Volley框架解析

        之前项目中用过Volley框架，看了一下源码，今天就写一个关于Volley的文章。

        Volley的使用是先创建一个RequestQueue，然后把自己的request add到RequestQueue里面，代码如下

mQueue = Volley.newRequestQueue(getApplicationContext());
...

StringRequest stringRequest = new StringRequest(requestType, url, new Response.Listener() {
    @Override
    public void onResponse(String response) {
     
    }
},
        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
         
        }) {

    @Override
    public Map getHeaders() throws AuthFailureError {
        
        return params;
    }

    @Override
    protected Map getParams() throws AuthFailureError {
        return map;

    }
};

...

mQueue.add(request);
就怎么简单。
那现在就从 mQueue.add(request); 开始解析，先看RequestQueue的属性

private AtomicInteger mSequenceGenerator;
private final Map> mWaitingRequests;
private final Set mCurrentRequests;
//这个是放从缓存取的线程任务的队列
private final PriorityBlockingQueue mCacheQueue;
//这个是放从网络请求的线程任务的队列
private final PriorityBlockingQueue mNetworkQueue;
//默认线程数，这个初始化NetworkDispatcher的时候用
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;
private final Cache mCache;
private final Network mNetwork;
private final ResponseDelivery mDelivery;
//从网络取的线程数组
private NetworkDispatcher[] mDispatchers;

//从缓存取的线程，就一个
private CacheDispatcher mCacheDispatcher;

再看创建 mQueue 的函数，最后调用这个函数，stack为null。

public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
    //缓存路径，这里就不讲了
    File cacheDir = new File(context.getCacheDir(), "volley");
    String userAgent = "volley/0";

    try {
        String network = context.getPackageName();
        PackageInfo queue = context.getPackageManager().getPackageInfo(network, 0);
        userAgent = network + "/" + queue.versionCode;
    } catch (NameNotFoundException var6) {
        ;
    }

    if(stack == null) {
        if(VERSION.SDK_INT >= 9) {
            //如果是9以上就用HTTPUrlConnection下载（为什么自己百度）
            stack = new HurlStack();
        } else {
            //如果是9一下就用HttpClient下载
            stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
        }
    }

    BasicNetwork network1 = new BasicNetwork((HttpStack)stack);
    RequestQueue queue1 = new RequestQueue(new DiskBasedCache(cacheDir), network1);
    queue1.start();
    return queue1;
}

再看 queue1.start() 里面的内容

public void start() {
    this.stop();
    //创建一个从缓存取的线程并start线程
    this.mCacheDispatcher = new CacheDispatcher(this.mCacheQueue, this.mNetworkQueue, this.mCache, this.mDelivery);
    this.mCacheDispatcher.start();
    
    //创建length个从网络取的线程并start所有线程，线程里面是死循环从队列里面去Request并处理Request
    for(int i = 0; i < this.mDispatchers.length; ++i) {
        NetworkDispatcher networkDispatcher = new NetworkDispatcher(this.mNetworkQueue, this.mNetwork, this.mCache, this.mDelivery);
        this.mDispatchers[i] = networkDispatcher;
        networkDispatcher.start();
    }

}

现在看add(Request request)函数

public Request add(Request request) {
    request.setRequestQueue(this);
    Set var2 = this.mCurrentRequests;
    synchronized(this.mCurrentRequests) {
        this.mCurrentRequests.add(request);
    }

    request.setSequence(this.getSequenceNumber());
    request.addMarker("add-to-queue");
    if(!request.shouldCache()) {
        //这个接口不缓存，所以不可能在缓存中，直接放在从网络下载的队列里面
        this.mNetworkQueue.add(request);
        return request;
    } else {
        Map var8 = this.mWaitingRequests;
        synchronized(this.mWaitingRequests) {
            String cacheKey = request.getCacheKey();
            if(this.mWaitingRequests.containsKey(cacheKey)) {
                //这个是已经有了一个请求同一个url的request
                Object stagedRequests = (Queue)this.mWaitingRequests.get(cacheKey);
                if(stagedRequests == null) {
                    stagedRequests = new LinkedList();
                }

                ((Queue)stagedRequests).add(request);
                this.mWaitingRequests.put(cacheKey, stagedRequests);
                if(VolleyLog.DEBUG) {
                    VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", new Object[]{cacheKey});
                }
            } else {
                this.mWaitingRequests.put(cacheKey, (Object)null);
                //在这里把请求添加到队列面
                this.mCacheQueue.add(request);
            }

            return request;
        }
    }
}  
    要想详细知道里面的内容，请阅读源码，这里只是你阅读源码之前的热身，有什么不对的地方请指出来，一起学习一起进。