Volley简单学习使用五—— 源代码分析三

一、Volley工作流程图：

二、Network

    在NetworkDispatcher中须要处理的网络请求。由以下进行处理：

    NetworkResponse networkResponse = mNetwork.performRequest(request);

看一下mNetwork的定义：（定义在NetworkDispatcher中）

    /** The network interface for processing requests. */
    private final Network mNetwork;

NetworkDispatcher.mNetwork初始化发生在RequestQueue.start()中：

    NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
         mCache, mDelivery);

而RequestQueue.mNetwork是在其构造函数中传入的：

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

由前面分析知RequestQueue的构建是在Volley.newRequestQueue中实现的：

    //创建以stack为參数的Network对象
    Network network = new BasicNetwork(stack);
    //创建RequestQueue对象
    RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
    queue.start();//继续向下分析的入口

    能够看出mNetwork事实上是BasicNetwork对象。

 
  则NetworkResponse中mNetwork实际上调用的是BasicNetwork.performRequest(),这是一个专门用来处理网络请求的函数，其作用为调用HttpStack处理请求，并将结果转换为可被ResponseDelivery处理的NetworkResponse。

看一下其源代码：

   @Override
    public NetworkResponse performRequest(Request<?> request) throws VolleyError {
        long requestStart = SystemClock.elapsedRealtime();
        while (true) {
            HttpResponse httpResponse = null;
            byte[] responseContents = null;
            Map<String, String> responseHeaders = new HashMap<String, String>();
            try {
              /** 忽略网络处理的细节*/
                // Gather headers.
                Map<String, String> headers = new HashMap<String, String>();
                addCacheHeaders(headers, request.getCacheEntry());

                /**运行网络请求
                 * 这里调用了HttpStack.performRequest。并得到一个HttpResponse返回结果*/
                httpResponse = mHttpStack.performRequest(request, headers);

                StatusLine statusLine = httpResponse.getStatusLine();
                int statusCode = statusLine.getStatusCode();
                responseHeaders = convertHeaders(httpResponse.getAllHeaders());

                /**新奇度验证：
                 * 304 Not Modified：client有缓冲的文件并发出了一个条件性的请求
                 * （通常是提供If-Modified-Since头表示客户仅仅想比指定日期更新的文档）。
                 * server告诉客户。原来缓冲的文档还能够继续使用。*/
                if (statusCode == HttpStatus.SC_NOT_MODIFIED) {
                  /** 解析成NetworkResponse，返回*/
                    return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED,
                            request.getCacheEntry().data, responseHeaders, true);
                }

                // 推断responses是否有实体信息，一些响应如204，并不包括content。所以须要验证
                if (httpResponse.getEntity() != null) {
                  //实体信息转化成byte[]
                    responseContents = entityToBytes(httpResponse.getEntity());
                } else {
                  // 无实体信息情况
                  responseContents = new byte[0];
                }

                // 超时情况处理.
                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);
            } 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) {
                ...
            }
        }
    }

总结一下Network.performRequest所做的工作：

1、由传入的HttpStack对象运行网络请求：mHttpStack.performRequest()

2、解析响应结果，将HttpResponse解析成NetworkResponse;

3、对返回结果进行新奇度验证（304）

4、将response的实体信息转化为byte数组

5、超时情况处理，假设发生超时，认证失败等错误。进行重试操作(attemptRetryOnException)。直到成功、抛出异常(不满足重试策略等)结束。

attemptRetryOnException()是依据重试策略进行请求重试操作：

    /**
     * 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.
     */
    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));
    }

三、HttpClientStack、HurlStack

    据上面源代码知。网络请求处理的逻辑实际上是交由传进来的參数HttpStack进行处理。

前面已经分析过。Android2.3之前使用 HttpClientStack，之后使用HurlStack。

1、先看两者的父类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 发起请求之前，加入额外的请求 Headers {@link Request#getHeaders()}
         */
        public HttpResponse performRequest(Request<?

> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError;
    }

2、HttpClientStack(使用HttpClient来实现)

    @Override
    public HttpResponse performRequest(Request<?

