我们先从Volley的入口分析，使用Volley的步骤：
//创建的请求队列
RequestQueue requestQueue = Volley.newRequestQueue(mContext);
//创建一个请求
Request request = new XXRequest(...);
//往队列添加请求
requestQueue.add(request);
我们先来看newRequestQueue(Context)方法:
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}
调用的是重载的方法：
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
//默认的缓存目录
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) {
//如果SDK >= 9 则采用HurlStack来做底层的网络请求
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
//SDK < 9则采用HttpClientStack来做底层的网络请求
stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}
//将HttpStack包装成一个Network
Network network = new BasicNetwork(stack);
//进一步包装Network，构建一个RequestQueue对象
RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
//调用RequestQueue的start方法来开启请求调度
queue.start();
return queue;
}
我们可以看到默认的缓存目录是context.getCacheDir()/volley,HttpStack是干什么用的呢？
/**
* An HTTP stack abstraction.
*/
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 additionalHeaders additional headers to be sent together with
* {@link Request#getHeaders()}
* @return the HTTP response
*/
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError;
}
HttpStack是一个接口，根据注释我们可以知道HttpStack是用于执行HTTP请求的.HttpClientStack的具体实现是HttpClient,网络请求都是通过HttpClient来进行处理的，而HurlStack的具体实现是HttpURLConnection,所有的网络请求都是交给HttpURLConnection去处理的。
/**
* An HttpStack that performs request over an {@link HttpClient}.
*/
public class HttpClientStack implements HttpStack {
protected final HttpClient mClient;
private final static String HEADER_CONTENT_TYPE = "Content-Type";
public HttpClientStack(HttpClient client) {
mClient = client;
}
private static void addHeaders(HttpUriRequest httpRequest, Map<String, String> headers) {
for (String key : headers.keySet()) {
httpRequest.setHeader(key, headers.get(key));
}
}
@SuppressWarnings("unused")
private static List<NameValuePair> getPostParameterPairs(Map<String, String> postParams) {
List<NameValuePair> result = new ArrayList<NameValuePair>(postParams.size());
for (String key : postParams.keySet()) {
result.add(new BasicNameValuePair(key, postParams.get(key)));
}
return result;
}
/*
*执行请求的方法
*/
@Override
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
HttpUriRequest httpRequest = createHttpRequest(request, additionalHeaders);
addHeaders(httpRequest, additionalHeaders);
addHeaders(httpRequest, request.getHeaders());
onPrepareRequest(httpRequest);
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);
}
/**
* Creates the appropriate subclass of HttpUriRequest for passed in request.
*/
@SuppressWarnings("deprecation")
/* protected */ 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());
}
}
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());
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;
}
case Method.HEAD:
return new HttpHead(request.getUrl());
case Method.OPTIONS:
return new HttpOptions(request.getUrl());
case Method.TRACE:
return new HttpTrace(request.getUrl());
case Method.PATCH: {
HttpPatch patchRequest = new HttpPatch(request.getUrl());
patchRequest.addHeader(HEADER_CONTENT_TYPE, request.getBodyContentType());
setEntityIfNonEmptyBody(patchRequest, request);
return patchRequest;
}
default:
throw new IllegalStateException("Unknown request method.");
}
}
private static void setEntityIfNonEmptyBody(HttpEntityEnclosingRequestBase httpRequest,
Request<?> request) throws AuthFailureError {
byte[] body = request.getBody();
if (body != null) {
HttpEntity entity = new ByteArrayEntity(body);
httpRequest.setEntity(entity);
}
}
/**
* Called before the request is executed using the underlying HttpClient.
*
* <p>Overwrite in subclasses to augment the request.</p>
*/
protected void onPrepareRequest(HttpUriRequest request) throws IOException {
// Nothing.
}
/**
* The HttpPatch class does not exist in the Android framework, so this has been defined here.
*/
public static final class HttpPatch extends HttpEntityEnclosingRequestBase {
public final static String METHOD_NAME = "PATCH";
public HttpPatch() {
super();
}
public HttpPatch(final URI uri) {
super();
setURI(uri);
}
/**
* @throws IllegalArgumentException if the uri is invalid.
