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社区首页 >专栏 >Google Volley框架源码走读

Google Volley框架源码走读

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张拭心 shixinzhang
发布于 2022-11-30 17:09:52
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发布于 2022-11-30 17:09:52
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文章被收录于专栏：拭心的安卓进阶之路

【工匠若水 http://blog.csdn.net/yanbober】 阅读前一篇《Google Volley使用之自定义》 http://blog.csdn.net/yanbober/article/details/45307099

开源项目链接

Volley自定义 Android Developer文档

Volley主页：https://android.googlesource.com/platform/frameworks/volley

Volley仓库：git clone https://android.googlesource.com/platform/frameworks/volley

Volley GitHub Demo：在GitHub主页搜索Volley会有很多，不过建议阅读Android Developer文档。

背景知识

在Volley使用基础那一篇最后一个知识点说到了Volley的请求架构，这里再搬过来说说。

在Android Developer上看到的这幅图：

RequestQueue会维护一个缓存调度线程（cache线程）和一个网络调度线程池（net线程），当一个Request被加到队列中的时候，cache线程会把这个请求进行筛选：如果这个请求的内容可以在缓存中找到，cache线程会亲自解析相应内容，并分发到主线程（UI）。如果缓存中没有，这个request就会被加入到另一个NetworkQueue，所有真正准备进行网络通信的request都在这里，第一个可用的net线程会从NetworkQueue中拿出一个request扔向服务器。当响应数据到的时候，这个net线程会解析原始响应数据，写入缓存，并把解析后的结果返回给主线程。

硬着头皮开始吧

直接这么看好空洞，所以直接把clone的工程导入IDE边看代码边看这个图吧。

还是按照前边的顺序分析吧，使用Volley的第一步首先是通过Volley.newRequestQueue(context)得到RequestQueue队列，那么先看下toolbox下的Volley.java中的这个方法吧。

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
 *
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> context A {@link Context} to use for creating the cache dir.
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> A started {@link RequestQueue} instance.
 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> RequestQueue <span class="hljs-title" style="box-sizing: border-box;">newRequestQueue</span>(Context context) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> newRequestQueue(context, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

先看如上注释所示，创建一个默认的worker pool，并且调运RequestQueue的start方法。在这个方法里又调运了该类的另一个连个参数的重载方法，如下所示。

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
 *
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> context A {@link Context} to use for creating the cache dir.
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> stack An {@link HttpStack} to use for the network, or null for default.
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> A started {@link RequestQueue} instance.
 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> RequestQueue <span class="hljs-title" style="box-sizing: border-box;">newRequestQueue</span>(Context context, HttpStack stack) {
    File cacheDir = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> File(context.getCacheDir(), DEFAULT_CACHE_DIR);

    String userAgent = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"volley/0"</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
        String packageName = context.getPackageName();
        PackageInfo info = context.getPackageManager().getPackageInfo(packageName, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
        userAgent = packageName + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/"</span> + info.versionCode;
    } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (PackageManager.NameNotFoundException e) {
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (stack == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (Build.VERSION.SDK_INT >= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>) {
            stack = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HurlStack();
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Prior to Gingerbread, HttpUrlConnection was unreliable.</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html</span>
            stack = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HttpClientStack(AndroidHttpClient.newInstance(userAgent));
        }
    }

    Network network = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> BasicNetwork(stack);

    RequestQueue queue = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> RequestQueue(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> DiskBasedCache(cacheDir), network);
    queue.start();

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> queue;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li></ul>

如上所示，该方法有两个参数，第一个Context是为了拿到当前App的Activity的一些信息，第二个参数在默认的RequestQueue newRequestQueue(Context context)方法中传递为null，也就是说在这段代码中的if (stack == null)会被执行。在这个if中对版本号进行了判断，如果版本号大于等于9就使得HttpStack对象的实例为HurlStack，如果小于9则实例为HttpClientStack。至于这里为何进行版本号判断，实际代码中的(Click Me To See)注释已经说明了。实际上HurlStack类也在toolbox中，他实现了toolbox的HttpStack接口中的HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)方法，其实现过程使用的是HttpURLConnection。而HttpClientStack也在toolbox中，他也实现了toolbox的HttpStack接口的HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)方法，不过其实现过程使用的是HttpClient而已。

其中的userAgent就是App的包名加版本号而已，传入new HttpClientStack(AndroidHttpClient.newInstance(userAgent));作为name TAG使用。

