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 Android教程網 >> Android技術 >> 關於Android編程 >> Android Volley完全解析(四),帶你從源碼的角度理解Volley

Android Volley完全解析(四),帶你從源碼的角度理解Volley

編輯:關於Android編程

 

經過前三篇文章的學習,Volley的用法我們已經掌握的差不多了,但是對於Volley的工作原理,恐怕有很多朋友還不是很清楚。因此,本篇文章中我們就來一起閱讀一下Volley的源碼,將它的工作流程整體地梳理一遍。同時,這也是Volley系列的最後一篇文章了。

其實,Volley的官方文檔中本身就附有了一張Volley的工作流程圖,如下圖所示。

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多數朋友突然看到一張這樣的圖,應該會和我一樣,感覺一頭霧水吧?沒錯,目前我們對Volley背後的工作原理還沒有一個概念性的理解,直接就來看這張圖自然會有些吃力。不過沒關系,下面我們就去分析一下Volley的源碼,之後再重新來看這張圖就會好理解多了。

說起分析源碼,那麼應該從哪兒開始看起呢?這就要回顧一下Volley的用法了,還記得嗎,使用Volley的第一步,首先要調用Volley.newRequestQueue(context)方法來獲取一個RequestQueue對象,那麼我們自然要從這個方法開始看起了,代碼如下所示:

public static RequestQueue newRequestQueue(Context context) {
    return newRequestQueue(context, null);
}
這個方法僅僅只有一行代碼,只是調用了newRequestQueue()的方法重載,並給第二個參數傳入null。那我們看下帶有兩個參數的newRequestQueue()方法中的代碼,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) {
        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,這裡在第10行判斷如果stack是等於null的,則去創建一個HttpStack對象,這裡會判斷如果手機系統版本號是大於9的,則創建一個HurlStack的實例,否則就創建一個HttpClientStack的實例。實際上HurlStack的內部就是使用HttpURLConnection進行網絡通訊的,而HttpClientStack的內部則是使用HttpClient進行網絡通訊的,這裡為什麼這樣選擇呢?可以參考我之前翻譯的一篇文章Android訪問網絡,使用HttpURLConnection還是HttpClient?

 

創建好了HttpStack之後,接下來又創建了一個Network對象,它是用於根據傳入的HttpStack對象來處理網絡請求的,緊接著new出一個RequestQueue對象,並調用它的start()方法進行啟動,然後將RequestQueue返回,這樣newRequestQueue()的方法就執行結束了。

那麼RequestQueue的start()方法內部到底執行了什麼東西呢?我們跟進去瞧一瞧:

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();
    }
}
這裡先是創建了一個CacheDispatcher的實例,然後調用了它的start()方法,接著在一個for循環裡去創建NetworkDispatcher的實例,並分別調用它們的start()方法。這裡的CacheDispatcher和NetworkDispatcher都是繼承自Thread的,而默認情況下for循環會執行四次,也就是說當調用了Volley.newRequestQueue(context)之後,就會有五個線程一直在後台運行,不斷等待網絡請求的到來,其中CacheDispatcher是緩存線程,NetworkDispatcher是網絡請求線程。

 

得到了RequestQueue之後,我們只需要構建出相應的Request,然後調用RequestQueue的add()方法將Request傳入就可以完成網絡請求操作了,那麼不用說,add()方法的內部肯定有著非常復雜的邏輯,我們來一起看一下:

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;
    }
    // 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> stagedRequests = mWaitingRequests.get(cacheKey);
            if (stagedRequests == null) {
                stagedRequests = new LinkedList>();
            }
            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);
        }
        return request;
    }
}
可以看到,在第11行的時候會判斷當前的請求是否可以緩存,如果不能緩存則在第12行直接將這條請求加入網絡請求隊列,可以緩存的話則在第33行將這條請求加入緩存隊列。在默認情況下,每條請求都是可以緩存的,當然我們也可以調用Request的setShouldCache(false)方法來改變這一默認行為。

OK,那麼既然默認每條請求都是可以緩存的,自然就被添加到了緩存隊列中,於是一直在後台等待的緩存線程就要開始運行起來了,我們看下CacheDispatcher中的run()方法,代碼如下所示:

