Volley框架源码解析

Volley 是 Google 在2013年 Google I/O 大会上推出的一款 Android 异步网络请求框架。名字的由来：

a burst or emission of many things or a large amount at once.

它的特点：适合 数据量小、通信频繁 的网络操作。我们可以从 github 上下载它的源码，地址：https://github.com/google/volley

Volley 整体设计

新建一个 Request 请求，将其添加到 RequestQueue 中，这里有一个判断,首先新建一个 CacheDispatcher(继承自 Thread) 判断内存中是否存在，如果存在就从内存中去；否则新建 NetworkDispatcher 去执行网络请求。最后通过 ResponseDelivery 处理反馈回来的数据。

Volley 中相关概念

Volley

一个工具类，它里面只有简单的几行代码，通过 newRequestQueue(…) 函数新建并启动一个请求队列 RequestQueue。

public class Volley {

    /** Default on-disk cache directory. */
    private static final String DEFAULT_CACHE_DIR = "volley";

    /**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @param context A {@link Context} to use for creating the cache dir.
     * @param stack An {@link HttpStack} to use for the network, or null for default.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);

        String userAgent = "volley/0";
        try {
            String packageName = context.getPackageName();
            PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);
            userAgent = packageName + "/" + info.versionCode;
        } catch (NameNotFoundException e) {
        }

        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                // Prior to Gingerbread, HttpUrlConnection was unreliable.
                // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);

        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    }

    /**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @param context A {@link Context} to use for creating the cache dir.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context) {
        return newRequestQueue(context, null);
    }
}

上述代码中，进行了系统版本的判断，如果版本号大于9，就选择 HurlStack,否则就 HttpClientStack，HurlStack 是基于 HttpUrlConnection 的，HttpClientStack 是基于 Apache HttpClient 的；最主要的两句代码：

RequestQueue queue = new RequestQueue(newDiskBasedCache(cacheDir), network);
queue.start();

在 RequestQueue 中需要传入两个参数，第一个参数是缓存路径，第二个参数是 Network。通过 start() 方法来完成队列的创建和启动。

Request

它是一个抽象类，如果要实现请求，必须继承于它，StringRequest、JsonRequest、ImageRequest 都是它的子类。

public abstract class Request<T> implements Comparable<Request<T>> {}

子类必须重写它的两个方法：

// 用于将网络返回的原生子节内容转换成合适的类型，在工作线程调用
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);
// 将解析成合适类型的内容传递给它们的监听回调
abstract protected void deliverResponse(T response);

Request 代码如下：

/**
 * Base class for all network requests.
 *
 * @param <T> The type of parsed response this request expects.
 */
public abstract class Request<T> implements Comparable<Request<T>> {

    /**
     * Default encoding for POST or PUT parameters. See {@link #getParamsEncoding()}.
     */
    private static final String DEFAULT_PARAMS_ENCODING = "UTF-8";

    /**
     * Supported request methods.
     */
    public interface Method {
        int DEPRECATED_GET_OR_POST = -1;
        int GET = 0;
        int POST = 1;
        int PUT = 2;
        int DELETE = 3;
        int HEAD = 4;
        int OPTIONS = 5;
        int TRACE = 6;
        int PATCH = 7;
    }

    /** An event log tracing the lifetime of this request; for debugging. */
    private final MarkerLog mEventLog = MarkerLog.ENABLED ? new MarkerLog() : null;

    /**
     * Request method of this request.  Currently supports GET, POST, PUT, DELETE, HEAD, OPTIONS,
     * TRACE, and PATCH.
     */
    private final int mMethod;

    /** URL of this request. */
    private final String mUrl;

    /** Default tag for {@link TrafficStats}. */
    private final int mDefaultTrafficStatsTag;

    /** Listener interface for errors. */
    private final Response.ErrorListener mErrorListener;

    /** Sequence number of this request, used to enforce FIFO ordering. */
    private Integer mSequence;

    /** The request queue this request is associated with. */
    private RequestQueue mRequestQueue;

    /** Whether or not responses to this request should be cached. */
    private boolean mShouldCache = true;

    /** Whether or not this request has been canceled. */
    private boolean mCanceled = false;

    /** Whether or not a response has been delivered for this request yet. */
    private boolean mResponseDelivered = false;

    /** Whether the request should be retried in the event of an HTTP 5xx (server) error. */
    private boolean mShouldRetryServerErrors = false;

    /** The retry policy for this request. */
    private RetryPolicy mRetryPolicy;

    /**
     * When a request can be retrieved from cache but must be refreshed from
     * the network, the cache entry will be stored here so that in the event of
     * a "Not Modified" response, we can be sure it hasn't been evicted from cache.
     */
    private Cache.Entry mCacheEntry = null;

    /** An opaque token tagging this request; used for bulk cancellation. */
    private Object mTag;

    /**
     * Creates a new request with the given 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.
     *
     * @deprecated Use {@link #Request(int, String, com.android.volley.Response.ErrorListener)}.
     */
    @Deprecated
    public Request(String url, Response.ErrorListener listener) {
        this(Method.DEPRECATED_GET_OR_POST, url, listener);
    }

    /**
     * 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);
    }

    /**
     * Return the method for this request.  Can be one of the values in {@link Method}.
     */
    public int getMethod() {
        return mMethod;
    }

    /**
     * Set a tag on this request. Can be used to cancel all requests with this
     * tag by {@link RequestQueue#cancelAll(Object)}.
     *
     * @return This Request object to allow for chaining.
     */
    public Request<?> setTag(Object tag) {
        mTag = tag;
        return this;
    }

    /**
     * Returns this request's tag.
     * @see Request#setTag(Object)
     */
    public Object getTag() {
        return mTag;
    }

