在Android开发中，网络请求是最重要的模块之一，Android中大部分网络请求使用的是HTTP连接，包括原生的HttpClient和HttpUrlConnection两种访问网络方式。需要注意的是，HttpClient方式在Android6.0以后，很多类已经不支持了。
目前主流的开源网络框架，主要有OkHttp，Volley，Retrofit三种，我本人在短暂的开发经历中基本也只接触过这几个，在此简单分析这三个框架，仅作为本人记录。
Volley框架
Volley框架是由Google在2013年发布的较为轻量级的网络框架，主要适用于处理请求次数较多，量级较小的网络请求。
一、Volley使用方式
Volley的使用方式十分简单，主要分为以下几步：
Step1: 引入Volley（Android Studio）
方法一：在项目的build.gradle 添加依赖
方法二：引入volley.jar包
方法三：通过git下载volley包，之后添加为项目的module，并为主工程添加依赖
需要注意的是，如果Volley返回Object，需要对其进行解析，转换为Gson对象，那么还需要引入Gson依赖
注：引入的版本根据sdk版本而定
Step2: 创建请求队列实例
一般而言，请求队列不需要每次进行网络请求时创建，通常一个Activity创建一个，或者对于较少请求的轻量级的应用，也可以一个应用只创建一个请求队列，主要视请求的多少而定。
Step3: 创建请求Request
Volley本身已封装好几种常用的Request，包括StringRequest，JsonRequest，JsonObjectRequest，JsonArrayRequest
值得一提的是，Volley封装了ImageLoader和ImageRequest，可以方便地支持图片的获取和加载，不需要额外自己添加图片的加载
以StringRequest为例，创建Request的代码也十分简单：
StringRequest stringRequest = new StringRequest(url, new Listener<String>() {
@Override
public void onResponse(String response) {
Log.e("xxxx", "onStringResponse: " + response);
}
}, new ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("xxxx", "onStringErrorResponse: " + error);
}
});
Step4: 将请求加入请求队列
经过以上几个步骤，就可以实现网络请求，成功和失败结果均返回。
二、Volley源码分析
Volley作为一个轻量级的网络框架，源码实际上并不复杂，接下来将针对其主要的代码进行分析。
1. Volley.java 的主要代码分析
图中截出的就是我们在使用Volley时的第一步创建请求队列的代码，事实上Volley文件中也只有这一个重要的函数，主要步骤如下：
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) {
stack = new HurlStack();
} else {
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
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;
}
- 创建了一个用于存储Volley缓存的文件，使用的是默认的缓存路径，然后获取包名、包的信息以及用户信息等
- 根据sdk版本，sdk版本大于等于9的，即Android系统2.3版本以上，创建HurlStack，9以下创建HttpClientStack
- 创建了请求队列，maxDiskCacheBytes指的是缓存的最大容量，在创建队列时如果定义了该参数，则按指定的容量，否则缓存容量为按照默认的-1
- RequestQueue.start()
2. RequestQueue的代码分析部分
RequestQueue最重要的是add和start两个函数。
start函数：
/**
* 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();
}
}
可以看到主要的步骤是创建并启动了两种分发器，其中mNetworkDispatchers包含多个NetworkDispatcher，数量默认为4。
add函数的主要代码如图所示：
/**
* 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;
}
}
- 将要add的请求加入当前请求的列表，并设置一个序列号码和一个已加入队列的tag，用来让分发器按请求的顺序处理请求。
- 如果请求设置了不缓存（默认是缓存），那么将其加入网络请求队列。
- 用cachekey来作为请求的唯一标识，如果有相同key的请求在waitingRequests中，标识有相同的请求已经执行并且还没有返回结果，为避免重复请求，则将该请求存入；如果没有，则将该请求加入缓存队列中。
3. CacheDispatcher 和 NetworkDispatcher
在RequestQueue的代码分析中，我们可以看到队列并没有对网络请求进行处理，接下来我们看看这两个分发器是如何处理加入队列的网络请求的。
① CacheDispatcher
run()函数解析：
@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) {
...
}
}
- 设置线程的优先级为最高，并初始化缓存
- 接下来进入一个无限循环，按序取出列表中的request，对request队列中的每一个request进行处理
对request进行处理的步骤：
// 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());
}
}
- 从请求列表中取出request，判断是否已取消，如未取消，则通过request的cacheKey从缓存中获取
- 如果缓存中没有对应key的request或者已经过期，则将该request发送到mNetworkQueue，等待NetworkDispatcher进行处理
- 如果有对应的缓存request，并且没有过期，那么CacheDispatcher会将该请求发送到主线程（发送到主线程的具体方法后面会说明）。
② NetworkDispatcher
同样，NetworkDispatcher的run()函数也有一个无限循环对request进行处理，在确认request未被中断（通常发生在请求超时的情况）并且未被取消后，进行处理。
@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) {
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);
}
}
}
- 取出request后，通过Network的performRequest请求网络
- 在获取到response后，通过parseNetworkResponse对response进行解析
- 如果request需要缓存，则存储到mCache中，等待CacheDispatcher进行处理
- 最后将获取到的response返回给主线程并标记request完成即可
4. NetworkDispatcher 请求网络方式
上面run函数中可以看到请求网络是通过mNetwork.performRequest函数请求网络，该函数在BasicNetwork中实现，实现方式如下图：
@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);
}
...
} 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);
}
}
}
}
- 获取headers，之后通过mHttpStack请求网络
- mHttpStack也是一个接口类，通过之前提到过的HurlStack和HttpClientStack实现
- 获取到 httpResponse后，根据响应的状态码，返回不同的NetworkResponse给NetworkDispatcher
5. 返回response给主线程的方式
在请求网络获取到NetworkResponse后，通过mDeliver.postResponse将结果返回给主线程，代码如下图：
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
ResponseRunnable代码：
@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();
}
}
根据Request请求的结果，Runnable分别调用了Request不同的函数。
以StringRequest为例：
@Override
protected void deliverResponse(String response) {
if (mListener != null) {
mListener.onResponse(response);
}
}
请求成功时调用mListener（创建StringRequest时的参数）的回调函数，在回调函数中对获取到的response进行需要的操作即可。
至此，Volley实现网络请求的基本流程就梳理完成了，最后再通过流程图进行总结：
对Volley框架的总结就到这里，下一篇将继续总结另一种重要的网络框架——OkHttp的相关内容。
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