Volley源码(一)--不考虑缓存

1.普及知识

PriorityBlockingQueue类是JDK提供的优先级队列 本身是线程安全的，通过take()方法取队列里的值,这个方法有个特点，如果没有取到值就一直阻塞在那里。Volley里就是在无限循环中从此队列取值,没有取到就一直阻塞,当请求比如StringRequest加到队列中时,循环才走的去

下面以StringRequest为例分析源码

2.先看看怎么使用

RequestQueue mQueue = Volley.newRequestQueue(context);StringRequest stringRequest = new StringRequest(Method.POST,"http://www.baidu.com",                          new Response.Listener<String>() {                              @Override                              public void onResponse(String response) {                                                             }                          }, new Response.ErrorListener() {                              @Override                              public void onErrorResponse(VolleyError error) {                                                              }                          });  mQueue.add(stringRequest);

3.再看看都做了些什么

3.1实例化RequestQueue

RequestQueue mQueue = Volley.newRequestQueue(context);

最终调用方法

 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(new BasicUrlRewriter(), userAgent);            } 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), new BasicUrlRewriter());            }        }        Network network = new BasicNetwork(stack);        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);        queue.start();        return queue;    }

其他的先不管，先看这两个

 RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network); queue.start();

在RequestQueue的构造方法中

public RequestQueue(Cache cache, Network network, int threadPoolSize,            ResponseDelivery delivery) {        mCache = cache;        mNetwork = network;        mDispatchers = new NetworkDispatcher[threadPoolSize];        mDelivery = delivery;    }

这里主要关注

mDispatchers = new NetworkDispatcher[threadPoolSize];mDelivery = delivery;

new了有4个NetworkDispatcher的数组，mDelivery是等到请求成功后将请求的结果通过listerner回调的。

然后queue.start();

3.2调用RequestQueue的start方法

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();        }    }

这里面开了一个CacheDispatcher线程和4个NetworkDispatcher线程。由于上面说了，我们先只考虑post请求，post请求默认是不会被缓存的，所以缓存线程CacheDispatcher我们先不管它，我们看NetworkDispatcher线程的run方法

public void run() {        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);        Request request;        while (true) {            try {                // Take a request from the queue.                request = mQueue.take();            } catch (InterruptedException e) {                // We may have been interrupted because it was time to quit.                if (mQuit) {                    return;                }                continue;            }......}

线程开启后,进入while(true)循环，由于一开始还没有请求，会一直阻塞在request = mQueue.take();好了，这块先只看到这里。

3.3将request加入到queue后做了什么

mQueue.add(stringRequest);

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

主要将request(这里就是我们实例化的StringRequest)加入到mCurrentRequests和mNetworkQueue(说好的,先不看缓存,那么shouldCache()返回false,下面直接return了request)。

而实例化NetworkDispatcher的时候就看到mNetworkQueue作为参数传进去了,NetworkDispatcher的run方法中request = mQueue.take();中的mQueue就是mNetworkQueue。所以等于我们把请求加到mNetworkQueue中,4个NetworkDispatcher线程在while(true)循环中不断的取数据。同样是NetworkDispatcher线程的run方法，看看取到数据后干了什么

3.4解析数据

 NetworkResponse networkResponse = mNetwork.performRequest(request);                request.addMarker("network-http-complete");                // If the server returned 304 AND we delivered a response already,                // we're done -- don't deliver a second identical response.                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.                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);

先调用了mNetwork的performRequest方法,这里先不细细研究。mNetwork是实例化RequestQueue时传入的BasiNetwork,它的performRequest方法就是将请求处理后封装为NetworkResponce返回回来。

再调用了request(StringRequest)的parseNetworkResponse方法，

protected Response<String> parseNetworkResponse(NetworkResponse response) {        String parsed;        try {            parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));        } catch (UnsupportedEncodingException e) {            parsed = new String(response.data);        }        return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response)); }

此方法中，将NetworkResponse转化为Response<String>(因为是StringRequest)。

3.5将解析好的数据在主线程中回调给listener

最后调用mDelivery的postResponse方法。从RequestQueue的构造方法中可以找到,传入的mDelivery就是ExecutorDelivery。那么就看一下ExecutorDelivery的postResponse方法(注意:,传入ExecutorDelivery时,ExecutorDelivery的构造方法中的Handle是和主线程的Looper相关联的

new ExecutorDelivery(new Handler(Looper.getMainLooper()))

)

public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {        request.markDelivered();        request.addMarker("post-response");        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));}

主要执行了mResponsePoster的execute方法,来看看mResponsePoster是什么

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);            }        };    }

最终execute方法里,执行了handle的post方法,不用说ResponseDeliveryRunnable肯定就是Runnable的继承类咯，而且会在主线程中调用它的run方法。来看一下ResponseDeliveryRunnable的run方法

public void run() {            // If this request has canceled, finish it and don't deliver.            if (mRequest.isCanceled()) {                mRequest.finish("canceled-at-delivery");                return;            }            // Deliver a normal response or error, depending.            if (mResponse.isSuccess()) {                mRequest.deliverResponse(mResponse.result);            } else {                mRequest.deliverError(mResponse.error);            }            // If this is an intermediate response, add a marker, otherwise we're done            // and the request can be finished.            if (mResponse.intermediate) {                mRequest.addMarker("intermediate-response");            } else {                mRequest.finish("done");            }            // If we have been provided a post-delivery runnable, run it.            if (mRunnable != null) {                mRunnable.run();            }       }

请求成功的话调用mRequest.deliverResponse(mResponse.result);否则调用mRequest.deliverError(mResponse.error);

请求成功,StringRequest里的deliverResponse方法

 protected void deliverResponse(String response) {        mListener.onResponse(response); }

可见将返回的String交给listener回调了。

请求失败,Request的deliverError方法(StringRequest没有重写deliverError方法，直接看父类)

 public void deliverError(VolleyError error) {        if (mErrorListener != null) {            mErrorListener.onErrorResponse(error);        } }

为什么会这样？

因为请求成功时，StringRequest返回的是String,JsonRequest返回的是Json,所以请求成功的方法子类需要重写,而请求失败的话返回的参数就是VolleyError,所有子类都一样，不需要重写。

写到这里,Volley的大致流程应该有个清楚的概念了吧