    /**
     * @return this request's {@link com.android.volley.Response.ErrorListener}.
     */
    public Response.ErrorListener getErrorListener() {
        return mErrorListener;
    }

    /**
     * @return A tag for use with {@link TrafficStats#setThreadStatsTag(int)}
     */
    public int getTrafficStatsTag() {
        return mDefaultTrafficStatsTag;
    }

    /**
     * @return The hashcode of the URL's host component, or 0 if there is none.
     */
    private static int findDefaultTrafficStatsTag(String url) {
        if (!TextUtils.isEmpty(url)) {
            Uri uri = Uri.parse(url);
            if (uri != null) {
                String host = uri.getHost();
                if (host != null) {
                    return host.hashCode();
                }
            }
        }
        return 0;
    }

    /**
     * Sets the retry policy for this request.
     *
     * @return This Request object to allow for chaining.
     */
    public Request<?> setRetryPolicy(RetryPolicy retryPolicy) {
        mRetryPolicy = retryPolicy;
        return this;
    }

    /**
     * Adds an event to this request's event log; for debugging.
     */
    public void addMarker(String tag) {
        if (MarkerLog.ENABLED) {
            mEventLog.add(tag, Thread.currentThread().getId());
        }
    }

    /**
     * Notifies the request queue that this request has finished (successfully or with error).
     *
     * <p>Also dumps all events from this request's event log; for debugging.</p>
     */
    void finish(final String tag) {
        if (mRequestQueue != null) {
            mRequestQueue.finish(this);
        }
        if (MarkerLog.ENABLED) {
            final long threadId = Thread.currentThread().getId();
            if (Looper.myLooper() != Looper.getMainLooper()) {
                // If we finish marking off of the main thread, we need to
                // actually do it on the main thread to ensure correct ordering.
                Handler mainThread = new Handler(Looper.getMainLooper());
                mainThread.post(new Runnable() {
                    @Override
                    public void run() {
                        mEventLog.add(tag, threadId);
                        mEventLog.finish(this.toString());
                    }
                });
                return;
            }

            mEventLog.add(tag, threadId);
            mEventLog.finish(this.toString());
        }
    }

    /**
     * Associates this request with the given queue. The request queue will be notified when this
     * request has finished.
     *
     * @return This Request object to allow for chaining.
     */
    public Request<?> setRequestQueue(RequestQueue requestQueue) {
        mRequestQueue = requestQueue;
        return this;
    }

    /**
     * Sets the sequence number of this request.  Used by {@link RequestQueue}.
     *
     * @return This Request object to allow for chaining.
     */
    public final Request<?> setSequence(int sequence) {
        mSequence = sequence;
        return this;
    }

    /**
     * Returns the sequence number of this request.
     */
    public final int getSequence() {
        if (mSequence == null) {
            throw new IllegalStateException("getSequence called before setSequence");
        }
        return mSequence;
    }

    /**
     * Returns the URL of this request.
     */
    public String getUrl() {
        return mUrl;
    }

    /**
     * Returns the cache key for this request.  By default, this is the URL.
     */
    public String getCacheKey() {
        return getUrl();
    }

    /**
     * Annotates this request with an entry retrieved for it from cache.
     * Used for cache coherency support.
     *
     * @return This Request object to allow for chaining.
     */
    public Request<?> setCacheEntry(Cache.Entry entry) {
        mCacheEntry = entry;
        return this;
    }

    /**
     * Returns the annotated cache entry, or null if there isn't one.
     */
    public Cache.Entry getCacheEntry() {
        return mCacheEntry;
    }

    /**
     * Mark this request as canceled.  No callback will be delivered.
     */
    public void cancel() {
        mCanceled = true;
    }

    /**
     * Returns true if this request has been canceled.
     */
    public boolean isCanceled() {
        return mCanceled;
    }

    /**
     * Returns a list of extra HTTP headers to go along with this request. Can
     * throw {@link AuthFailureError} as authentication may be required to
     * provide these values.
     * @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 which encoding should be used when converting POST parameters returned by
     * {@link #getPostParams()} into a raw POST body.
     *
     * <p>This controls both encodings:
     * <ol>
     *     <li>The string encoding used when converting parameter names and values into bytes prior
     *         to URL encoding them.</li>
     *     <li>The string encoding used when converting the URL encoded parameters into a raw
     *         byte array.</li>
     * </ol>
     *
     * @deprecated Use {@link #getParamsEncoding()} instead.
     */
    @Deprecated
    protected String getPostParamsEncoding() {
        return getParamsEncoding();
    }

    /**
     * @deprecated Use {@link #getBodyContentType()} instead.
     */
    @Deprecated
    public String getPostBodyContentType() {
        return getBodyContentType();
    }

    /**
     * Returns the raw POST body to be sent.
     *
     * @throws AuthFailureError In the event of auth failure
     *
     * @deprecated Use {@link #getBody()} instead.
     */
    @Deprecated
    public byte[] getPostBody() throws AuthFailureError {
        // Note: For compatibility with legacy clients of volley, this implementation must remain
        // here instead of simply calling the getBody() function because this function must
        // call getPostParams() and getPostParamsEncoding() since legacy clients would have
        // overridden these two member functions for POST requests.
        Map<String, String> postParams = getPostParams();
        if (postParams != null && postParams.size() > 0) {
            return encodeParameters(postParams, getPostParamsEncoding());
        }
        return null;
    }