> request, Map<String, String> additionalHeaders)
            throws IOException, AuthFailureError {
        HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders);//见附一
        addHeaders(httpRequest, additionalHeaders);
        addHeaders(httpRequest, request.getHeaders());
        onPrepareRequest(httpRequest);// Nothing.空函数，用于重写；该函数在request被excute之前被调用
        //一些网络设置
        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);
    }

附一：createHttpRequest函数：

    /**
     * 依据传进来的request来构造合适的HttpUriRequest
     */
    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());
                }
            }
            /***********一般较多使用的是POST与GET。其等同于HttpClient的一般使用流程***************/
            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());
                //这里就看到了前面实现Request时，重写getBodyContentType（）函数的意义
                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;
            }
            default:
                throw new IllegalStateException("Unknown request method.");
        }
    }

3、HurlStack(由HttpURLConnection来实现)

    @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);
        //UrlRewriter见附一
        if (mUrlRewriter != null) {
            String rewritten = mUrlRewriter.rewriteUrl(url);
            if (rewritten == null) {
                thrownew IOException("URL blocked by rewriter: " + url);
            }
            url = rewritten;
        }
        /**************HttpURLConnection的一般使用流程*******************/
        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.
            thrownew 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;
    }

附一：UrlRewriter

    /** 对URLs在使用前进行重写转换*/
    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);
    }

參数mUrlRewriter通过HttpStack的构造函数传入进来，故能够自行进行定义：

    public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) {
        mUrlRewriter = urlRewriter;
        mSslSocketFactory = sslSocketFactory;
    }

四、NetworkResponse

    回到起点NetworkDispatcher（Thread）中的run()函数，当中：

NetworkResponse networkResponse =
mNetwork.performRequest(request);

以下继续看NetworkResponse的源代码：

NetworkResponse类非常easy，仅是用以在多个类中传递数据，其成员变量：

1）成员变量

int statusCode Http
响应状态码

byte[]
data Body 数据

Map<String, String> headers 响应 Headers

boolean notModified 表示是否为 304 响应

long networkTimeMs 请求耗时

2）其主体仅仅为几个构造函数：

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

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

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

3）回想一下前面分析的设计NetworkResponse的类之间数据的传递关系：

这里的主体是依据NetworkDispatcher.run()函数进行分析的

0、函数中调用Network.performRequest();

     NetworkResponse networkResponse = mNetwork.performRequest(request);

     而Network.performRequest()是基于HttpStack实现的；

1、HttpClientStack与HurlStack（分别基于HttpClient与HttpURLConnection实现）中的public HttpResponse performRequest()函数返回HttpResponse ；

2、Network（实际为BasicNetwork）中performRequest()方法。使用1中的两个HttpStack类。获取到其返回值HttpResponse，然后将其解析成为NetworkResponse;

3、Request中 abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

    将NetworkResponse解析成Response;

    而该函数的调用是在NetworkDispatcher中的run()函数中调用的。                

4、在NetworkDispatcher.run(）的最后一步：

    mDelivery.postResponse(request, response);

    将response传递给了ResponseDelivery

后面继续看Delivery的逻辑；

ResponseDelivery mDelivery的实际类型是ExecutorDelivery：

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

}

能够看到非常easy，就是使用主线程的Looper构建一个Handler。以下全部的post操作都是调用这个Handler来运行Runnable;

比方：

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

将传递来的Response转化为ResponseDeliveryRunnable ，显然这是一个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;
        }

        // 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();
        }
   }

}

在这个子线程中，转而调用 Request来deliverResponse：

以StringRequest为例，来看这个函数：

@Override
protected void deliverResponse(String response) {
    if (mListener != null) {
        mListener.onResponse(response);
    }
}

这个Listener就是自己在定义Request的时候声明的ResponseListener，能够看到这个Listener工作在子线程中，所以假设要更新界面，注意使用Handler把消息传递主线程进行处理。

***************************************************** Volley图片载入的实现 *******************************************************

Volley的图片载入主要还是基于上面的原理来实现的。详细例如以下：

ImageLoader的使用：

//创建ImageLoader
imageLoader = new ImageLoader(httpUtils.getRequestQueue(), imageCache);
public ImageLoader(RequestQueue queue, ImageCache imageCache) {
    mRequestQueue = queue;
    mCache = imageCache;

}

这里面的ImageCache是自己定义的:

// 获取最大内存缓存大小
int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024);
MAX_CACHE_SIZE = maxMemory / 8; // 定义为应用最大缓存的1/8

mImageLruCache = new LruCache<String, Bitmap>(MAX_CACHE_SIZE){
    @Override
    protected int sizeOf(String url, Bitmap bitmap){
        return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
    }

};

// 创建ImageCache
imageCache = new ImageLoader.ImageCache() {
    @Override
    public void putBitmap(String url, Bitmap bitmap) {
        mImageLruCache.put(url, bitmap);
    }

    @Override
    public Bitmap getBitmap(String url) {
        return mImageLruCache.get(url);
    }
};

使用LruCache来实现ImageCache接口，实现图片的内存缓存：

public interface ImageCache {
    public Bitmap getBitmap(String url);
    public void putBitmap(String url, Bitmap bitmap);
}

