Android网络通信Volley框架源代码浅析（二）

尊重原创 http://write.blog.youkuaiyun.com/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<?

>>();

与之相应的分别有本地线程和网络线程。通过对RequestQueue源代码的分析，本地线程有一条。而网络线程默认有四条，我们能够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。须要把请求从本地队列增加网络队列
    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 { // 堵塞式从队列中取出请求 final Request<?> request = mCacheQueue.take(); request.addMarker("cache-queue-take"); // 推断request是否被取消了(调用cancel方法)。假设取消了就不运行，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，而且不须要刷新 mDelivery.postResponse(request, response); } else { //命中，须要刷新，将请求放入网络队列，这里面的代码事实上能够依据需求自己重写 // 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，依据前面的学习，他事实上使用的是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 { // 从队列中堵塞式取出一个请求. 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"); // 同理须要推断是否取消，假设取消运行下一个请求 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"); // 假设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"); // 假设须要缓存。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运行过程吧：
1、一个请求就是一个Request对象，首先将Request对象增加到RequestQueue中.
2、推断Request能否够缓存，假设能够。则增加到本地缓存队列，否则增加网络队列
3、本地线程不断监听本地队列是否有请求。假设有请求取出来
4、推断Request是否取消，假设取消，处理下一个请求
5、推断缓存是否命中，假设没有命中。将该请求增加网络队列
6、假设命中，可是过期，相同将该请求增加网络队列
7、假设命中。而且不用刷新，那么直接放回结果。不用增加网络队列
8、假设命中，而且须要刷新。那么放回结果，而且增加网络队列
9、相同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，推断是否取消，假设取消。那么取出下一个请求
10、假设没有取消，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、假设能够缓存，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，完毕了一个完整的请求

研究了这么久。我们还没有研究Request和Response是什么呢，假设熟悉http请求的同学相信非常好理解。
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。我仅仅介绍比較重要的几个方法:

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推断，假设取消了就不请求了 */
    private boolean mCanceled = false;

    /** 请求策略，比方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;
    }

    /**
     * 推断是否已经取消.
     */
    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();
    }

    
    

    /**
     * 设置是否能缓存
     *
     * @return This Request object to allow for chaining.
     */
    public final Request<?

> setShouldCache(boolean shouldCache) { mShouldCache = shouldCache; return this; } /** * 推断是否可以缓存 */ 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); } 以下继续看看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，我们须要知道例如以下知识：
Volley中的不论什么请求都是继承Request的。如Volley提供的StringRequest,JsonArrayRequest,JsonObjectRequest
ImageRequest等等。而且要实现当中的两个方法
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

abstract protected void deliverResponse(T response);

T是泛型，StringRequest中T表示String,后期我将会简介这几种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一个任务将怎样执行的机制（包含线程使用的细节、调度等）分离开来的方法。在线程池中经经常使用到 */
    private final Executor mResponsePoster;

    /**
     * 传入一个Handler,事实上就是执行在主线的Handler。我想你应该明确为什么他可以从子线程
	 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，大家有什么不明确的欢迎留言讨论....