否可用，但也有可能上一刻判断结果为可用，但下一刻发送数据包之前，连接就断了。

目前尚未找到优雅处理这种情况的方案，各位看官如果有好的解决方案，还望不吝赐教。拜谢！！！

断线重连

断线重连这里就不过多介绍，相信各位都知道是怎么回事。这里只说大致思路，然后直接上代码。

实现思路

客户端在监测到与服务器端的连接断开后，或者一开始就无法连接的情况下，使用指定的重连策略进行重连操作，直到重新建立连接或重试次数耗尽。

对于如何监测连接是否断开，则是通过重写ChannelInboundHandler#channelInactive来实现，但连接不可用，该方法会被触发，所以只需要在该方法做好重连工作即可。

代码实现

注：以下代码都是在上面心跳机制的基础上修改/添加的。

因为断线重连是客户端的工作，所以只需对客户端代码进行修改。

重试策略

RetryPolicy —— 重试策略接口

public interface RetryPolicy {
 
 boolean allowRetry(int retryCount);
 
 long getSleepTimeMs(int retryCount);
}

ExponentialBackOffRetry —— 重连策略的默认实现

public class ExponentialBackOffRetry implements RetryPolicy {
 private static final int MAX_RETRIES_LIMIT = 29;
 private static final int DEFAULT_MAX_SLEEP_MS = Integer.MAX_VALUE;
 private final Random random = new Random();
 private final long baseSleepTimeMs;
 private final int maxRetries;
 private final int maxSleepMs;
 public ExponentialBackOffRetry(int baseSleepTimeMs, int maxRetries) {
 this(baseSleepTimeMs, maxRetries, DEFAULT_MAX_SLEEP_MS);
 }
 public ExponentialBackOffRetry(int baseSleepTimeMs, int maxRetries, int maxSleepMs) {
 this.maxRetries = maxRetries;
 this.baseSleepTimeMs = baseSleepTimeMs;
 this.maxSleepMs = maxSleepMs;
 }
 @Override
 public boolean allowRetry(int retryCount) {
 if (retryCount < maxRetries) {
 return true;
 }
 return false;
 }
 @Override
 public long getSleepTimeMs(int retryCount) {
 if (retryCount < 0) {
 throw new IllegalArgumentException("retries count must greater than 0.");
 }
 if (retryCount > MAX_RETRIES_LIMIT) {
 System.out.println(String.format("maxRetries too large (%d). Pinning to %d", maxRetries, MAX_RETRIES_LIMIT));
 retryCount = MAX_RETRIES_LIMIT;
 }
 long sleepMs = baseSleepTimeMs * Math.max(1, random.nextInt(1 << retryCount));
 if (sleepMs > maxSleepMs) {
 System.out.println(String.format("Sleep extension too large (%d). Pinning to %d", sleepMs, maxSleepMs));
 sleepMs = maxSleepMs;
 }
 return sleepMs;
 }
}

ReconnectHandler—— 重连处理器

@ChannelHandler.Sharable
public class ReconnectHandler extends ChannelInboundHandlerAdapter {
 private int retries = 0;
 private RetryPolicy retryPolicy;
 private TcpClient tcpClient;
 public ReconnectHandler(TcpClient tcpClient) {
 this.tcpClient = tcpClient;
 }
 @Override
 public void channelActive(ChannelHandlerContext ctx) throws Exception {
 System.out.println("Successfully established a connection to the server.");
 retries = 0;
 ctx.fireChannelActive();
 }
 @Override
 public void channelInactive(ChannelHandlerContext ctx) throws Exception {
 if (retries == 0) {
 System.err.println("Lost the TCP connection with the server.");
 ctx.close();
 }
 boolean allowRetry = getRetryPolicy().allowRetry(retries);
 if (allowRetry) {
 long sleepTimeMs = getRetryPolicy().getSleepTimeMs(retries);
 System.out.println(String.format("Try to reconnect to the server after %dms. Retry count: %d.", sleepTimeMs, ++retries));
 final EventLoop eventLoop = ctx.channel().eventLoop();
 eventLoop.schedule(() -> {
 System.out.println("Reconnecting ...");
 tcpClient.connect();
 }, sleepTimeMs, TimeUnit.MILLISECONDS);
 }
 ctx.fireChannelInactive();
 }
 private RetryPolicy getRetryPolicy() {
 if (this.retryPolicy == null) {
 this.retryPolicy = tcpClient.getRetryPolicy();
 }
 return this.retryPolicy;
 }
}

