volley网络框架是Google在2013年Google I/O大会上推出了一个新的网络通信框架。Volley既可以访问网络取得数据，也可以加载图片，它的设计目标就是非常适合去进行数据量不大，但通信频繁的网络操作，而对于大数据量的网络操作，比如说下载文件等，Volley的表现就会非常糟糕。
一.volley的基本用法
volley内部实现是基于队列的，开发者只需要创建请求放到请求队列中就可以了，请求队列会依次执行队列中的请求。
一个应用程序如果请求不是太频繁的话完全可以使用一个请求队列（对应于application），也可以每个activity使用一个请求队列（对应于activity）。
1）创建一个请求队列
RequestQueue mQueue= Volley.newRequestQueue(getApplicationContext());
2).创建一个请求
volley提供了两个默认的request实现 StringRequest、JsonObjectRequest
StringRequest返回的数据类型是String类型，
//创建StringRequest请求
StringRequest mStringRequest = new StringRequest(Request.Method.GET, "http://www.baidu.com",
new Response.Listener<String>() {
@Override
public void onResponse(String response) {
Log.i("wangshu", response);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("wangshu", error.getMessage(), error);
}
});
//创建JsonObjectRequest请求
JsonObjectRequest mJsonObjectRequest = new JsonObjectRequest(Request.Method.POST,"http://api.1-blog.com/biz/bizserver/article/list.do",
new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
Log.d("wangshu", response.toString());
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("wangshu", error.getMessage(), error);
}
}
);
3).加入到请求队列中
mQueue.add(mJsonObjectRequest);
二、源码分析
首先看看Volley这个类，Volley位于volley.toolbox包下
public class Volley {
/** Default on-disk cache directory. */
private static final String DEFAULT_CACHE_DIR = "volley";
/**
* Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
*
* @param context A {@link Context} to use for creating the cache dir.
* @param stack An {@link HttpStack} to use for the network, or null for default.
* @return A started {@link RequestQueue} instance.
*/
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;
}
/**
* Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
*
* @param context A {@link Context} to use for creating the cache dir.
* @return A started {@link RequestQueue} instance.
*/
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
}
newRequestQeue(context,stack);方法会创建一个缓存目录，如果当前Android版本>=9就创建一个HurlStack实例（内部使用httpUrlContation实现）否则创建httpClintStack实例（内部使用HttpClint实现）,然后创建一个NetWork根据传入的Stack进行网络请求。最后创建RequestQueue实例然后调用start（）方法启动。
下面看看RequestQueue的start（）方法内部做了哪些事情：
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();
}
}
关闭当前的正在执行的调度器，创建一个缓存调度器CacheDispatcher并启动。然后在一个for循环中创建并启动NetWorkDispatcher (默认启动4个)
CacheDispatcher和NetWorkerDispatcher都是继承与线程。
下面看看CacheDispatcher内部做了些什么：
@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;
}
}
CacheDispatcher的run()方法是一个while(true)死循环，说明这个线程一直在执行，每次从请求队列中取出一个请求，根据这条请
求从缓存中取出对应的缓存如果=null 就把这条request加到NetWorkQueue中，不为空则判断请求是否过期，过期也加到
NetWorkQueue中，否则直接使用缓存数据不进行网络请求，直接解析缓存数据然后回调response接口。
NetWorkerDispatcher内部实现：
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
Request<?> request;
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());
VolleyError volleyError = new VolleyError(e);
mDelivery.postError(request, volleyError);
}
}
}
可见NetWorkerDispatcher调用了NetWorker.performRequest(),实际是NerWorker实现类BasicNetWorker的performRequest()方法，在其内部又调用了
mHttpStack.performRequest(request, headers);获取请求结果，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。
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，
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
三、自定义不同类型Request请求
自定义网络请求格式定义一个类继承与Request然后重写Request类中的deliverResponse()和parseNetworkResponse()方法，注意在构造函数中一定要调用super()方法将这几个参数传给父类，因为HTTP的请求和响应都是在父类中自动处理的。deliverResponse()方法仅仅是调用了mListener中的onResponse()方法，并将response内容传入即可，这样就可以将服务器响应的数据进行回调了。parseNetworkResponse()方法中则应该对服务器响应的数据进行解析，其中数据是以字节的形式存放在NetworkResponse的data变量中的，然后转换成需要的数据格式（json、xml等格式）就可以了。
四、图片Request