前提
上一次讲了Volley的核心RequestQueue，如果没有看过的话请参看Volley使用总结及源码分析（一），在其start方法中，维护了五个线程，一个缓存线程和四个网络线程，下面我们继续分析，在上一次提到注意线程初始化的时候传入的参数，那参数传入之后到底做了什么操作？
缓存线程
public class CacheDispatcher extends Thread {
private static final boolean DEBUG = VolleyLog.DEBUG;
private final BlockingQueue> mCacheQueue;
private final BlockingQueue> mNetworkQueue;
private final Cache mCache;
private final ResponseDelivery mDelivery;
private volatile boolean mQuit = false;
public CacheDispatcher(
BlockingQueue> cacheQueue, BlockingQueue> networkQueue,
Cache cache, ResponseDelivery delivery) {
//缓存请求队列
mCacheQueue = cacheQueue;
//网络请求队列
mNetworkQueue = networkQueue;
//DiskBasedCache类具体对象，用于本地缓存
mCache = cache;
//将响应传递回主线程
mDelivery = delivery;
}
//退出线程
public void quit() {
mQuit = true;
interrupt();
}
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher"); Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//初始化缓存
mCache.initialize();
while (true) {
try {
final Request request = mCacheQueue.take();
request.addMarker("cache-queue-take");
//判断是否取消
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
//当前请求是否有缓存，
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
request.addMarker("cache-miss");
//没有缓存，加入到网络请求队列
mNetworkQueue.put(request);
continue;
}
//判断缓存是否过期
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
//过期的话重新加入网络请求队列
mNetworkQueue.put(request);
continue;
}
//缓存没有过期的话，解析缓存为请求的响应
request.addMarker("cache-hit");
Response response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
//原始数据源是否需要刷新
if (!entry.refreshNeeded()) {
//将解析后的响应使用Delivery分发
mDelivery.postResponse(request, response);
} else {
//重新请求网络，刷新缓存
request.addMarker("cache-hit-refresh-needed");
request.setCacheEntry(entry);
response.intermediate = true;
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;
}
continue;
}
}
}
}
代码中加了中文注释，可以看到CacheDispatcher继承了Thread，实现了run方法。构造方法将传入的四个参数保存在成员变量中，quit方法设置取消标志，中断线程。run方法中首先对mCache进行初始化，然后不断从缓存请求队列中取请求处理mCacheQueue.take();，队列为空则等待，请求处理结束则将结果传递给ResponseDelivery 去执行后续处理。当结果未缓存过、缓存失效或缓存需要刷新的情况下，该请求都需要重新进入NetworkDispatcher去调度处理。
网络线程
public class NetworkDispatcher extends Thread {
private final BlockingQueue> mQueue;
private final Network mNetwork;
private final Cache mCache;
private final ResponseDelivery mDelivery;
private volatile boolean mQuit = false;
public NetworkDispatcher(BlockingQueue> queue,
Network network, Cache cache,
ResponseDelivery delivery) {
//网络请求队列
mQueue = queue;
//具体执行网络交互
mNetwork = network;
//DiskBasedCache类具体对象，用于本地缓存
mCache = cache;
//将响应传递回主线程
mDelivery = delivery;
}
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() {
//设置线程优先级
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
//记录开始时间
long startTimeMs = SystemClock.elapsedRealtime();
Request request;
try {
//从队列中取出网络请求
request = mQueue.take();
} catch (InterruptedException e) {
if (mQuit) {
return;
}
continue;
}
try {
request.addMarker("network-queue-take");
//判断是否取消
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
continue;
}
addTrafficStatsTag(request);
//performRequest中，调用HttpStack处理请求
NetworkResponse networkResponse = mNetwork.performRequest(request);
request.addMarker("network-http-complete");
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}
//使用request的parseNetworkResponse方法解析，请求的网络响应
Response response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
if (request.shouldCache() && response.cacheEntry != null) {
//添加缓存
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
request.markDelivered();
//将解析后的响应使用Delivery分发
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);
}
}
可以看到NetworkDispatcher与CacheDispatcher很像，主要是run方法中的流程不同。设置线程优先级，不断从网络请求队列中取出网络请求，调用BasicNetwork中的performRequest方法处理请求，在该方法中调用HttpStack处理请求，并将结果转换为可被ResponseDelivery处理的NetworkResponse。使用request.parseNetworkResponse处理网络返回的响应，加入缓存，返回处理后的响应结果。
总结
默认情况下，所有请求都会经过缓存线程判断，如果存在缓存且没有过期，则直接返回缓存的响应，否则加入网络线程中，在网络线程中，会调用HttpStack处理请求，返回的响应使用request的parseNetworkResponse方法进一步进行解析，并将结果缓存到本地。