    /**
     * 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;
    }

    /**
     * Returns which encoding should be used when converting POST or PUT parameters returned by
     * {@link #getParams()} into a raw POST or PUT body.
     *
     * <p>This controls both encodings:
     * <ol>
     *     <li>The string encoding used when converting parameter names and values into bytes prior
     *         to URL encoding them.</li>
     *     <li>The string encoding used when converting the URL encoded parameters into a raw
     *         byte array.</li>
     * </ol>
     */
    protected String getParamsEncoding() {
        return DEFAULT_PARAMS_ENCODING;
    }

    /**
     * Returns the content type of the POST or PUT body.
     */
    public String getBodyContentType() {
        return "application/x-www-form-urlencoded; charset=" + getParamsEncoding();
    }

    /**
     * Returns the raw POST or PUT body to be sent.
     *
     * <p>By default, the body consists of the request parameters in
     * application/x-www-form-urlencoded format. When overriding this method, consider overriding
     * {@link #getBodyContentType()} as well to match the new body format.
     *
     * @throws AuthFailureError in the event of auth failure
     */
    public byte[] getBody() throws AuthFailureError {
        Map<String, String> params = getParams();
        if (params != null && params.size() > 0) {
            return encodeParameters(params, getParamsEncoding());
        }
        return null;
    }

    /**
     * Converts <code>params</code> into an application/x-www-form-urlencoded encoded string.
     */
    private byte[] encodeParameters(Map<String, String> params, String paramsEncoding) {
        StringBuilder encodedParams = new StringBuilder();
        try {
            for (Map.Entry<String, String> entry : params.entrySet()) {
                encodedParams.append(URLEncoder.encode(entry.getKey(), paramsEncoding));
                encodedParams.append('=');
                encodedParams.append(URLEncoder.encode(entry.getValue(), paramsEncoding));
                encodedParams.append('&');
            }
            return encodedParams.toString().getBytes(paramsEncoding);
        } catch (UnsupportedEncodingException uee) {
            throw new RuntimeException("Encoding not supported: " + paramsEncoding, uee);
        }
    }

    /**
     * Set whether or not responses to this request should be cached.
     *
     * @return This Request object to allow for chaining.
     */
    public final Request<?> setShouldCache(boolean shouldCache) {
        mShouldCache = shouldCache;
        return this;
    }

    /**
     * Returns true if responses to this request should be cached.
     */
    public final boolean shouldCache() {
        return mShouldCache;
    }

    /**
     * Sets whether or not the request should be retried in the event of an HTTP 5xx (server) error.
     *
     * @return This Request object to allow for chaining.
     */
    public final Request<?> setShouldRetryServerErrors(boolean shouldRetryServerErrors) {
        mShouldRetryServerErrors = shouldRetryServerErrors;
        return this;
    }

    /**
     * Returns true if this request should be retried in the event of an HTTP 5xx (server) error.
     */
    public final boolean shouldRetryServerErrors() {
        return mShouldRetryServerErrors;
    }

    /**
     * Priority values.  Requests will be processed from higher priorities to
     * lower priorities, in FIFO order.
     */
    public enum Priority {
        LOW,
        NORMAL,
        HIGH,
        IMMEDIATE
    }

    /**
     * Returns the {@link Priority} of this request; {@link Priority#NORMAL} by default.
     */
    public Priority getPriority() {
        return Priority.NORMAL;
    }

    /**
     * Returns the socket timeout in milliseconds per retry attempt. (This value can be changed
     * per retry attempt if a backoff is specified via backoffTimeout()). If there are no retry
     * attempts remaining, this will cause delivery of a {@link TimeoutError} error.
     */
    public final int getTimeoutMs() {
        return mRetryPolicy.getCurrentTimeout();
    }

    /**
     * Returns the retry policy that should be used  for this request.
     */
    public RetryPolicy getRetryPolicy() {
        return mRetryPolicy;
    }

    /**
     * Mark this request as having a response delivered on it.  This can be used
     * later in the request's lifetime for suppressing identical responses.
     */
    public void markDelivered() {
        mResponseDelivered = true;
    }

    /**
     * Returns true if this request has had a response delivered for it.
     */
    public boolean hasHadResponseDelivered() {
        return mResponseDelivered;
    }

    /**
     * Subclasses must implement this to parse the raw network response
     * and return an appropriate response type. This method will be
     * called from a worker thread.  The response will not be delivered
     * if you return null.
     * @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);

    /**
     * Subclasses can override this method to parse 'networkError' and return a more specific error.
     *
     * <p>The default implementation just returns the passed 'networkError'.</p>
     *
     * @param volleyError the error retrieved from the network
     * @return an NetworkError augmented with additional information
     */
    protected VolleyError parseNetworkError(VolleyError volleyError) {
        return volleyError;
    }

    /**
     * Subclasses must implement this to perform delivery of the parsed
     * response to their listeners.  The given response is guaranteed to
     * be non-null; responses that fail to parse are not delivered.
     * @param response The parsed response returned by
     * {@link #parseNetworkResponse(NetworkResponse)}
     */
    abstract protected void deliverResponse(T response);

    /**
     * Delivers error message to the ErrorListener that the Request was
     * initialized with.
     *
     * @param error Error details
     */
    public void deliverError(VolleyError error) {
        if (mErrorListener != null) {
            mErrorListener.onErrorResponse(error);
        }
    }

    /**
     * Our comparator sorts from high to low priority, and secondarily by
     * sequence number to provide FIFO ordering.
     */
    @Override
    public int compareTo(Request<T> other) {
        Priority left = this.getPriority();
        Priority right = other.getPriority();

        // High-priority requests are "lesser" so they are sorted to the front.
        // Equal priorities are sorted by sequence number to provide FIFO ordering.
        return left == right ?
                this.mSequence - other.mSequence :
                right.ordinal() - left.ordinal();
    }

