Volley学习
参考文章
Android Volley完全解析(一)，初识Volley的基本用法
Android Volley完全解析(四)，带你从源码的角度理解Volley
Volley 源码解析
首先来看看官方给的一张流程图:
Volley流程图
Volley String的用法:
// 1. 创建Volley请求队列
RequestQueue mQueue = Volley.newRequestQueue(MainActivity.this);
// 2. 创建StringRequest
StringRequest stringRequest = new StringRequest("http://www.baidu.com/", new Response.Listener<String>() {
@Override
public void onResponse(String response) {
Logger.d(response);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Logger.d(error.getMessage());
}
});
// 3. 添加请求到队列
mQueue.add(stringRequest);
Volley源码分析
首先当然是看Volley # newRequestQueue()
public static newRequestQueue(Context context, HttpStack stack){
...
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
stack = new HurlStack();
} else {
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
Network network = new BasicNetwork(stack);// 根据传入的HttpStack对象来处理网络请求
// 创建一个请求队列
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
}
RequestQueue # start()
public void start() {
// 创建1个CacheDispatcher
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
// 创建4个NetworkDispatcher
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}
RequestQueue # add()
public <T> Request<T> add(Request<T> request) {
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request); // 添加到当前请求队列mCurrentRequests中
}
request.setSequence(getSequenceNumber());// 添加请求唯一序列号
request.addMarker("add-to-queue");
if (!request.shouldCache()) {// 如果不需要缓存, 直接添加到NetworkQueue
mNetworkQueue.add(request);
return request;
}
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
...
} else {
mWaitingRequests.put(cacheKey, null);
// 一般都需要缓存, 添加到缓存队列
mCacheQueue.add(request);
}
return request;
}
}
CacheDispatcher # run()
public void run() {
while (true) {
try {
// 尝试从缓存中检索此项
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;
}
// 我们有一个缓存命中; 解析其数据以传送回请求
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.
}
}
}); */
}
}
}
}
NetworkDispatcher # run()
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);
// 发送网络请求, 而Network是一个接口，这里具体的实现是BasicNetwork
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;
}
// 这里处理networkResponse
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");
}
// 调用ExecutorDelivery的postResponse()方法来回调解析出的数据
request.markDelivered();
mDelivery.postResponse(request, response);
}
...
}
}
BasicNetwork # performRequest()
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = Collections.emptyMap();
try {
// Gather headers.
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());
// HttpStack就是在一开始调用newRequestQueue()方法是创建的实例
httpResponse = mHttpStack.performRequest(request, headers);
StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();
responseHeaders = convertHeaders(httpResponse.getAllHeaders());
// Some responses such as 204s do not have content. We must check.
if (httpResponse.getEntity() != null) {
responseContents = entityToBytes(httpResponse.getEntity());
} else {
// Add 0 byte response as a way of honestly representing a
// no-content request.
responseContents = new byte[0];
}
// if the request is slow, log it.
long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
logSlowRequests(requestLifetime, request, responseContents, statusLine);
if (statusCode < 200 || statusCode > 299) {
throw new IOException();
}
// 将服务器返回的数据组装成一个NetworkResponse对象进行返回
return new NetworkResponse(statusCode, responseContents, responseHeaders, false,
SystemClock.elapsedRealtime() - requestStart);
}
}
}
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，代码如下所示:
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，我们看下run()方法中的代码是什么样的：
public void run() {
...
// 回调响应请求
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
...
}
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