Netty生产级的心跳和重连机制

今天研究的是，心跳和重连，虽然这次是大神写的代码，但是万变不离其宗，我们先回顾一下Netty应用心跳和重连的整个过程：

1）客户端连接服务端

2）在客户端的的ChannelPipeline中加入一个比较特殊的IdleStateHandler，设置一下客户端的写空闲时间，例如5s

3）当客户端的所有ChannelHandler中4s内没有write事件，则会触发userEventTriggered方法（上文介绍过）

4）我们在客户端的userEventTriggered中对应的触发事件下发送一个心跳包给服务端，检测服务端是否还存活，防止服务端已经宕机，客户端还不知道

5）同样，服务端要对心跳包做出响应，其实给客户端最好的回复就是“不回复”，这样可以服务端的压力，假如有10w个空闲Idle的连接，那么服务端光发送心跳回复，则也是费事的事情，那么怎么才能告诉客户端它还活着呢，其实很简单，因为5s服务端都会收到来自客户端的心跳信息，那么如果10秒内收不到，服务端可以认为客户端挂了，可以close链路

6）加入服务端因为什么因素导致宕机的话，就会关闭所有的链路链接，所以作为客户端要做的事情就是短线重连

以上描述的就是整个心跳和重连的整个过程，虽然很简单，上一篇blog也写了一个Demo,简单地做了一下上述功能

要写工业级的Netty心跳重连的代码，需要解决一下几个问题：

1）ChannelPipeline中的ChannelHandlers的维护，首次连接和重连都需要对ChannelHandlers进行管理

2）重连对象的管理，也就是bootstrap对象的管理

3）重连机制编写

完整的代码：https://github.com/BazingaLyn/netty-study/tree/master/src/main/java/com/lyncc/netty/idle

下面我们就看大神是如何解决这些问题的，首先先定义一个接口ChannelHandlerHolder，用来保管ChannelPipeline中的Handlers的

package com.lyncc.netty.idle;
import io.netty.channel.ChannelHandler;
/**
*
* 客户端的ChannelHandler集合，由子类实现，这样做的好处：
* 继承这个接口的所有子类可以很方便地获取ChannelPipeline中的Handlers
* 获取到handlers之后方便ChannelPipeline中的handler的初始化和在重连的时候也能很方便
* 地获取所有的handlers
*/
public interface ChannelHandlerHolder {
ChannelHandler[] handlers();
}

我们再来编写我们熟悉的服务端的ServerBootstrap的编写：

HeartBeatServer.java

package com.lyncc.netty.idle;
import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import io.netty.handler.timeout.IdleStateHandler;
import java.net.InetSocketAddress;
import java.util.concurrent.TimeUnit;
public class HeartBeatServer {
private final AcceptorIdleStateTrigger idleStateTrigger = new AcceptorIdleStateTrigger();
private int port;
public HeartBeatServer(int port) {
this.port = port;
}
public void start() {
EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();
try {
ServerBootstrap sbs = new ServerBootstrap().group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class).handler(new LoggingHandler(LogLevel.INFO))
.localAddress(new InetSocketAddress(port)).childHandler(new ChannelInitializer<SocketChannel>() {
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline().addLast(new IdleStateHandler(5, 0, 0, TimeUnit.SECONDS));
ch.pipeline().addLast(idleStateTrigger);
ch.pipeline().addLast("decoder", new StringDecoder());
ch.pipeline().addLast("encoder", new StringEncoder());
ch.pipeline().addLast(new HeartBeatServerHandler());
};
}).option(ChannelOption.SO_BACKLOG, 128).childOption(ChannelOption.SO_KEEPALIVE, true);
// 绑定端口，开始接收进来的连接
ChannelFuture future = sbs.bind(port).sync();
System.out.println("Server start listen at " + port);
future.channel().closeFuture().sync();
} catch (Exception e) {
bossGroup.shutdownGracefully();
workerGroup.shutdownGracefully();
}
}
public static void main(String[] args) throws Exception {
int port;
if (args.length > 0) {
port = Integer.parseInt(args[0]);
} else {
port = 8080;
}
new HeartBeatServer(port).start();
}
}

