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);
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
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。