netty 心跳机制示例，使用netty4，IdleStateHandler 实现。
本文假设你已经了解了Netty的使用，或者至少写过netty的helloworld，知道了netty的基本使用。我们知道使用netty的时候，大多数的东西都与Handler有关，我们的业务逻辑基本都是在Handler中实现的。Netty中自带了一个IdleStateHandler 可以用来实现心跳检测。
心跳检测的逻辑
本文中我们将要实现的心跳检测逻辑是这样的：服务端启动后，等待客户端连接，客户端连接之后，向服务端发送消息。如果客户端在“干活”那么服务端必定会收到数据，如果客户端“闲下来了”那么服务端就接收不到这个服务端的消息，既然客户端闲下来了，不干事，那么何必浪费连接资源呢？所以服务端检测到一定时间内客户端不活跃的时候，将客户端连接关闭。本文要实现的逻辑步骤为：
- 启动服务端，启动客户端
- 客户端向服务端发送"I am alive"，并sleep随机时间，用来模拟空闲。
- 服务端接收客户端消息，并返回"copy that"，客户端空闲时 计数+1.
- 服务端客户端继续通信
- 服务端检测客户端空闲太多，关闭连接。客户端发现连接关闭了，就退出了。
有了这个思路，我们先来编写服务端。
心跳检测服务端代码
public class HeartBeatServer { ???int port ; ???public HeartBeatServer(int port){ ???????this.port = port; ???} ???public void start(){ ???????ServerBootstrap bootstrap = new ServerBootstrap(); ???????EventLoopGroup boss = new NioEventLoopGroup(); ???????EventLoopGroup worker = new NioEventLoopGroup(); ???????try{ ???????????bootstrap.group(boss,worker) ???????????????????.handler(new LoggingHandler(LogLevel.INFO)) ???????????????????.channel(NioServerSocketChannel.class) ???????????????????.childHandler(new HeartBeatInitializer()); ???????????ChannelFuture future = bootstrap.bind(port).sync(); ???????????future.channel().closeFuture().sync(); ???????}catch(Exception e){ ???????????e.printStackTrace(); ???????}finally { ???????????worker.shutdownGracefully(); ???????????boss.shutdownGracefully(); ???????} ???} ???public static void main(String[] args) throws Exception { ???????HeartBeatServer server = new HeartBeatServer(8090); ???????server.start(); ???}}熟悉netty的同志，对于上面的模板一样的代码一定是在熟悉不过了。啥都不用看，只需要看childHandler(new HeartBeatInitializer()) 这一句。HeartBeatInitializer就是一个ChannelInitializer顾名思义，他就是在初始化channel的时做一些事情。我们所需要开发的业务逻辑Handler就是在这里添加的。其代码如下：
public class HeartBeatInitializer extends ChannelInitializer<Channel> { ???@Override ???protected void initChannel(Channel channel) throws Exception { ???????ChannelPipeline pipeline = channel.pipeline(); ???????pipeline.addLast("decoder", new StringDecoder()); ???????pipeline.addLast("encoder", new StringEncoder()); ???????pipeline.addLast(new IdleStateHandler(2,2,2, TimeUnit.SECONDS)); ???????pipeline.addLast(new HeartBeatHandler()); ???}}代码很简单，我们先添加了StringDecoder,和StringEncoder。这两个其实就是编解码用的，下面的IdleStateHandler才是本次心跳的核心组件。我们可以看到IdleStateHandler的构造函数中接收了4个参数，起定义如下：
public IdleStateHandler(long readerIdleTime, long writerIdleTime, long allIdleTime, TimeUnit unit);三个空闲时间参数，以及时间参数的格式。我们的例子中设置的是2,2,2，意思就是客户端2秒没有读/写，这个超时时间就会被触发。超时事件触发就需要我们来处理了，这就是上的HeartBeatInitializer中最后一行的HeartBeatHandler所做的事情。代码如下：
public class HeartBeatHandler extends SimpleChannelInboundHandler<String> { ???int readIdleTimes = 0; ???@Override ???protected void channelRead0(ChannelHandlerContext ctx, String s) throws Exception { ???????System.out.println(" ====== > [server] message received : " + s); ??????if("I am alive".equals(s)){ ???????????ctx.channel().writeAndFlush("copy that"); ???????}else { ??????????System.out.println(" 其他信息处理 ... "); ??????} ???} ???@Override ???public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception { ???????IdleStateEvent event = (IdleStateEvent)evt; ???????String eventType = null; ???????switch (event.state()){ ???????????case READER_IDLE: ???????????????eventType = "读空闲"; ???????????????readIdleTimes ++; // 读空闲的计数加1 ???????????????break; ???????????case WRITER_IDLE: ???????????????eventType = "写空闲"; ???????????????// 不处理 ???????????????break; ???????????case ALL_IDLE: ???????????????eventType ="读写空闲"; ???????????????// 不处理 ???????????????break; ???????} ???????System.out.println(ctx.channel().remoteAddress() + "超时事件：" +eventType); ???????if(readIdleTimes > 3){ ???????????System.out.println(" [server]读空闲超过3次，关闭连接"); ???????????ctx.channel().writeAndFlush("you are out"); ???????????ctx.channel().close(); ???????} ???} ???@Override ???public void channelActive(ChannelHandlerContext ctx) throws Exception { ???????System.err.println("=== " + ctx.channel().remoteAddress() + " is active ==="); ???}}至此，我们的服务端写好了。
