Android volley 解析（四）之缓存篇

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

@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
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();
while (true) {
...

2、获取缓存

while (true) {
try {
// Get a request from the cache triage queue, blocking until
// at least one is available.从缓存队列中获取缓存
final Request request = mCacheQueue.take();
request.addMarker("cache-queue-take");

// If the request has been canceled, don‘t bother dispatching it.
...

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

public Request add(Request request) {
// Tag the request as belonging to this queue and add it to the set of current requests.
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}

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

// If the request is uncacheable, skip the cache queue and go straight to the network.如果请求没有设置缓存，则把请求添加到网络队列中
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}

// Insert request into stage if there‘s already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
Queue<Request> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request>();
}

stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
}
} else {
// Insert ‘null‘ queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null);
//这里添加请求到缓存队列中
mCacheQueue.add(request);
}
return request;
}
}

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

//设置是否启用缓存
public final void setShouldCache(boolean shouldCache) {
mShouldCache = shouldCache;
}

设为 true，当然，我们看mShouldCache 的默认值

/**
* Whether or not responses to this request should be cached.
*/
private boolean mShouldCache = true;

volley默认启用缓存的。再往下看

// 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
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}

// Attempt to retrieve this item from cache. 通过缓存类获取缓存
Cache.Entry entry = mCache.get(request.getCacheKey());
//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。
if (entry == null) {
request.addMarker("cache-miss");
Log.i("CacheDispatcher", "没有缓存数据：" + request.getUrl());
mNetworkQueue.put(request);
continue;
}

// If it is completely expired, just send it to the network.缓存已经过期，则重新请求网络
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
Log.i("CacheDispatcher", "缓存数据过期：" + request.getUrl());
mNetworkQueue.put(request);
continue;
}

// We have a cache hit; parse its data for delivery back to the request.
request.addMarker("cache-hit");
//到这里表示已经成功获取缓存数据
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
//如果缓存需要刷新，则请求网络
if (!entry.refreshNeeded()) {
// Completely unexpired cache hit. Just deliver the response.
Log.i("CacheDispatcher", "获取缓存数据：" + request.getUrl());
mDelivery.postResponse(request, response);
} else {
// Soft-expired cache hit. We can deliver the cached response,
// but we need to also send the request to the network for
// refreshing.
request.addMarker("cache-hit-refresh-needed");
request.setCacheEntry(entry);

// Mark the response as intermediate.
response.intermediate = true;

// Post the intermediate response back to the user and have
// the delivery then forward the request along to the network.
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}

} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
}

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。

//省略部分代码
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}

// Parse the response here on the worker thread.
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");

// Write to cache if applicable.
// TODO: Only update cache metadata instead of entire record for 304s.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.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}

// Post the response back.
request.markDelivered();
mDelivery.postResponse(request, response);
} catch (VolleyError volleyError) {
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
mDelivery.postError(request, new VolleyError(e));
}
//省略部分代码

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？

*/
public static Cache.Entry parseCacheHeaders(NetworkResponse response) {
long now = System.currentTimeMillis();
//获取网络请求数据的头信息
Map<String, String> headers = response.headers;

long serverDate = 0;
long serverExpires = 0;
long softExpire = 0;
long maxAge = 0;
boolean hasCacheControl = false;

String serverEtag = null;
String headerValue;
//从头信息中获取 Date 数据
headerValue = headers.get("Date");
if (headerValue != null) {
serverDate = parseDateAsEpoch(headerValue);
}
//从头信息中获取 Cache-Control 数据，来控制缓存
headerValue = headers.get("Cache-Control");
if (headerValue != null) {
hasCacheControl = true;
String[] tokens = headerValue.split(",");
for (int i = 0; i < tokens.length; i++) {
String token = tokens[i].trim();
if (token.equals("no-cache") || token.equals("no-store")) {
return null;
} else if (token.startsWith("max-age=")) {
try {
maxAge = Long.parseLong(token.substring(8));
} catch (Exception e) {
}
} else if (token.equals("must-revalidate") || token.equals("proxy-revalidate")) {
maxAge = 0;
}
}
}

headerValue = headers.get("Expires");
if (headerValue != null) {
serverExpires = parseDateAsEpoch(headerValue);
}

serverEtag = headers.get("ETag");

