深入剖析Android的Volley框架中的图片加载功能

这里主要就是初始化HttpStack，对于HttpStack在API大于等于9的时候选择HttpUrlConnetcion，反之则选择HttpClient，这里我们并不关注Http相关代码。

接下来初始化了RequestQueue，然后调用了start（）方法。

接下来看RequestQueue的构造：

public RequestQueue(Cache cache, Network network) {
    this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
  }
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
    this(cache, network, threadPoolSize,
        new ExecutorDelivery(new Handler(Looper.getMainLooper())));
  }
public RequestQueue(Cache cache, Network network, int threadPoolSize,
      ResponseDelivery delivery) {
    mCache = cache;
    mNetwork = network;
    mDispatchers = new NetworkDispatcher[threadPoolSize];
    mDelivery = delivery;
  }

初始化主要就是4个参数：mCache、mNetwork、mDispatchers、mDelivery。第一个是硬盘缓存；第二个主要用于Http相关操作；第三个用于转发请求的；第四个参数用于把结果转发到UI线程（ps:你可以看到new Handler(Looper.getMainLooper())）。

接下来看start方法

#RequestQueue
 /**
   * Starts the dispatchers in this queue.
   */
  public void start() {
    stop(); // Make sure any currently running dispatchers are stopped.
    // Create the cache dispatcher and start it.
    mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
    mCacheDispatcher.start();

    // Create network dispatchers (and corresponding threads) up to the pool size.
    for (int i = 0; i < mDispatchers.length; i++) {
      NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
          mCache, mDelivery);
      mDispatchers[i] = networkDispatcher;
      networkDispatcher.start();
    }
  }

首先是stop，确保转发器退出，其实就是内部的几个线程退出，这里大家如果有兴趣可以看眼源码，参考下Volley中是怎么处理线程退出的（几个线程都是while(true）{//doSomething})。