• 目的： 
• 维持provider和consumer之间的长连接
• 实现： 
• dubbo心跳时间heartbeat默认是60s，超过heartbeat时间没有收到消息，就发送心跳消息(provider，consumer一样),如果连着3次(heartbeatTimeout为heartbeat*3)没有收到心跳响应，provider会关闭channel，而consumer会进行重连;不论是provider还是consumer的心跳检测都是通过启动定时任务的方式实现；
• provider绑定和consumer连接的入口：

public class HeaderExchanger implements Exchanger {    public static final String NAME = "header";    @Override    public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException {        return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))), true);    }    @Override    public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {        return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));    }}

• provider启动心跳检测：

public HeaderExchangeServer(Server server) {        if (server == null) {            throw new IllegalArgumentException("server == null");        }        this.server = server;        this.heartbeat = server.getUrl().getParameter(Constants.HEARTBEAT_KEY, 0);        //心跳超时时间默认为心跳时间的3倍        this.heartbeatTimeout = server.getUrl().getParameter(Constants.HEARTBEAT_TIMEOUT_KEY, heartbeat * 3);        //如果心跳超时时间小于心跳时间的两倍则抛异常        if (heartbeatTimeout < heartbeat * 2) {            throw new IllegalStateException("heartbeatTimeout < heartbeatInterval * 2");        }        startHeatbeatTimer();    }

• startHeatbeatTimer的实现 
• 先停止已有的定时任务，启动新的定时任务：

private void startHeatbeatTimer() {        // 停止原有定时任务        stopHeartbeatTimer();        // 发起新的定时任务        if (heartbeat > 0) {            heatbeatTimer = scheduled.scheduleWithFixedDelay(                    new HeartBeatTask(new HeartBeatTask.ChannelProvider() {                        public Collection
     
       getChannels() {                            return Collections.unmodifiableCollection(HeaderExchangeServer.this.getChannels());                        }                    }, heartbeat, heartbeatTimeout),                    heartbeat, heartbeat, TimeUnit.MILLISECONDS);        }    }
     

• HeartBeatTask的实现 
• 遍历所有的channel，检测心跳间隔,如果超过心跳间隔没有读或写，则发送需要回复的心跳消息,最有判断是否心跳超时(heartbeatTimeout),如果超时，provider关闭channel，consumer进行重连

public void run() {        try {            long now = System.currentTimeMillis();            for (Channel channel : channelProvider.getChannels()) {                if (channel.isClosed()) {                    continue;                }                try {                    Long lastRead = (Long) channel.getAttribute(HeaderExchangeHandler.KEY_READ_TIMESTAMP);                    Long lastWrite = (Long) channel.getAttribute(HeaderExchangeHandler.KEY_WRITE_TIMESTAMP);                    // 读写的时间，任一超过心跳间隔，发送心跳                    if ((lastRead != null && now - lastRead > heartbeat)                            || (lastWrite != null && now - lastWrite > heartbeat)) {                        Request req = new Request();                        req.setVersion("2.0.0");                        req.setTwoWay(true); // 需要响应的心跳事件                        req.setEvent(Request.HEARTBEAT_EVENT);                        channel.send(req);                        if (logger.isDebugEnabled()) {                            logger.debug("Send heartbeat to remote channel " + channel.getRemoteAddress()                                    + ", cause: The channel has no data-transmission exceeds a heartbeat period: " + heartbeat + "ms");                        }                    }                    // 最后读的时间，超过心跳超时时间                    if (lastRead != null && now - lastRead > heartbeatTimeout) {                        logger.warn("Close channel " + channel                                + ", because heartbeat read idle time out: " + heartbeatTimeout + "ms");                        // 客户端侧，重新连接服务端                        if (channel instanceof Client) {                            try {                                ((Client) channel).reconnect();                            } catch (Exception e) {                                //do nothing                            }                        // 服务端侧，关闭客户端连接                        } else {                            channel.close();                        }                    }                } catch (Throwable t) {                    logger.warn("Exception when heartbeat to remote channel " + channel.getRemoteAddress(), t);                }            }        } catch (Throwable t) {            logger.warn("Unhandled exception when heartbeat, cause: " + t.getMessage(), t);        }    }

• consumer端的实现 
• 默认需要心跳检测

public HeaderExchangeClient(Client client, boolean needHeartbeat) {        if (client == null) {            throw new IllegalArgumentException("client == null");        }        this.client = client;        // 创建 HeaderExchangeChannel 对象        this.channel = new HeaderExchangeChannel(client);        // 读取心跳相关配置        String dubbo = client.getUrl().getParameter(Constants.DUBBO_VERSION_KEY);        this.heartbeat = client.getUrl().getParameter(Constants.HEARTBEAT_KEY, dubbo != null && dubbo.startsWith("1.0.") ? Constants.DEFAULT_HEARTBEAT : 0);        this.heartbeatTimeout = client.getUrl().getParameter(Constants.HEARTBEAT_TIMEOUT_KEY, heartbeat * 3);        if (heartbeatTimeout < heartbeat * 2) { // 避免间隔太短            throw new IllegalStateException("heartbeatTimeout < heartbeatInterval * 2");        }        // 发起心跳定时器        if (needHeartbeat) {            startHeatbeatTimer();        }    }

我们可以看到dubbo的心跳检测，服务端会发送发送心跳包，客户端也会发送心跳包，与一般只有客户端发送心跳包，服务端接受心跳是有所不同的。