如上HttpStack创建完成之后创建了Network实例。BasicNetwork是Network接口的实现，他们都在toolbox中，BasicNetwork实现了public NetworkResponse performRequest(Request<?> request)方法，其作用是根据传入的HttpStack对象来处理网络请求。紧接着new出一个RequestQueue对象，并调用它的start()方法进行启动，然后将RequestQueue返回。RequestQueue是根目录下的一个类，其作用是一个请求调度队列调度程序的线程池。这样newRequestQueue()的方法就执行结束了。

现在再来看下根目录下RequestQueue队列的start方法，如下所示：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * Starts the dispatchers in this queue.
 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">start</span>() {
    stop();  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Make sure any currently running dispatchers are stopped.</span>
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Create the cache dispatcher and start it.</span>
    mCacheDispatcher = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
    mCacheDispatcher.start();

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Create network dispatchers (and corresponding threads) up to the pool size.</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < mDispatchers.length; i++) {
        NetworkDispatcher networkDispatcher = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkDispatcher(mNetworkQueue, mNetwork, mCache, mDelivery);
        mDispatchers[i] = networkDispatcher;
        networkDispatcher.start();
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

通过注释可以看出来这里在派发队列的事务。先是创建了一个CacheDispatcher的实例，然后调用了它的start()方法，接着在一个for循环里去创建NetworkDispatcher的实例，并分别调用它们的start()方法。这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的，而默认情况下for循环会执行（DEFAULT_NETWORK_THREAD_POOL_SIZE）四次，也就是说当调用了Volley.newRequestQueue(context)之后，就会有五个线程一直在后台运行，不断等待网络请求的到来，其中一个CacheDispatcher是缓存线程，四个NetworkDispatcher是网络请求线程。

按照之前使用Volley可以知道，得到了RequestQueue之后，我们只需要构建出相应的Request，然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作了。也就是说add()方法的内部是核心代码了。现在看下RequestQueue的add方法，具体如下：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * Adds a Request to the dispatch queue.
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> request The request to service
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> The passed-in request
 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <T> Request<T> <span class="hljs-title" style="box-sizing: border-box;">add</span>(Request<T> request) {
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Tag the request as belonging to this queue and add it to the set of current requests.</span>
    request.setRequestQueue(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">synchronized</span> (mCurrentRequests) {
        mCurrentRequests.add(request);
    }

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Process requests in the order they are added.</span>
    request.setSequence(getSequenceNumber());
    request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"add-to-queue"</span>);

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If the request is uncacheable, skip the cache queue and go straight to the network.</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!request.shouldCache()) {
        mNetworkQueue.add(request);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> request;
    }

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Insert request into stage if there's already a request with the same cache key in flight.</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">synchronized</span> (mWaitingRequests) {
        String cacheKey = request.getCacheKey();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mWaitingRequests.containsKey(cacheKey)) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// There is already a request in flight. Queue up.</span>
            Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (stagedRequests == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                stagedRequests = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> LinkedList<Request<?>>();
            }
            stagedRequests.add(request);
            mWaitingRequests.put(cacheKey, stagedRequests);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (VolleyLog.DEBUG) {
                VolleyLog.v(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Request for cacheKey=%s is in flight, putting on hold."</span>, cacheKey);
            }
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Insert 'null' queue for this cacheKey, indicating there is now a request in</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// flight.</span>
            mWaitingRequests.put(cacheKey, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>);
            mCacheQueue.add(request);
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> request;
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li></ul>

可以看到注释所示，添加一个Request到派发队列。Request是所有请求的基类，是一个抽象类。request.setRequestQueue(this);的作用就是将请求Request关联到当前RequestQueue。然后同步操作将当前Request添加到RequestQueue对象的mCurrentRequests HashSet中做记录。通过request.setSequence(getSequenceNumber());得到当前RequestQueue中请求的个数，然后关联到当前Request。request.addMarker("add-to-queue");添加调试的Debug标记。if (!request.shouldCache())判断当前的请求是否可以缓存，如果不能缓存则直接通过mNetworkQueue.add(request);将这条请求加入网络请求队列，然后返回request；如果可以缓存的话则在通过同步操作将这条请求加入缓存队列。在默认情况下，每条请求都是可以缓存的，当然我们也可以调用Request的setShouldCache(false)方法来改变这一默认行为。OK，那么既然默认每条请求都是可以缓存的（shouldCache返回为true），自然就被添加到了缓存队列中，于是一直在后台等待的缓存线程就要开始运行起来了。现在来看下CacheDispatcher中的run()方法，代码如下所示：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (DEBUG) VolleyLog.v(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"start new dispatcher"</span>);
    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Make a blocking call to initialize the cache.</span>
    mCache.initialize();

