Volley源码流程浅析

2016-10-29  本文已影响36人  Sanisy

我们先从Volley的入口分析,使用Volley的步骤:

 //创建的请求队列
 RequestQueue requestQueue = Volley.newRequestQueue(mContext);
 //创建一个请求
 Request request = new XXRequest(...);
 //往队列添加请求
 requestQueue.add(request);

我们先来看newRequestQueue(Context)方法:

  public static RequestQueue newRequestQueue(Context context) {
        return newRequestQueue(context, null);
  }

调用的是重载的方法:

  public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        //默认的缓存目录
        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) {
             //如果SDK >= 9 则采用HurlStack来做底层的网络请求
            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
                //SDK < 9则采用HttpClientStack来做底层的网络请求
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        //将HttpStack包装成一个Network
        Network network = new BasicNetwork(stack);
        //进一步包装Network,构建一个RequestQueue对象
        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        //调用RequestQueue的start方法来开启请求调度
        queue.start();

        return queue;
   }

我们可以看到默认的缓存目录是context.getCacheDir()/volley,HttpStack是干什么用的呢?

/**
 * An HTTP stack abstraction.
 */
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;

}

HttpStack是一个接口,根据注释我们可以知道HttpStack是用于执行HTTP请求的.HttpClientStack的具体实现是HttpClient,网络请求都是通过HttpClient来进行处理的,而HurlStack的具体实现是HttpURLConnection,所有的网络请求都是交给HttpURLConnection去处理的。


/**
 * An HttpStack that performs request over an {@link HttpClient}.
 */
public class HttpClientStack implements HttpStack {
    protected final HttpClient mClient;

    private final static String HEADER_CONTENT_TYPE = "Content-Type";

    public HttpClientStack(HttpClient client) {
        mClient = client;
    }

    private static void addHeaders(HttpUriRequest httpRequest, Map<String, String> headers) {
        for (String key : headers.keySet()) {
            httpRequest.setHeader(key, headers.get(key));
        }
    }

    @SuppressWarnings("unused")
    private static List<NameValuePair> getPostParameterPairs(Map<String, String> postParams) {
        List<NameValuePair> result = new ArrayList<NameValuePair>(postParams.size());
        for (String key : postParams.keySet()) {
            result.add(new BasicNameValuePair(key, postParams.get(key)));
        }
        return result;
    }
    
    /*
     *执行请求的方法
     */
    @Override
    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError {
        HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders);
        addHeaders(httpRequest, additionalHeaders);
        addHeaders(httpRequest, request.getHeaders());
        onPrepareRequest(httpRequest);
        HttpParams httpParams = httpRequest.getParams();
        int timeoutMs = request.getTimeoutMs();
        // TODO: Reevaluate this connection timeout based on more wide-scale
        // data collection and possibly different for wifi vs. 3G.
        HttpConnectionParams.setConnectionTimeout(httpParams, 5000);
        HttpConnectionParams.setSoTimeout(httpParams, timeoutMs);
        return mClient.execute(httpRequest);
    }

