Google Volley框架原始碼走讀

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

開源專案連結

Volley自定義 Android Developer文件

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

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

Volley GitHub Demo：在GitHub主頁搜尋Volley會有很多，不過建議閱讀Android Developer文件。

背景知識

在Volley使用基礎那一篇最後一個知識點說到了Volley的請求架構，這裡再搬過來說說。

在Android Developer上看到的這幅圖：

RequestQueue會維護一個快取排程執行緒（cache執行緒）和一個網路排程執行緒池（net執行緒），當一個Request被加到佇列中的時候，cache執行緒會把這個請求進行篩選：如果這個請求的內容可以在快取中找到，cache執行緒會親自解析相應內容，並分發到主執行緒（UI）。如果快取中沒有，這個request就會被加入到另一個NetworkQueue，所有真正準備進行網路通訊的request都在這裡，第一個可用的net執行緒會從NetworkQueue中拿出一個request扔向伺服器。當響應資料到的時候，這個net執行緒會解析原始響應資料，寫入快取，並把解析後的結果返回給主執行緒。

硬著頭皮開始吧

直接這麼看好空洞，所以直接把clone的工程匯入IDE邊看程式碼邊看這個圖吧。

還是按照前邊的順序分析吧，使用Volley的第一步首先是通過Volley.newRequestQueue(context)得到RequestQueue佇列，那麼先看下toolbox下的Volley.java中的這個方法吧。

<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>

先看如上註釋所示，建立一個預設的worker pool，並且調運RequestQueue的start方法。在這個方法裡又調運了該類的另一個連個引數的過載方法，如下所示。

<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，該方法有兩個引數，第一個Context是為了拿到當前App的Activity的一些資訊，第二個引數在預設的RequestQueue newRequestQueue(Context context)方法中傳遞為null，也就是說在這段程式碼中的if (stack == null)會被執行。在這個if中對版本號進行了判斷，如果版本號大於等於9就使得HttpStack物件的例項為HurlStack，如果小於9則例項為HttpClientStack。至於這裡為何進行版本號判斷，實際程式碼中的(Click Me To See)註釋已經說明了。實際上HurlStack類也在toolbox中，他實現了toolbox的HttpStack介面中的HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)方法，其實現過程使用的是HttpURLConnection。而HttpClientStack也在toolbox中，他也實現了toolbox的HttpStack介面的HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)方法，不過其實現過程使用的是HttpClient而已。

其中的userAgent就是App的包名加版本號而已，傳入new HttpClientStack(AndroidHttpClient.newInstance(userAgent));作為name TAG使用。

如上HttpStack建立完成之後建立了Network例項。BasicNetwork是Network介面的實現，他們都在toolbox中，BasicNetwork實現了public NetworkResponse performRequest(Request<?> request)方法，其作用是根據傳入的HttpStack物件來處理網路請求。緊接著new出一個RequestQueue物件，並呼叫它的start()方法進行啟動，然後將RequestQueue返回。RequestQueue是根目錄下的一個類，其作用是一個請求排程佇列排程程式的執行緒池。這樣newRequestQueue()的方法就執行結束了。

現在再來看下根目錄下RequestQueue佇列的start方法，如下所示：

<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>

通過註釋可以看出來這裡在派發佇列的事務。先是建立了一個CacheDispatcher的例項，然後呼叫了它的start()方法，接著在一個for迴圈裡去建立NetworkDispatcher的例項，並分別呼叫它們的start()方法。這裡的CacheDispatcher和NetworkDispatcher都是繼承自Thread的，而預設情況下for迴圈會執行（DEFAULT_NETWORK_THREAD_POOL_SIZE）四次，也就是說當呼叫了Volley.newRequestQueue(context)之後，就會有五個執行緒一直在後臺執行，不斷等待網路請求的到來，其中一個CacheDispatcher是快取執行緒，四個NetworkDispatcher是網路請求執行緒。

按照之前使用Volley可以知道，得到了RequestQueue之後，我們只需要構建出相應的Request，然後呼叫RequestQueue的add()方法將Request傳入就可以完成網路請求操作了。也就是說add()方法的內部是核心程式碼了。現在看下RequestQueue的add方法，具體如下：

<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>

可以看到註釋所示，新增一個Request到派發佇列。Request是所有請求的基類，是一個抽象類。request.setRequestQueue(this);的作用就是將請求Request關聯到當前RequestQueue。然後同步操作將當前Request新增到RequestQueue物件的mCurrentRequests HashSet中做記錄。通過request.setSequence(getSequenceNumber());得到當前RequestQueue中請求的個數，然後關聯到當前Request。request.addMarker("add-to-queue");新增除錯的Debug標記。if (!request.shouldCache())判斷當前的請求是否可以快取，如果不能快取則直接通過mNetworkQueue.add(request);將這條請求加入網路請求佇列，然後返回request；如果可以快取的話則在通過同步操作將這條請求加入快取佇列。在預設情況下，每條請求都是可以快取的，當然我們也可以呼叫Request的setShouldCache(false)方法來改變這一預設行為。OK，那麼既然預設每條請求都是可以快取的（shouldCache返回為true），自然就被新增到了快取佇列中，於是一直在後臺等待的快取執行緒就要開始執行起來了。現在來看下CacheDispatcher中的run()方法，程式碼如下所示：

<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>;
        }
    }
}</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>

首先通過Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);設定執行緒優先順序，然後通過mCache.initialize(); 初始化快取塊，其中mCache是由Volley.java中的newRequestQueue(Context context, HttpStack stack)方法中例項化傳入的，其Cache介面的實現為new DiskBasedCache(cacheDir)，其中cacheDir預設在Volley.java中不設定為/data/data/app-package/cache/volley/。

