源码：
1 /* 2 * Copyright (C) 2011 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.android.volley; 18 19 import android.os.Process; 20 21 import java.util.concurrent.BlockingQueue; 22 23 /** 24 * Provides a thread for performing cache triage on a queue of requests. 25 * 26 * Requests added to the specified cache queue are resolved from cache. 27 * Any deliverable response is posted back to the caller via a 28 * {@link ResponseDelivery}. Cache misses and responses that require 29 * refresh are enqueued on the specified network queue for processing 30 * by a {@link NetworkDispatcher}. 31 */ 32 public class CacheDispatcher extends Thread { 33 34 private static final boolean DEBUG = VolleyLog.DEBUG; 35 36 /** The queue of requests coming in for triage. */ 37 private final BlockingQueue > mCacheQueue; 38 39 /** The queue of requests going out to the network. */ 40 private final BlockingQueue > mNetworkQueue; 41 42 /** The cache to read from. */ 43 private final Cache mCache; 44 45 /** For posting responses. */ 46 private final ResponseDelivery mDelivery; 47 48 /** Used for telling us to die. */ 49 private volatile boolean mQuit = false; 50 51 /** 52 * Creates a new cache triage dispatcher thread. You must call {@link #start()} 53 * in order to begin processing. 54 * 55 * @param cacheQueue Queue of incoming requests for triage 56 * @param networkQueue Queue to post requests that require network to 57 * @param cache Cache interface to use for resolution 58 * @param delivery Delivery interface to use for posting responses 59 */ 60 public CacheDispatcher( 61 BlockingQueue > cacheQueue, BlockingQueue > networkQueue, 62 Cache cache, ResponseDelivery delivery) { 63 mCacheQueue = cacheQueue; 64 mNetworkQueue = networkQueue; 65 mCache = cache; 66 mDelivery = delivery; 67 } 68 69 /** 70 * Forces this dispatcher to quit immediately. If any requests are still in 71 * the queue, they are not guaranteed to be processed. 72 */ 73 public void quit() { 74 mQuit = true; 75 interrupt(); 76 } 77 78 @Override 79 public void run() { 80 if (DEBUG) VolleyLog.v("start new dispatcher"); 81 Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); 82 83 // Make a blocking call to initialize the cache. 84 mCache.initialize(); 85 86 Request request; 87 while (true) { 88 // release previous request object to avoid leaking request object when mQueue is drained. 89 request = null; 90 try { 91 // Take a request from the queue. 92 request = mCacheQueue.take(); 93 } catch (InterruptedException e) { 94 // We may have been interrupted because it was time to quit. 95 if (mQuit) { 96 return; 97 } 98 continue; 99 }100 try {101 request.addMarker("cache-queue-take");102 103 // If the request has been canceled, don't bother dispatching it.104 if (request.isCanceled()) {105 request.finish("cache-discard-canceled");106 continue;107 }108 109 // Attempt to retrieve this item from cache.110 Cache.Entry entry = mCache.get(request.getCacheKey());111 if (entry == null) {112 request.addMarker("cache-miss");113 // Cache miss; send off to the network dispatcher.114 mNetworkQueue.put(request);115 continue;116 }117 118 // If it is completely expired, just send it to the network.119 if (entry.isExpired()) {120 request.addMarker("cache-hit-expired");121 request.setCacheEntry(entry);122 mNetworkQueue.put(request);123 continue;124 }125 126 // We have a cache hit; parse its data for delivery back to the request.127 request.addMarker("cache-hit");128 Response response = request.parseNetworkResponse(129 new NetworkResponse(entry.data, entry.responseHeaders));130 request.addMarker("cache-hit-parsed");131 132 if (!entry.refreshNeeded()) {133 // Completely unexpired cache hit. Just deliver the response.134 mDelivery.postResponse(request, response);135 } else {136 // Soft-expired cache hit. We can deliver the cached response,137 // but we need to also send the request to the network for138 // refreshing.139 request.addMarker("cache-hit-refresh-needed");140 request.setCacheEntry(entry);141 142 // Mark the response as intermediate.143 response.intermediate = true;144 145 // Post the intermediate response back to the user and have146 // the delivery then forward the request along to the network.147 final Request finalRequest = request;148 mDelivery.postResponse(request, response, new Runnable() {149 @Override150 public void run() {151 try {152 mNetworkQueue.put(finalRequest);153 } catch (InterruptedException e) {154 // Not much we can do about this.155 }156 }157 });158 }159 } catch (Exception e) {160 VolleyLog.e(e, "Unhandled exception %s", e.toString());161 }162 }163 }164 } 111