    @Override
    public String toString() {
        String trafficStatsTag = "0x" + Integer.toHexString(getTrafficStatsTag());
        return (mCanceled ? "[X] " : "[ ] ") + getUrl() + " " + trafficStatsTag + " "
                + getPriority() + " " + mSequence;
    }
}

Request 实现了 Comparable 接口，说明 Request 是可以互相比较的，比较的内容就是两个 Request 的 priority。

RequestQueue

Volley 框架的核心类，将请求 Request 加入到 RequestQueue 中，来完成请求操作。代码如下：

/**
 * A request dispatch queue with a thread pool of dispatchers.
 *
 * Calling {@link #add(Request)} will enqueue the given Request for dispatch,
 * resolving from either cache or network on a worker thread, and then delivering
 * a parsed response on the main thread.
 */
public class RequestQueue {

    /** Callback interface for completed requests. */
    public interface RequestFinishedListener<T> {
        /** Called when a request has finished processing. */
        void onRequestFinished(Request<T> request);
    }

    /** Used for generating monotonically-increasing sequence numbers for requests. */
    private final AtomicInteger mSequenceGenerator = new AtomicInteger();

    /**
     * Staging area for requests that already have a duplicate request in flight.
     *
     * <ul>
     *     <li>containsKey(cacheKey) indicates that there is a request in flight for the given cache
     *          key.</li>
     *     <li>get(cacheKey) returns waiting requests for the given cache key. The in flight request
     *          is <em>not</em> contained in that list. Is null if no requests are staged.</li>
     * </ul>
     */
    private final Map<String, Queue<Request<?>>> mWaitingRequests =
            new HashMap<>();

    /**
     * The set of all requests currently being processed by this RequestQueue. A Request
     * will be in this set if it is waiting in any queue or currently being processed by
     * any dispatcher.
     */
    private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

    /** The cache triage queue. */
    private final PriorityBlockingQueue<Request<?>> mCacheQueue =
            new PriorityBlockingQueue<>();

    /** The queue of requests that are actually going out to the network. */
    private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
            new PriorityBlockingQueue<>();

    /** Number of network request dispatcher threads to start. */
    private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;

    /** Cache interface for retrieving and storing responses. */
    private final Cache mCache;

    /** Network interface for performing requests. */
    private final Network mNetwork;

    /** Response delivery mechanism. */
    private final ResponseDelivery mDelivery;

    /** The network dispatchers. */
    private final NetworkDispatcher[] mDispatchers;

    /** The cache dispatcher. */
    private CacheDispatcher mCacheDispatcher;

    private final List<RequestFinishedListener> mFinishedListeners =
            new ArrayList<>();

    /**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     * @param delivery A ResponseDelivery interface for posting responses and errors
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize,
            ResponseDelivery delivery) {
        mCache = cache;
        mNetwork = network;
        mDispatchers = new NetworkDispatcher[threadPoolSize];
        mDelivery = delivery;
    }

    /**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize) {
        this(cache, network, threadPoolSize,
                new ExecutorDelivery(new Handler(Looper.getMainLooper())));
    }

    /**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     */
    public RequestQueue(Cache cache, Network network) {
        this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
    }

    /**
     * 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();
        }
    }

    /**
     * Stops the cache and network dispatchers.
     */
    public void stop() {
        if (mCacheDispatcher != null) {
            mCacheDispatcher.quit();
        }
        for (final NetworkDispatcher mDispatcher : mDispatchers) {
            if (mDispatcher != null) {
                mDispatcher.quit();
            }
        }
    }

    /**
     * Gets a sequence number.
     */
    public int getSequenceNumber() {
        return mSequenceGenerator.incrementAndGet();
    }

    /**
     * Gets the {@link Cache} instance being used.
     */
    public Cache getCache() {
        return mCache;
    }

    /**
     * A simple predicate or filter interface for Requests, for use by
     * {@link RequestQueue#cancelAll(RequestFilter)}.
     */
    public interface RequestFilter {
        boolean apply(Request<?> request);
    }

    /**
     * Cancels all requests in this queue for which the given filter applies.
     * @param filter The filtering function to use
     */
    public void cancelAll(RequestFilter filter) {
        synchronized (mCurrentRequests) {
            for (Request<?> request : mCurrentRequests) {
                if (filter.apply(request)) {
                    request.cancel();
                }
            }
        }
    }

    /**
     * Cancels all requests in this queue with the given tag. Tag must be non-null
     * and equality is by identity.
     */
    public void cancelAll(final Object tag) {
        if (tag == null) {
            throw new IllegalArgumentException("Cannot cancelAll with a null tag");
        }
        cancelAll(new RequestFilter() {
            @Override
            public boolean apply(Request<?> request) {
                return request.getTag() == tag;
            }
        });
    }

    /**
     * Adds a Request to the dispatch queue.
     * @param request The request to service
     * @return The passed-in request
     */
    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);
        }

        // Process requests in the order they are added.
        request.setSequence(getSequenceNumber());
        request.addMarker("add-to-queue");

        // If the request is uncacheable, skip the cache queue and go straight to the network.
        if (!request.shouldCache()) {
            mNetworkQueue.add(request);
            return request;
        }

        // Insert request into stage if there's already a request with the same cache key in flight.
        synchronized (mWaitingRequests) {
            String cacheKey = request.getCacheKey();
            if (mWaitingRequests.containsKey(cacheKey)) {
                // There is already a request in flight. Queue up.
                Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
                if (stagedRequests == null) {
                    stagedRequests = new LinkedList<>();
                }
                stagedRequests.add(request);
                mWaitingRequests.put(cacheKey, stagedRequests);
                if (VolleyLog.DEBUG) {
                    VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
                }
            } else {
                // Insert 'null' queue for this cacheKey, indicating there is now a request in
                // flight.
                mWaitingRequests.put(cacheKey, null);
                mCacheQueue.add(request);
            }
            return request;
        }
    }