接下來由while (true)可以發現快取執行緒是一直在執行，其中通過mQuit標記進位制是否結束執行緒的操作。mCacheQueue.take()從阻塞佇列獲取最前面的一個request，沒有request就阻塞等待。接著通過mCache.get(request.getCacheKey());嘗試從快取中取出響應結果，如何為空的話則把這條請求加入到網路請求佇列中，如果不為空的話再判斷該快取是否已過期，如果已經過期了則同樣把這條請求加入到網路請求佇列中，否則就認為不需要重發網路請求，直接使用快取中的資料即可。在這個過程中調運了parseNetworkResponse()方法來對資料進行解析，再往後就是將解析出來的資料進行回撥了。現在先來看下Request抽象基類的這部分程式碼：

<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>

通過註釋可以看到他就是一個解析模組的功能。

上面說了，當呼叫了Volley.newRequestQueue(context)之後，就會有五個執行緒一直在後臺執行，不斷等待網路請求的到來，其中一個CacheDispatcher是快取執行緒，四個NetworkDispatcher是網路請求執行緒。CacheDispatcher的run方法剛才已經大致分析了，解析來看下NetworkDispatcher中是怎麼處理網路請求佇列的，具體程式碼如下所示：

<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，如上可以看見一個類似的while(true)迴圈，說明網路請求執行緒也是在不斷執行的。

如上通過mNetwork.performRequest(request);程式碼來傳送網路請求，而Network是一個介面，這裡具體的實現之前已經分析是BasicNetwork，所以先看下它的performRequest()方法，如下所示：

NetWork介面的程式碼：

<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>

上面說了，就是執行指定的請求。他的BasicNetwork實現子類如下：

<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);
            }
        }
    }
}</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>

這個方法是網路請求的具體實現，也是一個大while迴圈，其中mHttpStack.performRequest(request, headers);程式碼中的mHttpStack是Volley的newRequestQueue()方法中建立的例項，前面已經說過，這兩個物件的內部實際就是分別使用HttpURLConnection和HttpClient來傳送網路請求的，然後把伺服器返回的資料組裝成一個NetworkResponse物件進行返回。在NetworkDispatcher中收到了NetworkResponse這個返回值後又會呼叫Request的parseNetworkResponse()方法來解析NetworkResponse中的資料，同時將資料寫入到快取，這個方法的實現是交給Request的子類來完成的，因為不同種類的Request解析的方式也肯定不同。

前面你可以看到在NetWorkDispatcher的run中最後執行了mDelivery.postResponse(request, response);，也就是說在解析完了NetworkResponse中的資料之後，又會呼叫ExecutorDelivery（ResponseDelivery介面的實現類）的postResponse()方法來回撥解析出的資料，具體程式碼如下所示：

<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>

這裡可以看見在mResponsePoster的execute()方法中傳入了一個ResponseDeliveryRunnable物件，就可以保證該物件中的run()方法就是在主執行緒當中執行的了，我們看下run()方法中的程式碼是什麼樣的：

<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>

這段程式碼裡的run方法中可以看到如下一部分細節：

<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>

這段程式碼是最核心的，明顯可以看到通過mRequest的deliverResponse或者deliverError將反饋傳送到回撥到UI執行緒。這也是你重寫實現的介面方法。

再來整體看下

現在是該回過頭去看背景知識模組了，再看下那幅官方圖，對比就明白咋回事了。結合上圖和如上分析可以知道：

1. 當一個RequestQueue被成功申請後會開啟一個CacheDispatcher和4個預設的NetworkDispatcher。

2. CacheDispatcher快取排程器最為第一層緩衝，開始工作後阻塞的從快取序列mCacheQueue中取得請求；對於已經取消的請求，標記為跳過並結束這個請求；新的或者過期的請求，直接放入mNetworkQueue中由N個NetworkDispatcher進行處理；已獲得快取資訊（網路應答）卻沒有過期的請求，由Request的parseNetworkResponse進行解析，從而確定此應答是否成功。然後將請求和應答交由Delivery分發者進行處理，如果需要更新快取那麼該請求還會被放入mNetworkQueue中。

3. 將請求Request add到RequestQueue後對於不需要快取的請求（需要額外設定，預設是需要快取）直接丟入mNetworkQueue交給N個NetworkDispatcher處理；對於需要快取的，新的請求加到mCacheQueue中給CacheDispatcher處理；需要快取，但是快取列表中已經存在了相同URL的請求，放在mWaitingQueue中做暫時處理，等待之前請求完畢後，再重新新增到mCacheQueue中。

4. 網路請求排程器NetworkDispatcher作為網路請求真實發生的地方，對訊息交給BasicNetwork進行處理，同樣的，請求和結果都交由Delivery分發者進行處理。

5. Delivery分發者實際上已經是對網路請求處理的最後一層了，在Delivery對請求處理之前，Request已經對網路應答進行過解析，此時應答成功與否已經設定；而後Delivery根據請求所獲得的應答情況做不同處理；若應答成功，則觸發deliverResponse方法，最終會觸發開發者為Request設定的Listener；若應答失敗，則觸發deliverError方法，最終會觸發開發者為Request設定的ErrorListener；處理完後，一個Request的生命週期就結束了，Delivery會呼叫Request的finish操作，將其從mRequestQueue中移除，與此同時，如果等待列表中存在相同URL的請求，則會將剩餘的層級請求全部丟入mCacheQueue交由CacheDispatcher進行處理。

至此所有搞定。

PPPS一句：通過上面原理分析之後總結髮現，推薦整個App全域性持有一個RequestQueue的做法，這樣會有相對比較高的效能效率。