Dubbo分析之Exchange 层

前言

紧接着上文Dubbo分析之Transport层，本文继续介绍Exchange层，此层官方介绍为信息交换层：封装请求响应模式，同步转异步，以 Request, Response 为中心，扩展接口为 Exchanger, ExchangeChannel, ExchangeClient, ExchangeServer；下面分别进行介绍

Exchanger分析

Exchanger是此层的核心接口类，提供了connect()和bind()接口，分别返回ExchangeClient和ExchangeServer；dubbo提供了此接口的默认实现类HeaderExchanger，代码如下：

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))));
    }

}

在实现类中在connect和bind中分别实例化了HeaderExchangeClient和HeaderExchangeServer，传入的参数是Transporters，可以认为这里就是Transport层的入口类；这里的ExchangeClient/ExchangeServer其实就是对Client/Server的包装，同时传入了自己的ChannelHandler；ChannelHandler已经在Transport层介绍过了，提供了连接建立，连接端口，发送请求，接受请求等接口；已默认使用的Netty为例，这里就是对NettyClient和NettyServer的包装，同时传入DecodeHandler，在NettyHandler中被调用；

ExchangeClient分析

ExchangeClient本身也继承于Client，同时也继承于ExchangeChannel：

public interface ExchangeClient extends Client, ExchangeChannel {

}

public interface ExchangeChannel extends Channel {

    ResponseFuture request(Object request) throws RemotingException;

    ResponseFuture request(Object request, int timeout) throws RemotingException;

    ExchangeHandler getExchangeHandler();

    @Override
    void close(int timeout);

}

ExchangeChannel负责将上层的data包装成Request，然后发送给Transport层；具体的逻辑在HeaderExchangeChannel中：

public ResponseFuture request(Object request, int timeout) throws RemotingException {
        if (closed) {
            throw new RemotingException(this.getLocalAddress(), null, "Failed to send request " + request + ", cause: The channel " + this + " is closed!");
        }
        // create request.
        Request req = new Request();
        req.setVersion(Version.getProtocolVersion());
        req.setTwoWay(true);
        req.setData(request);
        DefaultFuture future = new DefaultFuture(channel, req, timeout);
        try {
            channel.send(req);
        } catch (RemotingException e) {
            future.cancel();
            throw e;
        }
        return future;
    }

创建了一个Request，在构造器中同时会产生一个RequestId；设置了协议版本，是否双向通信，最后设置了真实的业务数据；接下来实例化了一个DefaultFuture类，此类实现了同步转异步的方式，channel调用send发送请求之后，不需要等待结果，直接将DefaultFuture返回给上层，上层可以通过调用DefaultFuture的get方法来获取响应，get方法会阻塞等待获取服务器的响应才会返回；Client接收消息在handler里面，比如Netty在NettyHandler里面messageReceived方法介绍响应消息，NettyHandler最终会调用上面传入的DecodeHandler，DecodeHandler会先判断一下是否已经解码，如果解码就直接调用HeaderExchangeHandler，默认已经设置了编码解码器，所以会直接调用HeaderExchangeHandler里面的received方法：

public void received(Channel channel, Object message) throws RemotingException {
        channel.setAttribute(KEY_READ_TIMESTAMP, System.currentTimeMillis());
        ExchangeChannel exchangeChannel = HeaderExchangeChannel.getOrAddChannel(channel);
        try {
            if (message instanceof Request) {
                // handle request.
                Request request = (Request) message;
                if (request.isEvent()) {
                    handlerEvent(channel, request);
                } else {
                    if (request.isTwoWay()) {
                        Response response = handleRequest(exchangeChannel, request);
                        channel.send(response);
                    } else {
                        handler.received(exchangeChannel, request.getData());
                    }
                }
            } else if (message instanceof Response) {
                handleResponse(channel, (Response) message);
            } else if (message instanceof String) {
                if (isClientSide(channel)) {
                    Exception e = new Exception("Dubbo client can not supported string message: " + message + " in channel: " + channel + ", url: " + channel.getUrl());
                    logger.error(e.getMessage(), e);
                } else {
                    String echo = handler.telnet(channel, (String) message);
                    if (echo != null && echo.length() > 0) {
                        channel.send(echo);
                    }
                }
            } else {
                handler.received(exchangeChannel, message);
            }
        } finally {
            HeaderExchangeChannel.removeChannelIfDisconnected(channel);
        }
    }