    /**
     * Called from {@link Request#finish(String)}, indicating that processing of the given request
     * has finished.
     *
     * <p>Releases waiting requests for <code>request.getCacheKey()</code> if
     *      <code>request.shouldCache()</code>.</p>
     */
    <T> void finish(Request<T> request) {
        // Remove from the set of requests currently being processed.
        synchronized (mCurrentRequests) {
            mCurrentRequests.remove(request);
        }
        synchronized (mFinishedListeners) {
          for (RequestFinishedListener<T> listener : mFinishedListeners) {
            listener.onRequestFinished(request);
          }
        }

        if (request.shouldCache()) {
            synchronized (mWaitingRequests) {
                String cacheKey = request.getCacheKey();
                Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
                if (waitingRequests != null) {
                    if (VolleyLog.DEBUG) {
                        VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
                                waitingRequests.size(), cacheKey);
                    }
                    // Process all queued up requests. They won't be considered as in flight, but
                    // that's not a problem as the cache has been primed by 'request'.
                    mCacheQueue.addAll(waitingRequests);
                }
            }
        }
    }

    public  <T> void addRequestFinishedListener(RequestFinishedListener<T> listener) {
      synchronized (mFinishedListeners) {
        mFinishedListeners.add(listener);
      }
    }

    /**
     * Remove a RequestFinishedListener. Has no effect if listener was not previously added.
     */
    public  <T> void removeRequestFinishedListener(RequestFinishedListener<T> listener) {
      synchronized (mFinishedListeners) {
        mFinishedListeners.remove(listener);
      }
    }
}

• 1.主要成员变量

在 RequestQueue 中，维护了两个基于优先级的 Request 队列：缓存请求队列和网络请求队列。

／** The cache triage queue. */
    private final PriorityBlockingQueue<Request<?>> mCacheQueue =
            new PriorityBlockingQueue<>();

    /** The queue of requests that are actually going out to the network. */
    private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
            new PriorityBlockingQueue<>();

维护了一个正在进行中，尚未完成的请求集合：

/**
     * The set of all requests currently being processed by this RequestQueue. A Request
     * will be in this set if it is waiting in any queue or currently being processed by
     * any dispatcher.
     */
    private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

维护了一个等待请求的集合，如果一个请求正在被处理并且可以被缓存，后续的相同的 url 的请求，将进入到此等待队列中。

/**
     * Staging area for requests that already have a duplicate request in flight.
     *
     * <ul>
     *     <li>containsKey(cacheKey) indicates that there is a request in flight for the given cache
     *          key.</li>
     *     <li>get(cacheKey) returns waiting requests for the given cache key. The in flight request
     *          is <em>not</em> contained in that list. Is null if no requests are staged.</li>
     * </ul>
     */
    private final Map<String, Queue<Request<?>>> mWaitingRequests =
            new HashMap<>();

RequestQueue 的初始化，如下：

/**
     * Creates the worker pool. Processing will not begin until {@link #start()} is called.
     *
     * @param cache A Cache to use for persisting responses to disk
     * @param network A Network interface for performing HTTP requests
     * @param threadPoolSize Number of network dispatcher threads to create
     * @param delivery A ResponseDelivery interface for posting responses and errors
     */
    public RequestQueue(Cache cache, Network network, int threadPoolSize,
            ResponseDelivery delivery) {
        mCache = cache;
        mNetwork = network;
        mDispatchers = new NetworkDispatcher[threadPoolSize];
        mDelivery = delivery;
    }

• 2.启动队列

调用 start() 方法，启动队列。

/**
 * 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();
    }
}

在上述代码中，首先调用 stop() 结束所有的缓存请求和网络请求。之后开启一个 缓存调度线程 CacheDispatcher 和默认是4个的网络调度线程 NetworkDispatcher，缓存调度线程不断的从缓存请求队列中取出 Request 去处理，网络调度线程不断地从网络请求队列中取出 Request 去处理。stop() 代码如下：

/**
 * Stops the cache and network dispatchers.
 */
public void stop() {
    if (mCacheDispatcher != null) {
        mCacheDispatcher.quit();
    }
    for (final NetworkDispatcher mDispatcher : mDispatchers) {
        if (mDispatcher != null) {
            mDispatcher.quit();
        }
    }
}

• 3.加入请求

调用 add() 方法完成 Request 添加到 RequestQueue 的过程。代码如下：

/**
 * Adds a Request to the dispatch queue.
 * @param request The request to service
 * @return The passed-in request
 */
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);
    }

    // Process requests in the order they are added.
    request.setSequence(getSequenceNumber());
    request.addMarker("add-to-queue");

    // If the request is uncacheable, skip the cache queue and go straight to the network.
    if (!request.shouldCache()) {
        mNetworkQueue.add(request);
        return request;
    }

    // Insert request into stage if there's already a request with the same cache key in flight.
    synchronized (mWaitingRequests) {
        String cacheKey = request.getCacheKey();
        if (mWaitingRequests.containsKey(cacheKey)) {
            // There is already a request in flight. Queue up.
            Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
            if (stagedRequests == null) {
                stagedRequests = new LinkedList<>();
            }
            stagedRequests.add(request);
            mWaitingRequests.put(cacheKey, stagedRequests);
            if (VolleyLog.DEBUG) {
                VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
            }
        } else {
            // Insert 'null' queue for this cacheKey, indicating there is now a request in
            // flight.
            mWaitingRequests.put(cacheKey, null);
            mCacheQueue.add(request);
        }
        return request;
    }
}

