[置顶] Android网络通信Volley框架源码浅析（二）
尊重原创 http://write.blog.csdn.net/postedit/25921795
在前面的一片文章Volley框架浅析（一）中我们知道在RequestQueue这个类中，有两个队列:本地队列和网络队列
/** 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<?>>();
与之对应的分别有本地线程和网络线程，通过对RequestQueue源码的分析，本地线程有一条，而网络线程默认有四条，我们可以对网络线程的个数进行设置，我们首先来学习一下本地线程：
(1) CacheDispatcher.java
public class CacheDispatcher extends Thread {
private static final boolean DEBUG = VolleyLog.DEBUG;
//本地队列，从RequestQueue中传递进来的
private final BlockingQueue<Request<?>> mCacheQueue;
//网络请求队列，也是从RequestQueue中传递进来，当本地缓存没有命中时，需要把请求从本地队列加入网络队列
private final BlockingQueue<Request<?>> mNetworkQueue;
//磁盘缓存对象
private final Cache mCache;
//就是用于从子线程向Ui线程发送数据
private final ResponseDelivery mDelivery;
/** Used for telling us to die. */
private volatile boolean mQuit = false;
/**
* Creates a new cache triage dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param cacheQueue Queue of incoming requests for triage
* @param networkQueue Queue to post requests that require network to
* @param cache Cache interface to use for resolution
* @param delivery Delivery interface to use for posting responses
*/
public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// 缓存初始化，将磁盘中的数据读入内存
mCache.initialize();
while (true) {
try {
// 阻塞式从队列中取出请求
final Request<?> request = mCacheQueue.take();
request.addMarker("cache-queue-take");
// 判断request是否被取消了(调用cancel方法)，如果取消了就不执行，再次到队列中取请求
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
// 从缓存中读取数据
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
//没有命中
request.addMarker("cache-miss");
// 没有命中时，就将请求放入网络队列
mNetworkQueue.put(request);
continue;
}
// 数据已经过期，将请求放入网络队列
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
}
// 本地命中
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.
//命中，并且不需要刷新
mDelivery.postResponse(request, response);
} else {
//命中，需要刷新，将请求放入网络队列，这里面的代码其实可以根据需求自己重写
// 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;
}
}
}
}
(2) NetworkDispatcher.java
public class NetworkDispatcher extends Thread {
/** 网络队列 */
private final BlockingQueue<Request<?>> mQueue;
/** 用于Http请求，根据前面的学习，他其实使用的是HttpURLConnection或者HttpClient. */
private final Network mNetwork;
/** 本地缓存，网络请求成功后，放入缓存. */
private final Cache mCache;
/** For posting responses and errors. */
private final ResponseDelivery mDelivery;
/** Used for telling us to die. */
private volatile boolean mQuit = false;
/**
* Creates a new network dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param queue Queue of incoming requests for triage
* @param network Network interface to use for performing requests
* @param cache Cache interface to use for writing responses to cache
* @param delivery Delivery interface to use for posting responses
*/
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
private void addTrafficStatsTag(Request<?> request) {
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
}
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
Request<?> request;
while (true) {
try {
// 从队列中阻塞式取出一个请求.
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");
// 同理需要判断是否取消，如果取消执行下一个请求
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
addTrafficStatsTag(request);
// 通过NetWork的perfromRequest方法放回一个NetworkResponse对象
NetworkResponse networkResponse = mNetwork.performRequest(request);
request.addMarker("network-http-complete");
// 如果这个返回结果已经发送到了ui线程，就将它finish
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}
// 将NetworkResponse 解析成Response.
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
// 如果需要缓存，那么将结果存入缓存
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
// 标记为已经发送
request.markDelivered();
//将数据发送到Ui线程
mDelivery.postResponse(request, response);
} catch (VolleyError volleyError) {
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
mDelivery.postError(request, new VolleyError(e));
}
}
}
private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}
}
通过上面的代码，我们来总结一下一个请求的执行过程吧：1、一个请求就是一个Request对象，首先将Request对象加入到RequestQueue中.