    /**
     * Creates the appropriate subclass of HttpUriRequest for passed in request.
     */
    @SuppressWarnings("deprecation")
    /* protected */ static HttpUriRequest createHttpRequest(Request<?> request,
            Map<String, String> additionalHeaders) throws AuthFailureError {
        switch (request.getMethod()) {
            case Method.DEPRECATED_GET_OR_POST: {
                // This is the deprecated way that needs to be handled for backwards compatibility.
                // If the request's post body is null, then the assumption is that the request is
                // GET.  Otherwise, it is assumed that the request is a POST.
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    HttpPost postRequest = new HttpPost(request.getUrl());
                    postRequest.addHeader(HEADER_CONTENT_TYPE, request.getPostBodyContentType());
                    HttpEntity entity;
                    entity = new ByteArrayEntity(postBody);
                    postRequest.setEntity(entity);
                    return postRequest;
                } else {
                    return new HttpGet(request.getUrl());
                }
            }
            case Method.GET:
                return new HttpGet(request.getUrl());
            case Method.DELETE:
                return new HttpDelete(request.getUrl());
            case Method.POST: {
                HttpPost postRequest = new HttpPost(request.getUrl());
                postRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
                setEntityIfNonEmptyBody(postRequest, request);
                return postRequest;
            }
            case Method.PUT: {
                HttpPut putRequest = new HttpPut(request.getUrl());
                putRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
                setEntityIfNonEmptyBody(putRequest, request);
                return putRequest;
            }
            case Method.HEAD:
                return new HttpHead(request.getUrl());
            case Method.OPTIONS:
                return new HttpOptions(request.getUrl());
            case Method.TRACE:
                return new HttpTrace(request.getUrl());
            case Method.PATCH: {
                HttpPatch patchRequest = new HttpPatch(request.getUrl());
                patchRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
                setEntityIfNonEmptyBody(patchRequest, request);
                return patchRequest;
            }
            default:
                throw new IllegalStateException("Unknown request method.");
        }
    }

    private static void setEntityIfNonEmptyBody(HttpEntityEnclosingRequestBase httpRequest,
            Request<?> request) throws AuthFailureError {
        byte[] body = request.getBody();
        if (body != null) {
            HttpEntity entity = new ByteArrayEntity(body);
            httpRequest.setEntity(entity);
        }
    }

    /**
     * Called before the request is executed using the underlying HttpClient.
     *
     * <p>Overwrite in subclasses to augment the request.</p>
     */
    protected void onPrepareRequest(HttpUriRequest request) throws IOException {
        // Nothing.
    }

    /**
     * The HttpPatch class does not exist in the Android framework, so this has been defined here.
     */
    public static final class HttpPatch extends HttpEntityEnclosingRequestBase {

        public final static String METHOD_NAME = "PATCH";

        public HttpPatch() {
            super();
        }

        public HttpPatch(final URI uri) {
            super();
            setURI(uri);
        }

        /**
         * @throws IllegalArgumentException if the uri is invalid.
         */
        public HttpPatch(final String uri) {
            super();
            setURI(URI.create(uri));
        }

        @Override
        public String getMethod() {
            return METHOD_NAME;
        }

    }
}
public class HurlStack implements HttpStack {

    private static final String HEADER_CONTENT_TYPE = "Content-Type";

    /**
     * An interface for transforming URLs before use.
     */
    public interface UrlRewriter {
        /**
         * Returns a URL to use instead of the provided one, or null to indicate
         * this URL should not be used at all.
         */
        public String rewriteUrl(String originalUrl);
    }

    private final UrlRewriter mUrlRewriter;
    private final SSLSocketFactory mSslSocketFactory;

    public HurlStack() {
        this(null);
    }

    /**
     * @param urlRewriter Rewriter to use for request URLs
     */
    public HurlStack(UrlRewriter urlRewriter) {
        this(urlRewriter, null);
    }

    /**
     * @param urlRewriter Rewriter to use for request URLs
     * @param sslSocketFactory SSL factory to use for HTTPS connections
     */
    public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) {
        mUrlRewriter = urlRewriter;
        mSslSocketFactory = sslSocketFactory;
    }

    @Override
    public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError {
        String url = request.getUrl();
        HashMap<String, String> map = new HashMap<String, String>();
        map.putAll(request.getHeaders());
        map.putAll(additionalHeaders);
        if (mUrlRewriter != null) {
            String rewritten = mUrlRewriter.rewriteUrl(url);
            if (rewritten == null) {
                throw new IOException("URL blocked by rewriter: " + url);
            }
            url = rewritten;
        }
        URL parsedUrl = new URL(url);
        HttpURLConnection connection = openConnection(parsedUrl, request);
        for (String headerName : map.keySet()) {
            connection.addRequestProperty(headerName, map.get(headerName));
        }
        setConnectionParametersForRequest(connection, request);
        // Initialize HttpResponse with data from the HttpURLConnection.
        ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);
        int responseCode = connection.getResponseCode();
        if (responseCode == -1) {
            // -1 is returned by getResponseCode() if the response code could not be retrieved.
            // Signal to the caller that something was wrong with the connection.
            throw new IOException("Could not retrieve response code from HttpUrlConnection.");
        }
        StatusLine responseStatus = new BasicStatusLine(protocolVersion,
                connection.getResponseCode(), connection.getResponseMessage());
        BasicHttpResponse response = new BasicHttpResponse(responseStatus);
        response.setEntity(entityFromConnection(connection));
        for (Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
            if (header.getKey() != null) {
                Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
                response.addHeader(h);
            }
        }
        return response;
    }

