[Android]Volley源代码分析(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的源代码思路往下,来看下Volley对Cache的处理逻辑。

     我们回忆一下昨天的简单代码,我们的入口是从构造一个Request队列開始的,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;
    }

      參数HttpStack用于制定你的HttpStack实现机制,比方是採用apache的http-client还是HttpUrlConnection.当然假设你不指定,Volley也会依据你的sdk版本号给出不同的策略。

而这样的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,我们就能够生成request队列RequestQueue.可是,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採用生产者和消费者的模式来产生反应堆,而这中反应必需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;
    }

     如何？是不是认为Volley的代码写的很的浅显合理。好了,到RequestQueue.start開始,我们已经为我们的request构建好了它的上下文环境,我们接着仅仅须要将它add到这个队列中来就能够了；

 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,避免了反复的请求。我们add一个Request的时候,须要设置上这个RequestQueue。目的是为了结束的时候将自己从Queue中回收。我们这里还能够看到一个简单的状态机:

request.addMarker("add-to-queue");

    这种方法将在request不同的上下文中调用。方便以后查错。之后Request会检查是否须要进行Cache

        if (!request.shouldCache()) {
            mNetworkQueue.add(request);
            return request;
        }

    我们的观念里面,似乎文本数据是不须要Cache的,你能够通过这种方法来实现是否要cache住你的东西,当然不限制你的数据类型。

之后,假设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,并且在这个队列里面是能够进行优先级比較。ImageRequest对Request的优先级进行了指定:

com.android.volley.toolbox.ImageRequest：

    @Override
    public Priority getPriority() {
        return Priority.LOW;
    }

你能够自己指定你的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我们来看下它怎样实现的初始化：

 @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) { }
            }
        }
    }

我们能够看出,Volley差别于其它Cache的还有一个特点,就是存储元数据,或者说自己定义了数据格式。

在文件的头部添加了Volley的文件头。这样的做法不仅能从某方面保证了数据的安全性,也能非常好的存储数据元。

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自定义了自己的数据魔数,也依照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()) {//推断是否失效
                    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,当然因为queue是堵塞的,因此不会造成费电问题。

       Cache.Entry entry = mCache.get(request.getCacheKey());获得数据的时候假设数据存在,则会将真实数据读取出来。这就是Volley的LazyLoad。

       

if (entry.isExpired()) {//推断是否失效
                    request.addMarker("cache-hit-expired");
                    request.setCacheEntry(entry);
                    mNetworkQueue.put(request);
                    continue;
                }

      这段代码从时效性来推断是否进行淘汰。我们回想下刚才所示代码,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。

同一时候,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,不论你採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模式存在的意义之中的一个。

                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,到如今,对于Cache来说就差分发了,数据已经全然准备就绪了。我们上一讲说道Request终于会抛给Delivery对象用来异步分发,这样能有效避免分发造成的线程堵塞。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假设是UI线程的Handler。那么你的线程就是在UI线程中执行，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。

这样的命令跑在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,看官们能够自己阅读。