dubbo_远程同步调用原理

com.taobao.remoting.impl.DefaultClient.java
//同步调用远程接口
public Object invokeWithSync(Object appRequest, RequestControl control) throws RemotingException, InterruptedException {
        byte protocol = getProtocol(control);
        if (!TRConstants.isValidProtocol(protocol)) {
            throw new RemotingException("Invalid serialization protocol [" + protocol + "] on invokeWithSync.");
        }
        ResponseFuture future = invokeWithFuture(appRequest, control);
        return future.get();  //获取结果时让当前线程等待，ResponseFuture其实就是前面说的callback
}
public ResponseFuture invokeWithFuture(Object appRequest, RequestControl control) {
         byte protocol = getProtocol(control);
         long timeout = getTimeout(control);
         ConnectionRequest request = new ConnectionRequest(appRequest);
         request.setSerializeProtocol(protocol);
         Callback2FutureAdapter adapter = new Callback2FutureAdapter(request);
         connection.sendRequestWithCallback(request, adapter, timeout);
         return adapter;
}

Callback2FutureAdapter implements ResponseFuture
public Object get() throws RemotingException, InterruptedException {
 synchronized (this) {  // 旋锁
     while (!isDone) {  // 是否有结果了
  wait(); //没结果是释放锁，让当前线程处于等待状态
     }
 }
 if (errorCode == TRConstants.RESULT_TIMEOUT) {
     throw new TimeoutException("Wait response timeout, request["
      + connectionRequest.getAppRequest() + "].");
 }
 else if (errorCode > 0) {
     throw new RemotingException(errorMsg);
 }
 else {
     return appResp;
 }
}
客户端收到服务端结果后，回调时相关方法，即设置isDone = true并notifyAll()
public void handleResponse(Object _appResponse) {
         appResp = _appResponse; //将远程调用结果设置到callback中来
         setDone();
}
public void onRemotingException(int _errorType, String _errorMsg) {
         errorCode = _errorType;
         errorMsg = _errorMsg;
         setDone();
}
private void setDone() {
         isDone = true;
         synchronized (this) { //获取锁，因为前面wait()已经释放了callback的锁了
             notifyAll(); // 唤醒处于等待的线程
         }
}

CallbackExecutorTask
static private class CallbackExecutorTask implements Runnable {
         final ConnectionResponse resp;
         final ResponseCallback callback;
         final Thread createThread;
 
         CallbackExecutorTask(ConnectionResponse _resp, ResponseCallback _cb) {
             resp = _resp;
             callback = _cb;
             createThread = Thread.currentThread();
         }
 
         public void run() {
             // 预防这种情况：业务提供的Executor，让调用者线程来执行任务
             if (createThread == Thread.currentThread()
                       && callback.getExecutor() != DIYExecutor.getInstance()) {
                   StringBuilder sb = new StringBuilder();
                   sb.append("The network callback task [" + resp.getRequestId() + "] cancelled, cause:");
                   sb.append("Can not callback task on the network io thhread.");
                   LOGGER.warn(sb.toString());
                   return;
             }
 
             if (TRConstants.RESULT_SUCCESS == resp.getResult()) {
                   callback.handleResponse(resp.getAppResponse()); //设置调用结果
             }
             else {
                   callback.onRemotingException(resp.getResult(), resp
                            .getErrorMsg());  //处理调用异常
             }
         }
}