关于volley的一些分析-初始化

首先看下RequestQueue的实例化过程.从这里可以看出来，volley是自带有硬存的，并且当adk版本低于9时，网络交互运用的是HttpClient，大于9时运用的是HttpURLConnection

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 {
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);

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

return queue;
    }

这里配张图讲解下ResquestQueue的构成。

接下来看看RequestQueue的全局变量：

    //用于为每个request生成单调递增的编号
private AtomicInteger mSequenceGenerator = new AtomicInteger();

//暂存区域，存放已经有一个重复的请求request在发送的request。
//正在发送的request并不会保存在此队列中，当没有重复的request时，get(cacheKey)返回的是null。
//containsKey(cacheKey) 指向的是与正在发送的request拥有相同缓存键的队列
private final Map<String, Queue<Request<?>>> mWaitingRequests =
new HashMap<String, Queue<Request<?>>>();

//所有调用了RequestQueue.add的request都在此队列中。
private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

//缓存队列
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();

//存放正在与网络交互的request
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();

//默认与网络交互开启的线程数量
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;

//硬盘缓存
private final Cache mCache;

//用于执行网络请求
private final Network mNetwork;

//用于当网络请求返回时的响应
private final ResponseDelivery mDelivery;

//执行网络交互的线程
private NetworkDispatcher[] mDispatchers;

//缓存线程
private CacheDispatcher mCacheDispatcher;

private List<RequestFinishedListener> mFinishedListeners =
new ArrayList<RequestFinishedListener>();

接下来看一下ReqestQueue的add方法：

 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");

//当request并不进行缓存时，直接插入mNetworkQueue
if (!request.shouldCache()) {
            mNetworkQueue.add(request);
return request;
        }

// 当有与request的cacheKey相同的request正在与网络交互时，将后进入RequestQueue的request插入mWaitingRequest
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 {
// 当mWaitiingRequest中没有cacheKey这个键时，插入键值对cacheKey-null，表明这里已有request正在与网络进行交互
                mWaitingRequests.put(cacheKey, null);
                mCacheQueue.add(request);
            }
return request;
        }
    }

这个方法实际上是在调用Volley.newRequestQueue时调用的。可以看到的是这里仅是开启了线程。

   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();
        }
    }

停用RequestQueue也是调用各线程的interrupt方法

 public void stop() {
if (mCacheDispatcher != null) {
            mCacheDispatcher.quit();
        }
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
                mDispatchers[i].quit();
            }
        }
    }

接下来分析一下缓存线程CacheDispatcher ：
它接受的参数有四个：mCacheQueue（缓存队列）, mNetworkQueue（网络交互队列）, mCache（硬盘缓存）, mDelivery（交互完成后回调）。

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 {
// 从缓存队列中取出request，此处会堵塞
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;
                }

// 从硬盘缓存中取出该request对应的缓存
                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;
                }

// 缓存过期
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;
            }
        }
    }

网络交互线程mDispatchers接受的参数有四个：mNetworkQueue（网络交互队列）, mNetwork（执行网络交互）, mCache（硬盘缓存）, mDelivery（交互完成后回调）。

public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
            Request<?> request;
try {
//从队列中取出request，此处会堵塞。
                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);

// 执行网络交互
                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;
                }

//解析返回来的数据
                Response<?> response = request.parseNetworkResponse(networkResponse);
                request.addMarker("network-parse-complete");

//将缓存写入
// 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) {
                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);
            }
        }
    }

最后用一张图概括全流程：