2、判断Request是否可以缓存，如果可以，则加入到本地缓存队列，否则加入网络队列
3、本地线程不断监听本地队列是否有请求，如果有请求取出来
4、判断Request是否取消，如果取消，处理下一个请求
5、判断缓存是否命中，如果没有命中，将该请求加入网络队列
6、如果命中，但是过期，同样将该请求加入网络队列
7、如果命中，并且不用刷新，那么直接放回结果，不用加入网络队列
8、如果命中，并且需要刷新，那么放回结果，并且加入网络队列
9、同样4条网络线程也在不断监听网络队列是否有请求，一旦发现有请求，取出请求，判断是否取消，如果取消，那么取出下一个请求
10、如果没有取消，那么通过NetWork进行http请求，将请求结果封装成NetworkResponse,然后转换为Response
11、如果可以缓存，那么将数据写入缓存
12、通过Delivery将Response返回到ui线程
通过以上12步，完成了一个完整的请求
研究了这么久，我们还没有研究Request和Response是什么呢，如果熟悉http请求的同学相信很好理解，
Request就是一个http请求，Response就是http返回的内容，先看看Request这个类吧
Request是一个抽象类，我只介绍比较重要的几个方法:
public abstract class Request<T> implements Comparable<Request<T>> {
//Http 请求方法 POST,GET
private final int mMethod;
/** 请求URL*/
private final String mUrl;
//用于出错时的回调接口
private final Response.ErrorListener mErrorListener;
/** 这个请求在队列中的顺序 */
private Integer mSequence;
...
/** 是否可以缓存 */
private boolean mShouldCache = true;
/** 是否已经取消了，网络线程和本地线程都会对此判断，如果取消了就不请求了 */
private boolean mCanceled = false;
/** 请求策略，比如设置最大重试次数之类的*/
private RetryPolicy mRetryPolicy;
/**
* Creates a new request with the given method (one of the values from {@link Method}),
* URL, and error listener. Note that the normal response listener is not provided here as
* delivery of responses is provided by subclasses, who have a better idea of how to deliver
* an already-parsed response.
*/
public Request(int method, String url, Response.ErrorListener listener) {
mMethod = method;
mUrl = url;
mErrorListener = listener;
setRetryPolicy(new DefaultRetryPolicy());
mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}
/**
* Sets the retry policy for this request.
*
* @return This Request object to allow for chaining.
*/
public Request<?> setRetryPolicy(RetryPolicy retryPolicy) {
mRetryPolicy = retryPolicy;
return this;
}
...
/**
* 通过此方法取消一个请求
*/
public void cancel() {
mCanceled = true;
}
/**
* 判断是否已经取消.
*/
public boolean isCanceled() {
return mCanceled;
}
/**
* 获取请求头
* @throws AuthFailureError In the event of auth failure
*/
public Map<String, String> getHeaders() throws AuthFailureError {
return Collections.emptyMap();
}
/**
* Returns a Map of POST parameters to be used for this request, or null if
* a simple GET should be used. Can throw {@link AuthFailureError} as
* authentication may be required to provide these values.
*
* <p>Note that only one of getPostParams() and getPostBody() can return a non-null
* value.</p>
* @throws AuthFailureError In the event of auth failure
*
* @deprecated Use {@link #getParams()} instead.
*/
@Deprecated
protected Map<String, String> getPostParams() throws AuthFailureError {
return getParams();
}
/**
* Returns a Map of parameters to be used for a POST or PUT request. Can throw
* {@link AuthFailureError} as authentication may be required to provide these values.
*
* <p>Note that you can directly override {@link #getBody()} for custom data.</p>
*
* @throws AuthFailureError in the event of auth failure
*/
protected Map<String, String> getParams() throws AuthFailureError {
return null;
}
public String getBodyContentType() {
return "application/x-www-form-urlencoded; charset=" + getParamsEncoding();
}
/**
* 设置能否缓存
*
* @return This Request object to allow for chaining.
