Volley源码解析

Posted 2023-02-20 this.

Volley是Google开发的一个网络请求框架，该框架适合进行小而频繁的网络请求。
Volley的使用比较简单，只需几个简单的操作就可以实现发送请求。以StringRequest 为例：

//创建一个请求队列
RequestQueue requestQueue = Volley.newRequestQueue(this);
        String url = "request_url";
        //创建StringRequest对象，以Post数据为例
        StringRequest request = new StringRequest(Request.Method.POST, url, new Response.Listener<String>() 
            @Override
            public void onResponse(String response) 
                Log.i("TAG", response);
            
        , new Response.ErrorListener() 
            @Override
            public void onErrorResponse(VolleyError error) 
                 Log.i("TAG", error.toString());
            
        ) 
            //填写需要post的表单
            @Override
            protected Map<String, String> getParams() throws AuthFailureError 
                HashMap<String, String> map = new HashMap<>();
                map.put("userId", "0"); //测试数据
                map.put("sessionId", "0");
                return map;
            
        ;
requestQueue.add(request);

首先需要看看 Volley.newRequestQueue(this) 这句是如何实现的：

public static RequestQueue newRequestQueue(Context context, HttpStack stack) 
        //缓存路径
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);

        //代码省略

        if (stack == null) 
            if (Build.VERSION.SDK_INT >= 9) 
                stack = new HurlStack();
             else 
                stack = new HttpClientStack(androidHttpClient.newInstance(userAgent));
            
        

        //实现请求的类
        Network network = new BasicNetwork(stack);

        //创建一个请求队列
        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    

这里需要关注的是当SDK版本小于9时，就会使用HttpClientStack来完成请求，否则，则用HurlStack。前者使用的是HttpClient，而后者使用的是HttpURLConnection。
接着，执行了queue.start()语句：

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);
        //启动线程
        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);
                    //启动线程
            mDispatchers[i] = networkDispatcher;
            networkDispatcher.start();
        
    

上述代码中出现了两个分发器：CacheDispatcher和NetworkDispatcher。它们都是继承Thread，分别用于处理缓存和网络请求。其中处理网络请求的线程有4个。

private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;

既然是线程的子类，并且都调用了start方法启动线程，那么就需要看看run方法是如何实现的：

CacheDispatcher

 @Override
    public void run() 
       //代码省略
        while (true) 
            try 
                //不断地从队列中读取请求
                final Request<?> request = mCacheQueue.take();
                request.addMarker("cache-queue-take");

                // 如果请求被取消了，就不再处理
                if (request.isCanceled()) 
                    request.finish("cache-discard-canceled");
                    continue;
                

                // 取出网络请求的缓存
                Cache.Entry entry = mCache.get(request.getCacheKey());
                if (entry == null) 
                    request.addMarker("cache-miss");
                    // 如果找不到，就交由网络请求线程去处理
                    mNetworkQueue.put(request);
                    continue;
                

                // 缓存过期，交由网络请求线程去处理
                if (entry.isExpired()) 
                    request.addMarker("cache-hit-expired");
                    request.setCacheEntry(entry);
                    mNetworkQueue.put(request);
                    continue;
                

                // 命中缓存，写入数据
                request.addMarker("cache-hit");
                Response<?> response = request.parseNetworkResponse(
                        new NetworkResponse(entry.data, entry.responseHeaders));
                request.addMarker("cache-hit-parsed");
                if (!entry.refreshNeeded()) 
                    mDelivery.postResponse(request, response);
                 else 

                    request.addMarker("cache-hit-refresh-needed");
                    request.setCacheEntry(entry);
                    response.intermediate = true;

                    //发送请求结果
                    mDelivery.postResponse(request, response, new Runnable() 
                        @Override
                        public void run() 
                            try 
                                mNetworkQueue.put(request);
                             catch (InterruptedException e) 
                            
                        
                    );
                

             catch (InterruptedException e) 
                // 超时
                if (mQuit) 
                    return;
                
                continue;
            
        
    

NetworkDispatcher

public void run() 
     Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        while (true) 
            long startTimeMs = SystemClock.elapsedRealtime();
            Request<?> request;
            try 
                // 取出请求
                request = mQueue.take();
             catch (InterruptedException e) 
                // 超时
                if (mQuit) 
                    return;
                
                continue;
            

            try 
                request.addMarker("network-queue-take");

                // 如果取消，不做处理
                if (request.isCanceled()) 
                    request.finish("network-discard-cancelled");
                    continue;
                

                addTrafficStatsTag(request);

                // 执行网络请求，mNetwork就是之前创建队列时的BasicNetwork，里面具体实现了网络请求
                NetworkResponse networkResponse = mNetwork.performRequest(request);
                request.addMarker("network-http-complete");

                if (networkResponse.notModified && request.hasHadResponseDelivered()) 
                    request.finish("not-modified");
                    continue;
                

                // 解析返回结果
                Response<?> response = request.parseNetworkResponse(networkResponse);
                request.addMarker("network-parse-complete");

                // 如果可以缓存，则添加到缓存中
                if (request.shouldCache() && response.cacheEntry != null) 
                    mCache.put(request.getCacheKey(), response.cacheEntry);
                    request.addMarker("network-cache-written");
                

                // 回调结果
                request.markDelivered();
                mDelivery.postResponse(request, response);
             catch (VolleyError volleyError) 
                volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
                parseAndDeliverNetworkError(request, volleyError);
             catch (Exception e) 
                VolleyLog.e(e, "Unhandled exception %s", e.toString());
                VolleyError volleyError = new VolleyError(e);
                volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
                mDelivery.postError(request, volleyError);
            
        

接着需要看看请求结果是如何被回调的：
mDelivery.postResponse(request, response)，其实是调用了ExecutorDelivery中的postResponse方法：

@Override
    public void postResponse(Request<?> request, Response<?> response, Runnable runnable) 
        request.markDelivered();
        request.addMarker("post-response");
        //线程池添加任务
        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
    

再看看该任务中的run方法：

 @Override
        public void run() 
            if (mRequest.isCanceled()) 
                mRequest.finish("canceled-at-delivery");
                return;
            
            if (mResponse.isSuccess()) 
                //此处会调用Request的onResponse方法，实现回调
                mRequest.deliverResponse(mResponse.result);
             else 
                mRequest.deliverError(mResponse.error);
            

            if (mResponse.intermediate) 
                mRequest.addMarker("intermediate-response");
             else 
                mRequest.finish("done");
            


            if (mRunnable != null) 
                mRunnable.run();
            
       

至此，对于网络请求是如何处理的有了一定的了解。紧接着看看每次添加网络请求是如何实现的，即add方法：

public <T> Request<T> add(Request<T> request) 
        request.setRequestQueue(this);
        synchronized (mCurrentRequests) 
            //把请求添加到集合
            mCurrentRequests.add(request);
        

        // 添加顺序
        request.setSequence(getSequenceNumber());
        request.addMarker("add-to-queue");

        // 如果设置不缓存，直接走网络
        if (!request.shouldCache()) 
            mNetworkQueue.add(request);
            return request;
        

        // 否则，添加到缓存中，该缓存的键值对为cacheKey和request
        synchronized (mWaitingRequests) 
            String cacheKey = request.getCacheKey();
            if (mWaitingRequests.containsKey(cacheKey)) 
                //查找是否有相同键，取出对应请求
                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 
                //没有对应request就设置为null
                mWaitingRequests.put(cacheKey, null);
                //添加到处理缓存队列
                mCacheQueue.add(request);
            
            return request;