单独写一个AcceptorIdleStateTrigger，其实也是继承ChannelInboundHandlerAdapter，重写userEventTriggered方法，因为客户端是write，那么服务端自然是read，设置的状态就是IdleState.READER_IDLE，源码如下：

package com.lyncc.netty.idle;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.handler.timeout.IdleState;
import io.netty.handler.timeout.IdleStateEvent;
@ChannelHandler.Sharable
public class AcceptorIdleStateTrigger 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) {
throw new Exception("idle exception");
}
} else {
super.userEventTriggered(ctx, evt);
}
}
}

HeartBeatServerHandler就是一个很简单的自定义的Handler，不是重点：

package com.lyncc.netty.idle;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
public class HeartBeatServerHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
System.out.println("server channelRead..");
System.out.println(ctx.channel().remoteAddress() + "->Server :" + msg.toString());
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
cause.printStackTrace();
ctx.close();
}
}

接下来就是重点，我们需要写一个类，这个类可以去观察链路是否断了，如果断了，进行循环的断线重连操作，ConnectionWatchdog，顾名思义，链路检测狗，我们先看完整代码：

package com.lyncc.netty.idle;
import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelInitializer;
import io.netty.util.Timeout;
import io.netty.util.Timer;
import io.netty.util.TimerTask;
import java.util.concurrent.TimeUnit;
/**
*
* 重连检测狗，当发现当前的链路不稳定关闭之后，进行12次重连
*/
@Sharable
public abstract class ConnectionWatchdog extends ChannelInboundHandlerAdapter implements TimerTask ,ChannelHandlerHolder{
private final Bootstrap bootstrap;
private final Timer timer;
private final int port;
private final String host;
private volatile boolean reconnect = true;
private int attempts;
public ConnectionWatchdog(Bootstrap bootstrap, Timer timer, int port,String host, boolean reconnect) {
this.bootstrap = bootstrap;
this.timer = timer;
this.port = port;
this.host = host;
this.reconnect = reconnect;
}
/**
* channel链路每次active的时候，将其连接的次数重新☞ 0
*/
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
System.out.println("当前链路已经激活了，重连尝试次数重新置为0");
attempts = 0;
ctx.fireChannelActive();
}
@Override
public void channelInactive(ChannelHandlerContext ctx) throws Exception {
System.out.println("链接关闭");
if(reconnect){
System.out.println("链接关闭，将进行重连");
if (attempts < 12) {
attempts++;
//重连的间隔时间会越来越长
int timeout = 2 << attempts;
timer.newTimeout(this, timeout, TimeUnit.MILLISECONDS);
}
}
ctx.fireChannelInactive();
}
public void run(Timeout timeout) throws Exception {
ChannelFuture future;
//bootstrap已经初始化好了，只需要将handler填入就可以了
synchronized (bootstrap) {
bootstrap.handler(new ChannelInitializer<Channel>() {
@Override
protected void initChannel(Channel ch) throws Exception {
ch.pipeline().addLast(handlers());
}
});
future = bootstrap.connect(host,port);
}
//future对象
future.addListener(new ChannelFutureListener() {
public void operationComplete(ChannelFuture f) throws Exception {
boolean succeed = f.isSuccess();
//如果重连失败，则调用ChannelInactive方法，再次出发重连事件，一直尝试12次，如果失败则不再重连
if (!succeed) {
System.out.println("重连失败");
f.channel().pipeline().fireChannelInactive();
}else{
System.out.println("重连成功");
}
}
});
}
}

