这是 volley 的第四篇 blog 了，写完这篇，volley 的大部分用法也都算写了一遍，所以暂时不会写 volley 的文章了，如果想看我前面写的文章，可以点这里 Android volley 解析（三）之文件上传篇
为什么要用缓存
我们知道，当客户端在请求网络数据的时候，是需要消耗流量的，特别是对于移动端用户来说，对于流量的控制要求很高。所以在做网络请求的时候，如果对数据更新要求不是特别高，往往都会用到缓存机制，一方面能减少对服务端的请求，控制流量；另一方面，当客户端在没有网络的情况下也能看到上一次请求的数据，这样使用户体验更佳，如下图，微信朋友圈在没有网络的情况下，依然能看到朋友们的动态
volley 缓存的原理
看了我前面blog 的朋友一定还记得，我在讲 get 请求的时候，讲到了volley 的基本流程，首先启动的是缓存线程mCacheDispatcher来获取缓存；
那我们就从如何获取缓存开始分析；
1、初始化缓存
<code class="hljs r has-numbering"> @Override
public void run() {
<span class="hljs-keyword">if</span> (DEBUG) VolleyLog.v(<span class="hljs-string">"start new dispatcher"</span>);
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// Make a blocking call to initialize the cache.这里对缓存做初始化操作
mCache.initialize();
<span class="hljs-keyword">while</span> (true) {
<span class="hljs-keyword">...</span></code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li></ul>
2、获取缓存
<code class="hljs java has-numbering">
<span class="hljs-keyword">while</span> (<span class="hljs-keyword">true</span>) {
<span class="hljs-keyword">try</span> {
<span class="hljs-comment">// Get a request from the cache triage queue, blocking until</span>
<span class="hljs-comment">// at least one is available.从缓存队列中获取缓存</span>
<span class="hljs-keyword">final</span> Request request = mCacheQueue.take();
request.addMarker(<span class="hljs-string">"cache-queue-take"</span>);
<span class="hljs-comment">// If the request has been canceled, don't bother dispatching it.</span>
...</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li></ul>
这里有一个问题，缓存队列在什么时候添加缓存请求呢？我们回到最开始请求队列添加请求的地方
<code class="hljs java has-numbering"> <span class="hljs-keyword">public</span> Request <span class="hljs-title">add</span>(Request request) {
<span class="hljs-comment">// Tag the request as belonging to this queue and add it to the set of current requests.</span>
request.setRequestQueue(<span class="hljs-keyword">this</span>);
<span class="hljs-keyword">synchronized</span> (mCurrentRequests) {
mCurrentRequests.add(request);
}
<span class="hljs-comment">// Process requests in the order they are added.</span>
request.setSequence(getSequenceNumber());
request.addMarker(<span class="hljs-string">"add-to-queue"</span>);
<span class="hljs-comment">// If the request is uncacheable, skip the cache queue and go straight to the network.如果请求没有设置缓存，则把请求添加到网络队列中</span>
<span class="hljs-keyword">if</span> (!request.shouldCache()) {
mNetworkQueue.add(request);
<span class="hljs-keyword">return</span> request;
}
<span class="hljs-comment">// Insert request into stage if there's already a request with the same cache key in flight.</span>
<span class="hljs-keyword">synchronized</span> (mWaitingRequests) {
String cacheKey = request.getCacheKey();
<span class="hljs-keyword">if</span> (mWaitingRequests.containsKey(cacheKey)) {
<span class="hljs-comment">// There is already a request in flight. Queue up.</span>
Queue<Request> stagedRequests = mWaitingRequests.get(cacheKey);
<span class="hljs-keyword">if</span> (stagedRequests == <span class="hljs-keyword">null</span>) {
stagedRequests = <span class="hljs-keyword">new</span> LinkedList<Request>();
}
stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);
<span class="hljs-keyword">if</span> (VolleyLog.DEBUG) {
VolleyLog.v(<span class="hljs-string">"Request for cacheKey=%s is in flight, putting on hold."</span>, cacheKey);
}
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// Insert 'null' queue for this cacheKey, indicating there is now a request in</span>
