exchanger层默认实现类是HeaderExchange.

一 client

• DubboProtocol层调用exchanger接口获取client
  ExchangeClient client = Exchangers.connect(url, requestHandler)
• 接口返回HeaderExhangeClient

    public ExchangeClient connect(URL url, ExchangeHandler handler) throws RemotingException {
        return new HeaderExchangeClient(Transporters.connect(url, new DecodeHandler(new HeaderExchangeHandler(handler))), true);
    }

1.1 HeaderExhangeClient

• HeaderExhangeClient代理了下层transport层的发送接口
• 执行心跳报文发送
  heartbeat表示心跳间隔，
  heartbeat.timeout表示心跳超时时间，默认3倍心跳间隔。
  定时任务调用HeartBeatTask

private void startHeartbeatTimer() {
        stopHeartbeatTimer();
        if (heartbeat > 0) {
            heartbeatTimer = scheduled.scheduleWithFixedDelay(
                    new HeartBeatTask(new HeartBeatTask.ChannelProvider() {
                        @Override
                        public Collection<Channel> getChannels() {
                            return Collections.<Channel>singletonList(HeaderExchangeClient.this);
                        }
                    }, heartbeat, heartbeatTimeout),
                    heartbeat, heartbeat, TimeUnit.MILLISECONDS);
        }
    }

1.2 HeartBeatTask

• channel通信过程中，会分别设置读时间和写时间
• 上次读写时间距离现在超过心跳间隔，则构建心跳报文Request并发送
• 若事件超过心跳超时时间，client则connect重建连接，server端则close断开连接

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);
                    if ((lastRead != null && now - lastRead > heartbeat)
                            || (lastWrite != null && now - lastWrite > heartbeat)) {
                        Request req = new Request();
                        req.setVersion(Version.getProtocolVersion());
                        req.setTwoWay(true);
                        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");
                        }
                    }
                    if (lastRead != null && now - lastRead > heartbeatTimeout) {
                        logger.warn("Close channel " + channel
                                + ", because heartbeat read idle time out: " + heartbeatTimeout + "ms");
                        if (channel instanceof Client) {
                            try {
                                ((Client) channel).reconnect();
                            } catch (Exception e) {
                                //do nothing
                            }
                        } 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);
        }
    }

二 server

• DubboProtocol层调用exchanger接口获取server
  ExchangeServer server = Exchangers.bind(url, requestHandler)
• 和client层相同，支持心跳报文发送和处理。

三 收发报文

3.1发送报文

3.1.1 报文格式

组装Request格式的报文

Request.png

* data为上层传入的RpcInvocation类型数据或其他数据
* version为协议版本
* twoWay为是否需要响应报文
* id通过AtomicLong自增获取。
* hearbeat表示心跳报文，event表示心跳报文或其他事件报文
* broken表示报文解析失败，为垃圾报文

3.1.2 通信异步化

• 发送报文前，使用DefaultFuture存储请求信息。
  DefaultFuture future = new DefaultFuture(channel, req, timeout);
• 请求响应报文包含请求id，根据id获取请求的DefaultFuture，发送信号唤醒等待响应的线程，并调用回调函数。

private void doReceived(Response res) {
        lock.lock();
        try {
            response = res;
            if (done != null) {
                done.signal();
            }
        } finally {
            lock.unlock();
        }
        if (callback != null) {
            invokeCallback(callback);
        }
    }

3.2 收包handler

handler.png

3.2.1 HeaderExchangeHandler

HeaderExchangeHandler.png

3.2.2 DecodeHandler

DecodeableRpcInvocation和DecodeableRpcResult支持编解码的消息体，由DecodeHandler调用对应消息体的编解码函数。

3.2.3 HeartbeatHandler

心跳报文处理函数在transporter层封装，处理简单，即尽早处理心跳报文。

• 记录心跳时间
• 对心跳请求报文回发一个心跳响应。