Cache作为Volley最为核心的一部分,Volley花了重彩来实现它。本章我们顺着Volley的源码思路往下,来看下Volley对Cache的处理逻辑。
我们回想一下昨天的简单代码,我们的入口是从构造一个Request队列开始的,而我们并不直接调用new来构造,而是将控制权反转给Volley这个静态工厂来构造。
com.android.volley.toolbox.Volley：
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
String userAgent = "volley/0";
try {
String packageName = context.getPackageName();
PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException e) {
}
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
stack = new HurlStack();
} else {
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
Network network = new BasicNetwork(stack);
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
return queue;
}参数HttpStack用于制定你的HttpStack实现机制,比如是采用apache的http-client还是HttpUrlConnection.当然如果你不指定,Volley也会根据你的sdk版本给出不同的策略。而这种HttpStack对象被Network对象包装起来。上一节我们说过,为了构造平台统一的网络调用,Volley通过桥接的方式来实现网络调用,而桥接的接口就是这个Network.
Volley的核心在于Cache和Network。既然两个对象已经构造完了,我们就可以生成request队列RequestQueue.但是,为什么要开启queue.start呢？我们先看一下这个代码:
public void start() {
stop(); // Make sure any currently running dispatchers are stopped.
// Create the cache dispatcher and start it.
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
// Create network dispatchers (and corresponding threads) up to the pool size.
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}上一节体系结构我们已经说了,Volley采用生产者和消费者的模式来产生反应堆,而这中反应必须要通过线程的方式来实现。调用了RequestQueue的start之后,将开启一个Cache线程和一定数量的Network线程池。我们看到networkDispatcher的线程池数量由数组mDispatchers指定。而mDispatchers的赋值在RequestQueue的<init>中：
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}怎样？是不是觉得Volley的代码写的非常的浅显合理。好了,到RequestQueue.start开始,我们已经为我们的request构建好了它的上下文环境,我们接着只需要将它add到这个队列中来就可以了；
public <T> Request<T> add(Request<T> request) {
// Tag the request as belonging to this queue and add it to the set of current requests.
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}
// Process requests in the order they are added.
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
// Insert request into stage if there's already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
System.out.println("request.cacheKey = "+(cacheKey));
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
<span style="color:#33cc00;"> Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request<?>>();
}
stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);</span>
} else {
// Insert 'null' queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}这段代码绿色的部分是点睛之笔,有了暂存的概念,避免了重复的请求。我们add一个Request的时候,需要设置上这个RequestQueue。目的是为了结束的时候将自己从Queue中回收。我们这里还可以看到一个简单的状态机:
request.addMarker(“add-to-queue”);
这个方法将在request不同的上下文中调用。方便以后查错。之后Request会检查是否需要进行Cache
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}我们的观念里面,似乎文本数据是不需要Cache的,你可以通过这个方法来实现是否要cache住你的东西,当然不限制你的数据类型。之后,如果你的请求不被暂存的话,那就被投入Cache反应堆。我们来看下mCacheQueue这个对象:
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();我们看到mCacheQueue本质是一个PriorityBlockingQueue的线程安全队列,而且在这个队列里面是可以进行优先级比较。ImageRequest对Request的优先级进行了指定:
com.android.volley.toolbox.ImageRequest：
@Override
public Priority getPriority() {
return Priority.LOW;
}
你可以自己指定你的Request的优先级别.我们回到CacheDispatcher消费者,CacheDispatcher继承Thread。生成之后直接对Cache初始化mCache.initialize();初始化的目的在于获得Cache中已经存在的数据。Cache的实现类是DiskBasedCache.java我们来看下它如何实现的初始化：
@Override
public synchronized void initialize() {
if (!mRootDirectory.exists()) {
if (!mRootDirectory.mkdirs()) {
VolleyLog.e("Unable to create cache dir %s", mRootDirectory.getAbsolutePath());
}
return;
}
File[] files = mRootDirectory.listFiles();
if (files == null) {
return;
}
for (File file : files) {
FileInputStream fis = null;
try {
fis = new FileInputStream(file);
CacheHeader entry = CacheHeader.readHeader(fis);
entry.size = file.length();
putEntry(entry.key, entry);
} catch (IOException e) {
if (file != null) {
file.delete();
}
} finally {
try {
if (fis != null) {
fis.close();
}
} catch (IOException ignored) { }
}
}
}我们可以看出,Volley区别于其他Cache的另一个特点,就是存储元数据,或者说自定义了数据格式。在文件的头部增加了Volley的文件头。这种做法不仅能从某方面保证了数据的安全性,也能很好的存储数据元。
public static CacheHeader readHeader(InputStream is) throws IOException {
CacheHeader entry = new CacheHeader();
int magic = readInt(is);
if (magic != CACHE_MAGIC) {
// don't bother deleting, it'll get pruned eventually
throw new IOException();
}
entry.key = readString(is);
entry.etag = readString(is);
if (entry.etag.equals("")) {
entry.etag = null;
}
entry.serverDate = readLong(is);
entry.ttl = readLong(is);
entry.softTtl = readLong(is);
entry.responseHeaders = readStringStringMap(is);
return entry;
}我们从这段代码里面看到不少信息,Volley自己定义了自己的数据魔数,也按照Volley自己的规范来读取元数据。
好的,我们初始化了Cache接下来就是CacheDispatcher的核心了。
while (true) {
try {
// Get a request from the cache triage queue, blocking until
// at least one is available.
final Request<?> request = mCacheQueue.take();
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request);
continue;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) {//判断是否失效
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
}
// We have a cache hit; parse its data for delivery back to the request.
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
if (!entry.refreshNeeded()) {
// Completely unexpired cache hit. Just deliver the response.
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;
}
}线程通过while true的方式进行轮询,当然由于queue是阻塞的,因此不会造成费电问题。
Cache.Entry entry = mCache.get(request.getCacheKey());获得数据的时候如果数据存在,则会将真实数据读取出来。这就是Volley的LazyLoad。
if (entry.isExpired()) {//判断是否失效
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
}这段代码从时效性来判断是否进行淘汰。我们回顾下刚才所看到的代码,request在不同的上下文中总被标记为不同的状态,这对后期维护有及其重要的意义。同时,为了保证接口的统一性,CacheDispatcher将自己的结果伪装成为NetResponse。这样对外部接口来说,不论你采用的是那种方式获得数据,对我来说都当作网络来获取,这本身也是DAO模式存在的意义之一。
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(
<strong><span style="color:#006600;">new NetworkResponse</span></strong>(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed"); request.parseNetworkResponse的目的是为了让你的request转成自己的数据对象。好了,到现在,对于Cache来说就差分发了,数据已经完全准备就绪了。我们上一讲说道Request最终会抛给Delivery对象用来异步分发,这样能有效避免分发造成的线程阻塞。我刚才说了,Cache会伪装成为Netresponse来post数据,那也就是说对于Network的处理,这些部分也是一模一样的。因此,后一篇关于NetworkDispatcher的管理我将省略掉这些。Volley中
Delivery的实现类是:
com.android.volley.ExecutorDelivery.java
public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
handler.post(command);
}
};
}我们看到在它的<init>中传入了一个Handler,这个Handler如果是UI线程的Handler，那么你的线程就是在UI线程中运行，避免了你自己post UI线程消息的问题。post出来数据将被封装成为ResponseDeliveryRunnable 命令。这种命令跑在Handler所在的线程中.到此CacheDispatcher的基本流程就结束了,
ResponseDeliveryRunnable中除了分发以外也会进行一些收尾的工作,看官们可以自己阅读。