    /**
     * Initializes an {@link HttpEntity} from the given {@link HttpURLConnection}.
     * @param connection
     * @return an HttpEntity populated with data from <code>connection</code>.
     */
    private static HttpEntity entityFromConnection(HttpURLConnection connection) {
        BasicHttpEntity entity = new BasicHttpEntity();
        InputStream inputStream;
        try {
            inputStream = connection.getInputStream();
        } catch (IOException ioe) {
            inputStream = connection.getErrorStream();
        }
        entity.setContent(inputStream);
        entity.setContentLength(connection.getContentLength());
        entity.setContentEncoding(connection.getContentEncoding());
        entity.setContentType(connection.getContentType());
        return entity;
    }

    /**
     * Create an {@link HttpURLConnection} for the specified {@code url}.
     */
    protected HttpURLConnection createConnection(URL url) throws IOException {
        return (HttpURLConnection) url.openConnection();
    }

    /**
     * Opens an {@link HttpURLConnection} with parameters.
     * @param url
     * @return an open connection
     * @throws IOException
     */
    private HttpURLConnection openConnection(URL url, Request<?> request) throws IOException {
        HttpURLConnection connection = createConnection(url);

        int timeoutMs = request.getTimeoutMs();
        connection.setConnectTimeout(timeoutMs);
        connection.setReadTimeout(timeoutMs);
        connection.setUseCaches(false);
        connection.setDoInput(true);

        // use caller-provided custom SslSocketFactory, if any, for HTTPS
        if ("https".equals(url.getProtocol()) && mSslSocketFactory != null) {
            ((HttpsURLConnection)connection).setSSLSocketFactory(mSslSocketFactory);
        }

        return connection;
    }

    @SuppressWarnings("deprecation")
    /* package */ static void setConnectionParametersForRequest(HttpURLConnection connection,
            Request<?> request) throws IOException, AuthFailureError {
        switch (request.getMethod()) {
            case Method.DEPRECATED_GET_OR_POST:
                // This is the deprecated way that needs to be handled for backwards compatibility.
                // If the request's post body is null, then the assumption is that the request is
                // GET.  Otherwise, it is assumed that the request is a POST.
                byte[] postBody = request.getPostBody();
                if (postBody != null) {
                    // Prepare output. There is no need to set Content-Length explicitly,
                    // since this is handled by HttpURLConnection using the size of the prepared
                    // output stream.
                    connection.setDoOutput(true);
                    connection.setRequestMethod("POST");
                    connection.addRequestProperty(HEADER_CONTENT_TYPE,
                            request.getPostBodyContentType());
                    DataOutputStream out = new DataOutputStream(connection.getOutputStream());
                    out.write(postBody);
                    out.close();
                }
                break;
            case Method.GET:
                // Not necessary to set the request method because connection defaults to GET but
                // being explicit here.
                connection.setRequestMethod("GET");
                break;
            case Method.DELETE:
                connection.setRequestMethod("DELETE");
                break;
            case Method.POST:
                connection.setRequestMethod("POST");
                addBodyIfExists(connection, request);
                break;
            case Method.PUT:
                connection.setRequestMethod("PUT");
                addBodyIfExists(connection, request);
                break;
            case Method.HEAD:
                connection.setRequestMethod("HEAD");
                break;
            case Method.OPTIONS:
                connection.setRequestMethod("OPTIONS");
                break;
            case Method.TRACE:
                connection.setRequestMethod("TRACE");
                break;
            case Method.PATCH:
                connection.setRequestMethod("PATCH");
                addBodyIfExists(connection, request);
                break;
            default:
                throw new IllegalStateException("Unknown method type.");
        }
    }