<span class="hljs-comment">// flight.</span>
mWaitingRequests.put(cacheKey, <span class="hljs-keyword">null</span>);
<span class="hljs-comment">//这里添加请求到缓存队列中</span>
mCacheQueue.add(request);
}
<span class="hljs-keyword">return</span> request;
}
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li><li>33</li><li>34</li><li>35</li><li>36</li><li>37</li><li>38</li><li>39</li><li>40</li><li>41</li><li>42</li></ul>
代码不长，也很好理解，如果我们的请求没有设置了缓存，则请求添加到网络请求队列中，并直接返回了，不往下执行了，这时缓存队列中无法获取请求，所以这里我们知道了，想要用缓存则需要
在 Request 中把
<code class="hljs java has-numbering"> <span class="hljs-comment">//设置是否启用缓存</span>
<span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setShouldCache</span>(<span class="hljs-keyword">boolean</span> shouldCache) {
mShouldCache = shouldCache;
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>
设为 true，当然，我们看mShouldCache 的默认值
<code class="hljs java has-numbering"> <span class="hljs-javadoc">/**
* Whether or not responses to this request should be cached.
*/</span>
<span class="hljs-keyword">private</span> <span class="hljs-keyword">boolean</span> mShouldCache = <span class="hljs-keyword">true</span>;</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li></ul>
volley默认启用缓存的。再往下看
<code class="hljs cs has-numbering"> <span class="hljs-comment">// If the request has been canceled, don't bother dispatching it.如果已经取消请求，则结束本次请求的所有操作</span>
<span class="hljs-keyword">if</span> (request.isCanceled()) {
request.finish(<span class="hljs-string">"cache-discard-canceled"</span>);
<span class="hljs-keyword">continue</span>;
}
<span class="hljs-comment">// Attempt to retrieve this item from cache. 通过缓存类获取缓存</span>
Cache.Entry entry = mCache.<span class="hljs-keyword">get</span>(request.getCacheKey());
<span class="hljs-comment">//如果获取的缓存为空，这里有两种情况为空，一，第一次换取，没有情求过网络；二，缓存的数据达到了最大限度，此缓存已经被删除。</span>
<span class="hljs-keyword">if</span> (entry == <span class="hljs-keyword">null</span>) {
request.addMarker(<span class="hljs-string">"cache-miss"</span>);
Log.i(<span class="hljs-string">"CacheDispatcher"</span>, <span class="hljs-string">"没有缓存数据："</span> + request.getUrl());
mNetworkQueue.put(request);
<span class="hljs-keyword">continue</span>;
}
<span class="hljs-comment">// If it is completely expired, just send it to the network.缓存已经过期，则重新请求网络</span>
<span class="hljs-keyword">if</span> (entry.isExpired()) {
request.addMarker(<span class="hljs-string">"cache-hit-expired"</span>);
request.setCacheEntry(entry);
Log.i(<span class="hljs-string">"CacheDispatcher"</span>, <span class="hljs-string">"缓存数据过期："</span> + request.getUrl());
mNetworkQueue.put(request);
<span class="hljs-keyword">continue</span>;
}
<span class="hljs-comment">// We have a cache hit; parse its data for delivery back to the request.</span>
request.addMarker(<span class="hljs-string">"cache-hit"</span>);
<span class="hljs-comment">//到这里表示已经成功获取缓存数据</span>
Response<?> response = request.parseNetworkResponse(
<span class="hljs-keyword">new</span> NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker(<span class="hljs-string">"cache-hit-parsed"</span>);
<span class="hljs-comment">//如果缓存需要刷新，则请求网络</span>
<span class="hljs-keyword">if</span> (!entry.refreshNeeded()) {
<span class="hljs-comment">// Completely unexpired cache hit. Just deliver the response.</span>
Log.i(<span class="hljs-string">"CacheDispatcher"</span>, <span class="hljs-string">"获取缓存数据："</span> + request.getUrl());
