一、Volley使用简介
Volley是谷歌官方推出的网络请求框架，适合数据量小但通信频繁的网络操作。
1.首先需要获取一个RequestQueue对象
2.创建一个StringRequest对象
3.讲StringRequest对象添加到RequestQueue里面
private RequestQueue mQueue;
//第一步
mQueue = Volley.newRequestQueue(MainActivity.this);
//volley字符串请求
private void volleyStringRequest() {
//第二步
StringRequest stringRequest = new StringRequest("http://www.baidu.com",
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.e(TAG, "onErrorResponse: " + error.getMessage(), error);
}
});
//第三步
mQueue.add(stringRequest);
}
//volleyJson请求
private void volleyJsonRequest() {
//第二步
JsonObjectRequest jsonObjectRequest = new JsonObjectRequest("http://www.weather.com.cn/data/sk/101010100.html", null,
new Response.Listener<JSONObject>() {
@Override
public void onResponse(JSONObject response) {
Log.d(TAG, "onResponse: " + response.toString());
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e(TAG, "onErrorResponse: " + error.getMessage(), error);
}
});
//第三步
mQueue.add(jsonObjectRequest);
}
总结：通过newRequestQueue()函数新建并启动一个请求队列RequestQueue后，只需要往这个RequestQueue不断add Request即可。
二、Volley源码分析
在mQueue = Volley.newRequestQueue(MainActivity.this);中，其newRequestQueue(context);方法的实现如下：
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
可以看到它调用了有两个参数的newRequestQueue(context, stack)方法，该方法的实现如下：
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
return newRequestQueue(context, stack, -1);
}
可以看到它调用了有三个参数的newRequestQueue(context,stack,maxDiskCacheBytes)方法，该方法的实现如下：
public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR); //获取缓存目录
String userAgent = "volley/0";
try {
String packageName = context.getPackageName(); //获取包名
PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0); //获取包信息
userAgent = packageName + "/" + info.versionCode; //设置用户代理信息
} catch (NameNotFoundException e) {
}
if (stack == null) { //如果网络栈为空，为stack新建对应的栈
if (Build.VERSION.SDK_INT >= 9) { //如果SDK版本大于等于9
stack = new HurlStack();
} else { ////如果SDK版本小于9
// Prior to Gingerbread, HttpUrlConnection was unreliable.
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
Network network = new BasicNetwork(stack); //用网络栈新建Network
RequestQueue queue; //请求队列
//根据最大磁盘缓存字节生成对应的请求队列
if (maxDiskCacheBytes <= -1) //如果最大磁盘缓存字节小于等于-1，不指定最大缓存值
{
// No maximum size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
}
else
{ //为请求队列指定最大缓存值
// Disk cache size specified
queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
}
queue.start(); //开启请求队列
return queue; //返回请求队列
}
倒数第二行的queue.start();的start()方法的实现如下：
/**
* 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();
}
}
第二行的stop()方法的实现如下：
/**
* 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();
}
}
}
对于第三步的mQueue.add(stringRequest);的add方法，实现如下：
/**
* Adds a Request to the dispatch queue. 插入请求到分发队列
* @param request The request to service 请求的服务
* @return The passed-in request 通过的请求
*/
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); //把传入的请求加入请求集合中
}
// Process requests in the order they are added.
request.setSequence(getSequenceNumber()); //设置请求的顺序
request.addMarker("add-to-queue"); //添加标记
// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) { //如果传入的请求不能缓存
mNetworkQueue.add(request); //将传入的请求添加到网络队列
return request; //返回传入的请求
}
// Insert request into stage if there's already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey(); //获取缓存的key
if (mWaitingRequests.containsKey(cacheKey)) { //如果等待请求的map包含此key
// There is already a request in flight. Queue up.
Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey); //取出该请求队列
if (stagedRequests == null) { //如果请求队列为空
stagedRequests = new LinkedList<Request<?>>(); //将请求队列指定为新的请求链表
}
stagedRequests.add(request); //将当前的请求加入请求队列
mWaitingRequests.put(cacheKey, stagedRequests); //将更新后的请求队列存回等待请求的map
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
}
} else { //如果map不包含当前的key
// Insert 'null' queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null); //将此key存入等待请求的map中
mCacheQueue.add(request); //将传入的请求添加到快缓存队列
}
return request; //返回传入的请求
}
}
在上上段代码中出现的CacheDispatcher的代码如下：
public class CacheDispatcher extends Thread {
可以看到CacheDispatcher是Thread的子类，下面看其关键的run方法：
@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(); //初始化缓存
Request<?> request; //声明请求
while (true) {
// release previous request object to avoid leaking request object when mQueue is drained.
request = null; //将请求置空
try {
// Take a request from the queue.
request = mCacheQueue.take(); //从缓存队列中取出请求
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) { //如果临时中断就结束函数
return;
}
continue;
}
try {
request.addMarker("cache-queue-take"); //为请求设置标记
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) { //如果请求被取消
request.finish("cache-discard-canceled"); //结束请求
continue;
}
// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey()); //从缓存中取出entry
if (entry == null) { //如果entry为空
request.addMarker("cache-miss"); //设置缓存未命中标记
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request); //将请求网络队列
continue;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) { //entry过期
request.addMarker("cache-hit-expired"); //给请求设置缓存过期标志
request.setCacheEntry(entry); //给请求设置缓存entry
mNetworkQueue.put(request); //将请求添加到网络队列
continue;
}
// We have a cache hit; parse its data for delivery back to the request.
request.addMarker("cache-hit"); //给请求设置缓存命中标志
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders)); //解析网络响应
request.addMarker("cache-hit-parsed"); //给请求设置缓存命中解析标志
if (!entry.refreshNeeded()) { //如果entry不需要刷新
// Completely unexpired cache hit. Just deliver the response.
mDelivery.postResponse(request, response); //提交请求与响应
} 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"); //给请求设置缓存命中并需要刷新标志
request.setCacheEntry(entry); //给请求设置缓存entry
// Mark the response as intermediate.
response.intermediate = true; //设置响应马上执行
// Post the intermediate response back to the user and have
// the delivery then forward the request along to the network.
final Request<?> finalRequest = request;
mDelivery.postResponse(request, response, new Runnable() { //提交请求与响应并执行runnable
@Override
public void run() {
try {
mNetworkQueue.put(finalRequest); //把最终请求存入网络队列
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString()); //打印错误日志
}
}
}
可以看到run()方法中开启了while循环，先从缓存中取出entry，如果entry为空的话，将请求加入网络请求的的队列里；如果entry不为空则判断是否过期，如果过期把请求加入网络请求队列中。最后会调用parseNetworkResponse()方法进行数据的解析，之后将解析出来的数据进行回调。
与CacheDispatcher相对应的NetworkDispatcher类也继承自Thread，NetworkDispatcher的run方法如下：
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
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(); //取出响应
} 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); //执行网络请求
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); //解析响应
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); //提交响应
} 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);
}
}
}
可以看到run()方法中开启了while循环，先从缓存中取出request，并用performRequest()方法执行请求，获取响应之后，解析响应，最后用提交响应。
执行的流程图如下：
Volley流程
网友评论