RequestQueue mQueue = Volley.newRequestQueue(context);private void getJsonData() {String url=”http://172.17.202.36:8000/data.jason”;

JsonObjectRequest jsonRequest = new JsonObjectRequest(url,new Response.Listener<JSONObject>() {@Overridepublic void onResponse(JSONObject response) {Log.d(LOG_TAG, response.toString());}}, new Response.ErrorListener() {@Overridepublic void onErrorResponse(VolleyError error) {Log.e(LOG_TAG, error.getMessage(), error);}
});
mQueue.add(jsonRequest);
}

private RequestQueue mQueue = Volley.newRequestQueue(mContext);
private ImageLoader mImageLoader = new ImageLoader(mQueue, new BitmapCache());public class BitmapCache implements ImageLoader.ImageCache {private LruCache<String, Bitmap> cache;public BitmapCache() {cache = new LruCache<String, Bitmap>(8 * 1024 * 1024) {@Overrideprotected int sizeOf(String key, Bitmap bitmap) {return bitmap.getRowBytes() * bitmap.getHeight();}};}@Overridepublic Bitmap getBitmap(String url) {return cache.get(url);}@Overridepublic void putBitmap(String url, Bitmap bitmap) {cache.put(url, bitmap);}
}private void loadImage() {
String imageUrl = “http://172.17.202.36:8000/images/文字_0.png”;
ImageView iv = (ImageView)findViewById(R.id.iv);
ImageLoader.ImageListener listener = ImageLoader.getImageListener(iv, R.drawable.default_png, R.drawable.error_png);
mImageLoader.get(imageUrl, listener);
}

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

}public static RequestQueue newRequestQueue(Context context, HttpStack stack)
 {return newRequestQueue(context, stack, -1);
}

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

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

public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
    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) {  // API-level >= 9 
            stack = new HurlStack();
        } else {
            // Prior to Gingerbread, HttpUrlConnection was unreliable.
            stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
        }
    }

    Network network = new BasicNetwork(stack);

    RequestQueue queue;
    if (maxDiskCacheBytes <= -1)
    {
// No maximum size specified
        queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
    } else {
        // Disk cache size specified
        queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
    }

    queue.start();
return queue;
}

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

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

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

public RequestQueue(Cache cache, Network network, int threadPoolSize, ResponseDelivery delivery) {    mCache = cache;   
    mNetwork = network;
    mDispatchers = new NetworkDispatcher[threadPoolSize];
    mDelivery = delivery;
}

/**
 * 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();
    }
}

public class NetworkDispatcher extends Thread{
    ...
    public NetworkDispatcher(BlockingQueue<Request<?>> queue,
 Network network, Cache cache,
 ResponseDelivery delivery) {
        mQueue = queue;
        mNetwork = network;
        mCache = cache;
        mDelivery = delivery;
    }

    @Override
public void run() {
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        Request<?> request;
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
            // release previous request object to avoid leaking request object when mQueue is drained.
            request = null;
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) {
               ......            }
        }
    }
    ......
}

public class BasicNetwork implements Network {
    ......
    @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);
                    }

                    entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data, entry.responseHeaders, true,
 SystemClock.elapsedRealtime() - requestStart);
                }

                // Handle moved resources
if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY || statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
                    String newUrl = responseHeaders.get("Location");
                    request.setRedirectUrl(newUrl);
                }

// 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);
                }
if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY ||
 statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
                    VolleyLog.e("Request at %s has been redirected to %s", request.getOriginUrl(), request.getUrl());
                } else {
                    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 if (statusCode == HttpStatus.SC_MOVED_PERMANENTLY ||
 statusCode == HttpStatus.SC_MOVED_TEMPORARILY) {
                        attemptRetryOnException("redirect",
 request, new RedirectError(networkResponse));
                    } else {
// TODO: Only throw ServerError for 5xx status codes.
                        throw new ServerError(networkResponse);
                    }
                } else {
throw new NetworkError(e);
                }
            }
        }
    }
    ......
}

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;

public CacheDispatcher(BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
 Cache cache, ResponseDelivery delivery) {
        mCacheQueue = cacheQueue;        mNetworkQueue = networkQueue;
        mCache = cache;
        mDelivery = delivery;
    }

    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();

        Request<?> request;
        while (true) {
            // release previous request object to avoid leaking request object when mQueue is drained.
            request = null;
            try {
// Take a request from the queue.
                request = mCacheQueue.take();
            } catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
                if (mQuit) {
return;
                }
continue;
            }
try {
                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.
                    final Request<?> finalRequest = request;
                    mDelivery.postResponse(request, response, new Runnable() {
                    @Override
public void run() {
try {
                                mNetworkQueue.put(finalRequest);
                            } catch (InterruptedException e) {
// Not much we can do about this.
                            }
                        }
                    });
                }
            } catch (Exception e) {
                VolleyLog.e(e, "Unhandled exception %s", e.toString());
            }
        }
    }
}

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;
            }

            // 分发核心代码块
            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();
            }
        }
    }
}

/**
 * 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;
    }
}