Volley 源码分析

Volley 源码分析

图片分析

要说源码分析，我们得先看一下官方的配图：

从这张图中我们可以了解到 volley 工作流程：

1.请求加入优先队列

2.从缓存调度器中查看是否存在该请求，如果有（没有进入第三步）直接缓存中读取并解析数据，最后分发到 UI 线程（主线程）。

3.从网络中获取数据（如果设置可以缓存，则写入缓存）并解析数据，最后分发到 UI 线程（主线程）。

从图中，我们还可以看到 volley 的工作其实就是三个线程之间的数据传递 主线程 缓存线程 网络线程。

## 代码分析

既然是源码分析，当然是得从源码开始啦（怎么下载源码我就不说了！不会的自行谷歌）！那我们从哪段源码开始嘞？我们就从我们使用 volley 框架的第一句代码开始。

Volley.newRequestQueue(context) 是我们使用 volley 框架的第一句代码！这句代码是用创建个 RequestQueue (请求队列。对，这个就是 volley 工作流程中的第一步的铺垫。这样请求才能有容器装啊！)。那好，我们现在就来看一下 newRequestQueue(context) 这个静态方法：

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

从代码中可以看出，我们使用的是 newRequestQueue(Context context, HttpStack stack) 这个函数的重载方法！

我简要对这个方法说明下：在系统版本大于等于 9 的时候，我们创建 HurlStack(就是 HttpUrlConnection),在小于 9 的时候我们创建 HttpClientStack(就是 HttpClient) 至于为啥要要这么做。那就是 HttpUrlConnection 的性能要比HttpClient 的好。当然这里我还可以使用另外第三发的 HTTP 库 。比如说 okhttp 。这里我们只要调用 newRequestQueue(Context context, HttpStack stack) 这个方法就行了！当然 要对 okhttp 做简单的封装了！前提是要继承 HttpStack 这个接口啊！

接下来 我们直接看这句代码：RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network); 这就是我们要创建的请求队列啦！看 RequestQueue 这个类：

我们先看构造函数（直接看最复杂的！哈哈）

public RequestQueue(Cache cache, Network network, int threadPoolSize,
        ResponseDelivery delivery) {
    mCache = cache;
    mNetwork = network;
    mDispatchers = new NetworkDispatcher[threadPoolSize];
    mDelivery = delivery;
}

可以看到，需要传入 缓存 ，网络执行器，线程池大小，返回结果分发器。这四个参数！接着 我们再来看一下 queue.start() 这句代码的含义！还是一样先看代码：

 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  (mNetworkQueue, mNetwork,
                mCache, mDelivery);
        mDispatchers[i] = networkDispatcher;
        networkDispatcher.start();
    }
}

先来解释下这这段代码：先停止当前正在运行的所有的线程！接着 初始化 CacheDispatcher(缓存调度器) 并启动他！接着就是启动 NetworkDispatcher(网络调度器) 这有多个。他的个数全靠 threadPoolSize 这个变量控制（默认大小是4）。这样一来。请求队列就是初始化好了！就等待任务加入了啦！！那我们就趁热打铁，直接看加入任务队列的源码：

 public Request   {
    request.setRequestQueue(this);
    synchronized (mCurrentRequests) {
        mCurrentRequests.add(request);
    }
    request.setSequence(getSequenceNumber());
    request.addMarker("add-to-queue");
    if (!request.shouldCache()) {
        mNetworkQueue.add(request);
        return request;
    }
    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 {
            mWaitingRequests.put(cacheKey, null);
            mCacheQueue.add(request);
        }
        return request;
    }
}

依旧是来解读这段代码，在解读这该段代码的时候，我们先来弄清楚其中三个变量的含义：

//当前 RequestQueue 中所有的请求队列
private final Set<Request> mCurrentRequests = new HashSet<Request>();
//缓存队列
private final PriorityBlockingQueue<Request> mCacheQueue =
    new PriorityBlockingQueue<Request>();
//网络队列 正在进入的
private final PriorityBlockingQueue<Request> mNetworkQueue =
    new PriorityBlockingQueue<Request>();

含义如注释！那么我们接着看 add(Request request) 这个方法！首先还是先把任务加入 当前队列。之后request.shouldCache() 判断该任务需要被缓存。不需要的话 直接进入 网络队列！否则的话就加入缓存队列！！

分析完了怎么加入队列之后，我们要来分析下两外两个类了 CacheDispatcher 和 NetworkDispatcher 这里先说下这两个类的共同点：那就是都继承了 Thread 类！也就说他们都是线程类！可以被执行。这也就解释了 RequestQueue 类中的start() 方法中 mCacheDispatcher.start() 和 networkDispatcher.start() 这两句代码！

那么我们先来看一下 CacheDispatcher 这个类！我们直接该类最核心的方法 run() 方法！

 public void run() {
    if (DEBUG) VolleyLog.v("start new dispatcher");
    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
    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;
        }
    }
}

