Android网络通信Volley框架源代码浅析（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....