    private static void addBodyIfExists(HttpURLConnection connection, Request<?> request)
            throws IOException, AuthFailureError {
        byte[] body = request.getBody();
        if (body != null) {
            connection.setDoOutput(true);
            connection.addRequestProperty(HEADER_CONTENT_TYPE, request.getBodyContentType());
            DataOutputStream out = new DataOutputStream(connection.getOutputStream());
            out.write(body);
            out.close();
        }
    }
}

刚开始的时候我们知道,HttpStack是通过BasicNetwork封装成了一个Network对象,我们来看看BasicNetwork里面到底做了啥事情。

/**
 * A network performing Volley requests over an {@link HttpStack}.
 */
public class BasicNetwork implements Network {
    protected static final boolean DEBUG = VolleyLog.DEBUG;

    private static int SLOW_REQUEST_THRESHOLD_MS = 3000;

    private static int DEFAULT_POOL_SIZE = 4096;

    protected final HttpStack mHttpStack;

    protected final ByteArrayPool mPool;

    /**
     * @param httpStack HTTP stack to be used
     */
    public BasicNetwork(HttpStack httpStack) {
        // If a pool isn't passed in, then build a small default pool that will give us a lot of
        // benefit and not use too much memory.
        this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
    }

    /**
     * @param httpStack HTTP stack to be used
     * @param pool a buffer pool that improves GC performance in copy operations
     */
    public BasicNetwork(HttpStack httpStack, ByteArrayPool pool) {
        mHttpStack = httpStack;
        mPool = pool;
    }

    @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);
                }
            }
        }
    }
    /**省略代码*/

我们主要来看实现的performRequest方法,可以看到将请求头和请求交给了HttpStack进行了处理,调用的是HttpStackperformRequest(request,headers);并且将请求结果封装成NetworkResponse并返回,但是BasicNetwork的performRequest是什么时候被调用的呢?,我们进一步去看RequestQueue的实现。

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

    /** Used for generating monotonically-increasing sequence numbers for requests. */
    private 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<String, Queue<Request<?>>>();

    /**
     * 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<?>>();
    //需要缓存的请求的队列
    /** The cache triage queue. */
    private final PriorityBlockingQueue<Request<?>> mCacheQueue =
        new PriorityBlockingQueue<Request<?>>();
    //网络请求队列
    /** The queue of requests that are actually going out to the network. */
    private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
        new PriorityBlockingQueue<Request<?>>();
    //默认开启的网络请求线程数量
    /** 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;
    //网络请求调度线程组
    /** The network dispatchers. */
    private NetworkDispatcher[] mDispatchers;
    //缓存请求调度线程
    /** The cache dispatcher. */
    private CacheDispatcher mCacheDispatcher;

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

    /**
     * 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
     */
    public RequestQueue(Cache cache, Network network) {
        this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
    }

    //开始请求调度
    /**
     * 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();
        }
    }

    //停止所有的线程,包括缓存线程和网络请求线程
    /**
     * Stops the cache and network dispatchers.
     */
    public void stop() {
        if (mCacheDispatcher != null) {
            mCacheDispatcher.quit();
        }
        for (int i = 0; i < mDispatchers.length; i++) {
            if (mDispatchers[i] != null) {
                mDispatchers[i].quit();
            }
        }
    }

    //添加请求的方法
    /**
     * 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.
        //请求不需要缓存则,直接添加进网络请求队列
        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) {
            //需要进行缓存,并且已经有相同cacheKey存储与等待队列中,那么将请求用链表装载,然后添加进等待队列
            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);
                //等待队列中没有该cacheKey,则直接添加进缓存队列
                mCacheQueue.add(request);
            }
            return request;
        }
    }
}