载入图片时的使用方法：

imageListener = ImageLoader.getImageListener(myImageView, default_pg, failed_pg);
imageLoader.get(imageUrl, imageListener);

来到ImageLoader#get:

public ImageContainer get(String requestUrl, final ImageListener listener) {
    return get(requestUrl, listener, 0, 0);

}

public ImageContainer get(String requestUrl, ImageListener imageListener,
                          int maxWidth, int maxHeight) {
    return get(requestUrl, imageListener, maxWidth, maxHeight, ImageView.ScaleType.CENTER_INSIDE);

}

public ImageContainer get(String requestUrl, ImageListener imageListener,
                          int maxWidth, int maxHeight, ImageView.ScaleType scaleType) {

    // 假设操作不是在主线程，则直接抛出异常
    throwIfNotOnMainThread();

    // 为图片的URL创建一个特定的cacheKey，注意这个cache还和图片的大小及scaleType相关
    final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType);

    // 这里会使用自己定义的LruCache去获取一个Bitmap实例
    Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
    // 假设缓存中已经存在。则直接返回
    if (cachedBitmap != null) {
        // Return the cached bitmap.
        ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
        imageListener.onResponse(container, true);
        return container;
    }

    // 假设缓存中不存在，则进行获取
    ImageContainer imageContainer =
            new ImageContainer(null, requestUrl, cacheKey, imageListener);

    // 通知Observer这时能够使用默认的图片
    imageListener.onResponse(imageContainer, true);

    // 推断是否已经有了一个同样的请求在等待
    BatchedImageRequest request = mInFlightRequests.get(cacheKey);
    if (request != null) {
        // If it is, add this request to the list of listeners.
        request.addContainer(imageContainer);
        return imageContainer;
    }

    // 创建一个Request，反复之前的流程
    Request<Bitmap> newRequest = makeImageRequest(requestUrl, maxWidth, maxHeight, scaleType,
            cacheKey);

    mRequestQueue.add(newRequest);
    mInFlightRequests.put(cacheKey,
            new BatchedImageRequest(newRequest, imageContainer));
    return imageContainer;
}

处理逻辑大致和前面的addRequest同样。首先推断缓存中是否已经存在该url相应的bitmap。假设存在直接返回；假设不存在，先推断是否已经有了一个同样的请求在等待。假设是，把这个请求加入到监听者链表中。假设不存在，则创建一个Request<Bitmap>，加入到RequestQueue中，从网络中去获取；从网络中获取的流程和前面分析的同样。

先来看Request<Bitmap>:

protected Request<Bitmap> makeImageRequest(String requestUrl, int maxWidth, int maxHeight,
        ScaleType scaleType, final String cacheKey) {
    return new ImageRequest(requestUrl, new Listener<Bitmap>() {
        @Override
        public void onResponse(Bitmap response) {
            onGetImageSuccess(cacheKey, response);
        }
    }, maxWidth, maxHeight, scaleType, Config.RGB_565, new ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            onGetImageError(cacheKey, error);
        }
    });
}

实际上返回一个ImageRequest类型。来看其请求成功的响应：即把获得的图片存储到缓存中。

protected void onGetImageSuccess(String cacheKey, Bitmap response) {
    // 把获取到的图片存储到缓存中
    mCache.putBitmap(cacheKey, response);
    // 能够看到假设是多个同样请求在等待，则能够同一时候进行更新处理
    BatchedImageRequest request = mInFlightRequests.remove(cacheKey);

    if (request != null) {
        // Update the response bitmap.
        request.mResponseBitmap = response;
        // Send the batched response
        batchResponse(cacheKey, request);
    }
}

最后NetWork运行的结果会封装成NetWorkResponse。通过ResponseDelivery进行转发，这个类最后会调用Request中deliverResponse方法：

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

这个Listener就是最初定义的ImageListener:

public static ImageListener getImageListener(final ImageView view,
        final int defaultImageResId, final int errorImageResId) {
    return new ImageListener() {
        @Override
        public void onErrorResponse(VolleyError error) {
            if (errorImageResId != 0) {
                view.setImageResource(errorImageResId);
            }
        }

        @Override
        public void onResponse(ImageContainer response, boolean isImmediate) {
            if (response.getBitmap() != null) {
                view.setImageBitmap(response.getBitmap());
            } else if (defaultImageResId != 0) {
                view.setImageResource(defaultImageResId);
            }
        }
    };
}

能够看到这里终于给View空间设置了图片，以上就是Volley实现图片载入的流程。