稍微分析一下：

1）继承了ChannelInboundHandlerAdapter，说明它也是Handler，也对，作为一个检测对象，肯定会放在链路中，否则怎么检测

2）实现了2个接口，TimeTask，ChannelHandlerHolder

①TimeTask，我们就要写run方法，这应该是一个定时任务，这个定时任务做的事情应该是重连的工作

②ChannelHandlerHolder的接口，这个接口我们刚才说过是维护的所有的Handlers，因为在重连的时候需要获取Handlers

3）bootstrap对象，重连的时候依旧需要这个对象

4）当链路断开的时候会触发channelInactive这个方法，也就说触发重连的导火索是从这边开始的

好了，我们这边再写次核心的HeartBeatsClient的代码：

package com.lyncc.netty.idle;
import io.netty.bootstrap.Bootstrap;
import io.netty.channel.Channel;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.nio.NioSocketChannel;
import io.netty.handler.codec.string.StringDecoder;
import io.netty.handler.codec.string.StringEncoder;
import io.netty.handler.logging.LogLevel;
import io.netty.handler.logging.LoggingHandler;
import io.netty.handler.timeout.IdleStateHandler;
import io.netty.util.HashedWheelTimer;
import java.util.concurrent.TimeUnit;
public class HeartBeatsClient {
protected final HashedWheelTimer timer = new HashedWheelTimer();
private Bootstrap boot;
private final ConnectorIdleStateTrigger idleStateTrigger = new ConnectorIdleStateTrigger();
public void connect(int port, String host) throws Exception {
EventLoopGroup group = new NioEventLoopGroup();
boot = new Bootstrap();
boot.group(group).channel(NioSocketChannel.class).handler(new LoggingHandler(LogLevel.INFO));
final ConnectionWatchdog watchdog = new ConnectionWatchdog(boot, timer, port,host, true) {
public ChannelHandler[] handlers() {
return new ChannelHandler[] {
this,
new IdleStateHandler(0, 4, 0, TimeUnit.SECONDS),
idleStateTrigger,
new StringDecoder(),
new StringEncoder(),
new HeartBeatClientHandler()
};
}
};
ChannelFuture future;
//进行连接
try {
synchronized (boot) {
boot.handler(new ChannelInitializer<Channel>() {
//初始化channel
@Override
protected void initChannel(Channel ch) throws Exception {
ch.pipeline().addLast(watchdog.handlers());
}
});
future = boot.connect(host,port);
}
// 以下代码在synchronized同步块外面是安全的
future.sync();
} catch (Throwable t) {
throw new Exception("connects to fails", t);
}
}
/**
* @param args
* @throws Exception
*/
public static void main(String[] args) throws Exception {
int port = 8080;
if (args != null && args.length > 0) {
try {
port = Integer.valueOf(args[0]);
} catch (NumberFormatException e) {
// 采用默认值
}
}
new HeartBeatsClient().connect(port, "127.0.0.1");
}
}

也稍微说明一下：

1）创建了ConnectionWatchdog对象，自然要实现handlers方法

2）初始化好bootstrap对象

3）4秒内没有写操作，进行心跳触发，也就是IdleStateHandler这个方法

最后ConnectorIdleStateTrigger这个类

package com.lyncc.netty.idle;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.handler.timeout.IdleState;
import io.netty.handler.timeout.IdleStateEvent;
import io.netty.util.CharsetUtil;
@Sharable
public class ConnectorIdleStateTrigger extends ChannelInboundHandlerAdapter {
private static final ByteBuf HEARTBEAT_SEQUENCE = Unpooled.unreleasableBuffer(Unpooled.copiedBuffer("Heartbeat",
CharsetUtil.UTF_8));
@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(HEARTBEAT_SEQUENCE.duplicate());
}
} else {
super.userEventTriggered(ctx, evt);
}
}
}

HeartBeatClientHandler.java(不是重点)

package com.lyncc.netty.idle;
import io.netty.channel.ChannelHandler.Sharable;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.util.ReferenceCountUtil;
import java.util.Date;
@Sharable
public class HeartBeatClientHandler extends ChannelInboundHandlerAdapter {
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
System.out.println("激活时间是："+new Date());
System.out.println("HeartBeatClientHandler channelActive");
ctx.fireChannelActive();
}
@Override
public void channelInactive(ChannelHandlerContext ctx) throws Exception {
System.out.println("停止时间是："+new Date());
System.out.println("HeartBeatClientHandler channelInactive");
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
String message = (String) msg;
System.out.println(message);
if (message.equals("Heartbeat")) {
ctx.write("has read message from server");
ctx.flush();
}
ReferenceCountUtil.release(msg);
}
}

好了，到此为止，所有的代码都贴完了，我们做一个简单的测试，按照常理，如果不出任何状况的话，客户端4秒发送心跳，服务端5秒才验证是不会断连的，所以我们在启动之后，关闭服务端，然后再次重启服务端

首先启动服务端，控制台如下：

启动客户端，控制台如下：

客户端启动之后，服务端的控制台：

关闭服务端后，客户端控制台：

重启启动服务端：

重连成功~

巴特西

Netty生产级的心跳和重连机制

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