Android 网络框架 volley源码剖析

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经过前三篇文章的学习,Volley的用法我们已经掌握的差不多了,但是对于Volley的工作原理,恐怕有很多朋友还不是很清楚。因此,本篇文章中我们就来一起阅读一下Volley的源码,将它的工作流程整体地梳理一遍。同时,这也是Volley系列的最后一篇文章了。

其实,Volley的官方文档中本身就附有了一张Volley的工作流程图,如下图所示。

技术分享

多数朋友突然看到一张这样的图,应该会和我一样,感觉一头雾水吧?没错,目前我们对Volley背后的工作原理还没有一个概念性的理解,直接就来看这张图自然会有些吃力。不过没关系,下面我们就去分析一下Volley的源码,之后再重新来看这张图就会好理解多了。

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) {
    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,这里在第10行判断如果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()) {
    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<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);
    }
    return request;
  }
}

可以看到,在第11行的时候会判断当前的请求是否可以缓存,如果不能缓存则在第12行直接将这条请求加入网络请求队列,可以缓存的话则在第33行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) {
      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;
      }
    }
  }
}

代码有点长,我们只挑重点看。首先在11行可以看到一个while(true)循环,说明缓存线程始终是在运行的,接着在第23行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。之后会在第39行调用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) {
      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));
      }
    }
  }
}

同样地,在第7行我们看到了类似的while(true)循环,说明网络请求线程也是在不断运行的。在第28行的时候会调用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) {
      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);
        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,需要注意的是在第14行调用了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);
    } 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。其中在第22行调用了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。

好了,到这里我们就把Volley的完整执行流程全部梳理了一遍,你是不是已经感觉已经很清晰了呢?对了,还记得在文章一开始的那张流程图吗,刚才还不能理解,现在我们再来重新看下这张图:

技术分享

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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