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Get a request from the cache triage queue, blocking until</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// at least one is available.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Request<?> request = mCacheQueue.take();
            request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-queue-take"</span>);

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If the request has been canceled, don't bother dispatching it.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (request.isCanceled()) {
                request.finish(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-discard-canceled"</span>);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Attempt to retrieve this item from cache.</span>
            Cache.Entry entry = mCache.get(request.getCacheKey());
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (entry == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-miss"</span>);
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Cache miss; send off to the network dispatcher.</span>
                mNetworkQueue.put(request);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If it is completely expired, just send it to the network.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (entry.isExpired()) {
                request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-hit-expired"</span>);
                request.setCacheEntry(entry);
                mNetworkQueue.put(request);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We have a cache hit; parse its data for delivery back to the request.</span>
            request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-hit"</span>);
            Response<?> response = request.parseNetworkResponse(
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkResponse(entry.data, entry.responseHeaders));
            request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-hit-parsed"</span>);

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!entry.refreshNeeded()) {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Completely unexpired cache hit. Just deliver the response.</span>
                mDelivery.postResponse(request, response);
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Soft-expired cache hit. We can deliver the cached response,</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// but we need to also send the request to the network for</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// refreshing.</span>
                request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"cache-hit-refresh-needed"</span>);
                request.setCacheEntry(entry);

                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Mark the response as intermediate.</span>
                response.intermediate = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>;

                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Post the intermediate response back to the user and have</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// the delivery then forward the request along to the network.</span>
                mDelivery.postResponse(request, response, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable() {
                    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
                        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
                            mNetworkQueue.put(request);
                        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (InterruptedException e) {
                            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Not much we can do about this.</span>
                        }
                    }
                });
            }
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (InterruptedException e) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We may have been interrupted because it was time to quit.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mQuit) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
            }
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
        }
    }
/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释 
* }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li><li style="box-sizing: border-box; padding: 0px 5px;">76</li><li style="box-sizing: border-box; padding: 0px 5px;">77</li><li style="box-sizing: border-box; padding: 0px 5px;">78</li><li style="box-sizing: border-box; padding: 0px 5px;">79</li></ul>
*/

首先通过Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);设置线程优先级，然后通过mCache.initialize(); 初始化缓存块，其中mCache是由Volley.java中的newRequestQueue(Context context, HttpStack stack)方法中实例化传入的，其Cache接口的实现为new DiskBasedCache(cacheDir)，其中cacheDir默认在Volley.java中不设置为/data/data/app-package/cache/volley/。

接下来由while (true)可以发现缓存线程是一直在执行，其中通过mQuit标记进制是否结束线程的操作。mCacheQueue.take()从阻塞队列获取最前面的一个request，没有request就阻塞等待。接着通过mCache.get(request.getCacheKey());尝试从缓存中取出响应结果，如何为空的话则把这条请求加入到网络请求队列中，如果不为空的话再判断该缓存是否已过期，如果已经过期了则同样把这条请求加入到网络请求队列中，否则就认为不需要重发网络请求，直接使用缓存中的数据即可。在这个过程中调运了parseNetworkResponse()方法来对数据进行解析，再往后就是将解析出来的数据进行回调了。现在先来看下Request抽象基类的这部分代码：

代码语言：javascript
复制
<code class="language-android hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">/**
 * Subclasses must implement this <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">to</span> parse the raw network <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span>
 * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">and</span> return an appropriate <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span> type. This method will be
 * called from a worker thread.  The <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span> will <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">not</span> be delivered
 * <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> you return <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>.
 * @param <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Response</span> from the network
 * @return The parsed <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">or</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">in</span> the <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">case</span> of an <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">error</span>
 */
abstract protected <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Response</span><T> parseNetworkResponse(NetworkResponse <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span>);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

通过注释可以看到他就是一个解析模块的功能。

上面说了，当调用了Volley.newRequestQueue(context)之后，就会有五个线程一直在后台运行，不断等待网络请求的到来，其中一个CacheDispatcher是缓存线程，四个NetworkDispatcher是网络请求线程。CacheDispatcher的run方法刚才已经大致分析了，解析来看下NetworkDispatcher中是怎么处理网络请求队列的，具体代码如下所示：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> startTimeMs = SystemClock.elapsedRealtime();
        Request<?> request;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Take a request from the queue.</span>
            request = mQueue.take();
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (InterruptedException e) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// We may have been interrupted because it was time to quit.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mQuit) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
            }
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
        }

