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<?>>();

(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;
            }
        }
    }
}

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 one rrorResponse(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;
    }
}

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();
            }
       }
    }
}