上述代码的流程图如下：

• 4.请求完成

调用 finish()方法结束 Request 请求，代码如下：

/**
 * Called from {@link Request#finish(String)}, indicating that processing of the given request
 * has finished.
 *
 * <p>Releases waiting requests for <code>request.getCacheKey()</code> if
 *      <code>request.shouldCache()</code>.</p>
 */
<T> void finish(Request<T> request) {
    // Remove from the set of requests currently being processed.
    synchronized (mCurrentRequests) {
        mCurrentRequests.remove(request);
    }
    synchronized (mFinishedListeners) {
      for (RequestFinishedListener<T> listener : mFinishedListeners) {
        listener.onRequestFinished(request);
      }
    }

    if (request.shouldCache()) {
        synchronized (mWaitingRequests) {
            String cacheKey = request.getCacheKey();
            Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
            if (waitingRequests != null) {
                if (VolleyLog.DEBUG) {
                    VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
                            waitingRequests.size(), cacheKey);
                }
                // Process all queued up requests. They won't be considered as in flight, but
                // that's not a problem as the cache has been primed by 'request'.
                mCacheQueue.addAll(waitingRequests);
            }
        }
    }
}

在上述代码中，首先从正在进行中请求集合 mCurrentRequests 中移除该请求，然后查找请求等待集合 mWaitingRequests 中是否存在等待请求，如果存在，则将其在等待队列移除，并将等待队列所有的请求添加到缓存请求队列中，让缓存请求处理线程 CacheDispatcher 去处理。

• 5.请求取消

取消当前请求集合中所有符合条件的请求。

/**
     * Cancels all requests in this queue for which the given filter applies.
     * @param filter The filtering function to use
     */
    public void cancelAll(RequestFilter filter) {
        synchronized (mCurrentRequests) {
            for (Request<?> request : mCurrentRequests) {
                if (filter.apply(request)) {
                    request.cancel();
                }
            }
        }
    }

filter 参数表示按照自定义的过滤器需要取消请求。

/**
    * Cancels all requests in this queue with the given tag. Tag must be non-null
    * and equality is by identity.
    */
   public void cancelAll(final Object tag) {
       if (tag == null) {
           throw new IllegalArgumentException("Cannot cancelAll with a null tag");
       }
       cancelAll(new RequestFilter() {
           @Override
           public boolean apply(Request<?> request) {
               return request.getTag() == tag;
           }
       });
   }

tag 表示按照 Request.setTag() 设置好的 tag 取消请求。

CacheDispatcher

继承自Thread,用于处理缓存的请求，不断从缓存队列中取出请求，队列为空则等待，请求处理结束则将结果传递给 ReponseDelivery 去执行后续处理；当结果未缓存过、缓存失效或者缓存需要刷新的情况下，该请求将会交给 NetworkDispatcher 去处理。代码如下：

/**
 * Provides a thread for performing cache triage on a queue of requests.
 *
 * Requests added to the specified cache queue are resolved from cache.
 * Any deliverable response is posted back to the caller via a
 * {@link ResponseDelivery}.  Cache misses and responses that require
 * refresh are enqueued on the specified network queue for processing
 * by a {@link NetworkDispatcher}.
 */
public class CacheDispatcher extends Thread {

    private static final boolean DEBUG = VolleyLog.DEBUG;

    /** The queue of requests coming in for triage. */
    private final BlockingQueue<Request<?>> mCacheQueue;

    /** The queue of requests going out to the network. */
    private final BlockingQueue<Request<?>> mNetworkQueue;

    /** The cache to read from. */
    private final Cache mCache;

    /** For posting responses. */
    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);

        // Make a blocking call to initialize the cache.
        mCache.initialize();

        while (true) {
            try {
                // Get a request from the cache triage queue, blocking until
                // at least one is available.
                final Request<?> request = mCacheQueue.take();
                request.addMarker("cache-queue-take");

                // If the request has been canceled, don't bother dispatching it.
                if (request.isCanceled()) {
                    request.finish("cache-discard-canceled");
                    continue;
                }

                // Attempt to retrieve this item from cache.
                Cache.Entry entry = mCache.get(request.getCacheKey());
                if (entry == null) {
                    request.addMarker("cache-miss");
                    // Cache miss; send off to the network dispatcher.
                    mNetworkQueue.put(request);
                    continue;
                }

                // If it is completely expired, just send it to the network.
                if (entry.isExpired()) {
                    request.addMarker("cache-hit-expired");
                    request.setCacheEntry(entry);
                    mNetworkQueue.put(request);
                    continue;
                }

                // We have a cache hit; parse its data for delivery back to the request.
                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.
                    mDelivery.postResponse(request, response);
                } 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");
                    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;
                }
            }
        }
    }
}

从上述代码中，我们可以知道在 run() 方法中的处理流程如下：

NetworkDispatcher

继承 Thread,用于处理网络请求。启动之后不断的从网络请求队列中取出请求，队列为空则等待，请求处理结束则将结果传递给 ResponseDelivery 去执行后续处理，并判断结果是否需要进行缓存。代码如下：

/**
 * Provides a thread for performing network dispatch from a queue of requests.
 *
 * Requests added to the specified queue are processed from the network via a
 * specified {@link Network} interface. Responses are committed to cache, if
 * eligible, using a specified {@link Cache} interface. Valid responses and
 * errors are posted back to the caller via a {@link ResponseDelivery}.
 */
public class NetworkDispatcher extends Thread {
    /** The queue of requests to service. */
    private final BlockingQueue<Request<?>> mQueue;
    /** The network interface for processing requests. */
    private final Network mNetwork;
    /** The cache to write to. */
    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);
        while (true) {
            long startTimeMs = SystemClock.elapsedRealtime();
            Request<?> request;
            try {
                // Take a request from the queue.
                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");

