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 = 4;

    /** 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 = 0; 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 = 0; 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);
      }
    }
}

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 = 4;  
  
    /**  
     * 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 = 0; 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 = 0; 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。