Volley源码分析（三）
2017-09-26 本文已影响56人
被代码淹没的小伙子
1.Volley源码分析（一）
2.Volley源码分析（二）
3.Volley源码分析（三）
4.XVolley-基于Volley的封装的工具类
上一篇分析完了RequestQueue的大部分方法，add执行完后，Volley就会执行线程操作了，在第一篇博客中提到，star方法执行时会创建1个缓存线程（CacheDispatcher）和4个网络线程（NetworkDispatcher），并开始这5个线程。这里我们就先看缓存线程。
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;
/**
* Creates a new cache triage dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param cacheQueue Queue of incoming requests for triage
* @param networkQueue Queue to post requests that require network to
* @param cache Cache interface to use for resolution
* @param delivery Delivery interface to use for posting responses
*/
public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
//设置缓存线程的优先级
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
// Make a blocking call to initialize the cache.
//初始化缓存内容，对应的硬盘缓存-DiskBasedCache
mCache.initialize();
while (true) {
try {
// Get a request from the cache triage queue, blocking until
// at least one is available.
//BlockingQueue的take方法，取出队列中队首的request，如果没有则阻塞，等待到有request到来
final Request<?> request = mCacheQueue.take();
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
//如果request被取消，则结束当前这次，继续循环
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
//如果缓存里没有，则加入请求队列
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.
//如果缓存过期了，则加入请求队列
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.
//缓存中存在并且没有过期
request.addMarker("cache-hit");
//将数据包装成response
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.
//如果缓存不需要刷新，则直接将缓存回传
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.
//缓存需要刷新的话，先将缓存传回给客户，然后在将请求交给队列
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
}
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
}
}
}
}
首先从继承关系我们就可以看出，创建的是个线程。既然是个线程，无可厚非，肯定是看它的run方法，从源码我们也可以看出，这里面除了构造方法就两个方法，quit和run，quit就不用说了，这里重点看一下run方法。
可以看到第51行先设置了当前线程的优先级，保证线程的顺利进行。
第55行，初始化了缓存，这里要说明一下，Volley现在默认使用的是硬盘缓存，这一点从初始化requestqueue时就可以看出来。
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
后面可以看到，是个死循环，保证缓存线程一直执行。
第62行，可以看到从mCacheQueue.take取出请求。这里说明一下：
mCacheQueue是一个BlockingQueue，它的take方法，取出队列中队首的request，如果没有则阻塞，等待到有request到来
下面就是几种情况的判断：
1）如果该请求被取消------------------->结束当前这次循环
2）如果缓存中不存在这个请求------------>结束当前这次循环,并将请求加入网络请求队列
3）如果缓存过期了--------------------->结束当前这次循环,并将请求加入网络请求队列
当以上几种情况都不存在时，第95行，便要将缓存中这个request对应的请求结果封装成response
后面这个判断很奇妙，我一开始半天没理解，后来才懂了
这里判断缓存是否需要刷新，如果缓存不需要刷新，则将response回调给UI线程，如果需要刷新，同样先将response回调给UI线程，然后再将这个请求放入网络队列，进行请求并刷新缓存
缓存线程到这里基本上就看完了，现在来看网络线程
NetworkDispatcher。
public class NetworkDispatcher extends Thread {
/** The queue of requests to service. */
private final BlockingQueue<Request<?>> mQueue;
/** The network interface for processing requests. */
private final Network mNetwork;
/** The cache to write to. */
private final Cache mCache;
/** For posting responses and errors. */
private final ResponseDelivery mDelivery;
/** Used for telling us to die. */
private volatile boolean mQuit = false;
/**
* Creates a new network dispatcher thread. You must call {@link #start()}
* in order to begin processing.
*
* @param queue Queue of incoming requests for triage
* @param network Network interface to use for performing requests
* @param cache Cache interface to use for writing responses to cache
* @param delivery Delivery interface to use for posting responses
*/
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
/**
* Forces this dispatcher to quit immediately. If any requests are still in
* the queue, they are not guaranteed to be processed.
*/
public void quit() {
mQuit = true;
interrupt();
}
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
private void addTrafficStatsTag(Request<?> request) {
// Tag the request (if API >= 14)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
}
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
//记录开始时间
long startTimeMs = SystemClock.elapsedRealtime();
Request<?> request;
try {
// Take a request from the queue.
//从队首拿一个请求
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.
//如果被取消则结束当前这次循环
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
//添加流量统计标签
addTrafficStatsTag(request);
// Perform the network request.