mDelivery.postResponse(request, response);
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// Soft-expired cache hit. We can deliver the cached response,</span>
<span class="hljs-comment">// but we need to also send the request to the network for</span>
<span class="hljs-comment">// refreshing.</span>
request.addMarker(<span class="hljs-string">"cache-hit-refresh-needed"</span>);
request.setCacheEntry(entry);
<span class="hljs-comment">// Mark the response as intermediate.</span>
response.intermediate = <span class="hljs-keyword">true</span>;
<span class="hljs-comment">// Post the intermediate response back to the user and have</span>
<span class="hljs-comment">// the delivery then forward the request along to the network.</span>
mDelivery.postResponse(request, response, <span class="hljs-keyword">new</span> Runnable() {
@Override
<span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span>() {
<span class="hljs-keyword">try</span> {
mNetworkQueue.put(request);
} <span class="hljs-keyword">catch</span> (InterruptedException e) {
<span class="hljs-comment">// Not much we can do about this.</span>
}
}
});
}
} <span class="hljs-keyword">catch</span> (InterruptedException e) {
<span class="hljs-comment">// We may have been interrupted because it was time to quit.</span>
<span class="hljs-keyword">if</span> (mQuit) {
<span class="hljs-keyword">return</span>;
}
<span class="hljs-keyword">continue</span>;
}
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li><li>33</li><li>34</li><li>35</li><li>36</li><li>37</li><li>38</li><li>39</li><li>40</li><li>41</li><li>42</li><li>43</li><li>44</li><li>45</li><li>46</li><li>47</li><li>48</li><li>49</li><li>50</li><li>51</li><li>52</li><li>53</li><li>54</li><li>55</li><li>56</li><li>57</li><li>58</li><li>59</li><li>60</li><li>61</li><li>62</li><li>63</li><li>64</li><li>65</li><li>66</li><li>67</li><li>68</li></ul>
直接从缓存类Cache获取，并经过几次验证，如果缓存合法则解析然后交给 UI线程分发出去。下面来看看具体的流程
存储缓存
如果是第一次请求，或者缓存已过期，肯定是无法获取到缓存的，这时可根据上图分析，将会进入网络请求线程NetworkDispatcher，储存缓存毫无疑问也是在这里面实现的。
<code class="hljs avrasm has-numbering"> //省略部分代码
if (networkResponse<span class="hljs-preprocessor">.notModified</span> && request<span class="hljs-preprocessor">.hasHadResponseDelivered</span>()) {
request<span class="hljs-preprocessor">.finish</span>(<span class="hljs-string">"not-modified"</span>)<span class="hljs-comment">;</span>
continue<span class="hljs-comment">;</span>
}
// Parse the response here on the worker thread.
Response<?> response = request<span class="hljs-preprocessor">.parseNetworkResponse</span>(networkResponse)<span class="hljs-comment">;</span>
request<span class="hljs-preprocessor">.addMarker</span>(<span class="hljs-string">"network-parse-complete"</span>)<span class="hljs-comment">;</span>
// Write to cache if applicable.
// TODO: Only update cache metadata instead of entire record for <span class="hljs-number">304</span>s.如果需要缓存，并且用户已经把网络请求的数据转换成一份为缓存数据，则通过 Cache 类把缓存存储。
if (request<span class="hljs-preprocessor">.shouldCache</span>() && response<span class="hljs-preprocessor">.cacheEntry</span> != null) {
mCache<span class="hljs-preprocessor">.put</span>(request<span class="hljs-preprocessor">.getCacheKey</span>(), response<span class="hljs-preprocessor">.cacheEntry</span>)<span class="hljs-comment">;</span>
request<span class="hljs-preprocessor">.addMarker</span>(<span class="hljs-string">"network-cache-written"</span>)<span class="hljs-comment">;</span>
}
// Post the response back.