*/
public HttpPatch(final String uri) {
super();
setURI(URI.create(uri));
}
@Override
public String getMethod() {
return METHOD_NAME;
}
}
}
public class HurlStack implements HttpStack {
private static final String HEADER_CONTENT_TYPE = "Content-Type";
/**
* An interface for transforming URLs before use.
*/
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);
}
private final UrlRewriter mUrlRewriter;
private final SSLSocketFactory mSslSocketFactory;
public HurlStack() {
this(null);
}
/**
* @param urlRewriter Rewriter to use for request URLs
*/
public HurlStack(UrlRewriter urlRewriter) {
this(urlRewriter, null);
}
/**
* @param urlRewriter Rewriter to use for request URLs
* @param sslSocketFactory SSL factory to use for HTTPS connections
*/
public HurlStack(UrlRewriter urlRewriter, SSLSocketFactory sslSocketFactory) {
mUrlRewriter = urlRewriter;
mSslSocketFactory = sslSocketFactory;
}
@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);
if (mUrlRewriter != null) {
String rewritten = mUrlRewriter.rewriteUrl(url);
if (rewritten == null) {
throw new IOException("URL blocked by rewriter: " + url);
}
url = rewritten;
}
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.
throw new 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;
}
/**
* Initializes an {@link HttpEntity} from the given {@link HttpURLConnection}.
* @param connection
* @return an HttpEntity populated with data from <code>connection</code>.
*/
private static HttpEntity entityFromConnection(HttpURLConnection connection) {
BasicHttpEntity entity = new BasicHttpEntity();
InputStream inputStream;
try {
inputStream = connection.getInputStream();
} catch (IOException ioe) {
inputStream = connection.getErrorStream();
}
entity.setContent(inputStream);
entity.setContentLength(connection.getContentLength());
entity.setContentEncoding(connection.getContentEncoding());
entity.setContentType(connection.getContentType());
return entity;
}
/**
* Create an {@link HttpURLConnection} for the specified {@code url}.
*/
protected HttpURLConnection createConnection(URL url) throws IOException {
return (HttpURLConnection) url.openConnection();
}
/**
* Opens an {@link HttpURLConnection} with parameters.
* @param url
* @return an open connection
* @throws IOException
*/
private HttpURLConnection openConnection(URL url, Request<?> request) throws IOException {
HttpURLConnection connection = createConnection(url);
int timeoutMs = request.getTimeoutMs();
connection.setConnectTimeout(timeoutMs);
connection.setReadTimeout(timeoutMs);
connection.setUseCaches(false);
connection.setDoInput(true);
// use caller-provided custom SslSocketFactory, if any, for HTTPS
if ("https".equals(url.getProtocol()) && mSslSocketFactory != null) {
((HttpsURLConnection)connection).setSSLSocketFactory(mSslSocketFactory);
}
return connection;
}
@SuppressWarnings("deprecation")
/* package */ static void setConnectionParametersForRequest(HttpURLConnection connection,
Request<?> request) throws IOException, 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) {
// Prepare output. There is no need to set Content-Length explicitly,
// since this is handled by HttpURLConnection using the size of the prepared
// output stream.
connection.setDoOutput(true);
connection.setRequestMethod("POST");
connection.addRequestProperty(HEADER_CONTENT_TYPE,
request.getPostBodyContentType());
DataOutputStream out = new DataOutputStream(connection.getOutputStream());
out.write(postBody);
out.close();
}
break;
case Method.GET:
// Not necessary to set the request method because connection defaults to GET but
// being explicit here.
connection.setRequestMethod("GET");
break;
case Method.DELETE:
connection.setRequestMethod("DELETE");
break;
case Method.POST:
connection.setRequestMethod("POST");
addBodyIfExists(connection, request);
break;
case Method.PUT:
connection.setRequestMethod("PUT");
addBodyIfExists(connection, request);
break;
case Method.HEAD:
connection.setRequestMethod("HEAD");
break;
case Method.OPTIONS:
connection.setRequestMethod("OPTIONS");
break;
case Method.TRACE:
connection.setRequestMethod("TRACE");
break;
case Method.PATCH:
connection.setRequestMethod("PATCH");
addBodyIfExists(connection, request);
break;
default:
throw new IllegalStateException("Unknown method type.");
}
}
private static void addBodyIfExists(HttpURLConnection connection, Request<?> request)
throws IOException, AuthFailureError {
byte[] body = request.getBody();
if (body != null) {
connection.setDoOutput(true);
connection.addRequestProperty(HEADER_CONTENT_TYPE, request.getBodyContentType());
DataOutputStream out = new DataOutputStream(connection.getOutputStream());
out.write(body);
out.close();
}
}
}
刚开始的时候我们知道，HttpStack是通过BasicNetwork封装成了一个Network对象，我们来看看BasicNetwork里面到底做了啥事情。
/**
* A network performing Volley requests over an {@link HttpStack}.