//此处执行网络请求
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.
//如果服务器返回的304或者request已经存在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.
//设置request已经返回response
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);
}
}
}
private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}
}
同样，这里重点看run方法。
和缓存线程一样，这里先设置了线程的优先级，保证线程的进行，并且利用死循环，使线程一直进行，不会被回收。
第58行，首先从队首拿了一个请求。
第61行，这里就是java常用的中断线程的方式。
第73行，如果请求被取消的话，则结束当前这次循环。
这里重点说明，第82行，这里就是我们整个Volley真正执行网络请求的地方。
NetworkResponse networkResponse = mNetwork.performRequest(request);
可以看到，这里request被当做参数传入，最后返回了一个response。而方法是属于mNetwork，这个mNetwork是在volley初始化requestqueue时传入的。
/**
* 创建一个网络请求
*/
Network network = new BasicNetwork(stack);
/**
* 这里每次都会创建一个请求队列，可以优化，只创建一个全局队列吗
*/
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
还记得这里吗，第一篇博客的时候说过，后面会介绍这个Network，这里就很好理解了，这个mNetwork就是在这里传入，真正执行网络请求就是在这个类中，而这个类的构造函数需要我们传入一个HttpStack对象，这里就是我们最开始说版本判断策略模式那里。这里我们可以进入BasicNetwork类中，看一下performRequest方法，来验证我们的想法。
@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());
//执行网络请求
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);
}
// 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;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnectionError(e);
}
VolleyLog.e("Unexpected response code %d for %s", statusCode, request.getUrl());
NetworkResponse networkResponse;
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 >= 400 && statusCode <= 499) {
// Don't retry other client errors.
throw new ClientError(networkResponse);
} else if (statusCode >= 500 && statusCode <= 599) {
if (request.shouldRetryServerErrors()) {
attemptRetryOnException("server",
request, new ServerError(networkResponse));
} else {
throw new ServerError(networkResponse);
}
} else {
// 3xx? No reason to retry.
throw new ServerError(networkResponse);
}
} else {
attemptRetryOnException("network", request, new NetworkError());
}
}
}
}
可以看到，使用的其实也是Android原生的网络请求方式，只不过加入很多判断。
现在接着看NetworkDispatcher的run方法。
第88行，这里如果服务器返回了304，或者这个request已经返回了response则同样结束这次循环。
Response<?> response = request.parseNetworkResponse(networkResponse);
第94行，这里也是一个重点的地方，看到方法你会不会眼熟哪？如果你是熟练使用volley的话，你会发现这个方法就是我们自定义request中需要重写的方法。将网络请求返回的reponse封装转换为我们需要的response对象。
第99行，将请求的结果加入缓存，很好理解。
第106行，这里设置该request已经放回了response，对应的就是前面第88行的判断。
第107行，这里是我们接口回调的地方。这里需要详细看下mDelivery对象。
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
//Looper.getMainLooper()对应主线程，所以请求成功后的接口回调对应是在主线程中执行。
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
可以看到，在requestqueue的构造函数中，默认初始化了ExecutorDelivery类，这里需要注意一个地方Looper.getMainLooper()对应主线程，所以请求成功后的接口回调对应是在主线程中执行。
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);
}
};
}
可以看到，这里handler对应的是UI线程，执行的Runable。
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
}
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()) {
//请求成功，接口回调
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) {
//这里当请求成功后，对应情况：需要刷新缓存，先将缓存返回成response后，再异步请求，刷新缓存。
mRunnable.run();
}
}
这里只需要注意两个地方，首先接口回调的地方，可以看到deliverResponse这个方法是不是也很熟悉，自定义Request的时候，需要重写这个方法，执行我们的回调。
mRequest.deliverResponse(mResponse.result);
这里附上StringRequest的deliverResponse方法。
@Override
protected void deliverResponse(String response) {
if (mListener != null) {
mListener.onResponse(response);
}
}
这样一看就很清楚了。
最后需要注意的一点：
if (mRunnable != null) {
//这里当请求成功后，对应情况：需要刷新缓存，先将缓存返回成response后，再异步请求，刷新缓存。
mRunnable.run();
}
这里会的对应情况是什么那，还记不记得当我们缓存需要刷新时，会怎么做，Volley会先将缓存的response返回，然后执行一个网络请求，并刷新缓存。
//缓存需要刷新的话，先将缓存传回给客户，然后在将请求交给队列
mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Not much we can do about this.
}
}
});
