[Android]Volley源码分析(二)Cache

     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。