*/
public class BasicNetwork implements Network {
protected static final boolean DEBUG = VolleyLog.DEBUG;
private static int SLOW_REQUEST_THRESHOLD_MS = 3000;
private static int DEFAULT_POOL_SIZE = 4096;
protected final HttpStack mHttpStack;
protected final ByteArrayPool mPool;
/**
* @param httpStack HTTP stack to be used
*/
public BasicNetwork(HttpStack httpStack) {
// If a pool isn't passed in, then build a small default pool that will give us a lot of
// benefit and not use too much memory.
this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
}
/**
* @param httpStack HTTP stack to be used
* @param pool a buffer pool that improves GC performance in copy operations
*/
public BasicNetwork(HttpStack httpStack, ByteArrayPool pool) {
mHttpStack = httpStack;
mPool = pool;
}
@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);
}
// 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 = 0;
NetworkResponse networkResponse = null;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnectionError(e);
}
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 {
// TODO: Only throw ServerError for 5xx status codes.
throw new ServerError(networkResponse);
}
} else {
throw new NetworkError(networkResponse);
}
}
}
}
/**省略代码*/
我们主要来看实现的performRequest方法，可以看到将请求头和请求交给了HttpStack进行了处理，调用的是HttpStack的performRequest(request,headers);并且将请求结果封装成NetworkResponse并返回，但是BasicNetwork的performRequest是什么时候被调用的呢？，我们进一步去看RequestQueue的实现。
/**
* A request dispatch queue with a thread pool of dispatchers.
*
* Calling {@link #add(Request)} will enqueue the given Request for dispatch,
* resolving from either cache or network on a worker thread, and then delivering
* a parsed response on the main thread.
*/
public class RequestQueue {
/** Callback interface for completed requests. */
public static interface RequestFinishedListener<T> {
/** Called when a request has finished processing. */
public void onRequestFinished(Request<T> request);
}
/** Used for generating monotonically-increasing sequence numbers for requests. */
private AtomicInteger mSequenceGenerator = new AtomicInteger();
/**
* Staging area for requests that already have a duplicate request in flight.
*
* <ul>
* <li>containsKey(cacheKey) indicates that there is a request in flight for the given cache
* key.</li>
* <li>get(cacheKey) returns waiting requests for the given cache key. The in flight request
* is <em>not</em> contained in that list. Is null if no requests are staged.</li>
* </ul>
*/
private final Map<String, Queue<Request<?>>> mWaitingRequests =
new HashMap<String, Queue<Request<?>>>();
/**
* The set of all requests currently being processed by this RequestQueue. A Request
* will be in this set if it is waiting in any queue or currently being processed by
* any dispatcher.
*/
private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();
//需要缓存的请求的队列
/** The cache triage queue. */
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();
//网络请求队列
/** The queue of requests that are actually going out to the network. */
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();
//默认开启的网络请求线程数量
/** Number of network request dispatcher threads to start. */
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;
/** Cache interface for retrieving and storing responses. */
private final Cache mCache;
/** Network interface for performing requests. */
private final Network mNetwork;
/** Response delivery mechanism. */
private final ResponseDelivery mDelivery;
//网络请求调度线程组
/** The network dispatchers. */
private NetworkDispatcher[] mDispatchers;
//缓存请求调度线程
/** The cache dispatcher. */
private CacheDispatcher mCacheDispatcher;
private List<RequestFinishedListener> mFinishedListeners =
new ArrayList<RequestFinishedListener>();
/**
* Creates the worker pool. Processing will not begin until {@link #start()} is called.
*
* @param cache A Cache to use for persisting responses to disk
* @param network A Network interface for performing HTTP requests
* @param threadPoolSize Number of network dispatcher threads to create
* @param delivery A ResponseDelivery interface for posting responses and errors
*/
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
/**
* Creates the worker pool. Processing will not begin until {@link #start()} is called.
