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~