这段代码中有句代码非常抢眼，没错！那就是 while(true) 这句话了！有的童鞋可能已经想到了：这是个死循环(这不废话！)，另外肯定是在不同的从某个队列中取/存数据。没错，就是这样啊！其实很简单！我们接着细说：首先从缓存队列中去除队列，接着判断该请求是否已经取消 if (request.isCanceled()) 如果已经取消的话，就是不走下面的代码！继续从头循环！反之，从缓存中读取数据，如果没有的话就把该队列加入网络请求队列。如果有的但是缓存已经过期的话 也是加入网络请求队列（ if (entry == null) 和 if (entry.isExpired()) 这两个 if 下的语句就是处理上面两个功能的）。如果以上两个条件都不满足的话！就直接 request.parseNetworkResponse(new NetworkResponse(entry.data, entry.responseHeaders)) 解析缓存中的数据进行回调了！

下面我们看 NetworkDispatcher 类的代码，同样的我们直接看 核心代码:

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;
            }

            // Tag the request (if API >= 14)
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
                TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
            }

            // 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));
        }
    }
}

同样的是，先从网络请求队列中取出任务，接着在判断是否要取消，如果要则跳过下面的代码，重新取任务！调用 NetworkResponse networkResponse = mNetwork.performRequest(request); 这句代码，获取 网络返回结果！接着，代码和 CacheDispatcher 中差不多！唯一的区别就是：如果当前的请求需要加入缓存，则加入缓存！细心的同学可能发现了，CacheDispatcher 和 NetworkDispatcher 这两个类中有句相同的代码 Response<?> response = request.parseNetworkResponse(networkResponse); 就是这句！核心的就是parseNetworkResponse(networkResponse) 这个函数的实现我在上一篇 Volley 的使用以及自定义Request 中已经说过了！是有我们实现的！

这里的我们还得在注意一个类：那就是 BasicNetwork ！其实这个类没有什么可以细说的！他就是请求网络接着返回结果！我这里也把核心代码上一下：

  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().data, responseHeaders, true);
            }

            responseContents = entityToBytes(httpResponse.getEntity());
            // if the request is slow, log it.
            long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
            logSlowRequests(requestLifetime, request, responseContents, statusLine);

            if (statusCode != HttpStatus.SC_OK && statusCode != HttpStatus.SC_NO_CONTENT) {
                throw new IOException();
            }
            return new NetworkResponse(statusCode, responseContents, responseHeaders, false);
        } catch (SocketTimeoutException e) {
            attemptRetryOnException("socket", request, new TimeoutError());
        } catch (ConnectTimeoutException e) {
            attemptRetryOnException("connection", request, new TimeoutError());
        } catch (MalformedURLException e) {
            throw new RuntimeException("Bad URL " + request.getUrl(), e);
        } catch (IOException e) {
            int statusCode = 0;
            NetworkResponse networkResponse = null;
            if (httpResponse != null) {
                statusCode = httpResponse.getStatusLine().getStatusCode();
            } else {
                throw new NoConnectionError(e);
            }
            VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
            if (responseContents != null) {
                networkResponse = new NetworkResponse(statusCode, responseContents,
                        responseHeaders, false);
                if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
                        statusCode == HttpStatus.SC_FORBIDDEN) {
                    attemptRetryOnException("auth",
                            request, new AuthFailureError(networkResponse));
                } else {
                    // TODO: Only throw ServerError for 5xx status codes.
                    throw new ServerError(networkResponse);
                }
            } else {
                throw new NetworkError(networkResponse);
            }
        }
    }
}

这个类 我就真的不细讲啦！！

上面已经说了，在解析玩数据之后其实就分发数据了！让用户能够在 UI线程中调用了！现在我们就来看一下这个分发类：ExecutorDelivery。 我们还是先看这个类的构造函数：

 public  (final Handler handler) {
    // Make an Executor that just wraps the handler.
    mResponsePoster = new Executor() {
        @Override
        public void execute(Runnable command) {
            handler.post(command);
        }
    };
}

从代码中我们可以看到，我们需要传一个Handller进入，此时我们在回想一下这个类在 RequestQueue 类中是怎么初始化的？

 public RequestQueue(Cache cache, Network network, int threadPoolSize) {
    this(cache, network, threadPoolSize,
            new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}

没错！大家可以看到，是传入了一个主线中的handler！

当在 CacheDispatcher 和 NetworkDispatcher 这两个类中调用了 mDelivery.postResponse(request, response); 这个方法的时，我们来看一下 ExecutorDelivery 这个类都做了什么！

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

我们直接执行了了个Runnable-->ResponseDeliveryRunnable。那他又做了什么呢？

 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();
        }
   }

我们看最直接的这句代码 mRequest.deliverResponse(mResponse.result); 我们调用了 Request的中的一个方法（这个方法，依然是要我们自己实现！）。接着，我们在看 ExecutorDelivery 的构造函数时，我们就会豁然开朗！数据终于到了 UI线程了！

至此 volley 框架的整体流程分析完毕！！！！

还在说几句：volley 框架的架构设计非常优美！扩展性极高！这个大概得益于 volley 面向接口的设计方案吧！面向接口的架构设计 也是我不断努力的方向！！！！