服务端和客户端都会使用此方法，这里是客户端接受的是Response，直接调用handleResponse方法：

static void handleResponse(Channel channel, Response response) throws RemotingException {
        if (response != null && !response.isHeartbeat()) {
            DefaultFuture.received(channel, response);
        }
    }

接收到响应之后，再去告诉DefaultFuture已经收到响应，DefaultFuture本身存放了requestId对应DefaultFuture的一个ConcurrentHashMap；具体怎么映射过去，Response也包含一个responseId，此responseId和requestId是相同的；

private final Lock lock = new ReentrantLock();
  private final Condition done = lock.newCondition();
	
  public static void received(Channel channel, Response response) {
        try {
            DefaultFuture future = FUTURES.remove(response.getId());
            if (future != null) {
                future.doReceived(response);
            } else {
                logger.warn("The timeout response finally returned at "
                        + (new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS").format(new Date()))
                        + ", response " + response
                        + (channel == null ? "" : ", channel: " + channel.getLocalAddress()
                        + " -> " + channel.getRemoteAddress()));
            }
        } finally {
            CHANNELS.remove(response.getId());
        }
    }
	
    private void doReceived(Response res) {
        lock.lock();
        try {
            response = res;
            if (done != null) {
                done.signal();
            }
        } finally {
            lock.unlock();
        }
        if (callback != null) {
            invokeCallback(callback);
        }
    }

通过responseId获取了之前请求时创建的DefaultFuture，然后再更新DefaultFuture内部的response对象，更新完之后在调用Condition的signal方法，用户唤起通过DefaultFuture的get方法获取响应的阻塞线程：

public Object get(int timeout) throws RemotingException {
        if (timeout <= 0) {
            timeout = Constants.DEFAULT_TIMEOUT;
        }
        if (!isDone()) {
            long start = System.currentTimeMillis();
            lock.lock();
            try {
                while (!isDone()) {
                    done.await(timeout, TimeUnit.MILLISECONDS);
                    if (isDone() || System.currentTimeMillis() - start > timeout) {
                        break;
                    }
                }
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            } finally {
                lock.unlock();
            }
            if (!isDone()) {
                throw new TimeoutException(sent > 0, channel, getTimeoutMessage(false));
            }
        }
        return returnFromResponse();
    }

可以发现阻塞要么被获取被signal方法唤醒，要么等待超时；以上大致是客户端发送获取响应的流程，下面看看服务器端流程

ExchangeServer分析

ExchangeServer继承于Server，同时提供了两个包装服务端Channel的方法

public interface ExchangeServer extends Server {

    Collection<ExchangeChannel> getExchangeChannels();

    ExchangeChannel getExchangeChannel(InetSocketAddress remoteAddress);
}

服务器端主要用于接收Request消息，然后处理消息，最后把响应发送给客户端，相关接收消息已经在上面介绍过了，同样是在HeaderExchangeHandler里面的received方法中，只不过这里的消息类型为Request；

Response handleRequest(ExchangeChannel channel, Request req) throws RemotingException {
        Response res = new Response(req.getId(), req.getVersion());
        if (req.isBroken()) {
            Object data = req.getData();

            String msg;
            if (data == null) msg = null;
            else if (data instanceof Throwable) msg = StringUtils.toString((Throwable) data);
            else msg = data.toString();
            res.setErrorMessage("Fail to decode request due to: " + msg);
            res.setStatus(Response.BAD_REQUEST);

            return res;
        }
        // find handler by message class.
        Object msg = req.getData();
        try {
            // handle data.
            Object result = handler.reply(channel, msg);
            res.setStatus(Response.OK);
            res.setResult(result);
        } catch (Throwable e) {
            res.setStatus(Response.SERVICE_ERROR);
            res.setErrorMessage(StringUtils.toString(e));
        }
        return res;
    }

首先创建了一个Response，并且指定responseId为requestId，方便在客户端定位到具体的DefaultFuture；然后调用handler的reply方法处理消息，返回结果，如何处理的将在后面的protocol层介绍，大致就是通过Request的信息，反射调用Server端的服务，然后返回结果，然后将结果放入Response对象中，通过channel将消息发送客户端；

总结

本文介绍了Exchange层的大体流程，围绕Exchanger，ExchangeClient和ExchangeServer展开；请求封装成Request，响应封装成Response，客户端通过异步的方式接收服务器请求；