Volley作为轻量级网络请求框架已经被广泛使用，这篇文章就从源码角度深层次了解Volley的构成，立足于熟练使用Volley的基础之上
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先来看一下请求流程图
Volley框架
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从一个简单是
Request来探究这个Request生命过程。
RequestQueue queue = Volley.newRequestQueue(this); // 1 创建全局请求队列（类似队列的类）
StringRequest request = new StringRequest(Request.Method.GET, "https://blog.csdn.net/guolin_blog/article/details/17482095/", new Response.Listener<String>() {
@Override
public void onResponse(String response) {
Log.d(TAG, "onResponse: " + response);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.d(TAG, "onErrorResponse: " + error);
}
}); // 创建一个Request
queue.add(request); //2 添加到请求队列
Volley的网络请求其实就是三步走战略：
- 创建全局请求队列管理类
RequestQueue
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
// 创建一个用于缓存数据的文件夹 data/data/包名/cache/volley
String userAgent = "volley/0"; // 请求者的标识
try {
String packageName = context.getPackageName();
PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException e) {
}
// HttpStack 是执行一个http请求的最终执行对象。这里会判断android版本
// api 9 以后是HttpUrlConnection ，以前是用HttpClient作为 android的网络 请求 ,这两者的区别就自行百度了
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));
}
}
// 创建一个Network 对象那，NetWork用来将stack的相应数据解析分装成Volly自己要用的NetworkResponse
Network network = new BasicNetwork(stack);
RequestQueue queue; //创建RequestQueue对象，插入用于网络请求的network和缓存对象
if (maxDiskCacheBytes <= -1)
{
// No maximum size specified 自定义缓存大小
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}
else
{
// Disk cache size specified 默认缓存大小
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start(); // 开启线程
return queue;
}
”
queue.start() 这是开启网络请求的开关
/**
* 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();
}
}
这个方法首先就是暂停所有网络请求，创建一个CacheDispatcher和四个NetWorkDispatcher，其实这两个Dispatcher 就是继承线程的子类。所以start后就会开始执行他们中的run（） 方法了，既然是线程，那么方法刚进来的stop 方法来看一下：
/**
* Stops the cache and network dispatchers.
*/
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}
遍历缓存和网络线程调用他们各自的quit方法
public void quit() {// 设置一个当前线程退出的标志位，然后打断当前线程，会抛出InterruptedException
mQuit = true;
interrupt();
}
既然网络请求的执行者HttpStack、解析Response 的NetWork 、一个CacheDispatcher 、四个NetWorkDispatcher 都已经准备好了，而且这五个线程已经开始执行，接下来就是将构建的StringRequest 加入到开始创建的RequestQueue 中
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queue.add(request);将一个请求添加到队列，开始请求数据逻辑：
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); // request 关联全局Requestqueue
synchronized (mCurrentRequests) { // 添加同步锁，最新的请求添加到mCurrentRequest（Set<Request> 保存所有未请求的Request ,为了让用户可以cancel某一个或者取消所有request）
mCurrentRequests.add(request);
}
// Process requests in the order they are added.
request.setSequence(getSequenceNumber()); //设置Request请求顺序
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) { //判断该请求是否可以缓存
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(); //以该请求的 method+url 作为缓存key
if (mWaitingRequests.containsKey(cacheKey)) {
// 判断请求等待map中是否包含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. 如果等待请求缓存map中不包含该cachekey 则在等待map中占个坑 表示当前有正在执行一个有缓存的请求
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request); // 添加该请求到缓存队列 由CacheDispatcher执行该请求
}
return request;
}
}
接下来看一下，执行缓存的CacheDispatcher和NetworkDispatcher这两个线程内部如何执行请求：
- CacheDispatcher
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); // 设置
线程为后台处理
// Make a blocking call to initialize the cache.
mCache.initialize(); // 初始化缓存相关的操作
Request<?> request;
while (true) { //不断循环从cacheQueue中取数据
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
try {
// Take a request from the queue.
request = mCacheQueue.take(); // 获取一个queuest 如果被打断 跳出循环
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) { // 判断该请求是否已经被取消
request.finish("cache-discard-canceled");
continue;
}
// Attempt to retrieve this item from cache. 从缓存恢复获取缓存实体
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {// 如果该缓存实体被清除了，将该request添加到网络请求队列
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()) { // 如果缓存过期，标注该缓存已经过期 添加到网络请求
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"); // 缓存命中 从缓存中获取该缓存实体解析成Response
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
if (!entry.refreshNeeded()) { // 缓存不需要refersh 通过mDelivery发送解析数据到，Request创建的线程
// 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.
final Request<?> finalRequest = request;
// 先发送数据响应，后去将整该请求添加到网络请求队列中
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
}
}
}
- NetWorkDispatcher
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND); //设置优先级
Request<?> request;
while (true) { // 循环遍历网络队列中的Request
long startTimeMs = SystemClock.elapsedRealtime();
// release previous request object to avoid leaking request object when mQueue is drained.
request = null;
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. // 执行网络请求
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()) { // 是否需要缓存
request.finish("not-modified"); // 移除Request从RequestQueue中
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) { // 缓存Response
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);
}
}
}
由于涉及到多线程问题，在RequestQueue中包含两个队列：
/** The cache triage queue. */
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();
/** The queue of requests that are actually going out to the network. */
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();
PriorityBlockingQueue是一个阻塞队列，继承自BlockingQueue，内部使用ReentrantBlock非公平竞争锁机制，完成在多个线程正常获取队列中的Request。
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