request<span class="hljs-preprocessor">.markDelivered</span>()<span class="hljs-comment">;</span>
mDelivery<span class="hljs-preprocessor">.postResponse</span>(request, response)<span class="hljs-comment">;</span>
} catch (VolleyError volleyError) {
parseAndDeliverNetworkError(request, volleyError)<span class="hljs-comment">;</span>
} catch (Exception e) {
VolleyLog<span class="hljs-preprocessor">.e</span>(e, <span class="hljs-string">"Unhandled exception %s"</span>, e<span class="hljs-preprocessor">.toString</span>())<span class="hljs-comment">;</span>
mDelivery<span class="hljs-preprocessor">.postError</span>(request, new VolleyError(e))<span class="hljs-comment">;</span>
}
//省略部分代码</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li></ul>
通过以上代码可以知道，在网络请求线程请求到数据以后，会交给用户解析，并把数据转换一份成缓存数据，通过 Cache 缓存操作类，把数据缓存起来。
网络数据转换成缓存数据
上面提到了，把网络数据转化成缓存数据，那么，volley 是如何转换的？
<code class="hljs cs has-numbering"> */
<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> Cache.Entry <span class="hljs-title">parseCacheHeaders</span>(NetworkResponse response) {
<span class="hljs-keyword">long</span> now = System.currentTimeMillis();
<span class="hljs-comment">//获取网络请求数据的头信息</span>
Map<String, String> headers = response.headers;
<span class="hljs-keyword">long</span> serverDate = <span class="hljs-number">0</span>;
<span class="hljs-keyword">long</span> serverExpires = <span class="hljs-number">0</span>;
<span class="hljs-keyword">long</span> softExpire = <span class="hljs-number">0</span>;
<span class="hljs-keyword">long</span> maxAge = <span class="hljs-number">0</span>;
boolean hasCacheControl = <span class="hljs-keyword">false</span>;
String serverEtag = <span class="hljs-keyword">null</span>;
String headerValue;
<span class="hljs-comment">//从头信息中获取 Date 数据</span>
headerValue = headers.<span class="hljs-keyword">get</span>(<span class="hljs-string">"Date"</span>);
<span class="hljs-keyword">if</span> (headerValue != <span class="hljs-keyword">null</span>) {
serverDate = parseDateAsEpoch(headerValue);
}
<span class="hljs-comment">//从头信息中获取 Cache-Control 数据，来控制缓存</span>
headerValue = headers.<span class="hljs-keyword">get</span>(<span class="hljs-string">"Cache-Control"</span>);
<span class="hljs-keyword">if</span> (headerValue != <span class="hljs-keyword">null</span>) {
hasCacheControl = <span class="hljs-keyword">true</span>;
String[] tokens = headerValue.split(<span class="hljs-string">","</span>);
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i < tokens.length; i++) {
String token = tokens[i].trim();
<span class="hljs-keyword">if</span> (token.equals(<span class="hljs-string">"no-cache"</span>) || token.equals(<span class="hljs-string">"no-store"</span>)) {
<span class="hljs-keyword">return</span> <span class="hljs-keyword">null</span>;
} <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (token.startsWith(<span class="hljs-string">"max-age="</span>)) {
<span class="hljs-keyword">try</span> {
maxAge = Long.parseLong(token.substring(<span class="hljs-number">8</span>));
} <span class="hljs-keyword">catch</span> (Exception e) {
}
} <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (token.equals(<span class="hljs-string">"must-revalidate"</span>) || token.equals(<span class="hljs-string">"proxy-revalidate"</span>)) {
maxAge = <span class="hljs-number">0</span>;
}
}
}
headerValue = headers.<span class="hljs-keyword">get</span>(<span class="hljs-string">"Expires"</span>);
<span class="hljs-keyword">if</span> (headerValue != <span class="hljs-keyword">null</span>) {
serverExpires = parseDateAsEpoch(headerValue);
}
serverEtag = headers.<span class="hljs-keyword">get</span>(<span class="hljs-string">"ETag"</span>);
