Android Volley完全解析

作者: 一只好奇的茂 | 来源:发表于2017-06-28 16:41 被阅读259次

    特点

    适合数据量小的频率高的请求,不适合下载上传大文件

    1. Volley的网络请求线程池默认大小为4。意味着可以并发进行4个请求,大于4个,会排在队列中。
    2. Request#getBody() 方法返回byte[]类型,作为 Http.POST 和 Http.PUT Request body 中的数据。这就意味着需要把用 http 传输的数据一股脑读取到内存中。如果文件过大,有可能导致OOM。

    考虑这样一个场景:
    你同时上传4个文件,这四个文件都很大,这时候你的内存占用就很高,很容易oom。
    这时候,你发网络请求,调用普通api。
    所有的网络线程都被上传文件的任务占满了,你的网络请求只有在文件上传完毕后才能得到执行。体验就是,很慢!

    所以Volley适合数据量小,频率快的请求。

    用法

    StringRequest

    public class MainActivity extends AppCompatActivity {
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            final RequestQueue mQueue = Volley.newRequestQueue(this);
            StringRequest stringRequest = new StringRequest("http://www.jianshu.com",
                    new Response.Listener<String>() {
                        @Override
                        public void onResponse(String response) {
                            Log.d("TAG", response);
                        }
                    }, new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    Log.e("TAG", error.getMessage(), error);
                }
            });
            mQueue.add(stringRequest);
        }
    }
    

    源码解析

    说起分析源码,那么应该从哪儿开始看起呢?这就要回顾一下Volley的用法了,还记得吗,使用Volley的第一步,首先要调用Volley.newRequestQueue(context)方法来获取一个RequestQueue对象,那么我们自然要从这个方法开始看起了,代码如下所示:

    public static RequestQueue newRequestQueue(Context context) {  
        return newRequestQueue(context, null);  
    }  
    

    这个方法仅仅只有一行代码,只是调用了newRequestQueue()的方法重载,并给第二个参数传入null。那我们看下带有两个参数的newRequestQueue()方法中的代码,如下所示:

    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) {                                     //  ①
            if (Build.VERSION.SDK_INT >= 9) {  
                stack = new HurlStack();  
            } else {  
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));  
            }  
        }  
        Network network = new BasicNetwork(stack);  
        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);  
        queue.start();  
        return queue;  
    }  
    

    可以看到,这里在①判断如果stack是等于null的,则去创建一个HttpStack对象,这里会判断如果手机系统版本号是大于9的,则创建一个HurlStack的实例,否则就创建一个HttpClientStack的实例。实际上HurlStack的内部就是使用HttpURLConnection进行网络通讯的,而HttpClientStack的内部则是使用HttpClient进行网络通讯的,这里为什么这样选择呢?可以参考我之前翻译的一篇文章Android访问网络,使用HttpURLConnection还是HttpClient?
    创建好了HttpStack之后,接下来又创建了一个Network对象,它是用于根据传入的HttpStack对象来处理网络请求的,紧接着new出一个RequestQueue对象,并调用它的start()方法进行启动,然后将RequestQueue返回,这样newRequestQueue()的方法就执行结束了。

    那么RequestQueue的start()方法内部到底执行了什么东西呢?我们跟进去瞧一瞧:

    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();  
        }  
    }  
    

    这里先是创建了一个CacheDispatcher的实例,然后调用了它的start()方法,接着在一个for循环里去创建NetworkDispatcher的实例,并分别调用它们的start()方法。这里的CacheDispatcher和NetworkDispatcher都是继承自Thread的,而默认情况下for循环会执行四次,也就是说当调用了Volley.newRequestQueue(context)之后,就会有五个线程一直在后台运行,不断等待网络请求的到来,其中CacheDispatcher是缓存线程,NetworkDispatcher是网络请求线程。

    得到了RequestQueue之后,我们只需要构建出相应的Request,然后调用RequestQueue的add()方法将Request传入就可以完成网络请求操作了,那么不用说,add()方法的内部肯定有着非常复杂的逻辑,我们来一起看一下:

    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()) {                          //  ②
            mNetworkQueue.add(request);                //  ③
            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();  
            if (mWaitingRequests.containsKey(cacheKey)) {  
                // There is already a request in flight. Queue up.  
                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 {  
                // Insert 'null' queue for this cacheKey, indicating there is now a request in  
                // flight.  
                mWaitingRequests.put(cacheKey, null);  
                mCacheQueue.add(request);                 //  ④
            }  
            return request;  
        }  
    }  
    

    可以看到,在②会判断当前的请求是否可以缓存,如果不能缓存则在③直接将这条请求加入网络请求队列,可以缓存的话则在④将这条请求加入缓存队列。在默认情况下,每条请求都是可以缓存的,当然我们也可以调用Request的setShouldCache(false)方法来改变这一默认行为。

    OK,那么既然默认每条请求都是可以缓存的,自然就被添加到了缓存队列中,于是一直在后台等待的缓存线程就要开始运行起来了,我们看下CacheDispatcher中的run()方法,代码如下所示:

    public class CacheDispatcher extends Thread {  
      
        ……  
      
        @Override  
        public void run() {  
            if (DEBUG) VolleyLog.v("start new dispatcher");  
            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);  
            // Make a blocking call to initialize the cache.  
            mCache.initialize();  
            while (true) {                  //   ⑤
                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());    //  ⑥
                    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()) {  
                        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(   //  ⑦ 
                       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;  
                }  
            }  
        }  
    }  
    

    代码有点长,我们只挑重点看。首先在⑤可以看到一个while(true)循环,说明缓存线程始终是在运行的,接着在⑥会尝试从缓存当中取出响应结果,如何为空的话则把这条请求加入到网络请求队列中,如果不为空的话再判断该缓存是否已过期,如果已经过期了则同样把这条请求加入到网络请求队列中,否则就认为不需要重发网络请求,直接使用缓存中的数据即可。之后会在⑦调用Request的parseNetworkResponse()方法来对数据进行解析,再往后就是将解析出来的数据进行回调了,这部分代码我们先跳过,因为它的逻辑和NetworkDispatcher后半部分的逻辑是基本相同的,那么我们等下合并在一起看就好了,先来看一下NetworkDispatcher中是怎么处理网络请求队列的,代码如下所示:

    public class NetworkDispatcher extends Thread {  
        ……  
        @Override  
        public void run() {  
            Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);  
            Request<?> request;  
            while (true) {                  //  ⑧
                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;  
                }  
                try {  
                    request.addMarker("network-queue-take");  
                    // If the request was cancelled already, do not perform the  
                    // network request.  
                    if (request.isCanceled()) {  
                        request.finish("network-discard-cancelled");  
                        continue;  
                    }  
                    addTrafficStatsTag(request);  
                    // Perform the network request.  
                    NetworkResponse networkResponse = mNetwork.performRequest(request);       //   ⑨
                    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);  
                } catch (VolleyError volleyError) {  
                    parseAndDeliverNetworkError(request, volleyError);  
                } catch (Exception e) {  
                    VolleyLog.e(e, "Unhandled exception %s", e.toString());  
                    mDelivery.postError(request, new VolleyError(e));  
                }  
            }  
        }  
    }  
    

    同样地,在⑧我们看到了类似的while(true)循环,说明网络请求线程也是在不断运行的。在⑨会调用Network的performRequest()方法来去发送网络请求,而Network是一个接口,这里具体的实现是BasicNetwork,我们来看下它的performRequest()方法,如下所示:

    public class BasicNetwork implements Network {  
        ……  
        @Override  
        public NetworkResponse performRequest(Request<?> request) throws VolleyError {  
            long requestStart = SystemClock.elapsedRealtime();  
            while (true) {                   // ⑤
                HttpResponse httpResponse = null;  
                byte[] responseContents = null;  
                Map<String, String> responseHeaders = new HashMap<String, String>();  
                try {  
                    // Gather headers.  
                    Map<String, String> headers = new HashMap<String, String>();  
                    addCacheHeaders(headers, request.getCacheEntry());  
                    httpResponse = mHttpStack.performRequest(request, headers); //⑩  
                    StatusLine statusLine = httpResponse.getStatusLine();  
                    int statusCode = statusLine.getStatusCode();  
                    responseHeaders = convertHeaders(httpResponse.getAllHeaders());  
                    // Handle cache validation.  
                    if (statusCode == HttpStatus.SC_NOT_MODIFIED) {  
                        return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,  
                                request.getCacheEntry() == null ? null : request.getCacheEntry().data,  
                                responseHeaders, true);  
                    }  
                    // Some responses such as 204s do not have content.  We must check.  
                    if (httpResponse.getEntity() != null) {  
                      responseContents = entityToBytes(httpResponse.getEntity());  
                    } else {  
                      // Add 0 byte response as a way of honestly representing a  
                      // no-content request.  
                      responseContents = new byte[0];  
                    }  
                    // if the request is slow, log it.  
                    long requestLifetime = SystemClock.elapsedRealtime() - requestStart;  
                    logSlowRequests(requestLifetime, request, responseContents, statusLine);  
                    if (statusCode < 200 || statusCode > 299) {  
                        throw new IOException();  
                    }  
                    return new NetworkResponse(statusCode, responseContents, responseHeaders, false);  
                } catch (Exception e) {  
                    ……  
                }  
            }  
        }  
    }  
    

    这段方法中大多都是一些网络请求细节方面的东西,我们并不需要太多关心,需要注意的是在⑩调用了HttpStack的performRequest()方法,这里的HttpStack就是在一开始调用newRequestQueue()方法是创建的实例,默认情况下如果系统版本号大于9就创建的HurlStack对象,否则创建HttpClientStack对象。前面已经说过,这两个对象的内部实际就是分别使用HttpURLConnection和HttpClient来发送网络请求的,我们就不再跟进去阅读了,之后会将服务器返回的数据组装成一个NetworkResponse对象进行返回。

    在NetworkDispatcher中收到了NetworkResponse这个返回值后又会调用Request的parseNetworkResponse()方法来解析NetworkResponse中的数据,以及将数据写入到缓存,这个方法的实现是交给Request的子类来完成的,因为不同种类的Request解析的方式也肯定不同。还记得我们在上一篇文章中学习的自定义Request的方式吗?其中parseNetworkResponse()这个方法就是必须要重写的。

    在解析完了NetworkResponse中的数据之后,又会调用ExecutorDelivery的postResponse()方法来回调解析出的数据,代码如下所示:

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

    其中,在mResponsePoster的execute()方法中传入了一个ResponseDeliveryRunnable对象,就可以保证该对象中的run()方法就是在主线程当中运行的了,我们看下run()方法中的代码是什么样的:

    private class ResponseDeliveryRunnable implements Runnable {  
        private final Request mRequest;  
        private final Response mResponse;  
        private final Runnable mRunnable;  
      
        public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {  
            mRequest = request;  
            mResponse = response;  
            mRunnable = runnable;  
        }  
      
        @SuppressWarnings("unchecked")  
        @Override  
        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);              //  ⑪
            } 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();  
            }  
       }  
    }  
    

    代码虽然不多,但我们并不需要行行阅读,抓住重点看即可。其中在⑪调用了Request的deliverResponse()方法,有没有感觉很熟悉?没错,这个就是我们在自定义Request时需要重写的另外一个方法,每一条网络请求的响应都是回调到这个方法中,最后我们再在这个方法中将响应的数据回调到Response.Listener的onResponse()方法中就可以了。

    总结

    其中蓝色部分代表主线程,绿色部分代表缓存线程,橙色部分代表网络线程。我们在主线程中调用RequestQueue的add()方法来添加一条网络请求,这条请求会先被加入到缓存队列当中,如果发现可以找到相应的缓存结果就直接读取缓存并解析,然后回调给主线程。如果在缓存中没有找到结果,则将这条请求加入到网络请求队列中,然后处理发送HTTP请求,解析响应结果,写入缓存,并回调主线程。

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