// Cache-Control takes precedence over an Expires header, even if both exist and Expires
// is more restrictive.
if (hasCacheControl) {
softExpire = now + maxAge * 1000;
} else if (serverDate > 0 && serverExpires >= serverDate) {
// Default semantic for Expire header in HTTP specification is softExpire.
softExpire = now + (serverExpires - serverDate);
}

Cache.Entry entry = new Cache.Entry();
entry.data = response.data;
entry.etag = serverEtag;
entry.softTtl = softExpire;
entry.ttl = entry.softTtl;
entry.serverDate = serverDate;
entry.responseHeaders = headers;

return entry;
}

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

@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = new HashMap<String, String>();
try {
// Gather headers.
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
httpResponse = mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();

responseHeaders = convertHeaders(httpResponse.getAllHeaders());
//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。
if(request.getCacheTime() != 0){
responseHeaders.put("Cache-Control","max-age=" + request.getCacheTime());
}
// Handle cache validation.
if (statusCode == HttpStatus.SC_NOT_MODIFIED) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
request.getCacheEntry().data, responseHeaders, true);
}

responseContents = entityToBytes(httpResponse.getEntity());
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);

if (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {
throw new IOException();
}
return new NetworkResponse(statusCode, responseContents, responseHeaders, false);
} catch (SocketTimeoutException e) {
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException e) {
attemptRetryOnException("connection", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode = 0;
NetworkResponse networkResponse = null;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnectionError(e);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
if (responseContents != null) {
networkResponse = new NetworkResponse(statusCode, responseContents,
responseHeaders, false);
if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException("auth",
request, new AuthFailureError(networkResponse));
} else {
// TODO: Only throw ServerError for 5xx status codes.
throw new ServerError(networkResponse);
}
} else {
throw new NetworkError(networkResponse);
}
}
}
}

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

public final void setShouldCache(boolean shouldCache) {
mShouldCache = shouldCache;
}

不过这个条件默认情况下是开启的。

2、设置缓存的时间

public void setCacheTime(long cacheTime) {
mCacheTime = cacheTime;
}

这里 cacheTime 的单位是秒。

接下来看看具体用法

public class CacheRequestActivity extends ActionBarActivity {

/*数据显示的View*/
private TextView mIdTxt,mNameTxt,mDownloadTxt,mLogoTxt,mVersionTxt ;
/*弹出等待对话框*/
private ProgressDialog mDialog ;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.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 = new ProgressDialog(this) ;
mDialog.setMessage("get请求中...");
mDialog.show();
/*请求网络获取数据*/
MiNongApi.CacheObjectMiNongApi("minongbang", new ResponseListener<TestBean>() {
@Override
public void onErrorResponse(VolleyError error) {
mDialog.dismiss();
}

@Override
public void onResponse(TestBean response) {
Log.v("zgy","=======response======="+response) ;
mDialog.dismiss();
/*显示数据*/
mIdTxt.setText(response.getId() + "");
mNameTxt.setText(response.getName());
mDownloadTxt.setText(response.getDownload() + "");
mLogoTxt.setText(response.getLogo());
mVersionTxt.setText(response.getVersion() + "");
}
});
}

}

cache api

public static void CacheObjectMiNongApi(String value,ResponseListener listener){
String url ;
try {
url = Constant.MinongHost +"?test="+ URLEncoder.encode(value, "utf-8") ;
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
url = Constant.MinongHost +"?test="+ URLEncoder.encode(value) ;
}
Request request = new GetObjectRequest(url,new TypeToken<TestBean>(){}.getType(),listener) ;
//请用缓存
request.setShouldCache(true);
//设置缓存时间10分钟
request.setCacheTime(10*60);
VolleyUtil.getRequestQueue().add(request) ;
}

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。

源码下载地址