                // 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);
                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);
                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);
            } 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);
            }
        }
    }

    private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
        error = request.parseNetworkError(error);
        mDelivery.postError(request, error);
    }
}

上述代码中，处理逻辑如下：

Cache

它是一个接口，用于将请求结果进行缓存。里面主要有 put/get/remove/clear() 方法，还有一个实体类 Entry 用于存储该类的成员变量和方法。代码如下：

/**
 * An interface for a cache keyed by a String with a byte array as data.
 */
public interface Cache {
    /**
     * Retrieves an entry from the cache.
     * @param key Cache key
     * @return An {@link Entry} or null in the event of a cache miss
     */
    Entry get(String key);

    /**
     * Adds or replaces an entry to the cache.
     * @param key Cache key
     * @param entry Data to store and metadata for cache coherency, TTL, etc.
     */
    void put(String key, Entry entry);

    /**
     * Performs any potentially long-running actions needed to initialize the cache;
     * will be called from a worker thread.
     */
    void initialize();

    /**
     * Invalidates an entry in the cache.
     * @param key Cache key
     * @param fullExpire True to fully expire the entry, false to soft expire
     */
    void invalidate(String key, boolean fullExpire);

    /**
     * Removes an entry from the cache.
     * @param key Cache key
     */
    void remove(String key);

    /**
     * Empties the cache.
     */
    void clear();

    /**
     * Data and metadata for an entry returned by the cache.
     */
    class Entry {
        /** The data returned from cache. */
        public byte[] data;

        /** ETag for cache coherency. */
        public String etag;

        /** Date of this response as reported by the server. */
        public long serverDate;

        /** The last modified date for the requested object. */
        public long lastModified;

        /** TTL for this record. */
        public long ttl;

        /** Soft TTL for this record. */
        public long softTtl;

        /** Immutable response headers as received from server; must be non-null. */
        public Map<String, String> responseHeaders = Collections.emptyMap();

        /** True if the entry is expired. */
        boolean isExpired() {
            return this.ttl < System.currentTimeMillis();
        }

        /** True if a refresh is needed from the original data source. */
        boolean refreshNeeded() {
            return this.softTtl < System.currentTimeMillis();
        }
    }

}

其中，

• tag — Http 响应首部中用于缓存新鲜度验证的 ETag
• serverDate — Http 响应首部中的响应产生时间
• ttl — 缓存的过期时间
• softTtl — 缓存的新鲜时间
• isExpired — 判断缓存是否过期，过期缓存将不再使用
• refreshNeeded() — 判断缓存是否新鲜，不新鲜的缓存需要发到服务端坐新鲜度的检测。

Network

表示网络请求的接口，处理网络请求，里面只有唯一的方法：

/**
 * An interface for performing requests.
 */
public interface Network {
    /**
     * Performs the specified request.
     * @param request Request to process
     * @return A {@link NetworkResponse} with data and caching metadata; will never be null
     * @throws VolleyError on errors
     */
    NetworkResponse performRequest(Request<?> request) throws VolleyError;
}

NetworkResponse

它是网络请求和缓存请求回来的额结果，是 Request 的 parseNetworkResponse(…) 的参数，它里面封装了网络请求响应的 StatusCode、Headers和 Body 等。如下：

/**
 * Data and headers returned from {@link Network#performRequest(Request)}.
 */
public class NetworkResponse {
    /**
     * Creates a new network response.
     * @param statusCode the HTTP status code
     * @param data Response body
     * @param headers Headers returned with this response, or null for none
     * @param notModified True if the server returned a 304 and the data was already in cache
     * @param networkTimeMs Round-trip network time to receive network response
     */
    public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
            boolean notModified, long networkTimeMs) {
        this.statusCode = statusCode;
        this.data = data;
        this.headers = headers;
        this.notModified = notModified;
        this.networkTimeMs = networkTimeMs;
    }

    public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
            boolean notModified) {
        this(statusCode, data, headers, notModified, 0);
    }

    public NetworkResponse(byte[] data) {
        this(HttpStatus.SC_OK, data, Collections.<String, String>emptyMap(), false, 0);
    }

    public NetworkResponse(byte[] data, Map<String, String> headers) {
        this(HttpStatus.SC_OK, data, headers, false, 0);
    }

    /** The HTTP status code. */
    public final int statusCode;

    /** Raw data from this response. */
    public final byte[] data;

    /** Response headers. */
    public final Map<String, String> headers;

    /** True if the server returned a 304 (Not Modified). */
    public final boolean notModified;

    /** Network roundtrip time in milliseconds. */
    public final long networkTimeMs;
}

NetworkResponse 的流程图如下：

Response

封装了经过解析之后的数据，用于传输。在内部有两个接口 Listener 和 ErrorListener 分别表示请求成功和失败的回调。 Response 的构造函数被私有化，而通过两个静态方法 success() 和 error() 来创建对象。代码如下：

/**
 * Encapsulates a parsed response for delivery.
 *
 * @param <T> Parsed type of this response
 */
public class Response<T> {

    /** Callback interface for delivering parsed responses. */
    public interface Listener<T> {
        /** Called when a response is received. */
        void onResponse(T response);
    }

    /** Callback interface for delivering error responses. */
    public interface ErrorListener {
        /**
         * Callback method that an error has been occurred with the
         * provided error code and optional user-readable message.
         */
        void onErrorResponse(VolleyError error);
    }

    /** Returns a successful response containing the parsed result. */
    public static <T> Response<T> success(T result, Cache.Entry cacheEntry) {
        return new Response<T>(result, cacheEntry);
    }

