volley4--RequestQueue

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.Handler;
 import android.os.Looper;

 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 import java.util.Queue;
 import java.util.Set;
 import java.util.concurrent.PriorityBlockingQueue;
 import java.util.concurrent.atomic.AtomicInteger;

 /**
  * A request dispatch queue with a thread pool of dispatchers.
  *
  * Calling {@link #add(Request)} will enqueue the given Request for dispatch,
  * resolving from either cache or network on a worker thread, and then delivering
  * a parsed response on the main thread.
  */
 public class RequestQueue {

     /** Callback interface for completed requests. */
     public static interface RequestFinishedListener<T> {
         /** Called when a request has finished processing. */
         public void onRequestFinished(Request<T> request);
     }

     /** Used for generating monotonically-increasing sequence numbers for requests. */
     private AtomicInteger mSequenceGenerator = new AtomicInteger();

     /**
      * Staging area for requests that already have a duplicate request in flight.
      *
      * <ul>
      *     <li>containsKey(cacheKey) indicates that there is a request in flight for the given cache
      *          key.</li>
      *     <li>get(cacheKey) returns waiting requests for the given cache key. The in flight request
      *          is <em>not</em> contained in that list. Is null if no requests are staged.</li>
      * </ul>
      */
     private final Map<String, Queue<Request<?>>> mWaitingRequests =
             new HashMap<String, Queue<Request<?>>>();

     /**
      * The set of all requests currently being processed by this RequestQueue. A Request
      * will be in this set if it is waiting in any queue or currently being processed by
      * any dispatcher.
      */
     private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

     /** The cache triage queue. */
     private final PriorityBlockingQueue<Request<?>> mCacheQueue =
         new PriorityBlockingQueue<Request<?>>();

     /** The queue of requests that are actually going out to the network. */
     private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
         new PriorityBlockingQueue<Request<?>>();

     /** Number of network request dispatcher threads to start. */
     private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = ;

     /** Cache interface for retrieving and storing responses. */
     private final Cache mCache;

     /** Network interface for performing requests. */
     private final Network mNetwork;

     /** Response delivery mechanism. */
     private final ResponseDelivery mDelivery;

     /** The network dispatchers. */
     private NetworkDispatcher[] mDispatchers;

     /** The cache dispatcher. */
     private CacheDispatcher mCacheDispatcher;

     private List<RequestFinishedListener> mFinishedListeners =
             new ArrayList<RequestFinishedListener>();

     /**
      * Creates the worker pool. Processing will not begin until {@link #start()} is called.
      *
      * @param cache A Cache to use for persisting responses to disk
      * @param network A Network interface for performing HTTP requests
      * @param threadPoolSize Number of network dispatcher threads to create
      * @param delivery A ResponseDelivery interface for posting responses and errors
      */
     public RequestQueue(Cache cache, Network network, int threadPoolSize,
             ResponseDelivery delivery) {
         mCache = cache;
         mNetwork = network;
         mDispatchers = new NetworkDispatcher[threadPoolSize];
         mDelivery = delivery;
     }

     /**
      * Creates the worker pool. Processing will not begin until {@link #start()} is called.
      *
      * @param cache A Cache to use for persisting responses to disk
      * @param network A Network interface for performing HTTP requests
      * @param threadPoolSize Number of network dispatcher threads to create
      */
     public RequestQueue(Cache cache, Network network, int threadPoolSize) {
         this(cache, network, threadPoolSize,
                 new ExecutorDelivery(new Handler(Looper.getMainLooper())));
     }

     /**
      * Creates the worker pool. Processing will not begin until {@link #start()} is called.
      *
      * @param cache A Cache to use for persisting responses to disk
      * @param network A Network interface for performing HTTP requests
      */
     public RequestQueue(Cache cache, Network network) {
         this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
     }

     /**
      * 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 = ; i < mDispatchers.length; i++) {
             NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
                     mCache, mDelivery);
             mDispatchers[i] = networkDispatcher;
             networkDispatcher.start();
         }
     }

     /**
      * Stops the cache and network dispatchers.
      */
     public void stop() {
         if (mCacheDispatcher != null) {
             mCacheDispatcher.quit();
         }
         for (int i = ; i < mDispatchers.length; i++) {
             if (mDispatchers[i] != null) {
                 mDispatchers[i].quit();
             }
         }
     }