        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"network-queue-take"</span>);

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If the request was cancelled already, do not perform the</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// network request.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (request.isCanceled()) {
                request.finish(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"network-discard-cancelled"</span>);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
            }

            addTrafficStatsTag(request);

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Perform the network request.</span>
            NetworkResponse networkResponse = mNetwork.performRequest(request);
            request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"network-http-complete"</span>);

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If the server returned 304 AND we delivered a response already,</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// we're done -- don't deliver a second identical response.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (networkResponse.notModified && request.hasHadResponseDelivered()) {
                request.finish(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"not-modified"</span>);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Parse the response here on the worker thread.</span>
            Response<?> response = request.parseNetworkResponse(networkResponse);
            request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"network-parse-complete"</span>);

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Write to cache if applicable.</span>
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// TODO: Only update cache metadata instead of entire record for 304s.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (request.shouldCache() && response.cacheEntry != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                mCache.put(request.getCacheKey(), response.cacheEntry);
                request.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"network-cache-written"</span>);
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Post the response back.</span>
            request.markDelivered();
            mDelivery.postResponse(request, response);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (VolleyError volleyError) {
            volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
            parseAndDeliverNetworkError(request, volleyError);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (Exception e) {
            VolleyLog.e(e, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Unhandled exception %s"</span>, e.toString());
            VolleyError volleyError = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> VolleyError(e);
            volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
            mDelivery.postError(request, volleyError);
        }
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li></ul>

和CacheDispatcher差不多，如上可以看见一个类似的while(true)循环，说明网络请求线程也是在不断运行的。

如上通过mNetwork.performRequest(request);代码来发送网络请求，而Network是一个接口，这里具体的实现之前已经分析是BasicNetwork，所以先看下它的performRequest()方法，如下所示：

NetWork接口的代码：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Network</span> {</span>
    <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
     * Performs the specified request.
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @param</span> request Request to process
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span> A {@link NetworkResponse} with data and caching metadata; will never be null
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @throws</span> VolleyError on errors
     */</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> NetworkResponse <span class="hljs-title" style="box-sizing: border-box;">performRequest</span>(Request<?> request) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throws</span> VolleyError;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

上面说了，就是执行指定的请求。他的BasicNetwork实现子类如下：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> NetworkResponse <span class="hljs-title" style="box-sizing: border-box;">performRequest</span>(Request<?> request) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throws</span> VolleyError {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> requestStart = SystemClock.elapsedRealtime();
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
        HttpResponse httpResponse = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">byte</span>[] responseContents = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;
        Map<String, String> responseHeaders = Collections.emptyMap();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Gather headers.</span>
            Map<String, String> headers = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<String, String>();
            addCacheHeaders(headers, request.getCacheEntry());
            httpResponse = mHttpStack.performRequest(request, headers);
            StatusLine statusLine = httpResponse.getStatusLine();
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> statusCode = statusLine.getStatusCode();

            responseHeaders = convertHeaders(httpResponse.getAllHeaders());
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Handle cache validation.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (statusCode == HttpStatus.SC_NOT_MODIFIED) {

                Entry entry = request.getCacheEntry();
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (entry == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkResponse(HttpStatus.SC_NOT_MODIFIED, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>,
                            responseHeaders, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>,
                            SystemClock.elapsedRealtime() - requestStart);
                }

                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// A HTTP 304 response does not have all header fields. We</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// have to use the header fields from the cache entry plus</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// the new ones from the response.</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5</span>
                entry.responseHeaders.putAll(responseHeaders);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
                        entry.responseHeaders, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>,
                        SystemClock.elapsedRealtime() - requestStart);
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Some responses such as 204s do not have content.  We must check.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (httpResponse.getEntity() != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                responseContents = entityToBytes(httpResponse.getEntity());
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Add 0 byte response as a way of honestly representing a</span>
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// no-content request.</span>
                responseContents = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">byte</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>];
            }

            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// if the request is slow, log it.</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> requestLifetime = SystemClock.elapsedRealtime() - requestStart;
            logSlowRequests(requestLifetime, request, responseContents, statusLine);