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) {
            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()) {
                    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;
            }
        }
    }
}
代碼有點長,我們只挑重點看。首先在11行可以看到一個while(true)循環,說明緩存線程始終是在運行的,接著在第23行會嘗試從緩存當中取出響應結果,如何為空的話則把這條請求加入到網絡請求隊列中,如果不為空的話再判斷該緩存是否已過期,如果已經過期了則同樣把這條請求加入到網絡請求隊列中,否則就認為不需要重發網絡請求,直接使用緩存中的數據即可。之後會在第39行調用Request的parseNetworkResponse()方法來對數據進行解析,再往後就是將解析出來的數據進行回調了,這部分代碼我們先跳過,因為它的邏輯和NetworkDispatcher後半部分的邏輯是基本相同的,那麼我們等下合並在一起看就好了,先來看一下NetworkDispatcher中是怎麼處理網絡請求隊列的,代碼如下所示:
public class NetworkDispatcher extends Thread {
	……
    @Override
    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;
            }
            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);
                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));
            }
        }
    }
}
同樣地,在第7行我們看到了類似的while(true)循環,說明網絡請求線程也是在不斷運行的。在第28行的時候會調用Network的performRequest()方法來去發送網絡請求,而Network是一個接口,這裡具體的實現是BasicNetwork,我們來看下它的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) {
            HttpResponse httpResponse = null;
            byte[] responseContents = null;
            Map responseHeaders = new HashMap();
            try {
                // Gather headers.
                Map headers = new HashMap();
                addCacheHeaders(headers, request.getCacheEntry());
                httpResponse = mHttpStack.performRequest(request, headers);
                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) {
                ……
            }
        }
    }
}

這段方法中大多都是一些網絡請求細節方面的東西,我們並不需要太多關心,需要注意的是在第14行調用了HttpStack的performRequest()方法,這裡的HttpStack就是在一開始調用newRequestQueue()方法是創建的實例,默認情況下如果系統版本號大於9就創建的HurlStack對象,否則創建HttpClientStack對象。前面已經說過,這兩個對象的內部實際就是分別使用HttpURLConnection和HttpClient來發送網絡請求的,我們就不再跟進去閱讀了,之後會將服務器返回的數據組裝成一個NetworkResponse對象進行返回。

在NetworkDispatcher中收到了NetworkResponse這個返回值後又會調用Request的parseNetworkResponse()方法來解析NetworkResponse中的數據,以及將數據寫入到緩存,這個方法的實現是交給Request的子類來完成的,因為不同種類的Request解析的方式也肯定不同。還記得我們在上一篇文章中學習的自定義Request的方式嗎?其中parseNetworkResponse()這個方法就是必須要重寫的。

在解析完了NetworkResponse中的數據之後,又會調用ExecutorDelivery的postResponse()方法來回調解析出的數據,代碼如下所示:

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()方法中傳入了一個ResponseDeliveryRunnable對象,就可以保證該對象中的run()方法就是在主線程當中運行的了,我們看下run()方法中的代碼是什麼樣的:
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);
        } 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();
        }
   }
}

代碼雖然不多,但我們並不需要行行閱讀,抓住重點看即可。其中在第22行調用了Request的deliverResponse()方法,有沒有感覺很熟悉?沒錯,這個就是我們在自定義Request時需要重寫的另外一個方法,每一條網絡請求的響應都是回調到這個方法中,最後我們再在這個方法中將響應的數據回調到Response.Listener的onResponse()方法中就可以了。

好了,到這裡我們就把Volley的完整執行流程全部梳理了一遍,你是不是已經感覺已經很清晰了呢?對了,還記得在文章一開始的那張流程圖嗎,剛才還不能理解,現在我們再來重新看下這張圖:

/

其中藍色部分代表主線程,綠色部分代表緩存線程,橙色部分代表網絡線程。我們在主線程中調用RequestQueue的add()方法來添加一條網絡請求,這條請求會先被加入到緩存隊列當中,如果發現可以找到相應的緩存結果就直接讀取緩存並解析,然後回調給主線程。如果在緩存中沒有找到結果,則將這條請求加入到網絡請求隊列中,然後處理發送HTTP請求,解析響應結果,寫入緩存,並回調主線程。

怎麼樣,是不是感覺現在理解這張圖已經變得輕松簡單了?好了,到此為止我們就把Volley的用法和源碼全部學習完了,相信你已經對Volley非常熟悉並可以將它應用到實際項目當中了,那麼Volley完全解析系列的文章到此結束,感謝大家有耐心看到最後。

 

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