doc: 阅读netty心跳机制

netty/netty-doc.md

@@ -141,3 +141,87 @@ outbound event会按照自顶向下的顺序被outbound进行处理，outbound h
 ### ThreadSafety
 channelhandler可以在任何时刻添加到pipeline或从pipeline中移除，ChannelPipeline是线程安全的。例如，可以在交换敏感信息前添加encryption handler并且在交换完信息后将encryption handler移除。
 
+## Heartbeat机制
+在netty中，可以通过使用`IdleStateHandler`来实现心跳机制，可以向pipeline中添加`IdleStateHandler`作为心跳检测processor，并且可以添加一个自定义handler并实现`userEventTriggered`接口作为对超时事件的处理。
+
+### 服务端heartbeat实现
+服务端实现示例如下：
+```java
+ServerBootstrap b= new ServerBootstrap();
+b.group(bossGroup,workerGroup).channel(NioServerSocketChannel.class)
+        .option(ChannelOption.SO_BACKLOG,1024)
+        .childHandler(new ChannelInitializer<SocketChannel>() {
+            @Override
+            protected void initChannel(SocketChannel socketChannel) throws Exception {
+            　socketChannel.pipeline().addLast(new IdleStateHandler(5, 0, 0, TimeUnit.SECONDS));
+                socketChannel.pipeline().addLast(new StringDecoder());
+                socketChannel.pipeline().addLast(new HeartBeatServerHandler())；
+            }
+        });
+```
+自定义的心跳超时处理handler，其逻辑则如下所示。当每当timeout event被触发时，都会调用`userEventTriggered`事件，`timeout event`包含`read idle timeout`或`write idle timeout`。
+```java
+class HeartBeatServerHandler extends ChannelInboundHandlerAdapter {
+    private int lossConnectCount = 0;
+
+    @Override
+    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
+        System.out.println("No message from the client has been received for 5 seconds!");
+        if (evt instanceof IdleStateEvent){
+            IdleStateEvent event = (IdleStateEvent)evt;
+            if (event.state()== IdleState.READER_IDLE){
+                lossConnectCount++;
+                if (lossConnectCount>2){
+                    System.out.println("Close this inactive channel!");
+                    ctx.channel().close();
+                }
+            }
+        }else {
+            super.userEventTriggered(ctx,evt);
+        }
+    }
+
+    @Override
+    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
+        lossConnectCount = 0;
+        System.out.println("client says: "+msg.toString());
+    }
+
+    @Override
+    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
+        ctx.close();
+    }
+}
+```
+
+### 客户端heartbeat实现
+客户端也会将IdleStateHandler作为心跳检测processor提娜佳到pipeline中，并且添加自定义handler作为超时事件处理。
+
+如下示例将IdleStateHandler的心跳检测设置为了每4s一次，并且自定义handler的`userEventTriggered`方法用于处理write idle的场景，代码示例如下：
+```java
+Bootstrap b = new Bootstrap();
+b.group(group).channel(NioSocketChannel.class)
+        .handler(new ChannelInitializer<SocketChannel>() {
+            @Override
+            protected void initChannel(SocketChannel socketChannel) throws Exception {
+                socketChannel.pipeline().addLast(new IdleStateHandler(0,4,0, TimeUnit.SECONDS));
+                socketChannel.pipeline().addLast(new StringEncoder());
+                socketChannel.pipeline().addLast(new HeartBeatClientHandler());
+            }
+        });
+```
+
+```java
+public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
+    System.out.println("Client cyclic heartbeat monitoring sending: "+new Date());
+    if (evt instanceof IdleStateEvent){
+        IdleStateEvent event = (IdleStateEvent)evt;
+        if (event.state()== IdleState.WRITER_IDLE){
+            if (curTime<beatTime){
+                curTime++;
+                ctx.writeAndFlush("biubiu");
+            }
+        }
+    }
+}
+```