android的volley框架

1.Volley概述

在项目开发过程中，博主曾写过大量的访问网络重复代码，特别是ListView adapter很难避免getView()方法不被重复调用，如果ImageView不利用缓存机制，那么网络的负荷就会更大！曾将访问网络代码和缓存封装起来使用，但是中间仍存在不少瑕疵！今年的Google I/O 2013上，Volley发布了！Volley是Android平台上的网络通信库，能使网络通信更快，更简单，更健壮

Volley特别适合数据量不大但是通信频繁的场景，现在android提供的源码已经包含Volley，以后在项目中，可以根据需求引入Volley jar文件！

2.Volley源码分析

(1).Volley.java

Volley.newRequestQueue()方法在一个app最好执行一次，可以使用单例设计模式或者在application完成初始化，具体原因请查看代码分析

/**
 * @author zimo2013
 * @see http://blog.csdn.net/zimo2013
 */
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 {
            stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
        }
    }

    Network network = new BasicNetwork(stack);

    //cacheDir 缓存路径 /data/data//cache/
    RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
    queue.start();
    /*
     * 实例化一个RequestQueue，其中start()主要完成相关工作线程的开启，
     * 比如开启缓存线程CacheDispatcher先完成缓存文件的扫描， 还包括开启多个NetworkDispatcher访问网络线程，
     * 该多个网络线程将从 同一个 网络阻塞队列中读取消息
     *
     * 此处可见，start()已经开启，所有我们不用手动的去调用该方法，在start()方法中如果存在工作线程应该首先终止，并重新实例化工作线程并开启
     * 在访问网络很频繁，而又重复调用start()，势必会导致性能的消耗；但是如果在访问网络很少时，调用stop()方法，停止多个线程，然后调用start(),反而又可以提高性能，具体可折中选择
     */

    return queue;
}

(2).RequestQueue.java

/**
 * RequestQueue类存在2个非常重要的PriorityBlockingQueue类型的成员字段mCacheQueue mNetworkQueue ，该PriorityBlockingQueue为java1.5并发库提供的新类
 * 其中有几个重要的方法，比如take()为从队列中取得对象，如果队列不存在对象，将会被阻塞，直到队列中存在有对象，类似于Looper.loop()
 *
 * 实例化一个request对象，调用RequestQueue.add(request),该request如果不允许被缓存，将会被添加至mNetworkQueue队列中，待多个NetworkDispatcher线程take()取出对象
 * 如果该request可以被缓存，该request将会被添加至mCacheQueue队列中，待mCacheDispatcher线程从mCacheQueue.take()取出对象，
 * 如果该request在mCache中不存在匹配的缓存时，该request将会被移交添加至mNetworkQueue队列中，待网络访问完成后，将关键头信息添加至mCache缓存中去！
 *
 * @author zimo2013
 * @see http://blog.csdn.net/zimo2013
 */
public void start() {
    stop();

    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();
    }
}

 (3).CacheDispatcher.java

/**
 * @author zimo2013
 * @see http://blog.csdn.net/zimo2013
 */
@Override
public void run() {
    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

    //缓存初始化，会遍历整个缓存文件夹
    mCache.initialize();
    {
        //执行代码
        /*if (!mRootDirectory.exists()) {
            if (!mRootDirectory.mkdirs()) {
                VolleyLog.e("Unable to create cache dir %s", mRootDirectory.getAbsolutePath());
            }
            return;
        }

        File[] files = mRootDirectory.listFiles();
        if (files == null) {
            return;
        }
        for (File file : files) {
            FileInputStream fis = null;
            try {
                fis = new FileInputStream(file);
                CacheHeader entry = CacheHeader.readHeader(fis);
                entry.size = file.length();
                putEntry(entry.key, entry);
            } catch (IOException e) {
                if (file != null) {
                   file.delete();
                }
            } finally {
                try {
                    if (fis != null) {
                        fis.close();
                    }
                } catch (IOException ignored) { }
            }
        }*/
    }

    while (true) {
        try {
            //该方法可能会被阻塞
            final Request request = mCacheQueue.take();

            Cache.Entry entry = mCache.get(request.getCacheKey());
            if (entry == null) {
                //缓存不存在，则将该request添加至网络队列中
                mNetworkQueue.put(request);
                continue;
            }

            //是否已经过期
            if (entry.isExpired()) {
                request.setCacheEntry(entry);
                mNetworkQueue.put(request);
                continue;
            }

            Response response = request.parseNetworkResponse(
                    new NetworkResponse(entry.data, entry.responseHeaders));

            //存在缓存，执行相关操作

        } catch (InterruptedException e) {
        }
    }
}

(4).NetworkDispatcher.java

/**
 * @author zimo2013
 * @see http://blog.csdn.net/zimo2013
 */
@Override
public void run() {
    Request request;
    while (true) {
        try {
            //可能会被
            request = mQueue.take();
        } catch (InterruptedException e) {
            // We may have been interrupted because it was time to quit.
            if (mQuit) {
                return;
            }
            continue;
        }

        try {

            //访问网络，得到数据
            NetworkResponse networkResponse = mNetwork.performRequest(request);

            if (networkResponse.notModified && request.hasHadResponseDelivered()) {
                request.finish("not-modified");
                continue;
            }