<span class="hljs-comment">// Cache-Control takes precedence over an Expires header, even if both exist and Expires</span>
<span class="hljs-comment">// is more restrictive.</span>
<span class="hljs-keyword">if</span> (hasCacheControl) {
softExpire = now + maxAge * <span class="hljs-number">1000</span>;
} <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> (serverDate > <span class="hljs-number">0</span> && serverExpires >= serverDate) {
<span class="hljs-comment">// Default semantic for Expire header in HTTP specification is softExpire.</span>
softExpire = now + (serverExpires - serverDate);
}
Cache.Entry entry = <span class="hljs-keyword">new</span> Cache.Entry();
entry.data = response.data;
entry.etag = serverEtag;
entry.softTtl = softExpire;
entry.ttl = entry.softTtl;
entry.serverDate = serverDate;
entry.responseHeaders = headers;
<span class="hljs-keyword">return</span> entry;
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li><li>33</li><li>34</li><li>35</li><li>36</li><li>37</li><li>38</li><li>39</li><li>40</li><li>41</li><li>42</li><li>43</li><li>44</li><li>45</li><li>46</li><li>47</li><li>48</li><li>49</li><li>50</li><li>51</li><li>52</li><li>53</li><li>54</li><li>55</li><li>56</li><li>57</li><li>58</li><li>59</li><li>60</li><li>61</li><li>62</li><li>63</li><li>64</li><li>65</li></ul>
前面我们提到的缓存是否过期和是否需要刷新，都是通过 response 的头部信息来判断，但是在BasicNetwork中只实现了缓存是否过期的操作，没有实现缓存是否需要刷新
<code class="hljs java has-numbering"> <span class="hljs-annotation">@Override</span>
<span class="hljs-keyword">public</span> NetworkResponse <span class="hljs-title">performRequest</span>(Request<?> request) <span class="hljs-keyword">throws</span> VolleyError {
<span class="hljs-keyword">long</span> requestStart = SystemClock.elapsedRealtime();
<span class="hljs-keyword">while</span> (<span class="hljs-keyword">true</span>) {
HttpResponse httpResponse = <span class="hljs-keyword">null</span>;
<span class="hljs-keyword">byte</span>[] responseContents = <span class="hljs-keyword">null</span>;
Map<String, String> responseHeaders = <span class="hljs-keyword">new</span> HashMap<String, String>();
<span class="hljs-keyword">try</span> {
<span class="hljs-comment">// Gather headers.</span>
Map<String, String> headers = <span class="hljs-keyword">new</span> HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
httpResponse = mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
<span class="hljs-keyword">int</span> statusCode = statusLine.getStatusCode();
responseHeaders = convertHeaders(httpResponse.getAllHeaders());
<span class="hljs-comment">//从这里开始设置缓存信息，如果设置了缓存的缓存时间，则把它添加到头部信息中，但是没有实现是否需要刷新缓存的操作，如果有需要，也可以在这里实现，这是就需要修改源码。</span>
<span class="hljs-keyword">if</span>(request.getCacheTime() != <span class="hljs-number">0</span>){
responseHeaders.put(<span class="hljs-string">"Cache-Control"</span>,<span class="hljs-string">"max-age="</span> + request.getCacheTime());
}
<span class="hljs-comment">// Handle cache validation.</span>
<span class="hljs-keyword">if</span> (statusCode == HttpStatus.SC_NOT_MODIFIED) {
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
request.getCacheEntry().data, responseHeaders, <span class="hljs-keyword">true</span>);
}
responseContents = entityToBytes(httpResponse.getEntity());
<span class="hljs-comment">// if the request is slow, log it.</span>
<span class="hljs-keyword">long</span> requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);
<span class="hljs-keyword">if</span> (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {
<span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> IOException();
}
