OkHttp源码分析

异步请求流程

• 从这样一句话开始发出一个异步的请求

okHttpClient.newCall(request).enqueue(callback);

• 实际上调用的是 RealCall 的 enqueue 方法，将回调包装成一个 AsyncCall(NamedRunnabled的子类) 传递给了 Client 的 Dispatcher

//@see OkHttpClient
  @Override public Call newCall(Request request) {
    return new RealCall(this, request);
  }
  
  //@see RealCall
  @Override public void enqueue(Callback responseCallback) {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    client.dispatcher().enqueue(new AsyncCall(responseCallback));
  }

• 每当 Dispatcher 收到一个新的 AsyncCall
  1. 若是有空闲的线程就去将其放入执行队列，并执行它
  2. 没有的话，就放入等待队列

//@see Dispatcher
  synchronized void enqueue(AsyncCall call) {
    if (runningAsyncCalls.size() < maxRequests && runningCallsForHost(call) < maxRequestsPerHost) {
      runningAsyncCalls.add(call);
      executorService().execute(call);
    } else {
      readyAsyncCalls.add(call);
    }
  }

• AsyncCall 的执行过程经过 getResponseWithInterceptorChain() 来链式调用client.interceptors()，从上到下层层包装了各类缓存、重试等机制，并添加部分默认的Headers参数等，最后调用到CallServerInterceptor.intercept(Chain chain)方法和服务器创建 Socket 链接取得 Response，而后再把 Response 沿着来时的顺序由下往上层层返回，并进行相应的处理，最后返回到 AsyncCall 的 execute 方法中。

• 而后相应的调用成功或失败的回调（仍是在执行线程中，不会切换到主线程）

//@see RealCall.AsyncCall
    @Override protected void execute() {
      boolean signalledCallback = false;
      try {
        Response response = getResponseWithInterceptorChain();
        if (retryAndFollowUpInterceptor.isCanceled()) {
          signalledCallback = true;
          responseCallback.onFailure(RealCall.this, new IOException("Canceled"));
        } else {
          signalledCallback = true;
          responseCallback.onResponse(RealCall.this, response);
        }
      } catch (IOException e) {
        if (signalledCallback) {
          // Do not signal the callback twice!
          Platform.get().log(INFO, "Callback failure for " + toLoggableString(), e);
        } else {
          responseCallback.onFailure(RealCall.this, e);
        }
      } finally {
        client.dispatcher().finished(this);
      }
    }
  
  private Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!retryAndFollowUpInterceptor.isForWebSocket()) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(
        retryAndFollowUpInterceptor.isForWebSocket()));

    Interceptor.Chain chain = new RealInterceptorChain(
        interceptors, null, null, null, 0, originalRequest);
    return chain.proceed(originalRequest);
  }

• 最后在client.dispatcher().finished()方法中会
  1. 将该 AsyncCall 从执行队列中移除
  2. 调用Dispatcher.promoteCalls()方法来将等待队列中的 AsyncCall 放入执行队列中并执行

//@see Dispatcher
  private void promoteCalls() {
    if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
    if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.

    for (Iterator<AsyncCall> i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall call = i.next();

      if (runningCallsForHost(call) < maxRequestsPerHost) {
        i.remove();
        runningAsyncCalls.add(call);
        executorService().execute(call);
      }

      if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
    }
  }

同步请求流程

这个会把 realCall 放到 dispatcher 的 runningSyncCalls 中，用于对请求的统一管理

并直接在当前线程调用getResponseWithInterceptorChain()得到 Response

//@see RealCall
  @Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    try {
      client.dispatcher().executed(this);
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } finally {
      client.dispatcher().finished(this);
    }
  }