            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (statusCode < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">200</span> || statusCode > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">299</span>) {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> IOException();
            }
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkResponse(statusCode, responseContents, responseHeaders, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>,
                    SystemClock.elapsedRealtime() - requestStart);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (SocketTimeoutException e) {
            attemptRetryOnException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"socket"</span>, request, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimeoutError());
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (ConnectTimeoutException e) {
            attemptRetryOnException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"connection"</span>, request, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimeoutError());
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (MalformedURLException e) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> RuntimeException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Bad URL "</span> + request.getUrl(), e);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span> (IOException e) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> statusCode = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;
            NetworkResponse networkResponse = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>;
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (httpResponse != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                statusCode = httpResponse.getStatusLine().getStatusCode();
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NoConnectionError(e);
            }
            VolleyLog.e(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Unexpected response code %d for %s"</span>, statusCode, request.getUrl());
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (responseContents != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
                networkResponse = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkResponse(statusCode, responseContents,
                        responseHeaders, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>, SystemClock.elapsedRealtime() - requestStart);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (statusCode == HttpStatus.SC_UNAUTHORIZED ||
                        statusCode == HttpStatus.SC_FORBIDDEN) {
                    attemptRetryOnException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"auth"</span>,
                            request, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> AuthFailureError(networkResponse));
                } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// TODO: Only throw ServerError for 5xx status codes.</span>
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ServerError(networkResponse);
                }
            } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> NetworkError(networkResponse);
            }
        }
    }
/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释 
* }</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li><li style="box-sizing: border-box; padding: 0px 5px;">62</li><li style="box-sizing: border-box; padding: 0px 5px;">63</li><li style="box-sizing: border-box; padding: 0px 5px;">64</li><li style="box-sizing: border-box; padding: 0px 5px;">65</li><li style="box-sizing: border-box; padding: 0px 5px;">66</li><li style="box-sizing: border-box; padding: 0px 5px;">67</li><li style="box-sizing: border-box; padding: 0px 5px;">68</li><li style="box-sizing: border-box; padding: 0px 5px;">69</li><li style="box-sizing: border-box; padding: 0px 5px;">70</li><li style="box-sizing: border-box; padding: 0px 5px;">71</li><li style="box-sizing: border-box; padding: 0px 5px;">72</li><li style="box-sizing: border-box; padding: 0px 5px;">73</li><li style="box-sizing: border-box; padding: 0px 5px;">74</li><li style="box-sizing: border-box; padding: 0px 5px;">75</li><li style="box-sizing: border-box; padding: 0px 5px;">76</li><li style="box-sizing: border-box; padding: 0px 5px;">77</li><li style="box-sizing: border-box; padding: 0px 5px;">78</li><li style="box-sizing: border-box; padding: 0px 5px;">79</li><li style="box-sizing: border-box; padding: 0px 5px;">80</li><li style="box-sizing: border-box; padding: 0px 5px;">81</li><li style="box-sizing: border-box; padding: 0px 5px;">82</li><li style="box-sizing: border-box; padding: 0px 5px;">83</li><li style="box-sizing: border-box; padding: 0px 5px;">84</li><li style="box-sizing: border-box; padding: 0px 5px;">85</li><li style="box-sizing: border-box; padding: 0px 5px;">86</li></ul>
*/

这个方法是网络请求的具体实现，也是一个大while循环，其中mHttpStack.performRequest(request, headers);代码中的mHttpStack是Volley的newRequestQueue()方法中创建的实例，前面已经说过，这两个对象的内部实际就是分别使用HttpURLConnection和HttpClient来发送网络请求的，然后把服务器返回的数据组装成一个NetworkResponse对象进行返回。在NetworkDispatcher中收到了NetworkResponse这个返回值后又会调用Request的parseNetworkResponse()方法来解析NetworkResponse中的数据，同时将数据写入到缓存，这个方法的实现是交给Request的子类来完成的，因为不同种类的Request解析的方式也肯定不同。

前面你可以看到在NetWorkDispatcher的run中最后执行了mDelivery.postResponse(request, response);，也就是说在解析完了NetworkResponse中的数据之后，又会调用ExecutorDelivery（ResponseDelivery接口的实现类）的postResponse()方法来回调解析出的数据，具体代码如下所示：

代码语言：javascript
复制
<code class="language-android hljs vbscript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">@Override
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> void postResponse(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Request</span><?> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">request</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Response</span><?> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span>, Runnable runnable) {
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">request</span>.markDelivered();
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">request</span>.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"post-response"</span>);
    mResponsePoster.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">execute</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ResponseDeliveryRunnable(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">request</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">response</span>, runnable));
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