     /**
      * Gets a sequence number.
      */
     public int getSequenceNumber() {
         return mSequenceGenerator.incrementAndGet();
     }

     /**
      * Gets the {@link Cache} instance being used.
      */
     public Cache getCache() {
         return mCache;
     }

     /**
      * A simple predicate or filter interface for Requests, for use by
      * {@link RequestQueue#cancelAll(RequestFilter)}.
      */
     public interface RequestFilter {
         public boolean apply(Request<?> request);
     }

     /**
      * Cancels all requests in this queue for which the given filter applies.
      * @param filter The filtering function to use
      */
     public void cancelAll(RequestFilter filter) {
         synchronized (mCurrentRequests) {
             for (Request<?> request : mCurrentRequests) {
                 if (filter.apply(request)) {
                     request.cancel();
                 }
             }
         }
     }

     /**
      * Cancels all requests in this queue with the given tag. Tag must be non-null
      * and equality is by identity.
      */
     public void cancelAll(final Object tag) {
         if (tag == null) {
             throw new IllegalArgumentException("Cannot cancelAll with a null tag");
         }
         cancelAll(new RequestFilter() {
             @Override
             public boolean apply(Request<?> request) {
                 return request.getTag() == tag;
             }
         });
     }

     /**
      * Adds a Request to the dispatch queue.
      * @param request The request to service
      * @return The passed-in request
      */
     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;
         }
     }

     /**
      * Called from {@link Request#finish(String)}, indicating that processing of the given request
      * has finished.
      *
      * <p>Releases waiting requests for <code>request.getCacheKey()</code> if
      *      <code>request.shouldCache()</code>.</p>
      */
     <T> void finish(Request<T> request) {
         // Remove from the set of requests currently being processed.
         synchronized (mCurrentRequests) {
             mCurrentRequests.remove(request);
         }
         synchronized (mFinishedListeners) {
           for (RequestFinishedListener<T> listener : mFinishedListeners) {
             listener.onRequestFinished(request);
           }
         }

         if (request.shouldCache()) {
             synchronized (mWaitingRequests) {
                 String cacheKey = request.getCacheKey();
                 Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
                 if (waitingRequests != null) {
                     if (VolleyLog.DEBUG) {
                         VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
                                 waitingRequests.size(), cacheKey);
                     }
                     // Process all queued up requests. They won't be considered as in flight, but
                     // that's not a problem as the cache has been primed by 'request'.
                     mCacheQueue.addAll(waitingRequests);
                 }
             }
         }
     }

     public  <T> void addRequestFinishedListener(RequestFinishedListener<T> listener) {
       synchronized (mFinishedListeners) {
         mFinishedListeners.add(listener);
       }
     }

     /**
      * Remove a RequestFinishedListener. Has no effect if listener was not previously added.
      */
     public  <T> void removeRequestFinishedListener(RequestFinishedListener<T> listener) {
       synchronized (mFinishedListeners) {
         mFinishedListeners.remove(listener);
       }
     }
 }

RequestQueue

1.

其实RequestQueue里面有两个队列，一个我称为缓存队列mCacheQueue，一个称为网络队列mNetworkQueue

如果请求要求加入缓存队列(例如我们给request设置一个属性ShouldCache，然后提供set方法来设置)，将会试图从硬盘缓存中获取数据，如果没有缓存，这个请求将被放入网络队列

如果请求不要求缓存，则直接加入网络队列。

加入队列以后，我们开启线程，从队列中取出请求。

可想而知，我们最好有一个线程CacheDispatcher从缓存队列中取，一个NetworkDispatcher从网络队列中取，然而网络请求往往大量，所以volley实际上有多个线程同时从网络队列中取出请求(这里涉及线程同步，volley使用PriorityBlockingQueue解决)

为什么要先建立几个线程，从队列中取，而不是每个request开启一个线程呢？这样做的好处是避免重复大量创建线程所带来的开销，另外由于所有的request都存在在一个RequestQueue里面，便于我们对request的管理，例如我们要关闭某个request,又或者我们请求了很多相同的request，对应这些操作，我们如果将request分散，是很难统一解决的，所以这样用类似线程池的思想，统一管理线程。

