(Android) OkHttp3.10 源码学习笔记 3 Dispatcher分析

本章我们介绍OkHttp的任务调度器Dispatcher，dispatcher的作用为维护请求的状态，并维护一个线程池。Dispatcher包含了三个队列和一个线程池，看注释大家应该能明白他们是做什么的

  /** Executes calls. Created lazily. */
  private @Nullable ExecutorService executorService;

  /** Ready async calls in the order they'll be run. */
  private final Deque<AsyncCall> readyAsyncCalls = new ArrayDeque<>();

  /** Running asynchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<AsyncCall> runningAsyncCalls = new ArrayDeque<>();

  /** Running synchronous calls. Includes canceled calls that haven't finished yet. */
  private final Deque<RealCall> runningSyncCalls = new ArrayDeque<>();

同步请求只有一个队列，在执行executed方法时把网络请求call放入runningSyncCalls队列

  synchronized void executed(RealCall call) {
    runningSyncCalls.add(call);
  }

异步请求执行的是dispatcher的enqueue方法，异步请求为什么需要两个队列？这里其实是一个生产者消费者模型

Dispatcher 生产者

ExecutorService 消费者池

因此，一个生产者消费者模型需要对应两个队列，一个是正在执行的请求队列，一个是等待执行的请求队列。

下面这张图描述了异步请求的过程，每当一个请求入队时，首先会判断正在执行的队列是否已满，没满则放入线程池执行，否则将call放入就绪等待队列

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

同时我们之前提到，AsyncCall在execute方法执行完后，会调用dispatcher的finished方法，这里会调用promoteCalls方法

 private <T> void finished(Deque<T> calls, T call, boolean promoteCalls) {
    int runningCallsCount;
    Runnable idleCallback;
    synchronized (this) {
      if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
      if (promoteCalls) promoteCalls();
      runningCallsCount = runningCallsCount();
      idleCallback = this.idleCallback;
    }

    if (runningCallsCount == 0 && idleCallback != null) {
      idleCallback.run();
    }
  }

对等待队列和正在执行的队列进行管理，这边会根据具体条件把ready队列的放入running队列等待执行，注释其实写的非常清楚了

 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.
    }
  }

总结

不论是同步还是异步请求，封装好的Call都会在dispatcher里被调度执行，这里我们要理解dispatcher里的三个队列和执行线程池，以及dispatcher是如何对队列进行操作的。

(Android) OkHttp3.10 源码学习笔记 3 Dispatcher分析

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