Dubbo分析之Exchange層

系列文章

Dubbo分析Serialize層
Dubbo分析之Transport層
Dubbo分析之Exchange 層

前言

緊接着上文Dubbo分析之Transport層，本文繼續介紹Exchange層，此層官方介紹爲信息交換層：封裝請求響應模式，同步轉異步，以 Request, Response 爲中心，擴展接口爲 Exchanger, ExchangeChannel, ExchangeClient, ExchangeServer；下面分別進行介紹

Exchanger分析

Exchanger是此層的核心接口類，提供了connect()和bind()接口，分別返回ExchangeClient和ExchangeServer；dubbo提供了此接口的默認實現類HeaderExchanger，代碼以下：

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))));
    }
 
}

在實現類中在connect和bind中分別實例化了HeaderExchangeClient和HeaderExchangeServer，傳入的參數是Transporters，能夠認爲這裏就是Transport層的入口類；這裏的ExchangeClient/ExchangeServer其實就是對Client/Server的包裝，同時傳入了本身的ChannelHandler；ChannelHandler已經在Transport層介紹過了，提供了鏈接創建，鏈接端口，發送請求，接受請求等接口；已默認使用的Netty爲例，這裏就是對NettyClient和NettyServer的包裝，同時傳入DecodeHandler，在NettyHandler中被調用；

ExchangeClient分析

ExchangeClient自己也繼承於Client，同時也繼承於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負責將上層的data包裝成Request，而後發送給Transport層；具體的邏輯在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;
   }

建立了一個Request，在構造器中同時會產生一個RequestId；設置了協議版本，是否雙向通訊，最後設置了真實的業務數據；接下來實例化了一個DefaultFuture類，此類實現了同步轉異步的方式，channel調用send發送請求以後，不須要等待結果，直接將DefaultFuture返回給上層，上層能夠經過調用DefaultFuture的get方法來獲取響應，get方法會阻塞等待獲取服務器的響應纔會返回；Client接收消息在handler裏面，好比Netty在NettyHandler裏面messageReceived方法介紹響應消息，NettyHandler最終會調用上面傳入的DecodeHandler，DecodeHandler會先判斷一下是否已經解碼，若是解碼就直接調用HeaderExchangeHandler，默認已經設置了編碼解碼器，因此會直接調用HeaderExchangeHandler裏面的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);
       }
   }

服務端和客戶端都會使用此方法，這裏是客戶端接受的是Response，直接調用handleResponse方法：

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

接收到響應以後，再去告訴DefaultFuture已經收到響應，DefaultFuture自己存放了requestId對應DefaultFuture的一個ConcurrentHashMap；具體怎麼映射過去，Response也包含一個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);
      }
  }

經過responseId獲取了以前請求時建立的DefaultFuture，而後再更新DefaultFuture內部的response對象，更新完以後在調用Condition的signal方法，用戶喚起經過DefaultFuture的get方法獲取響應的阻塞線程：

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();
    }

能夠發現阻塞要麼被獲取被signal方法喚醒，要麼等待超時；以上大體是客戶端發送獲取響應的流程，下面看看服務器端流程

ExchangeServer分析

ExchangeServer繼承於Server，同時提供了兩個包裝服務端Channel的方法

public interface ExchangeServer extends Server {
 
    Collection<ExchangeChannel> getExchangeChannels();
 
    ExchangeChannel getExchangeChannel(InetSocketAddress remoteAddress);
}

服務器端主要用於接收Request消息，而後處理消息，最後把響應發送給客戶端，相關接收消息已經在上面介紹過了，一樣是在HeaderExchangeHandler裏面的received方法中，只不過這裏的消息類型爲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;
  }

首先建立了一個Response，而且指定responseId爲requestId，方便在客戶端定位到具體的DefaultFuture；而後調用handler的reply方法處理消息，返回結果，如何處理的將在後面的protocol層介紹，大體就是經過Request的信息，反射調用Server端的服務，而後返回結果，而後將結果放入Response對象中，經過channel將消息發送客戶端；

總結

本文介紹了Exchange層的大致流程，圍繞Exchanger，ExchangeClient和ExchangeServer展開；請求封裝成Request，響應封裝成Response，客戶端經過異步的方式接收服務器請求；

示例代碼地址

https://github.com/ksfzhaohui...
https://gitee.com/OutOfMemory...