Volley源码解析（一）
Volley基本用法
本文章目的不在于介绍Volley的用法，仅对基本的用法进行介绍，意在引出源码解析的入口
Volley的基本用法通常为3步
- 创建RequestQueue
- 创建Request
- 将Request加入RequestQueue
以Josn请求为例
RequestQueue mQueue = Volley.newRequestQueue(context);
JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(url, null,
new Response.Listener() {
@Override
public void onResponse(JSONObject response) {
Log.d("TAG", response.toString());
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("TAG", error.getMessage(), error);
}
});
mQueue.add(JsonObjectRequest);
Volley源码解读
此处先贴一张Volley的架构图，从guolin大神处搬来
首先从我们使用Volley的入口出开始
RequestQueue mQueue = Volley.newRequestQueue(context);
/**
* 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;
}
代码十分简洁，首先声明一个缓存文件cacheDir，http请求头userAgent ，
分别初始化声明HttpStack，Network，RequestQueue ，然后启动RequestQueue ，下面分别介绍一下各个类。
HttpStack
/**
* Performs an HTTP request with the given parameters.
*
* A GET request is sent if request.getPostBody() == null. A POST request is sent otherwise,
* and the Content-Type header is set to request.getPostBodyContentType().
*
* @param request the request to perform
* @param additionalHeaders additional headers to be sent together with
* {@link Request#getHeaders()}
* @return the HTTP response
*/
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError;
HttpStack接口只有一个方法performRequest，根据注释，可知这个方法为实际请求http访问的方法。该接口有两个实现类，分别对应使用HttpClient实现的HttpClientStack和使用HttpUrlConnection实现的HurlStack
Network
/**
* Performs the specified request.
* @param request Request to process
* @return A {@link NetworkResponse} with data and caching metadata; will never be null
* @throws VolleyError on errors
*/
public NetworkResponse performRequest(Request<?> request) throws VolleyError;
BasicNetwork是Network的实现类之一，大略看一下子类重写的performRequest，可以看到其调用了HttpStack的performRequest，并返回了NetworkResponse
RequestQueue
构造方法只是初始化了几个对象，主要看start()方法
/**
* Starts the dispatchers in this queue.
*/
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();
}
}
CacheDispatcher和NetworkDispatcher均为Thread子类，即启动了一个缓存线程和默认的4个Network线程。
各个类主要方法就这些，然后我们继续跟着start()方法追踪。
首先看 mCacheDispatcher.start(); 实际执行的方法为其run()方法，省略一些其他方法，如下
public void run() {
...
// 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.
...
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
}
}
}
首先是Cache的相关初始化，然后此线程一直循环执行如下内容：
从mCacheQueue中取出一个Request对象，判断此Request对象是否被取消，是否已被缓存，如若未被缓存或者缓存已过期，就将之放入NetworkQueue等待重新请求，否则使用已缓存的内容，并根据是否需要刷新，决定是否将之加入NetworkQueue。缓存的使用待会在看。
继续看NetworkDispatcher的run()方法
@Override
public void run() {
while (true) {
...
request = mQueue.take();
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;
}
...
// 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);
...
}
}
此线程也是一直循环取NetworkQueue中的请求，如果request未被取消，则调用BasicNetwork请求访问，返回NetworkResponse ，并解析为用户需要的Response，如果需要缓存，则将此Response放入缓存队列中，最后调用mDelivery.postResponse,返回给用户。
例子的第二步为构建一个Request，就不在看了。
第三步是将这个Request加入mQueue中,看一下add方法
/**
* Adds a Request to the dispatch queue.
* @param request The request to service
* @return The passed-in request
*/
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;
}
}
首先将Request设置为属于此Queue，然后判断是否需要缓存，不需要的话就直接加入NetworkQueue等待被使用，否则的话，继续判断该Request是否已请求过，若为新请求，则将之加入缓存队列CacheQueue以及重复队列WaitingRequests,若为重复请求，就在重复请求队列中，该CacheKey对应的位置中加入该请求。
整个流程到此结束，下一篇分析请求过程，回调过程，缓存过程。