同时，这样做又会带来不利，因为实际请求线程的线程数目是固定的，意味着当request数目大于线程数目时，有的线程将被阻塞，造成效率下降，更多的问题，会在接下来的文章提到。

至于CacheDispatcher和NetworkDispatcher是怎么请求数据的呢？

对于NetworkDispatcher而言，必然是开启网络连接，然后获取数据的(例如url.openConnection)，这是我们的常用实现，先不做详细解释(volley对这些实现进行了更详细的封装)

再来考虑，获得结果以后，我们怎么回调。

还是面向对象的思路，volley将响应结果封装成一个repsonse类(和request对应)

对应NetworkDispatcher而言，在它的run()方法里面，取得request以后，根据url请求数据，将数据封装成respsonse对象，再有一个分发器ResponseDelivery分发到对应的request

有人会问?解析到response以后，我们给request设计一个方法(例如将parseRespose(Respsonse respsonse))用于使用response，同时在这个方法内，回调监听器不就好了吗？为什么要多此一举，创建一个分发器呢？

原因是这样更灵活，但是还有一个重要的原因是，注意到我们回调，往往是在主线程中进行的(因为很可能要操作UI)，如果我们在NetworkDispatcher(子线程)里面，直接回调，可能造成错误，这是ResponseDelivery存在的另外一个原因。

根据上面的结论，最后来看一张简单的流程图

根据流程分析，我们可以体会到，volley设计框架的基本思路，对比于我们简单的实现，volley的实现方式耦合更加松散，使用面向接口编程，同时使用更多组合方式而不是继承。使用了代理等设计模式，同时提高了线程的利用率。总之volley的架构设计又各种各样的好处。

我在这里介绍几个volley的功能，以及它考虑到的，而我们很可能没有考虑到的问题。这些问题或者说功能上的优势，会伴随着本专栏的深入让大家逐渐体会。

---

下面从源码角度看RequestQueue类，首先当然是属性

 /**
  * A request dispatch queue with a thread pool of dispatchers.
  *
  * Calling {@link #add(Request)} will enqueue the given Request for dispatch,
  * resolving from either cache or network on a worker thread, and then delivering
  * a parsed response on the main thread.
  * 一个拥有线程池的请求队列
  * 调用add()分发，将添加一个用于分发的请求
  * worker线程从缓存或网络获取响应，然后将该响应提供给主线程
  */
 public class RequestQueue {  

     /**
      * Callback interface for completed requests.
      * 任务完成的回调接口
      */
     public static interface RequestFinishedListener<T> {
         /** Called when a request has finished processing. */
         public void onRequestFinished(Request<T> request);
     }  

     /**
      * Used for generating monotonically-increasing sequence numbers for requests.
      * 使用原子类，记录队列中当前的请求数目
      */
     private AtomicInteger mSequenceGenerator = new AtomicInteger();  

     /**
      * Staging area for requests that already have a duplicate request in flight.<br>
      * 等候缓存队列，重复请求集结map,每个queue里面都是相同的请求
      * <ul>
      *     <li>containsKey(cacheKey) indicates that there is a request in flight for the given cache
      *          key.</li>
      *     <li>get(cacheKey) returns waiting requests for the given cache key. The in flight request
      *          is <em>not</em> contained in that list. Is null if no requests are staged.</li>
      * </ul>
      * 如果map里面包含该请求的cachekey，说明已经有相同key的请求在执行
      * get(cacheKey)根据cachekey返回对应的请求
      */
     private final Map<String, Queue<Request<?>>> mWaitingRequests =
             new HashMap<String, Queue<Request<?>>>();  

     /**
      * The set of all requests currently being processed by this RequestQueue. A Request
      * will be in this set if it is waiting in any queue or currently being processed by
      * any dispatcher.
      * 队列当前拥有的所以请求的集合
      * 请求在队列中，或者正被调度，都会在这个集合中
      */
     private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();  

     /**
      * The cache triage queue.
      * 缓存队列
      */
     private final PriorityBlockingQueue<Request<?>> mCacheQueue =
         new PriorityBlockingQueue<Request<?>>();  

