Volley 知识梳理 Volley解析
1、网络请求核心
1.1 Network
Volley与网络请求相关的接口有两个：html
/**
* An interface for performing requests.
*/
public interface Network {
/**
* Performs the specified request.
* @param request Request to process
* @return A {@link NetworkResponse} with data and caching metadata; will never be null
* @throws VolleyError on errors
*/
public NetworkResponse performRequest(Request<?> request) throws VolleyError;
}
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1.2 HttpStack
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;
}
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1.3 具体实现
对于上面的两个接口，它们各有对应的实现类：java
Network：BasicNetworkHttpStack：在SDK大于等于九时，其实现类是HurlStack，小于九时，对应的实现类是HttpClientStack。
BasicNetwork和HttpStack的两个实现类的关系是：HttpStack是BasicNetwork的一个成员变量，当Volley传入Request，调用BasicNetwork#performRequest后，在它的内部实际是经过HttpStack#performRequest发起网络请求，并把标准网络请求返回结果HttpResponse封装成Volley的NetworkResponse。android
在看具体的请求以前，咱们须要先了解一下和请求相关的两个类Request和NetworkResponse。缓存
1.3.1 Request
public abstract class Request<T> implements Comparable<Request<T>> {
/**
* Default encoding for POST or PUT parameters. See {@link #getParamsEncoding()}.
*/
private static final String DEFAULT_PARAMS_ENCODING = "UTF-8";
/**
* Supported request methods.
*/
public interface Method {
int DEPRECATED_GET_OR_POST = -1;
int GET = 0;
int POST = 1;
int PUT = 2;
int DELETE = 3;
int HEAD = 4;
int OPTIONS = 5;
int TRACE = 6;
int PATCH = 7;
}
/** An event log tracing the lifetime of this request; for debugging. */
private final MarkerLog mEventLog = MarkerLog.ENABLED ? new MarkerLog() : null;
/**
* Request method of this request. Currently supports GET, POST, PUT, DELETE, HEAD, OPTIONS,
* TRACE, and PATCH.
*/
private final int mMethod;
/** URL of this request. */
private final String mUrl;
/** Default tag for {@link TrafficStats}. */
private final int mDefaultTrafficStatsTag;
/** Listener interface for errors. */
private final Response.ErrorListener mErrorListener;
/** Sequence number of this request, used to enforce FIFO ordering. */
private Integer mSequence;
/** The request queue this request is associated with. */
private RequestQueue mRequestQueue;
/** Whether or not responses to this request should be cached. */
private boolean mShouldCache = true;
/** Whether or not this request has been canceled. */
private boolean mCanceled = false;
/** Whether or not a response has been delivered for this request yet. */
private boolean mResponseDelivered = false;
// A cheap variant of request tracing used to dump slow requests.
private long mRequestBirthTime = 0;
/** Threshold at which we should log the request (even when debug logging is not enabled). */
private static final long SLOW_REQUEST_THRESHOLD_MS = 3000;
/** The retry policy for this request. */
private RetryPolicy mRetryPolicy;
/**
* When a request can be retrieved from cache but must be refreshed from
* the network, the cache entry will be stored here so that in the event of
* a "Not Modified" response, we can be sure it hasn't been evicted from cache. */ private Cache.Entry mCacheEntry = null; /** An opaque token tagging this request; used for bulk cancellation. */ private Object mTag; } 复制代码
Request<T>实现了Comparable<Request<T>>接口，它是全部网络请求的基类，实现这个接口是为了比较各个请求之间的优先级。 它定义了内部接口Method，表示支持的方法：bash
public interface Method {
int DEPRECATED_GET_OR_POST = -1;
int GET = 0;
int POST = 1;
int PUT = 2;
int DELETE = 3;
int HEAD = 4;
int OPTIONS = 5;
int TRACE = 6;
int PATCH = 7;
}
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除此以外还定义了成员变量包括：服务器
mMethod：该请求所对应的方法。mUrl：请求的Url。mSequence：序列号。mRequestQueue：该请求所加入的队列。mShouldCache：是否须要缓存。mCanceled：该请求是否已经取消。mResponseDelivered：该请求是否已经delivered。mRequestBirthTime：请求开始的时间。RetryPolicy mRetryPolicy：重试策略。Cache.Entry mCacheEntry：当一个请求结果能够从缓存中得到，但必须经过网络请求来刷新，缓存就能够存放在这里，当出现Not Modified时，将它返回。mTag：标识，用来一次性取消多个请求。
它有两个抽象方法：网络
/**
* Subclasses must implement this to parse the raw network response
* and return an appropriate response type. This method will be
* called from a worker thread. The response will not be delivered
* if you return null.