            // Parse the response here on the worker thread.
            Response response = request.parseNetworkResponse(networkResponse);

            // 写入缓存
            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) {
            parseAndDeliverNetworkError(request, volleyError);
        } catch (Exception e) {
            VolleyLog.e(e, "Unhandled exception %s", e.toString());
            mDelivery.postError(request, new VolleyError(e));
        }
    }
}

(5).StringRequest.java

其中在parseNetworkResponse()中，完成将byte[]到String的转化,可能会出现字符乱码，HttpHeaderParser.parseCharset(response.headers)方法在尚未指定是返回为ISO-8859-1，可以修改为utf-8

public class StringRequest extends Request {
    private final Listener mListener;

    /**
     * Creates a new request with the given method.
     *
     * @param method the request {@link Method} to use
     * @param url URL to fetch the string at
     * @param listener Listener to receive the String response
     * @param errorListener Error listener, or null to ignore errors
     */
    public StringRequest(int method, String url, Listener listener,
            ErrorListener errorListener) {
        super(method, url, errorListener);
        mListener = listener;
    }

    public StringRequest(String url, Listener listener, ErrorListener errorListener) {
        this(Method.GET, url, listener, errorListener);
    }

    @Override
    protected void deliverResponse(String response) {
        mListener.onResponse(response);
    }

    @Override
    protected Response parseNetworkResponse(NetworkResponse response) {
        String parsed;
        try {
            //将data字节数据转化为String对象
            parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
        } catch (UnsupportedEncodingException e) {
            parsed = new String(response.data);
        }
        //返回Response对象，其中该对象包含访问相关数据
        return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
    }
}

(6).ImageLoader.java

/**
 * @author zimo2013
 * @see http://blog.csdn.net/zimo2013
 */
public ImageContainer get(String requestUrl, ImageListener imageListener,
        int maxWidth, int maxHeight) {
    throwIfNotOnMainThread();
    final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight);

    //从mCache得到bitmap对象，因此可以覆写ImageCache，完成图片的三级缓存，即在原有的LruCache添加一个软引用缓存
    Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
    if (cachedBitmap != null) {
        //得到缓存对象
        ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
        imageListener.onResponse(container, true);
        return container;
    }

    ImageContainer imageContainer =
            new ImageContainer(null, requestUrl, cacheKey, imageListener);

    // 首先更新该view，其指定了defaultImage
    imageListener.onResponse(imageContainer, true);

    // 根据可以去检查该请求是否已经发起过
    BatchedImageRequest request = mInFlightRequests.get(cacheKey);
    if (request != null) {
        request.addContainer(imageContainer);
        return imageContainer;
    }

    Request newRequest =
        new ImageRequest(requestUrl, new Listener() {
            @Override
            public void onResponse(Bitmap response) {
                //如果请求成功
                onGetImageSuccess(cacheKey, response);
            }
        }, maxWidth, maxHeight,
        Config.RGB_565, new ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                onGetImageError(cacheKey, error);
            }
        });
    //添加至请求队列中
    mRequestQueue.add(newRequest);
    //同一添加进map集合，以方便检查该request是否正在请求网络，可以节约资源
    mInFlightRequests.put(cacheKey, new BatchedImageRequest(newRequest, imageContainer));
    return imageContainer;
}

private void onGetImageSuccess(String cacheKey, Bitmap response) {
    //缓存对象
    mCache.putBitmap(cacheKey, response);

    // 请求完成，不需要检测
    BatchedImageRequest request = mInFlightRequests.remove(cacheKey);

    if (request != null) {
        request.mResponseBitmap = response;
        //处理结果
        batchResponse(cacheKey, request);
    }
}

private void batchResponse(String cacheKey, BatchedImageRequest request) {
    mBatchedResponses.put(cacheKey, request);
    //通过handler，发送一个操作
    if (mRunnable == null) {
        mRunnable = new Runnable() {
            @Override
            public void run() {
                for (BatchedImageRequest bir : mBatchedResponses.values()) {
                    for (ImageContainer container : bir.mContainers) {
                        if (container.mListener == null) {
                            continue;
                        }
                        if (bir.getError() == null) {
                            container.mBitmap = bir.mResponseBitmap;
                            //更新结果
                            container.mListener.onResponse(container, false);
                        } else {
                            container.mListener.onErrorResponse(bir.getError());
                        }
                    }
                }
                mBatchedResponses.clear();
                mRunnable = null;
            }

        };
        // mHandler对应的looper是MainLooper，因此被MainLooper.loop()得到该message，故该runnable操作在主线程中执行，
        mHandler.postDelayed(mRunnable, mBatchResponseDelayMs);
    }
}