这里可以看见在mResponsePoster的execute()方法中传入了一个ResponseDeliveryRunnable对象，就可以保证该对象中的run()方法就是在主线程当中运行的了，我们看下run()方法中的代码是什么样的：

代码语言：javascript
复制
<code class="language-android hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * A Runnable used for delivering network responses to a listener on the
 * main thread.
 */</span>
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@SuppressWarnings</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"rawtypes"</span>)
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ResponseDeliveryRunnable</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Runnable</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Request mRequest;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Response mResponse;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Runnable mRunnable;

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">ResponseDeliveryRunnable</span>(Request request, Response response, Runnable runnable) {
        mRequest = request;
        mResponse = response;
        mRunnable = runnable;
    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@SuppressWarnings</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"unchecked"</span>)
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If this request has canceled, finish it and don't deliver.</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mRequest.isCanceled()) {
            mRequest.finish(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"canceled-at-delivery"</span>);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span>;
        }

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Deliver a normal response or error, depending.</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mResponse.isSuccess()) {
            mRequest.deliverResponse(mResponse.result);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
            mRequest.deliverError(mResponse.error);
        }

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If this is an intermediate response, add a marker, otherwise we're done</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// and the request can be finished.</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mResponse.intermediate) {
            mRequest.addMarker(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"intermediate-response"</span>);
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
            mRequest.finish(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"done"</span>);
        }

        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// If we have been provided a post-delivery runnable, run it.</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mRunnable != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>) {
            mRunnable.run();
        }
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li></ul>

这段代码里的run方法中可以看到如下一部分细节：

代码语言：javascript
复制
<code class="language-android hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">// Deliver a normal response <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">or</span> error, depending.
if (mResponse<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.isSuccess</span>()) {
    mRequest<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.deliverResponse</span>(mResponse<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.result</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
} else {
    mRequest<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.deliverError</span>(mResponse<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.error</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

这段代码是最核心的，明显可以看到通过mRequest的deliverResponse或者deliverError将反馈发送到回调到UI线程。这也是你重写实现的接口方法。

再来整体看下

现在是该回过头去看背景知识模块了，再看下那幅官方图，对比就明白咋回事了。结合上图和如上分析可以知道：

  1. 当一个RequestQueue被成功申请后会开启一个CacheDispatcher和4个默认的NetworkDispatcher。
  2. CacheDispatcher缓存调度器最为第一层缓冲，开始工作后阻塞的从缓存序列mCacheQueue中取得请求；对于已经取消的请求，标记为跳过并结束这个请求；新的或者过期的请求，直接放入mNetworkQueue中由N个NetworkDispatcher进行处理；已获得缓存信息（网络应答）却没有过期的请求，由Request的parseNetworkResponse进行解析，从而确定此应答是否成功。然后将请求和应答交由Delivery分发者进行处理，如果需要更新缓存那么该请求还会被放入mNetworkQueue中。
  3. 将请求Request add到RequestQueue后对于不需要缓存的请求（需要额外设置，默认是需要缓存）直接丢入mNetworkQueue交给N个NetworkDispatcher处理；对于需要缓存的，新的请求加到mCacheQueue中给CacheDispatcher处理；需要缓存，但是缓存列表中已经存在了相同URL的请求，放在mWaitingQueue中做暂时处理，等待之前请求完毕后，再重新添加到mCacheQueue中。
  4. 网络请求调度器NetworkDispatcher作为网络请求真实发生的地方，对消息交给BasicNetwork进行处理，同样的，请求和结果都交由Delivery分发者进行处理。
  5. Delivery分发者实际上已经是对网络请求处理的最后一层了，在Delivery对请求处理之前，Request已经对网络应答进行过解析，此时应答成功与否已经设定；而后Delivery根据请求所获得的应答情况做不同处理；若应答成功，则触发deliverResponse方法，最终会触发开发者为Request设定的Listener；若应答失败，则触发deliverError方法，最终会触发开发者为Request设定的ErrorListener；处理完后，一个Request的生命周期就结束了，Delivery会调用Request的finish操作，将其从mRequestQueue中移除，与此同时，如果等待列表中存在相同URL的请求，则会将剩余的层级请求全部丢入mCacheQueue交由CacheDispatcher进行处理。

至此所有搞定。

PPPS一句：通过上面原理分析之后总结发现，推荐整个App全局持有一个RequestQueue的做法，这样会有相对比较高的性能效率。

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