这是 volley 的第四篇 blog 了，写完这篇，volley 的大部分用法也都算写了一遍，所以暂时不会写 volley 的文章了，如果想看我前面写的文章，可以点这里 Android volley 解析（三）之文件上传篇
为什么要用缓存
我们知道，当客户端在请求网络数据的时候，是需要消耗流量的，特别是对于移动端用户来说，对于流量的控制要求很高。所以在做网络请求的时候，如果对数据更新要求不是特别高，往往都会用到缓存机制，一方面能减少对服务端的请求，控制流量；另一方面，当客户端在没有网络的情况下也能看到上一次请求的数据，这样使用户体验更佳，如下图，微信朋友圈在没有网络的情况下，依然能看到朋友们的动态
volley 缓存的原理
看了我前面blog 的朋友一定还记得，我在讲 get 请求的时候，讲到了volley 的基本流程，首先启动的是缓存线程mCacheDispatcher来获取缓存； 那我们就从如何获取缓存开始分析； 1、初始化缓存
@Overridepublic void run() {if (DEBUG) VolleyLog.v(“start new dispatcher”);Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);// Make a blocking call to initialize the cache.这里对缓存做初始化操作mCache.initialize();while (true) {…
2、获取缓存
while (true) {try {Request request = mCacheQueue.take();request.addMarker(“cache-queue-take”);// If the request has been canceled, don’t bother dispatching it….
这里有一个问题，缓存队列在什么时候添加缓存请求呢？我们回到最开始请求队列添加请求的地方
public Request add(Request 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.如果请求没有设置缓存，则把请求添加到网络队列中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);//这里添加请求到缓存队列中mCacheQueue.add(request);}return request;}}
代码不长，也很好理解，如果我们的请求没有设置了缓存，则请求添加到网络请求队列中，并直接返回了，不往下执行了，这时缓存队列中无法获取请求，所以这里我们知道了，想要用缓存则需要 在 Request 中把
(boolean shouldCache) {mShouldCache = shouldCache;}
设为 true，，当然，我们看mShouldCache 的默认值
/*** Whether or not responses to this request should be cached.*/private boolean mShouldCache = true;
volley默认启用缓存的。再往下看
// If the request has been canceled, don’t bother dispatching it.如果已经取消请求，则结束本次请求的所有操作if (request.isCanceled()) {request.finish(“cache-discard-canceled”);continue;}// Attempt to retrieve this item from cache. 通过缓存类获取缓存Cache.Entry entry = mCache.get(request.getCacheKey());//如果获取的缓存为空，这里有两种情况为空，一，第一次换取，没有情求过网络；二，缓存的数据达到了最大限度，此缓存已经被删除。if (entry == null) {request.addMarker(“cache-miss”);Log.i(“CacheDispatcher”, “没有缓存数据：” + request.getUrl());mNetworkQueue.put(request);continue;}// If it is completely expired, just send it to the network.缓存已经过期，则重新请求网络if (entry.isExpired()) {request.addMarker(“cache-hit-expired”);request.setCacheEntry(entry);Log.i(“CacheDispatcher”, “缓存数据过期：” + request.getUrl());mNetworkQueue.put(request);continue;}// We have a cache hit; parse its data for delivery back to the request.request.addMarker(“cache-hit”);//到这里表示已经成功获取缓存数据Response<?> response = request.parseNetworkResponse(new NetworkResponse(entry.data, entry.responseHeaders));request.addMarker(“cache-hit-parsed”);//如果缓存需要刷新，则请求网络if (!entry.refreshNeeded()) {// Completely unexpired cache hit. Just deliver the response.Log.i(“CacheDispatcher”, “获取缓存数据：” + request.getUrl());mDelivery.postResponse(request, response);} else {request.addMarker(“cache-hit-refresh-needed”);request.setCacheEntry(entry);// Mark the response as intermediate.response.intermediate = true;// Post the intermediate response back to the user and have// the delivery then forward the request along to the network.mDelivery.postResponse(request, response, new Runnable() {@Override() {try {mNetworkQueue.put(request);} catch (InterruptedException e) {// Not much we can do about this.}}});}} catch (InterruptedException e) {// We may have been interrupted because it was time to quit.if (mQuit) {return;}continue;}}人生如果错了方向，停止就是进步”。