ClientHandlersInitializer

在之前的基础上，添加了重连处理器ReconnectHandler。

public class ClientHandlersInitializer extends ChannelInitializer<SocketChannel> {
 private ReconnectHandler reconnectHandler;
 private EchoHandler echoHandler;
 public ClientHandlersInitializer(TcpClient tcpClient) {
 Assert.notNull(tcpClient, "TcpClient can not be null.");
 this.reconnectHandler = new ReconnectHandler(tcpClient);
 this.echoHandler = new EchoHandler();
 }
 @Override
 protected void initChannel(SocketChannel ch) throws Exception {
 ChannelPipeline pipeline = ch.pipeline();
 pipeline.addLast(this.reconnectHandler);
 pipeline.addLast(new LengthFieldBasedFrameDecoder(Integer.MAX_VALUE, 0, 4, 0, 4));
 pipeline.addLast(new LengthFieldPrepender(4));
 pipeline.addLast(new StringDecoder(CharsetUtil.UTF_8));
 pipeline.addLast(new StringEncoder(CharsetUtil.UTF_8));
 pipeline.addLast(new Pinger());
 }
}

TcpClient

在之前的基础上添加重连、重连策略的支持。

public class TcpClient {
 private String host;
 private int port;
 private Bootstrap bootstrap;
 
 private RetryPolicy retryPolicy;
 
 private Channel channel;
 public TcpClient(String host, int port) {
 this(host, port, new ExponentialBackOffRetry(1000, Integer.MAX_VALUE, 60 * 1000));
 }
 public TcpClient(String host, int port, RetryPolicy retryPolicy) {
 this.host = host;
 this.port = port;
 this.retryPolicy = retryPolicy;
 init();
 }
 
 public void connect() {
 synchronized (bootstrap) {
 ChannelFuture future = bootstrap.connect(host, port);
 future.addListener(getConnectionListener());
 this.channel = future.channel();
 }
 }
 public RetryPolicy getRetryPolicy() {
 return retryPolicy;
 }
 private void init() {
 EventLoopGroup group = new NioEventLoopGroup();
 // bootstrap 可重用, 只需在TcpClient实例化的时候初始化即可.
 bootstrap = new Bootstrap();
 bootstrap.group(group)
 .channel(NioSocketChannel.class)
 .handler(new ClientHandlersInitializer(TcpClient.this));
 }
 private ChannelFutureListener getConnectionListener() {
 return new ChannelFutureListener() {
 @Override
 public void operationComplete(ChannelFuture future) throws Exception {
 if (!future.isSuccess()) {
 future.channel().pipeline().fireChannelInactive();
 }
 }
 };
 }
 public static void main(String[] args) {
 TcpClient tcpClient = new TcpClient("localhost", 2222);
 tcpClient.connect();
 }
}

测试

在测试之前，为了避开 Connection reset by peer 异常，可以稍微修改Pinger的ping()方法，添加if (second == 5)的条件判断。如下：

private void ping(Channel channel) {
 int second = Math.max(1, random.nextInt(baseRandom));
 if (second == 5) {
 second = 6;
 }
 System.out.println("next heart beat will send after " + second + "s.");
 ScheduledFuture<?> future = channel.eventLoop().schedule(new Runnable() {
 @Override
 public void run() {
 if (channel.isActive()) {
 System.out.println("sending heart beat to the server...");
 channel.writeAndFlush(ClientIdleStateTrigger.HEART_BEAT);
 } else {
 System.err.println("The connection had broken, cancel the task that will send a heart beat.");
 channel.closeFuture();
 throw new RuntimeException();
 }
 }
 }, second, TimeUnit.SECONDS);
 future.addListener(new GenericFutureListener() {
 @Override
 public void operationComplete(Future future) throws Exception {
 if (future.isSuccess()) {
 ping(channel);
 }
 }
 });
 }

启动客户端

先只启动客户端，观察控制台输出，可以看到类似如下日志：

断线重连测试——客户端控制台输出

可以看到，当客户端发现无法连接到服务器端，所以一直尝试重连。随着重试次数增加，重试时间间隔越大，但又不想无限增大下去，所以需要定一个阈值，比如60s。如上图所示，当下一次重试时间超过60s时，会打印Sleep extension too large(*). Pinning to 60000，单位为ms。出现这句话的意思是，计算出来的时间超过阈值（60s），所以把真正睡眠的时间重置为阈值（60s）。

启动服务器端

接着启动服务器端，然后继续观察客户端控制台输出。

断线重连测试——服务器端启动后客户端控制台输出

可以看到，在第9次重试失败后，第10次重试之前，启动的服务器，所以第10次重连的结果为Successfully established a connection to the server.，即成功连接到服务器。接下来因为还是不定时ping服务器，所以出现断线重连、断线重连的循环。

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