一、網路請求核心

1.1 Network

Volley與網路請求相關的介面有兩個：

/**
 * 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
     */
    public NetworkResponse performRequest(Request<?> request) throws VolleyError;
}
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1.2 HttpStack

public interface HttpStack {
    /**
     * Performs an HTTP request with the given parameters.
     *
     * <p>A GET request is sent if request.getPostBody() == null. A POST request is sent otherwise,
     * and the Content-Type header is set to request.getPostBodyContentType().</p>
     *
     * @param request the request to perform
     * @param additionalHeaders additional headers to be sent together with
     *         {@link Request#getHeaders()}
     * @return the HTTP response
     */
    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
        throws IOException, AuthFailureError;
}
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1.3 具體實現

對於上面的兩個介面，它們各有對應的實現類：

• Network：BasicNetwork
• HttpStack：在SDK大於等於九時，其實現類是HurlStack，小於九時，對應的實現類是HttpClientStack。

BasicNetwork和HttpStack的兩個實現類的關係是：HttpStack是BasicNetwork的一個成員變數，當Volley傳入Request，呼叫BasicNetwork#performRequest後，在它的內部實際是通過HttpStack#performRequest發起網路請求，並把標準網路請求返回結果HttpResponse封裝成Volley的NetworkResponse。

在看具體的請求之前，我們需要先了解一下和請求相關的兩個類Request和NetworkResponse。

1.3.1 Request

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;

    // A cheap variant of request tracing used to dump slow requests.
    private long mRequestBirthTime = 0;

    /** Threshold at which we should log the request (even when debug logging is not enabled). */
    private static final long SLOW_REQUEST_THRESHOLD_MS = 3000;

    /** 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;

}
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Request<T>實現了Comparable<Request<T>>介面，它是所有網路請求的基類，實現這個介面是為了比較各個請求之間的優先順序。 它定義了內部介面Method，表示支援的方法：

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;
}
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除此之外還定義了成員變數包括：

• mMethod：該請求所對應的方法。
• mUrl：請求的Url。
• mSequence：序列號。
• mRequestQueue：該請求所加入的佇列。
• mShouldCache：是否需要快取。
• mCanceled：該請求是否已經取消。
• mResponseDelivered：該請求是否已經delivered。
• mRequestBirthTime：請求開始的時間。
• RetryPolicy mRetryPolicy：重試策略。
• Cache.Entry mCacheEntry：當一個請求結果可以從快取中獲得，但必須通過網路請求來重新整理，快取就可以存放在這裡，當出現Not Modified時，將它返回。
• mTag：標識，用來一次性取消多個請求。

它有兩個抽象方法：

    /**
     * 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 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);
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1.3.2 NetworkResponse

NetworkResponse是BasicNetwork#performRequest返回的，它僅僅包含返回的content和Header，這樣就便於後面對Response進行轉換。

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;
}
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它包含以下成員變數：

• statusCode：HTTP狀態碼。
• byte[] data：返回資料。
• Map<String, String> headers：返回的頭部。
• boolean notModified：如果伺服器返回了304，那麼為true。
• long networkTimesMs：網路請求所消耗的時間。

1.3.3 下面我們來看一下BasicNetwork的實現程式碼：

@Override
    public NetworkResponse performRequest(Request<?> request) throws VolleyError {
        long requestStart = SystemClock.elapsedRealtime();
        while (true) {
            HttpResponse httpResponse = null;
            byte[] responseContents = null;
            Map<String, String> responseHeaders = Collections.emptyMap();
            try {
                // Gather headers.
                Map<String, String> headers = new HashMap<String, String>();
                addCacheHeaders(headers, request.getCacheEntry());
                httpResponse = mHttpStack.performRequest(request, headers);
                StatusLine statusLine = httpResponse.getStatusLine();
                int statusCode = statusLine.getStatusCode();

                responseHeaders = convertHeaders(httpResponse.getAllHeaders());
                // Handle cache validation.
                if (statusCode == HttpStatus.SC_NOT_MODIFIED) {

                    Entry entry = request.getCacheEntry();
                    if (entry == null) {
                        return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
                                responseHeaders, true,
                                SystemClock.elapsedRealtime() - requestStart);
                    }

                    // A HTTP 304 response does not have all header fields. We
                    // have to use the header fields from the cache entry plus
                    // the new ones from the response.
                    // http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.5
                    entry.responseHeaders.putAll(responseHeaders);
                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
                            entry.responseHeaders, true,
                            SystemClock.elapsedRealtime() - requestStart);
                }

                // Some responses such as 204s do not have content.  We must check.
                if (httpResponse.getEntity() != null) {
                  responseContents = entityToBytes(httpResponse.getEntity());
                } else {
                  // Add 0 byte response as a way of honestly representing a
                  // no-content request.
                  responseContents = new byte[0];
                }