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> NetworkResponse(statusCode, responseContents, responseHeaders, <span class="hljs-keyword">false</span>);
} <span class="hljs-keyword">catch</span> (SocketTimeoutException e) {
attemptRetryOnException(<span class="hljs-string">"socket"</span>, request, <span class="hljs-keyword">new</span> TimeoutError());
} <span class="hljs-keyword">catch</span> (ConnectTimeoutException e) {
attemptRetryOnException(<span class="hljs-string">"connection"</span>, request, <span class="hljs-keyword">new</span> TimeoutError());
} <span class="hljs-keyword">catch</span> (MalformedURLException e) {
<span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException(<span class="hljs-string">"Bad URL "</span> + request.getUrl(), e);
} <span class="hljs-keyword">catch</span> (IOException e) {
<span class="hljs-keyword">int</span> statusCode = <span class="hljs-number">0</span>;
NetworkResponse networkResponse = <span class="hljs-keyword">null</span>;
<span class="hljs-keyword">if</span> (httpResponse != <span class="hljs-keyword">null</span>) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} <span class="hljs-keyword">else</span> {
<span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> NoConnectionError(e);
}
VolleyLog.e(<span class="hljs-string">"Unexpected response code %d for %s"</span>, statusCode, request.getUrl());
<span class="hljs-keyword">if</span> (responseContents != <span class="hljs-keyword">null</span>) {
networkResponse = <span class="hljs-keyword">new</span> NetworkResponse(statusCode, responseContents,
responseHeaders, <span class="hljs-keyword">false</span>);
<span class="hljs-keyword">if</span> (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException(<span class="hljs-string">"auth"</span>,
request, <span class="hljs-keyword">new</span> AuthFailureError(networkResponse));
} <span class="hljs-keyword">else</span> {
<span class="hljs-comment">// TODO: Only throw ServerError for 5xx status codes.</span>
<span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> ServerError(networkResponse);
}
} <span class="hljs-keyword">else</span> {
<span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> NetworkError(networkResponse);
}
}
}
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li><li>33</li><li>34</li><li>35</li><li>36</li><li>37</li><li>38</li><li>39</li><li>40</li><li>41</li><li>42</li><li>43</li><li>44</li><li>45</li><li>46</li><li>47</li><li>48</li><li>49</li><li>50</li><li>51</li><li>52</li><li>53</li><li>54</li><li>55</li><li>56</li><li>57</li><li>58</li><li>59</li><li>60</li><li>61</li><li>62</li><li>63</li><li>64</li><li>65</li><li>66</li><li>67</li></ul>
如何使用缓存
根据上面分析不难发现，要使用缓存，得具备两个条件，
1、启用缓存
<code class="hljs java has-numbering"><span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setShouldCache</span>(<span class="hljs-keyword">boolean</span> shouldCache) {
mShouldCache = shouldCache;
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li></ul>
不过这个条件默认情况下是开启的。
2、设置缓存的时间
<code class="hljs cs has-numbering"> <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setCacheTime</span>(<span class="hljs-keyword">long</span> cacheTime) {
mCacheTime = cacheTime;
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li></ul>
这里 cacheTime 的单位是秒。
接下来看看具体用法
<code class="hljs java has-numbering"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">CacheRequestActivity</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">ActionBarActivity</span> {</span>
<span class="hljs-comment">/*数据显示的View*/</span>
<span class="hljs-keyword">private</span> TextView mIdTxt,mNameTxt,mDownloadTxt,mLogoTxt,mVersionTxt ;
<span class="hljs-comment">/*弹出等待对话框*/</span>
<span class="hljs-keyword">private</span> ProgressDialog mDialog ;
