源码：
/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/ package com.android.volley; import android.os.Process; import java.util.concurrent.BlockingQueue; /**
* Provides a thread for performing cache triage on a queue of requests.
*
* Requests added to the specified cache queue are resolved from cache.
* Any deliverable response is posted back to the caller via a
* {@link ResponseDelivery}. Cache misses and responses that require
* refresh are enqueued on the specified network queue for processing
* by a {@link NetworkDispatcher}.
*/
public class CacheDispatcher extends Thread { private static final boolean DEBUG = VolleyLog.DEBUG; /** The queue of requests coming in for triage. */
private final BlockingQueue<Request<?>> mCacheQueue; /** The queue of requests going out to the network. */
private final BlockingQueue<Request<?>> mNetworkQueue; /** The cache to read from. */
private final Cache mCache; /** For posting responses. */
private final ResponseDelivery mDelivery; /** Used for telling us to die. */
private volatile boolean mQuit = false; /**
* Creates a new cache triage dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param cacheQueue Queue of incoming requests for triage
* @param networkQueue Queue to post requests that require network to
* @param cache Cache interface to use for resolution
* @param delivery Delivery interface to use for posting responses
*/
public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
} /**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
} @Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); // Make a blocking call to initialize the cache.
mCache.initialize(); Request<?> request;
while (true) {
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("cache-queue-take"); // If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
} // Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request);
continue;
} // If it is completely expired, just send it to the network.
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
} // We have a cache hit; parse its data for delivery back to the request.
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed"); if (!entry.refreshNeeded()) {
// Completely unexpired cache hit. Just deliver the response.
mDelivery.postResponse(request, response);
} else {
// Soft-expired cache hit. We can deliver the cached response,
// but we need to also send the request to the network for
// refreshing.
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.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
}
}
CacheDispatcher
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