可以看到get方法，首先通过throwIfNotOnMainThread()方法限制必须在UI线程调用；
然后根据传入的参数计算cacheKey，获取cache;
=>如果cache存在，直接将返回结果封装为一个ImageContainer(cachedBitmap, requestUrl)，然后直接回调imageListener.onResponse(container, true);我们就可以设置图片了。
=>如果cache不存在，初始化一个ImageContainer（没有bitmap），然后直接回调，imageListener.onResponse(imageContainer, true);，这里为了让大家在回调中判断，然后设置默认图片（所以，大家在自己实现listener的时候，别忘了判断resp.getBitmap()!=null）；
接下来检查该url是否早已加入了请求对了，如果早已加入呢，则将刚初始化的ImageContainer加入BatchedImageRequest，返回结束。
如果是一个新的请求，则通过makeImageRequest创建一个新的请求，然后将这个请求分别加入mRequestQueue和mInFlightRequests，注意mInFlightRequests中会初始化一个BatchedImageRequest，存储相同的请求队列。
这里注意mRequestQueue是个对象，并不是队列数据结构，所以我们要看下add方法
#RequestQueue
public <T> Request<T> add(Request<T> request) {
// Tag the request as belonging to this queue and add it to the set of current requests.
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}
// Process requests in the order they are added.
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
// Insert request into stage if there's already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request<?>>();
}
stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
}
} else {
// Insert 'null' queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}