                // if the request is slow, log it.
                long requestLifetime = SystemClock.elapsedRealtime() - requestStart;
                logSlowRequests(requestLifetime, request, responseContents, statusLine);

                if (statusCode < 200 || statusCode > 299) {
                    throw new IOException();
                }
                return new NetworkResponse(statusCode, responseContents, responseHeaders, false,
                        SystemClock.elapsedRealtime() - requestStart);
            } catch (SocketTimeoutException e) {
                attemptRetryOnException("socket", request, new TimeoutError());
            } catch (ConnectTimeoutException e) {
                attemptRetryOnException("connection", request, new TimeoutError());
            } catch (MalformedURLException e) {
                throw new RuntimeException("Bad URL " + request.getUrl(), e);
            } catch (IOException e) {
                int statusCode = 0;
                NetworkResponse networkResponse = null;
                if (httpResponse != null) {
                    statusCode = httpResponse.getStatusLine().getStatusCode();
                } else {
                    throw new NoConnectionError(e);
                }
                VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
                if (responseContents != null) {
                    networkResponse = new NetworkResponse(statusCode, responseContents,
                            responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);
                    if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
                            statusCode == HttpStatus.SC_FORBIDDEN) {
                        attemptRetryOnException("auth",
                                request, new AuthFailureError(networkResponse));
                    } else {
                        // TODO: Only throw ServerError for 5xx status codes.
                        throw new ServerError(networkResponse);
                    }
                } else {
                    throw new NetworkError(networkResponse);
                }
            }
        }
    }
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上面的程式碼對應的具體流程如下：

• 記錄請求開始時間
• 通過Request#mCacheEntry來構建新的請求的Header
• 把Request和Header傳給HttpStack
• HttpStack發起請求之後，獲得HttpResponse的StatusCode和Header
• 根據StatusCode來構建NetworkResponse，判斷是否是304：
• 是，那麼再判斷mCacheEntry是否為空，如果為空，那麼僅僅傳入頭部構建；否則更新mCacheEntry的頭部，傳入mCacheEntry.data來構建。
• 否，從Entity中獲得byte[]構建NetworkResponse。
• 如果在請求或者處理過程當中，發生了異常，那麼會根據情況，返回不同的異常，最終都是一個VolleyError，它的子類包括：
• NetworkError：用來表示這是在發起請求過程中所產生的錯誤。
• ParseError：用來表示伺服器的資料不能被解析。
• TimeoutError：用來表示socket連線超時。
• ServerError：用來表示伺服器返回了一個異常響應。
• NoConnectionError：表示沒有可建立的連線。

1.3.4 RetryPolicy

在上面的請求過程當中，我們看到當發生SocketTimeoutException/ConnectTimeoutException，會呼叫 attemptRetryOnException，而在Request的建構函式中，會給它傳入一個DefaultRetryPolicy：

    //BasicNetwork.java
    /**
     * Attempts to prepare the request for a retry. If there are no more attempts remaining in the
     * request's retry policy, a timeout exception is thrown.
     * @param request The request to use.
     */
    private static void attemptRetryOnException(String logPrefix, Request<?> request,
            VolleyError exception) throws VolleyError {
        RetryPolicy retryPolicy = request.getRetryPolicy();
        int oldTimeout = request.getTimeoutMs();

        try {
            retryPolicy.retry(exception);
        } catch (VolleyError e) {
            request.addMarker(
                    String.format("%s-timeout-giveup [timeout=%s]", logPrefix, oldTimeout));
            throw e;
        }
        request.addMarker(String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout));
    }

    //DefaultRetryPolicy.java
        /**
     * 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;
        }
    }
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我們可以看到，它其實是修改當前Request對應的RetryPolicy中的超時時間，這樣再下次請求時，該Request所允許超時的時間就會變長，從而減少超時情況的發生。

1.4 小結

在外界看來，BasicNetwork就是接受Request，返回NetworkResponse，在執行過程當中，有可能丟擲VolleyError。

二、NetworkResponse轉換為Response

在上面的例子當中，我們看到Request必須要實現一個抽象方法，將網路請求的返回結果NetworkResponse轉換成為Response，來遞交給下一級，它包含以下幾個成員變數：

    /** 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 errorCode != OK */
    public final VolleyError error;
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下面是StringRequest的實現：

@Override
protected Response<String> parseNetworkResponse(NetworkResponse response) {
    String parsed;
    try {
        parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
    } catch (UnsupportedEncodingException e) {
        parsed = new String(response.data);
    }
    return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
}
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在NetworkDispatcher中，當Request解析完請求後，會把cacheEntry儲存起來，而它的Key來自於Request#getCacheKey：

if (request.shouldCache() && response.cacheEntry != null) {
    mCache.put(request.getCacheKey(), response.cacheEntry);
    request.addMarker("network-cache-written");
}
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這樣就把result和cache分開來了。