* @param response Response from the network
* @return The parsed response, or null in the case of an error
*/
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);
/**
* Subclasses must implement this to perform delivery of the parsed
* response to their listeners. The given response is guaranteed to
* be non-null; responses that fail to parse are not delivered.
* @param response The parsed response returned by
* {@link #parseNetworkResponse(NetworkResponse)}
*/
abstract protected void deliverResponse(T response);
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1.3.2 NetworkResponse
NetworkResponse是BasicNetwork#performRequest返回的，它仅仅包含返回的content和Header，这样就便于后面对Response进行转换。数据结构
public class NetworkResponse {
/**
* Creates a new network response.
* @param statusCode the HTTP status code
* @param data Response body
* @param headers Headers returned with this response, or null for none
* @param notModified True if the server returned a 304 and the data was already in cache
* @param networkTimeMs Round-trip network time to receive network response
*/
public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified, long networkTimeMs) {
this.statusCode = statusCode;
this.data = data;
this.headers = headers;
this.notModified = notModified;
this.networkTimeMs = networkTimeMs;
}
public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified) {
this(statusCode, data, headers, notModified, 0);
}
public NetworkResponse(byte[] data) {
this(HttpStatus.SC_OK, data, Collections.<String, String>emptyMap(), false, 0);
}
public NetworkResponse(byte[] data, Map<String, String> headers) {
this(HttpStatus.SC_OK, data, headers, false, 0);
}
/** The HTTP status code. */
public final int statusCode;
/** Raw data from this response. */
public final byte[] data;
/** Response headers. */
public final Map<String, String> headers;
/** True if the server returned a 304 (Not Modified). */
public final boolean notModified;
/** Network roundtrip time in milliseconds. */
public final long networkTimeMs;
}
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它包含如下成员变量：app
statusCode：HTTP状态码。byte[] data：返回数据。Map<String, String> headers：返回的头部。boolean notModified：若是服务器返回了304，那么为true。long networkTimesMs：网络请求所消耗的时间。
1.3.3 下面咱们来看一下BasicNetwork的实现代码：
@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);
}
}
}
}
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上面的代码对应的具体流程以下：框架
- 记录请求开始时间
- 经过
Request#mCacheEntry来构建新的请求的Header - 把
Request和Header传给HttpStack HttpStack发起请求以后，得到HttpResponse的StatusCode和Header- 根据
StatusCode来构建NetworkResponse，判断是不是304： - 是，那么再判断
mCacheEntry是否为空，若是为空，那么仅仅传入头部构建；不然更新mCacheEntry的头部，传入mCacheEntry.data来构建。 - 否，从
Entity中得到byte[]构建NetworkResponse。 - 若是在请求或者处理过程中，发生了异常，那么会根据状况，返回不一样的异常，最终都是一个
VolleyError，它的子类包括： NetworkError：用来表示这是在发起请求过程当中所产生的错误。ParseError：用来表示服务器的数据不能被解析。TimeoutError：用来表示socket链接超时。ServerError：用来表示服务器返回了一个异常响应。NoConnectionError：表示没有可创建的链接。
1.3.4 RetryPolicy
在上面的请求过程中，咱们看到当发生SocketTimeoutException/ConnectTimeoutException，会调用 attemptRetryOnException，而在Request的构造函数中，会给它传入一个DefaultRetryPolicy：
//BasicNetwork.java
/**
* Attempts to prepare the request for a retry. If there are no more attempts remaining in the
