作者：sprinkle_liz

www.jianshu.com/p/1a28e48edd92

提醒：本篇适合有一定netty基础的读者阅读

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。相关链接：

https://blog.csdn.net/linuu/article/details/51385682

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!";@Overridepublic 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 serverctx.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;@Overridepublic 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() {@Overridepublic 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() {@Overridepublic void operationComplete(Future future) throws Exception {if (future.isSuccess()) {ping(channel);}}});}@Overridepublic 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();}@Overrideprotected 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;/** 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 {@Overridepublic 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.Sharable
public class ServerBizHandler extends SimpleChannelInboundHandler<String> {private final String REC_HEART_BEAT = "I had received the heart beat!";@Overrideprotected 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();}}@Overridepublic void channelActive(ChannelHandlerContext ctx) throws Exception {System.out.println("Established connection with the remote client.");// do somethingctx.fireChannelActive();}@Overridepublic void channelInactive(ChannelHandlerContext ctx) throws Exception {System.out.println("Disconnected with the remote client.");// do somethingctx.fireChannelInactive();}@Overridepublic 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());}}

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：

在服务器端可以看到控制台输出了类似如下的日志：

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：

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 {/*** 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;}@Overridepublic boolean allowRetry(int retryCount) {if (retryCount < maxRetries) {return true;}return false;}@Overridepublic 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;}@Overridepublic void channelActive(ChannelHandlerContext ctx) throws Exception {System.out.println("Successfully established a connection to the server.");retries = 0;ctx.fireChannelActive();}@Overridepublic 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();}@Overrideprotected 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() {@Overridepublic 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() {@Overridepublic 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() {@Overridepublic 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：

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。

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！！！

END

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