三、快取相關Cache

• Cache是一個介面，它定義了一個快取管理類應當實現的方法，在Volley當中，我們預設實現的快取管理類是DiskBasedCache，在後面的分析中，它分別被傳給了NetworkDispatcher和CacheDispatcher，在前者當中寫入快取，在後者當中讀出快取。
• Cache中有一個內部類Entry，它定義快取的資料結構，當使用者希望快取請求結果的時候，那麼就需要構建一個Cache.Entry，然後在構建Response的時候將它傳入，這樣在NetworkDispatcher中就可以寫入到永續性儲存當中。

  /**
     * Data and metadata for an entry returned by the cache.
     */
    public static 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. */
        public boolean isExpired() {
            return this.ttl < System.currentTimeMillis();
        }

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

4.1 CacheDispatcher

它是一個執行緒，它的建構函式中傳入了：

• cacheQueue：快取的處理佇列
• networkQueue：網路的處理佇列
• cache：快取管理類
• delivery：遞送類。

上面這兩個佇列的型別為無界佇列BlockingQueue<Request<?>>，我們來看一下，在其run()方法中的具體操作：

    @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;
                }
                continue;
            }
        }
    }
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• 從mCacheQueue中阻塞地獲取Request，直到該佇列中有元素。
• 如果在輪到該Request執行時，它已經被Cancel了，那麼呼叫Request#finish方法，並取佇列的下一個Request進行操作。
• 嘗試從快取管理類mCache中取出該Request對應的快取，如果快取為空(entry == null)，那麼將Request加入到網路佇列；如果快取過期(entry.isExpired())，那麼給Request設定該快取，之後加入到網路佇列，這兩種情況最終都會繼續取快取佇列的下一個Request進行操作。
• 如果都不是上面的情況，那麼通過Cache.Entry中的data和header解析構建NetworkResponse，然後回撥給Request解析，最終得到Response
• 如果快取需要重新整理(!entry.refreshNeeded())，那麼給Request設定cacheEntry，在給使用者回撥該結果後，還要再把該Request加入到網路佇列。
• 如果快取不需要重新整理，那麼直接返回即可。

4.2 NetworkDispatcher

網路執行緒的處理方式和快取執行緒類似，它的建構函式包括：

• queue：網路佇列
• network：網路框架
• cache：快取管理類
• delivery：遞送類

    @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);
    }
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• 先判斷Request是否被Cancel，如果是，那麼直接回撥#finish，這和上面類似。
• 呼叫Network請求，得到NetworkResponse。
• 判斷這個NetworkResponse是否是304並且已經遞送給使用者了，如果是，那麼直接呼叫#finish，退出。
• 通過Request#parseNetworkResponse將NetworkResponse解析為Response。
• 判斷Request是否需要快取，如果需要快取，那麼呼叫快取管理類mCache儲存快取。
• 通過mDelivery遞送最終的結果。
• 如果在上述的請求中發生了異常，那麼會通過mDelivery傳送錯誤給使用者。

五、返回結果

從上面的兩個Thread的處理過程來看，對於從佇列中取出的Request，最終的處理方式無非有這兩種：一種是呼叫Request#finish(xxx)，另一種是通過mDelivery。

5.1 Request#finish(xxx)

    //Request.java
   void finish(final String tag) {
        if (mRequestQueue != null) {
            mRequestQueue.finish(this);
        }
    }

    //RequestQueue.java
    <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);
                }
            }
        }
    }
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這裡面涉及到兩個集合mCurrentRequests和mWaitingRequests：

• Set<Request<?>>：所有被新增的Request都進入這一集合，這樣在RequestQueue#cancelAll時，就是取消這個集合當中的佇列。
• Map<String, Queue<Request<?>>>：它的Key是每個Request的getCacheKey，具有相同key的Request會被放在同一個佇列當中。

5.2 ResponseDelivery

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

    /**
     * Posts an error for the given request.
     */
    public void postError(Request<?> request, VolleyError error);
}
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它的預設實現是ExecutorDelivery，它負責將請求的結果返回給使用者，它所有的方法，最終都會走到內部的ResponseDeliveryRunnable的run()方法當中，該run()方法執行所在的執行緒和構建ExecutorDelivery所使用的handler有關：

    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();
            }
       }
    }
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六、RequestQueue

先看下Volley，它其中有一個靜態方法RequestQueue newRequestQueue(Context, HttpStack)，最終會呼叫RequestQueue#start()。

    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;
    }
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首先看一下RequestQueue的建構函式，它會構造四個NetworkDispatcher。

    /**
     * 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;
    }
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之後再看一下start()方法，它會啟動快取執行緒，並依次啟動網路執行緒。

    /**
     * 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();
        }
    }
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當我們呼叫這個RequestQueue的新增方法時：

    /**
     * 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<Request<?>>();
                }
                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;
        }
    }
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