     /**
      * The queue of requests that are actually going out to the network.
      * 网络队列，有阻塞和fifo功能
      */
     private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
         new PriorityBlockingQueue<Request<?>>();  

     /**
      * Number of network request dispatcher threads to start.
      * 默认用于调度的线程池数目
      */
     private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = ;  

     /**
      * Cache interface for retrieving and storing responses.
      * 缓存
      */
     private final Cache mCache;  

     /**
      * Network interface for performing requests.
      * 执行请求的网络
      */
     private final Network mNetwork;  

     /** Response delivery mechanism. */
     private final ResponseDelivery mDelivery;  

     /**
      * The network dispatchers.
      * 该队列的所有网络调度器
      */
     private NetworkDispatcher[] mDispatchers;  

     /**
      * The cache dispatcher.
      * 缓存调度器
      */
     private CacheDispatcher mCacheDispatcher;  

     /**
      * 任务完成监听器队列
      */
     private List<RequestFinishedListener> mFinishedListeners =
             new ArrayList<RequestFinishedListener>();  

属性很多，而且耦合的类也比较多，我挑重要的讲，这里大家只要先记住某个属性是什么就可以，至于它的具体实现我们先不管

1，首先看List<RequestFinishedListener> mFinishedListeners任务完成监听器队列，这个队列保留了很多监听器，这些监听器都是监听RequestQueue请求队列的，而不是监听单独的某个请求。RequestQueue中每个请求完成后，都会回调这个监听队列里面的所有监听器。这是RequestQueue的统一管理的体现。

2，AtomicInteger mSequenceGenerator原子类，对java多线程熟悉的朋友应该知道，这个是为了线程安全而创造的类，不了解的朋友，可以把它认识是int类型，用于记录当前队列中的请求数目

3，PriorityBlockingQueue<Request<?>> mCacheQueue缓存队列，用于存放向请求缓存的request,线程安全，有阻塞功能，也就是说当队列里面没有东西的时候，线程试图从队列取请求，这个线程就会阻塞

4,PriorityBlockingQueue<Request<?>> mNetworkQueue网络队列，用于存放准备发起网络请求的request，功能同上

5,CacheDispatcher mCacheDispatcher缓存调度器,继承了Thread类,本质是一个线程，这个线程将会被开启进入一个死循环，不断从mCacheQueue缓存队列取出请求，然后去缓存Cache中查找结果

6，NetworkDispatcher[] mDispatchers网络调度器数组，继承了Thread类，本质是多个线程，所以线程都将被开启进入死循环，不断从mNetworkQueue网络队列取出请求，然后去网络Network请求数据

7，Set<Request<?>> mCurrentRequests记录队列中的所有请求，也就是上面mCacheQueue缓存队列与mNetworkQueue网络队列的总和，用于统一管理

8，Cache mCache缓存对象，面向对象的思想，把缓存看成一个实体

9，Network mNetwork网络对象，面向对象的思想，把网络看成一个实体

10,ResponseDelivery mDelivery分发器，就是这个分发器，负责把响应发给对应的请求，分发器存在的意义之前已经提到了，主要是为了耦合更加送并且能在主线程中操作UI

11，Map<String, Queue<Request<?>>> mWaitingRequests等候缓存队列，重复请求集结map,每个queue里面都是相同的请求。为什么需要这个map呢？map的key其实是request的url，如果我们有多个请求的url都是相同的，也就是说请求的资源是相同的，volley就把这些请求放入一个队列，在用url做key将队列放入map中。

因为这些请求都是相同的，可以说结果也是相同的。那么我们只要获得一个请求的结果，其他相同的请求，从缓存中取就可以了。

所以等候缓存队列的作用就是，当其中的一个request获得响应，我们就将这个队列放入缓存队列mCacheQueue中，让这些request去缓存获取结果就好了。

这是volley处理重复请求的思路。

其实看懂上面的属性就可以了解RequestQueue类的作用，大家结合上面的属性，看一下流程图

ok，我们还是从构造函数开始看起吧

/**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     * 创建一个工作池，在调用start()方法以后，开始执行
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     * @param delivery A ResponseDelivery interface for posting responses and errors
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize,
            ResponseDelivery delivery) {
        mCache = cache;//缓存，用于保留响应到硬盘
        mNetwork = network;//网络接口，用于执行http请求
        mDispatchers = new NetworkDispatcher[threadPoolSize];//根据线程池大小，创建调度器数组
        mDelivery = delivery;//一个分发接口，用于响应和错误
    }  