*/
public final Request<?> setShouldCache(boolean shouldCache) {
mShouldCache = shouldCache;
return this;
}
/**
* 判断是否能够缓存
*/
public final boolean shouldCache() {
return mShouldCache;
}
/**
* 这是个抽象方法，我们必须实现，用于将NetworkResponse 转化为Response
* @param response Response from the network
* @return The parsed response, or null in the case of an error
*/
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);
/**
* 这个我们也必须实现，用于将Response发送到ui线程
* @param response The parsed response returned by
* {@link #parseNetworkResponse(NetworkResponse)}
*/
abstract protected void deliverResponse(T response);
}
下面继续看看Response这个类：
public class Response<T> {
/** 成功的时候回调. */
public interface Listener<T> {
/** Called when a response is received. */
public void onResponse(T response);
}
/** 失败的时候回调 */
public interface ErrorListener {
/**
* Callback method that an error has been occurred with the
* provided error code and optional user-readable message.
*/
public void onErrorResponse(VolleyError error);
}
/** 成功的时候创建一个Response. */
public static <T> Response<T> success(T result, Cache.Entry cacheEntry) {
return new Response<T>(result, cacheEntry);
}
/**
* 失败的时候创建一个Response
*/
public static <T> Response<T> error(VolleyError error) {
return new Response<T>(error);
}
/** Parsed response, or null in the case of error. */
public final T result;
/**
* Returns whether this response is considered successful.
*/
public boolean isSuccess() {
return error == null;
}
//私有的，我们无法调用
private Response(T result, Cache.Entry cacheEntry) {
this.result = result;
this.cacheEntry = cacheEntry;
this.error = null;
}
private Response(VolleyError error) {
this.result = null;
this.cacheEntry = null;
this.error = error;
}
}
学习了上面两个类后，我们需要知道如下知识：
Volley中的任何请求都是继承Request的，如Volley提供的StringRequest,JsonArrayRequest,JsonObjectRequest
ImageRequest等等，并且要实现其中的两个方法
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);
abstract protected void deliverResponse(T response);
T是泛型，StringRequest中T表示String,后期我将会简单介绍这几种Request的使用，敬请大家期待。。。
最后在介绍一个接口，就是ResponseDelivery.java
它的一个实现类是ExecutorDelivery.java
public class ExecutorDelivery implements ResponseDelivery {
/** 执行已提交的 Runnable 任务的对象。此接口提供一种将任务提交与每个任务将如何运行的机制（包括线程使用的细节、调度等）分离开来的方法，在线程池中经常用到 */
private final Executor mResponsePoster;
/**
* 传入一个Handler,其实就是运行在主线的Handler，我想你应该明白为什么他能够从子线程
将数据传入ui线程了
* @param handler {@link Handler} to post responses on
*/
public ExecutorDelivery(final Handler handler) {
// Make an Executor that just wraps the handler.
mResponsePoster = new Executor() {
@Override
public void execute(Runnable command) {
//这里调用了handler的post方法
handler.post(command);
}
};
}
/**
* Creates a new response delivery interface, mockable version
* for testing.
* @param executor For running delivery tasks
*/
public ExecutorDelivery(Executor executor) {
mResponsePoster = executor;
}
@Override
public void postResponse(Request<?> request, Response<?> response) {
postResponse(request, response, null);
}
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
/**
* A Runnable used for delivering network responses to a listener on the
* main thread.
*/
@SuppressWarnings("rawtypes")
private class ResponseDeliveryRunnable implements Runnable {
private final Request mRequest;
private final Response mResponse;
private final Runnable mRunnable;
public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
mRequest = request;
mResponse = response;
mRunnable = runnable;
}
@SuppressWarnings("unchecked")
@Override
public void run() {
// If this request has canceled, finish it and don't deliver.
if (mRequest.isCanceled()) {
mRequest.finish("canceled-at-delivery");
return;
}
// Deliver a normal response or error, depending.
if (mResponse.isSuccess()) {
//在这里调用了deliverResponse
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}
// If this is an intermediate response, add a marker, otherwise we're done
// and the request can be finished.
if (mResponse.intermediate) {
mRequest.addMarker("intermediate-response");
} else {
mRequest.finish("done");
}
// If we have been provided a post-delivery runnable, run it.
if (mRunnable != null) {
mRunnable.run();
}
}
}
}
好了，今天就写到这里吧，大家有什么不明白的欢迎留言讨论....