*
* @param cache A Cache to use for persisting responses to disk
* @param network A Network interface for performing HTTP requests
*/
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}
//开始请求调度
/**
* 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();
}
}
//停止所有的线程，包括缓存线程和网络请求线程
/**
* Stops the cache and network dispatchers.
*/
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}
//添加请求的方法
/**
* 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.
//请求不需要缓存则，直接添加进网络请求队列
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) {
//需要进行缓存，并且已经有相同cacheKey存储与等待队列中，那么将请求用链表装载，然后添加进等待队列
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);
//等待队列中没有该cacheKey,则直接添加进缓存队列
mCacheQueue.add(request);
}
return request;
}
}
}
主要来看add(Request<?>)，start以及stop方法。
add方法
首先判断是否是需要缓存的请求，如果是，则进一步判断等待队列中是否已经存在过同样cacheKey的请求，如果存在，那么将所有包含该cacheKey的请求封装成一个LinkedList添加进等待请求队列。等待队列中的请求能被调度的条件是：原来与该cacheKey一致的请求已经被取消，或者服务器返回的是304状态码并且原来的请求已经得到了响应，满足条件的才会将等待请求队列中的所有请求添加进缓存请求队列mCacheQueue。如果不存在，那么将请求添加进缓存请求队列，等待线程来调度。
start方法
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();
}
}
该方法主要创建了一个缓存调度线程和4个网络调度线程，并且启动了这些线程然后等待请求的到来。start方法是创建并启动线程，那么stop方法应该就是停止掉这些线程了。
/**
* Stops the cache and network dispatchers.
*/
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}
接下来分析CacheDispatcher和NetworkDispatcher,看看是如何调度网络请求的。由于是线程我们最主要关注的点是run方法。
CacheDispatcher的run方法代码：
@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();
while (true) {
try {
// Get a request from the cache triage queue, blocking until
// at least one is available.
final Request<?> request = mCacheQueue.take();
request.addMarker("cache-queue-take");
//上面已经说过如果cancel了，那么就会将等待请求队列的请求(具有相同cacheKey的请求)添加进mCacheQueue
// If the request has been canceled, don't bother dispatching it.
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}
//缓存检查
// 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;
}
//检查缓存是否已经过期
// 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");
//检查缓存是否需要刷新
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.
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;
}
}
}
首先进行缓存判断，如果没有缓存，那么直接将请求Request交给网络请求队列mNetworkQueue；如果该请求已经存在缓存中，那么判断缓存是否已经过期，过期同样交给网络请求队列，没过期则判断是否需要刷新缓存，不需要则返回缓存的响应结果，如果需要刷新，会先将缓存的响应结果返回，返回再讲该请求交给网络请求队列去刷新缓存。
@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
request.addMarker("post-response");
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, 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;
}
//先将缓存返回
// 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");
}
//最后去网络刷新缓存
// If we have been provided a post-delivery runnable, run it.
if (mRunnable != null) {
mRunnable.run();
}
}
}
NetworkDispatcher
@Override
public void run() {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
Request<?> request;
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);
}
}
}
网络请求线程就比较简单了，直接请求，然后判断是否需要刷新缓存(返回304并且缓存已经被返回过了，那么不需要刷新缓存)，该请求是一个缓存请求(也就是说shouldCache()为ture，这个由服务器规定max-age，如果该请求在上一次请求的时间+max-age时间 < 当前的系统时间，则不会去向服务器去请求而是直接返回本地的缓存，即shouldCache为false)，那么就刷新缓存。最后是响应结果。
大体的流程如下：
volley.png
至于响应是如何回到主线程的呢？我们来看看RequestQueue的其中一个构造方法：
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
我们可以看到new ExecutorDelivery(new Handler(Looper.getMainLooper())，ExecutorDelivery实现的是ResponseDelivery接口，ResponseDelivery其实就是用于返回请求结果的，看到ExecutorDelivery构造参数Looper.getMainLooper()，我们就应该可以猜到最后的结果是通过Handler机制来传递到主线程的。
其实现在网络请求库有OkHttp和Retrofit等，如果想要使用OkHttp作为Volley的底层请求处理库的话，需要定义一个类实现HttpStack，并在performRequest方法中使用OkHttp来做相关的网络请求。
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