    /**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize) {
        this(cache, network, threadPoolSize,
                new ExecutorDelivery(new Handler(Looper.getMainLooper())));
    }

对于RequestQueue来说，必须有的参数是缓存，网络，分发器，网络线程的数目

对应上面的属性可以知道，原来这些东西都是外部传进来的，参照本专栏的开篇，可以知道，是在Volley这个类里面传进来的，同时在外部，我们也是通过Volley.newRequestQueue()方法来创建并且开启queue队列的。

紧接着来看start()方法，这个方法用于启动队列

 /**
      * Starts the dispatchers in this queue.
      */
     public void start() {
         stop();  //保证当前所有运行的分发停止 Make sure any currently running dispatchers are stopped.
         // Create the cache dispatcher and start it.
         //创建新的缓存调度器，并且启动它
         mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
         mCacheDispatcher.start();  

         // Create network dispatchers (and corresponding threads) up to the pool size.
         //创建网络调度器,并且启动它们
         for (int i = ; i < mDispatchers.length; i++) {
             NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
                     mCache, mDelivery);
             mDispatchers[i] = networkDispatcher;
             networkDispatcher.start();
         }  

可以看到，所谓启动队列，就是创建了CacheDispatcher缓存调度器，和mDispatchers[]网络调度器数组，根据前面的介绍我们知道，它们都是线程，所以start()方法里面，其实就是调用了它们的start()方法。也就是说RequestQueue启动的本质，是这些调度器的启动，这些调度器启动以后，会进入死循环，不断从队列中取出request来进行数据请求。

由于Dispatcher调度器的数目有限(是根据我们给构造方法传入的参数threadPoolSize决定的)，意味着Volley框架，同时在执行数据请求的线程数目是有限的，这样避免了重复创建线程所带来的开销，同时可能会带来效率的下降。

所以threadPoolSize对不同的应用，设置的大小大家不同，大家要根据自己项目实际情况，经过测试来确定这个值。

说完开启，我们再来看RequestQueue的关闭

 /**
      * Stops the cache and network dispatchers.
      * 停止调度器(包括缓存和网络)
      */
     public void stop() {
         if (mCacheDispatcher != null) {
             mCacheDispatcher.quit();
         }
         for (int i = ; i < mDispatchers.length; i++) {
             if (mDispatchers[i] != null) {
                 mDispatchers[i].quit();
             }
         }
     }  

对比开启，其实stop()的本质也是关闭所有的调度器，调用了它们的quit()方法，至于这个方法做的是什么，很容易想到，是把它们内部while循环的标志设成false

再来看add()方法，这方法用于将request加入队列，也是一个非常重要方法

 /**
      * Adds a Request to the dispatch queue.
      * @param request The request to service
      * @return The passed-in request
      * 向请求队列添加请求
      */
     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);//为请求设置其请求队列
         synchronized (mCurrentRequests) {
             mCurrentRequests.add(request);
         }  

         // Process requests in the order they are added.
         request.setSequence(getSequenceNumber());//设置请求序号
         request.addMarker("add-to-queue");  

         // If the request is uncacheable, skip the cache queue and go straight to the network.
         //如果该请求不缓存，添加到网络队列
         if (!request.shouldCache()) {
             mNetworkQueue.add(request);
             return request;
         }
         //如果该请求要求缓存
         // 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.
                 //如果已经有一个请求在工作，则排队等候
                 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.
                 //为该key插入null,表明现在有一个请求在工作
                 mWaitingRequests.put(cacheKey, null);
                 mCacheQueue.add(request);
             }
             return request;
         }
     }  

对于一个request而言，首先它会被加入mCurrentRequests，这是用于request的统一管理

然后，调用shouldCache()判断是从缓存中取还是网络请求，如果是网络请求，则加入mNetworkQueue，然后改方法返回

如果请求缓存，根据mWaitingRequests是否已经有相同的请求在进行，如果是，则将该request加入mWaitingRequests

如果不是，则将request加入mCacheQueue去进行缓存查询

到目前为止，我们知道了调度器会从队列里面拿请求，至于具体是怎么请求的，我们还不清楚。这也体现了volley设计的合理性，通过组合来分配各个职责，每个类的职责都比较单一。

