Netty 如何通过心跳检测机制实现空闲自动断开？

## 什么是心跳检测 心跳检测指的是在客户端和服务端维护一种特殊的数据包，客户端通过这个数据包告诉服务端自己还是存活的，然后服务端可以通过这个心跳检测机制去实现一些业务功能，如：空闲自动断开、判断客户端是否在线等 ## 如何实现心跳检测机制 其实只需要引入IdleStateHandler，就搞定了 ``` try { ServerBootstrap bootstrap = new ServerBootstrap(); bootstrap.group(bossGroup, workerGroup) //设置两个线程组 .channel(NioServerSocketChannel.class) .option(ChannelOption.SO_BACKLOG, 128) .childOption(ChannelOption.SO_KEEPALIVE, true) .childHandler(new ChannelInitializer<SocketChannel>() { @Override protected void initChannel(SocketChannel ch) throws Exception { log.info("客户端channel 初始化"); ch.pipeline().addLast(new IdleStateHandler(3,3,3, TimeUnit.SECONDS)); ch.pipeline().addLast(new NettyServerHandler()); } }); ChannelFuture cf = bootstrap.bind(9999).sync(); log.info("服务启动成功...."); cf.addListener((ChannelFutureListener) future -> log.info("监听端口:{}",future.isSuccess())); cf.channel().closeFuture().sync(); } finally { bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); } ``` ## 心跳检测机制工作原理 进入源码之后，我们可以发现IdleStateHandler内部定义的一些结构 ### 初始化构造器后 定义了读、写、读写 事件 ### 触发channelActive后 ``` @Override public void channelActive(ChannelHandlerContext ctx) throws Exception { // This method will be invoked only if this handler was added // before channelActive() event is fired. If a user adds this handler // after the channelActive() event, initialize() will be called by beforeAdd(). initialize(ctx); super.channelActive(ctx); } ``` 调用了initialize(ctx);，我们点进去看看里面做了什么 ``` private void initialize(ChannelHandlerContext ctx) { // Avoid the case where destroy() is called before scheduling timeouts. // See: https://github.com/netty/netty/issues/143 switch (state) { case 1: case 2: return; } state = 1; initOutputChanged(ctx); lastReadTime = lastWriteTime = ticksInNanos(); if (readerIdleTimeNanos > 0) { readerIdleTimeout = schedule(ctx, new ReaderIdleTimeoutTask(ctx), readerIdleTimeNanos, TimeUnit.NANOSECONDS); } if (writerIdleTimeNanos > 0) { writerIdleTimeout = schedule(ctx, new WriterIdleTimeoutTask(ctx), writerIdleTimeNanos, TimeUnit.NANOSECONDS); } if (allIdleTimeNanos > 0) { allIdleTimeout = schedule(ctx, new AllIdleTimeoutTask(ctx), allIdleTimeNanos, TimeUnit.NANOSECONDS); } } ``` 通过我们的配置，分别schedule三个任务：ReaderIdleTimeoutTask、WriterIdleTimeoutTask、AllIdleTimeoutTask ## 定时执行task 这里拿ReaderIdleTimeoutTask进行说明 ``` private final class ReaderIdleTimeoutTask extends AbstractIdleTask { ReaderIdleTimeoutTask(ChannelHandlerContext ctx) { super(ctx); } @Override protected void run(ChannelHandlerContext ctx) { long nextDelay = readerIdleTimeNanos; if (!reading) { nextDelay -= ticksInNanos() - lastReadTime; } if (nextDelay <= 0) { // Reader is idle - set a new timeout and notify the callback. readerIdleTimeout = schedule(ctx, this, readerIdleTimeNanos, TimeUnit.NANOSECONDS); boolean first = firstReaderIdleEvent; firstReaderIdleEvent = false; try { IdleStateEvent event = newIdleStateEvent(IdleState.READER_IDLE, first); channelIdle(ctx, event); } catch (Throwable t) { ctx.fireExceptionCaught(t); } } else { // Read occurred before the timeout - set a new timeout with shorter delay. readerIdleTimeout = schedule(ctx, this, nextDelay, TimeUnit.NANOSECONDS); } } } ``` - 获取读空闲的时长 - 如果当前没有在读，则将当前时间剪去上次读取channel的时间做差值，然后将配置的空闲时长剪去这个差值，得到 新的nextDelay - 如果nextDelay小于0，则说明，距离上次读取channel的时间已经超过了程序配置的时间 - 重新schedule一个新的task，时间重置 - 触发IdleStateEvent 时间，一般业务逻辑回去订阅这个事件 - 如果大于0，则按照新的delay重新schedule一个task ## 编写业务逻辑，实现空闲自动断开 ``` @Slf4j class NettyServerHandler extends ChannelInboundHandlerAdapter { int i=0; @Override public void channelRead(ChannelHandlerContext ctx, Object msg) { ByteBuf byteBuf = (ByteBuf) msg; log.info("第{}次收到客户端消息:{}\n",i++,byteBuf.readableBytes()); } @Override public void channelReadComplete(ChannelHandlerContext ctx) { } @Override public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception { if (evt instanceof IdleStateEvent) { IdleStateEvent idleStateEvent = (IdleStateEvent) evt; switch (idleStateEvent.state()) { case ALL_IDLE: log.error("读写都空闲"); break; case READER_IDLE: log.error("读空闲"); break; case WRITER_IDLE: log.error("写空闲"); break; } } } } ```