作者：sprinkle_liz
www.jianshu.com/p/1a28e48edd92
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.
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.
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。
/** * <p> * 用于捕获{@link IdleState#WRITER_IDLE}事件（未在指定时间内向服务器发送数据），然后向<code>Server</code>端发送一个心跳包。 * </p> */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
/** * <p>客户端连接到服务器端后，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。</p> */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()); }}TcpClient —— TCP连接的客户端
public class TcpClient { private String host; private int port; private Bootstrap bootstrap; /** 将<code>Channel</code>保存起来, 可用于在其他非handler的地方发送数据 */ 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(); } /** * 向远程TCP服务器请求连接 */ 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
/** * <p>在规定时间内未收到客户端的任何数据包, 将主动断开该连接</p> */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
/** * <p>收到来自客户端的数据包后, 直接在控制台打印出来.</p> */@ChannelHandler.Sharablepublic 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
/** * <p>用于初始化服务器端涉及到的所有<code>Handler</code></p> */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()); }}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
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 { /** * Called when an operation has failed for some reason. This method should return * true to make another attempt. * * @param retryCount the number of times retried so far (0 the first time) * @return true/false */ boolean allowRetry(int retryCount); /** * get sleep time in ms of current retry count. * * @param retryCount current retry count * @return the time to sleep */ long getSleepTimeMs(int retryCount);}ExponentialBackOffRetry —— 重连策略的默认实现
/** * <p>Retry policy that retries a set number of times with increasing sleep time between retries</p> */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.Sharablepublic 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; /** 重连策略 */ private RetryPolicy retryPolicy; /** 将<code>Channel</code>保存起来, 可用于在其他非handler的地方发送数据 */ 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(); } /** * 向远程TCP服务器请求连接 */ 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。
扩展
在不同环境，可能会有不同的重连需求。有不同的重连需求的，只需自己实现RetryPolicy接口，然后在创建TcpClient的时候覆盖默认的重连策略即可。
完！！！
关注Java技术栈微信公众号，在后台回复关键字：Java，可以获取一份栈长整理的 Java 最新技术干货。
最近干货分享
点击「阅读原文」一起搞技术，爽～