心跳检测客户端代码
netty的api设计使得编码的模式非常具有通用性，所以客户端代码和服务端的代码几乎一样：启动client端的代码几乎一样，也需要一个ChannelInitializer，也需要Handler。改动的地方很少，因此本文不对客户端代码进行详细解释。下面给出client端的完整代码：
public class HeartBeatClient ?{ ???int port; ???Channel channel; ???Random random ; ???public HeartBeatClient(int port){ ???????this.port = port; ???????random = new Random(); ???} ???public static void main(String[] args) throws Exception{ ???????HeartBeatClient client = new HeartBeatClient(8090); ???????client.start(); ???} ???public void start() { ???????EventLoopGroup eventLoopGroup = new NioEventLoopGroup(); ???????try{ ???????????Bootstrap bootstrap = new Bootstrap(); ???????????bootstrap.group(eventLoopGroup).channel(NioSocketChannel.class) ???????????????????.handler(new HeartBeatClientInitializer()); ???????????connect(bootstrap,port); ???????????String ?text = "I am alive"; ???????????while (channel.isActive()){ ???????????????sendMsg(text); ???????????} ???????}catch(Exception e){ ???????????// do something ???????}finally { ???????????eventLoopGroup.shutdownGracefully(); ???????} ???} ???public void connect(Bootstrap bootstrap,int port) throws Exception{ ???????channel = bootstrap.connect("localhost",8090).sync().channel(); ???} ???public void sendMsg(String text) throws Exception{ ???????int num = random.nextInt(10); ???????Thread.sleep(num * 1000); ???????channel.writeAndFlush(text); ???} ???static class HeartBeatClientInitializer extends ChannelInitializer<Channel> { ???????@Override ???????protected void initChannel(Channel ch) throws Exception { ???????????ChannelPipeline pipeline = ch.pipeline(); ???????????pipeline.addLast("decoder", new StringDecoder()); ???????????pipeline.addLast("encoder", new StringEncoder()); ???????????pipeline.addLast(new HeartBeatClientHandler()); ???????} ???} ???static class HeartBeatClientHandler extends SimpleChannelInboundHandler<String> { ???????@Override ???????protected void channelRead0(ChannelHandlerContext ctx, String msg) throws Exception { ???????????System.out.println(" client received :" +msg); ???????????if(msg!= null && msg.equals("you are out")) { ???????????????System.out.println(" server closed connection , so client will close too"); ???????????????ctx.channel().closeFuture(); ???????????} ???????} ???}}运行代码
在上面的代码写好之后，我们先启动服务端，然后在启动客户端。运行日志如下：
server端：
=== /127.0.0.1:57700 is active === ====== > [server] message received : I am alive ====== > [server] message received : I am alive/127.0.0.1:57700超时事件：写空闲/127.0.0.1:57700超时事件：读空闲/127.0.0.1:57700超时事件：读写空闲/127.0.0.1:57700超时事件：写空闲/127.0.0.1:57700超时事件：读空闲/127.0.0.1:57700超时事件：读写空闲/127.0.0.1:57700超时事件：写空闲 ====== > [server] message received : I am alive/127.0.0.1:57700超时事件：写空闲/127.0.0.1:57700超时事件：读写空闲/127.0.0.1:57700超时事件：读空闲/127.0.0.1:57700超时事件：写空闲/127.0.0.1:57700超时事件：读写空闲/127.0.0.1:57700超时事件：读空闲 [server]读空闲超过3次，关闭连接client端：
client sent msg and sleep 2 client received :copy that client received :copy that client sent msg and sleep 6 client sent msg and sleep 6 client received :copy that client received :you are out server closed connection , so client will close tooProcess finished with exit code 0通过上面的运行日志，我们可以看到：
1.客户端在与服务器成功建立之后，发送了3次‘I am alive‘，服务端也回应了3次：‘copy that‘
2.由于客户端消极怠工，超时了多次，服务端关闭了链接。
3.客户端知道服务端抛弃自己之后，也关闭了连接，程序退出。
以上简单了演示了一下，netty的心跳机制，其实主要就是使用了IdleStateHandler。源码下载：https://gitee.com/dimixu/netty_learn.git
Netty 实现心跳机制.md
原文地址：https://www.cnblogs.com/demingblog/p/9957143.html