主要来看add(Request<?>),start以及stop方法。
add方法
首先判断是否是需要缓存的请求,如果是,则进一步判断等待队列中是否已经存在过同样cacheKey的请求,如果存在,那么将所有包含该cacheKey的请求封装成一个LinkedList添加进等待请求队列。等待队列中的请求能被调度的条件是:原来与该cacheKey一致的请求已经被取消,或者服务器返回的是304状态码并且原来的请求已经得到了响应,满足条件的才会将等待请求队列中的所有请求添加进缓存请求队列mCacheQueue。如果不存在,那么将请求添加进缓存请求队列,等待线程来调度。

start方法

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();
        }
    }

该方法主要创建了一个缓存调度线程和4个网络调度线程,并且启动了这些线程然后等待请求的到来。start方法是创建并启动线程,那么stop方法应该就是停止掉这些线程了。

    /**
     * Stops the cache and network dispatchers.
     */
    public void stop() {
        if (mCacheDispatcher != null) {
            mCacheDispatcher.quit();
        }
        for (int i = 0; i < mDispatchers.length; i++) {
            if (mDispatchers[i] != null) {
                mDispatchers[i].quit();
            }
        }
    }

接下来分析CacheDispatcher和NetworkDispatcher,看看是如何调度网络请求的。由于是线程我们最主要关注的点是run方法。
CacheDispatcher的run方法代码:

@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");
                //上面已经说过如果cancel了,那么就会将等待请求队列的请求(具有相同cacheKey的请求)添加进mCacheQueue
                // If the request has been canceled, don't bother dispatching it.
                if (request.isCanceled()) {
                    request.finish("cache-discard-canceled");
                    continue;
                }
                //缓存检查
                // 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;
                }
                //检查缓存是否已经过期
                // 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");
                //检查缓存是否需要刷新
                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;
            }
        }
    }

首先进行缓存判断,如果没有缓存,那么直接将请求Request交给网络请求队列mNetworkQueue;如果该请求已经存在缓存中,那么判断缓存是否已经过期,过期同样交给网络请求队列,没过期则判断是否需要刷新缓存,不需要则返回缓存的响应结果,如果需要刷新,会先将缓存的响应结果返回,返回再讲该请求交给网络请求队列去刷新缓存。

    @Override
    public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
        request.markDelivered();
        request.addMarker("post-response");
        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
    }

    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;
            }
            //先将缓存返回
            // 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");
            }
            //最后去网络刷新缓存
            // If we have been provided a post-delivery runnable, run it.
            if (mRunnable != null) {
                mRunnable.run();
            }
       }
    }

NetworkDispatcher

@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);
            }
        }
    }

网络请求线程就比较简单了,直接请求,然后判断是否需要刷新缓存(返回304并且缓存已经被返回过了,那么不需要刷新缓存),该请求是一个缓存请求(也就是说shouldCache()为ture,这个由服务器规定max-age,如果该请求在上一次请求的时间+max-age时间 < 当前的系统时间,则不会去向服务器去请求而是直接返回本地的缓存,即shouldCache为false),那么就刷新缓存。最后是响应结果。

大体的流程如下:

volley.png

至于响应是如何回到主线程的呢?我们来看看RequestQueue的其中一个构造方法:

  public RequestQueue(Cache cache, Network network, int threadPoolSize) {
        this(cache, network, threadPoolSize,
                new ExecutorDelivery(new Handler(Looper.getMainLooper())));
  }

我们可以看到new ExecutorDelivery(new Handler(Looper.getMainLooper()),ExecutorDelivery实现的是ResponseDelivery接口,ResponseDelivery其实就是用于返回请求结果的,看到ExecutorDelivery构造参数Looper.getMainLooper(),我们就应该可以猜到最后的结果是通过Handler机制来传递到主线程的。

其实现在网络请求库有OkHttp和Retrofit等,如果想要使用OkHttp作为Volley的底层请求处理库的话,需要定义一个类实现HttpStack,并在performRequest方法中使用OkHttp来做相关的网络请求。

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