前言
这段时间入职新公司，发现网络请求库使用的是Volley，由于对Volley还不是很熟悉，于是有了今天这篇文章。
Volley的基本使用
//Volley的回调是在主线程的
Log.e("TAG", "volley: -----------"+ Thread.currentThread().getName());
//1.创建请求队列
RequestQueue requestQueue = Volley.newRequestQueue(this);
String url = "https://api.apiopen.top/musicBroadcasting";
//2.创建请求
StringRequest request = new StringRequest(Request.Method.GET, url, new Response.Listener<String>() {
@Override
public void onResponse(String response) {
Log.e("TAG", "onResponse: ---" + Thread.currentThread().getName());
Log.e("TAG", "onResponse: ---" + response);
}
}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
Log.e("TAG", "onErrorResponse: ---" + Thread.currentThread().getName());
}
});
//3.将创建的请求添加到队列中
requestQueue.add(request);
- 注意： Volley的回调是在主线程的所以可以直接在回调中进行UI操作
分析
创建请求队列 Volley.newRequestQueue(this);做了些什么
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, (BaseHttpStack) null);
}
//调用重载方法 BaseHttpStack 为null
public static RequestQueue newRequestQueue(Context context, BaseHttpStack stack) {
BasicNetwork network;
if (stack == null) {
//安卓2.3及以上使用HttpURLConnection
if (Build.VERSION.SDK_INT >= 9) {
network = new BasicNetwork(new HurlStack());
} else {
//下面这段话的意思是安卓2.3以前HttpURLConnection不可靠，所以使用的Apache的HttpClient，后面如果不再兼容2.3以下会考虑把Apache的Http移除
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
// At some point in the future we'll move our minSdkVersion past Froyo and can
// delete this fallback (along with all Apache HTTP code).
String userAgent = "volley/0";
try {
String packageName = context.getPackageName();
PackageInfo info =
context.getPackageManager().getPackageInfo(packageName, /* flags= */ 0);
userAgent = packageName + "/" + info.versionCode;
} catch (NameNotFoundException e) {
}
//采用HttpClient
network =
new BasicNetwork(
new HttpClientStack(AndroidHttpClient.newInstance(userAgent)));
}
} else {
network = new BasicNetwork(stack);
}
return newRequestQueue(context, network);
}
private static RequestQueue newRequestQueue(Context context, Network network) {
//创建缓存路径
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
// 创建请求队列
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
return queue;
}
public void start() {
//一个Queue里面有一个缓存线程和默认4个网络请求线程。也就是说内部其实会启动5个线程
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();
}
}
Request
- Request就是封装了一些请求相关的东西，这里暂时略过
requestQueue.add(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.
//这里的意思就是把请求和队列关联起来，这个请求属于这个队列
request.setRequestQueue(this);
synchronized (mCurrentRequests) {//添加到请求集合里面 set集合
mCurrentRequests.add(request);
}
// Process requests in the order they are added. 这一步是可以让请求按照添加顺序去处理 直接维护一个队列不是更好吗？？？
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");
// If the request is uncacheable, skip the cache queue and go straight to the network.
//判断是否需要缓存responses 如果不需要就跳过缓存队列 直接到网络请求队列 值得注意的是Volley默认是做缓存的
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
mCacheQueue.add(request);
return request;
}
- 接下来就是看这几个线程NetworkDispatcher和CacheDispatcher做了些什么
直接看他们的run方法
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();
//一个死循环处理请求
while (true) {
try {
processRequest();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
Thread.currentThread().interrupt();
return;
}
VolleyLog.e(
"Ignoring spurious interrupt of CacheDispatcher thread; "
+ "use quit() to terminate it");
}
}
}
//从队列里面拿出请求然后处理
private void processRequest() throws InterruptedException {
// Get a request from the cache triage queue, blocking until
// at least one is available.
final Request<?> request = mCacheQueue.take();
processRequest(request);
}
@VisibleForTesting
void processRequest(final Request<?> request) throws InterruptedException {
request.addMarker("cache-queue-take");
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
return;
}
// 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.
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// 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;
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
// Post the intermediate response back to the user and have
// the delivery then forward the request along to the network.
mDelivery.postResponse(
request,
response,
new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
// Restore the interrupted status
Thread.currentThread().interrupt();
}
}
});
} else {
// request has been added to list of waiting requests
// to receive the network response from the first request once it returns.
mDelivery.postResponse(request, response);
}
}
}
void processRequest(Request<?> request) {
long startTimeMs = SystemClock.elapsedRealtime();
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");
request.notifyListenerResponseNotUsable();
return;
}
addTrafficStatsTag(request);
//执行网络请求的地方 通过HttpUrlConnection 或者HttpClient来请求数据
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");
request.notifyListenerResponseNotUsable();
return;
}
// 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);
request.notifyListenerResponseReceived(response);
} catch (VolleyError volleyError) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
request.notifyListenerResponseNotUsable();
} 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);
request.notifyListenerResponseNotUsable();
}
}
缓存处理
- Volley判断是否需要刷新缓存是使用服务端设置的，会考虑服务端返回header里的Cache-Control的Expires。但是有时候接口并不返回这些东西，这种情况下，volley设置的缓存ttl就是0,也就是相当于没有缓存，每次都会从网络请求
- 如果需要缓存，应该怎么设置？
- 重写请求头信息原理就是增加过期时间
public class CacheControlHttpHeaderParser extends HttpHeaderParser {
/**
* @param response
* @param cacheTime 设置缓存时间
* @return
*/
public static Cache.Entry parseCacheHeaders(NetworkResponse response, long cacheTime) {
long now = System.currentTimeMillis();
Map<String, String> headers = response.headers;
long serverDate = 0;
long lastModified = 0;
String serverEtag = null;
String headerValue;
headerValue = headers.get("Date");
if (headerValue != null) {
serverDate = parseDateAsEpoch(headerValue);
}
headerValue = headers.get("Last-Modified");
if (headerValue != null) {
lastModified = parseDateAsEpoch(headerValue);
}
serverEtag = headers.get("ETag");
Cache.Entry entry = new Cache.Entry();
entry.data = response.data;
entry.etag = serverEtag;
entry.softTtl = now + cacheTime;
entry.ttl = now + cacheTime;
entry.serverDate = serverDate;
entry.lastModified = lastModified;
entry.responseHeaders = headers;
entry.allResponseHeaders = response.allHeaders;
return entry;
}
}
- 在Request里重写parseNetworkResponse
public class StringCacheRequest extends StringRequest {
private static final long CACHE_TIME = 1000 * 60 * 5;//5分钟
public StringCacheRequest(int method, String url, Response.Listener<String> listener, @Nullable Response.ErrorListener errorListener) {
super(method, url, listener, errorListener);
}
public StringCacheRequest(String url, Response.Listener<String> listener, @Nullable Response.ErrorListener errorListener) {
super(url, listener, errorListener);
}
@Override
protected Response<String> parseNetworkResponse(NetworkResponse response) {
String parsed;
try {
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
return Response.success(parsed, CacheControlHttpHeaderParser.parseCacheHeaders(response, CACHE_TIME));
}
}
总结
Volley的源码对于开发者还是比较友好的，没什么地方绕弯子。看起来也是行云流水，酣畅淋漓。对于安卓开发者从长远看还是推荐使用OkHttp，OkHttp效率更高，设计更为合理。并且支持Http2、SPDY。