    /**
     * Returns a failed response containing the given error code and an optional
     * localized message displayed to the user.
     */
    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;

    /** Cache metadata for this response, or null in the case of error. */
    public final Cache.Entry cacheEntry;

    /** Detailed error information if <code>errorCode != OK</code>. */
    public final VolleyError error;

    /** True if this response was a soft-expired one and a second one MAY be coming. */
    public boolean intermediate = false;

    /**
     * Returns whether this response is considered successful.
     */
    public boolean isSuccess() {
        return error == null;
    }


    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;
    }
}

ResponseDelivery

请求结果的传输接口，用于传递请求结果或者请求错误。如下：

public interface ResponseDelivery {
    /**
     * Parses a response from the network or cache and delivers it.
     */
    void postResponse(Request<?> request, Response<?> response);

    /**
     * Parses a response from the network or cache and delivers it. The provided
     * Runnable will be executed after delivery.
     */
    void postResponse(Request<?> request, Response<?> response, Runnable runnable);

    /**
     * Posts an error for the given request.
     */
    void postError(Request<?> request, VolleyError error);
}

ExecutorDelivery

请求结果传输接口具体实现类，在 handler 对应线程中传输缓存调度线程或者网络调度线程中产生的请求结果或者请求错误，请求成功时调用 Request.deliverResponse() 方法，失败时调用 Request.deliverError() 方法。代码如下：

/**
 * Delivers responses and errors.
 */
public class ExecutorDelivery implements ResponseDelivery {
    /** Used for posting responses, typically to the main thread. */
    private final Executor mResponsePoster;

    /**
     * Creates a new response delivery interface.
     * @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) {
                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));
    }

    @Override
    public void postError(Request<?> request, VolleyError error) {
        request.addMarker("post-error");
        Response<?> response = Response.error(error);
        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
    }

    /**
     * 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()) {
                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();
            }
       }
    }
}

RetryPolicy

重试策略接口，里面有三个方法，但只有 retry() 方法是有用的。如下：

/**
 * Retry policy for a request.
 */
public interface RetryPolicy {

    /**
     * Returns the current timeout (used for logging).
     */
    int getCurrentTimeout();

    /**
     * Returns the current retry count (used for logging).
     */
    int getCurrentRetryCount();

    /**
     * Prepares for the next retry by applying a backoff to the timeout.
     * @param error The error code of the last attempt.
     * @throws VolleyError In the event that the retry could not be performed (for example if we
     * ran out of attempts), the passed in error is thrown.
     */
    void retry(VolleyError error) throws VolleyError;
}

retry() 方法，表示是否重试。在请求异常时此接口会被调用，可在此函数实现中抛出错误的异常表示停止重试。

DefaultRetryPolicy

RetryPolicy 的实现类，Volley 默认的重试策略主要是通过在 retry() 函数中红判断重试次数是否达到上限确定是否需要重试。代码如下：

/**
 * Default retry policy for requests.
 */
public class DefaultRetryPolicy implements RetryPolicy {
    /** The current timeout in milliseconds. */
    private int mCurrentTimeoutMs;

    /** The current retry count. */
    private int mCurrentRetryCount;

    /** The maximum number of attempts. */
    private final int mMaxNumRetries;

    /** The backoff multiplier for the policy. */
    private final float mBackoffMultiplier;

    /** The default socket timeout in milliseconds */
    public static final int DEFAULT_TIMEOUT_MS = 2500;

    /** The default number of retries */
    public static final int DEFAULT_MAX_RETRIES = 1;

    /** The default backoff multiplier */
    public static final float DEFAULT_BACKOFF_MULT = 1f;

    /**
     * Constructs a new retry policy using the default timeouts.
     */
    public DefaultRetryPolicy() {
        this(DEFAULT_TIMEOUT_MS, DEFAULT_MAX_RETRIES, DEFAULT_BACKOFF_MULT);
    }

    /**
     * Constructs a new retry policy.
     * @param initialTimeoutMs The initial timeout for the policy.
     * @param maxNumRetries The maximum number of retries.
     * @param backoffMultiplier Backoff multiplier for the policy.
     */
    public DefaultRetryPolicy(int initialTimeoutMs, int maxNumRetries, float backoffMultiplier) {
        mCurrentTimeoutMs = initialTimeoutMs;
        mMaxNumRetries = maxNumRetries;
        mBackoffMultiplier = backoffMultiplier;
    }

    /**
     * Returns the current timeout.
     */
    @Override
    public int getCurrentTimeout() {
        return mCurrentTimeoutMs;
    }

    /**
     * Returns the current retry count.
     */
    @Override
    public int getCurrentRetryCount() {
        return mCurrentRetryCount;
    }

    /**
     * Returns the backoff multiplier for the policy.
     */
    public float getBackoffMultiplier() {
        return mBackoffMultiplier;
    }

    /**
     * Prepares for the next retry by applying a backoff to the timeout.
     * @param error The error code of the last attempt.
     */
    @Override
    public void retry(VolleyError error) throws VolleyError {
        mCurrentRetryCount++;
        mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier);
        if (!hasAttemptRemaining()) {
            throw error;
        }
    }

    /**
     * Returns true if this policy has attempts remaining, false otherwise.
     */
    protected boolean hasAttemptRemaining() {
        return mCurrentRetryCount <= mMaxNumRetries;
    }
}

到此为止，Volley 的主要类的源码就分析完啦，它里面还有很多工具类，很值得深入了解，下次继续努力。

参考文章：

1. Volley 源码解析

   http://p.codekk.com/blogs/detail/54cfab086c4761e5001b2542