<span class="hljs-annotation">@Override</span>
<span class="hljs-keyword">protected</span> <span class="hljs-keyword">void</span> <span class="hljs-title">onCreate</span>(Bundle savedInstanceState) {
<span class="hljs-keyword">super</span>.onCreate(savedInstanceState);
setContentView(R.layout.activity_get);
mIdTxt = (TextView) findViewById(R.id.id_id) ;
mNameTxt = (TextView) findViewById(R.id.id_name) ;
mDownloadTxt = (TextView) findViewById(R.id.id_download) ;
mLogoTxt = (TextView) findViewById(R.id.id_logo) ;
mVersionTxt = (TextView) findViewById(R.id.id_version) ;
mDialog = <span class="hljs-keyword">new</span> ProgressDialog(<span class="hljs-keyword">this</span>) ;
mDialog.setMessage(<span class="hljs-string">"get请求中..."</span>);
mDialog.show();
<span class="hljs-comment">/*请求网络获取数据*/</span>
MiNongApi.CacheObjectMiNongApi(<span class="hljs-string">"minongbang"</span>, <span class="hljs-keyword">new</span> ResponseListener<TestBean>() {
<span class="hljs-annotation">@Override</span>
<span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">onErrorResponse</span>(VolleyError error) {
mDialog.dismiss();
}
<span class="hljs-annotation">@Override</span>
<span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">onResponse</span>(TestBean response) {
Log.v(<span class="hljs-string">"zgy"</span>,<span class="hljs-string">"=======response======="</span>+response) ;
mDialog.dismiss();
<span class="hljs-comment">/*显示数据*/</span>
mIdTxt.setText(response.getId() + <span class="hljs-string">""</span>);
mNameTxt.setText(response.getName());
mDownloadTxt.setText(response.getDownload() + <span class="hljs-string">""</span>);
mLogoTxt.setText(response.getLogo());
mVersionTxt.setText(response.getVersion() + <span class="hljs-string">""</span>);
}
});
}
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li><li>16</li><li>17</li><li>18</li><li>19</li><li>20</li><li>21</li><li>22</li><li>23</li><li>24</li><li>25</li><li>26</li><li>27</li><li>28</li><li>29</li><li>30</li><li>31</li><li>32</li><li>33</li><li>34</li><li>35</li><li>36</li><li>37</li><li>38</li><li>39</li><li>40</li></ul>
cache api
<code class="hljs cs has-numbering"> <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">CacheObjectMiNongApi</span>(String <span class="hljs-keyword">value</span>,ResponseListener listener){
String url ;
<span class="hljs-keyword">try</span> {
url = Constant.MinongHost +<span class="hljs-string">"?test="</span>+ URLEncoder.encode(<span class="hljs-keyword">value</span>, <span class="hljs-string">"utf-8"</span>) ;
} <span class="hljs-keyword">catch</span> (UnsupportedEncodingException e) {
e.printStackTrace();
url = Constant.MinongHost +<span class="hljs-string">"?test="</span>+ URLEncoder.encode(<span class="hljs-keyword">value</span>) ;
}
Request request = <span class="hljs-keyword">new</span> GetObjectRequest(url,<span class="hljs-keyword">new</span> TypeToken<TestBean>(){}.getType(),listener) ;
<span class="hljs-comment">//请用缓存</span>
request.setShouldCache(<span class="hljs-keyword">true</span>);
<span class="hljs-comment">//设置缓存时间10分钟</span>
request.setCacheTime(<span class="hljs-number">10</span>*<span class="hljs-number">60</span>);
VolleyUtil.getRequestQueue().add(request) ;
}</code><ul style="display: block;" class="pre-numbering"><li>1</li><li>2</li><li>3</li><li>4</li><li>5</li><li>6</li><li>7</li><li>8</li><li>9</li><li>10</li><li>11</li><li>12</li><li>13</li><li>14</li><li>15</li></ul>
再来看看效果图，在缓存存在的情况下当把网络连接断开的时候，依然能够获取到数据
有一种情况需要注意：当需要获取缓存，且希望缓存刷新的时候，这种情况就需要修改 Volley 的源码，前面已经提到一点点，相信大家都能实现的。
转载自：http://blog.youkuaiyun.com/jxxfzgy/article/details/44088299
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