我们提到，RequestQueue的一个重要作用，就是对request的统一管理，其实所谓的管理，更多是对request的关闭，下面我来看一下这些方法

 /**
      * Called from {@link Request#finish(String)}, indicating that processing of the given request
      * has finished.
      * 在request类的finish()方法里面，会调用这个方法，说明该请求结束
      * <p>Releases waiting requests for <code>request.getCacheKey()</code> if
      *      <code>request.shouldCache()</code>.</p>
      */
     public <T> void finish(Request<T> request) {
         // Remove from the set of requests currently being processed.
         synchronized (mCurrentRequests) {//从当前请求队列中移除
             mCurrentRequests.remove(request);
         }
         synchronized (mFinishedListeners) {//回调监听器
           for (RequestFinishedListener<T> listener : mFinishedListeners) {
             listener.onRequestFinished(request);
           }
         }  

         if (request.shouldCache()) {//如果该请求要被缓存
             synchronized (mWaitingRequests) {
                 String cacheKey = request.getCacheKey();
                 Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);//移除该缓存
                 if (waitingRequests != null) {//如果存在缓存等候队列
                     if (VolleyLog.DEBUG) {
                         VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
                                 waitingRequests.size(), cacheKey);
                     }
                     // Process all queued up requests. They won't be considered as in flight, but
                     // that's not a problem as the cache has been primed by 'request'.
                     // 处理所有队列中的请求
                     mCacheQueue.addAll(waitingRequests);//
                 }
             }
         }
     }  

finish()用于表示某个特定的request完成了,只有将要完成的request传进来就好了，然后会在各个队列中移除它

这里需要注意，一个request完成以后，会将waitingRequests里面所有相同的请求，都加入到mCacheQueue缓存队列中，这就意味着，这些请求从缓存中取出结果就好了，这样就避免了频繁相同网络请求的开销。这也是Volley的亮点之一。

然后我们再来看一些取消方法

 /**
      * A simple predicate or filter interface for Requests, for use by
      * {@link RequestQueue#cancelAll(RequestFilter)}.
      * 一个简单的过滤接口,在cancelAll()方法里面被使用
      */
     public interface RequestFilter {
         public boolean apply(Request<?> request);
     }  

     /**
      * Cancels all requests in this queue for which the given filter applies.
      * @param filter The filtering function to use
      * 根据过滤器规则，取消相应请求
      */
     public void cancelAll(RequestFilter filter) {
         synchronized (mCurrentRequests) {
             for (Request<?> request : mCurrentRequests) {
                 if (filter.apply(request)) {
                     request.cancel();
                 }
             }
         }
     }  

     /**
      * Cancels all requests in this queue with the given tag. Tag must be non-null
      * and equality is by identity.
      * 根据标记取消相应请求
      */
     public void cancelAll(final Object tag) {
         if (tag == null) {
             throw new IllegalArgumentException("Cannot cancelAll with a null tag");
         }
         cancelAll(new RequestFilter() {
             @Override
             public boolean apply(Request<?> request) {
                 return request.getTag() == tag;
             }
         });
     }  

上面的设计可以说是非常巧妙的，为了增加取消的灵活性，创建了一个RequestFilter来自定义取消request的规则

在cancelAll(RequestFilter filter)方法里面，我们传入过滤器，就可以根据需要取消我想要取消的一类request,这种形式类似文件遍历的FileFilter

而这种形式，volley还为我们提供了一个具体的实现cancelAll(final Object tag)，来根据标签取消request，这里我们也就明白了request<T>类中mTag属性的用处了

可以说volley处处都体现了设计模式的美感。

Ok,RequestQueue介绍到这里，就介绍了整个的基本结构，剩下的困惑，是CacheDispatcher，networkDispatcher怎么从队列里面取出request的问题了，但是这些问题跟队列的关系没有那么紧，也就是说具体实现的任务，又交到了这两个类的身上，总而言之，这里也体现了单一责任原则。

接下来的文章，将会分类讲述这两个功能的实现。