* request's retry policy, a timeout exception is thrown. * @param request The request to use. */ private static void attemptRetryOnException(String logPrefix, Request<?> request, VolleyError exception) throws VolleyError { RetryPolicy retryPolicy = request.getRetryPolicy(); int oldTimeout = request.getTimeoutMs(); try { retryPolicy.retry(exception); } catch (VolleyError e) { request.addMarker( String.format("%s-timeout-giveup [timeout=%s]", logPrefix, oldTimeout)); throw e; } request.addMarker(String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout)); } //DefaultRetryPolicy.java /** * Prepares for the next retry by applying a backoff to the timeout. * @param error The error code of the last attempt. */ @Override public void retry(VolleyError error) throws VolleyError { mCurrentRetryCount++; mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier); if (!hasAttemptRemaining()) { throw error; } } 复制代码
咱们能够看到，它实际上是修改当前Request对应的RetryPolicy中的超时时间，这样再下次请求时，该Request所容许超时的时间就会变长，从而减小超时状况的发生。
1.4 小结
在外界看来，BasicNetwork就是接受Request，返回NetworkResponse，在执行过程中，有可能抛出VolleyError。
2、NetworkResponse转换为Response
在上面的例子当中，咱们看到Request必需要实现一个抽象方法，将网络请求的返回结果NetworkResponse转换成为Response，来递交给下一级，它包含如下几个成员变量：
/** Parsed response, or null in the case of error. */
public final T result;
/** Cache metadata for this response, or null in the case of error. */
public final Cache.Entry cacheEntry;
/** Detailed error information if errorCode != OK */
public final VolleyError error;
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下面是StringRequest的实现：
@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, HttpHeaderParser.parseCacheHeaders(response));
}
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在NetworkDispatcher中，当Request解析完请求后，会把cacheEntry保存起来，而它的Key来自于Request#getCacheKey：
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
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这样就把result和cache分开来了。
3、缓存相关Cache
Cache是一个接口，它定义了一个缓存管理类应当实现的方法，在Volley当中，咱们默认实现的缓存管理类是DiskBasedCache，在后面的分析中，它分别被传给了NetworkDispatcher和CacheDispatcher，在前者当中写入缓存，在后者当中读出缓存。Cache中有一个内部类Entry，它定义缓存的数据结构，当使用者但愿缓存请求结果的时候，那么就须要构建一个Cache.Entry，而后在构建Response的时候将它传入，这样在NetworkDispatcher中就能够写入到持久性存储当中。
/**
* Data and metadata for an entry returned by the cache.
*/
public static class Entry {
/** The data returned from cache. */
public byte[] data;
/** ETag for cache coherency. */
public String etag;
/** Date of this response as reported by the server. */
public long serverDate;
/** The last modified date for the requested object. */
public long lastModified;
/** TTL for this record. */
public long ttl;
/** Soft TTL for this record. */
public long softTtl;
/** Immutable response headers as received from server; must be non-null. */
public Map<String, String> responseHeaders = Collections.emptyMap();
/** True if the entry is expired. */
public boolean isExpired() {
return this.ttl < System.currentTimeMillis();
}
/** True if a refresh is needed from the original data source. */
public boolean refreshNeeded() {
return this.softTtl < System.currentTimeMillis();
}
}
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4、NetworkDispatcher和CacheDispatcher
4.1 CacheDispatcher
它是一个线程，它的构造函数中传入了：
cacheQueue：缓存的处理队列networkQueue：网络的处理队列cache：缓存管理类delivery：递送类。
上面这两个队列的类型为无界队列BlockingQueue<Request<?>>，咱们来看一下，在其run()方法中的具体操做：
@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");
// 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. 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; } } } 复制代码
- 从
mCacheQueue中阻塞地获取Request，直到该队列中有元素。 - 若是在轮到该
Request执行时，它已经被Cancel了，那么调用Request#finish方法，并取队列的下一个Request进行操做。 - 尝试从缓存管理类
mCache中取出该Request对应的缓存，若是缓存为空(entry == null)，那么将Request加入到网络队列；若是缓存过时(entry.isExpired())，那么给Request设置该缓存，以后加入到网络队列，这两种状况最终都会继续取缓存队列的下一个Request进行操做。 - 若是都不是上面的状况，那么经过
Cache.Entry中的data和header解析构建NetworkResponse，而后回调给Request解析，最终获得Response - 若是缓存须要刷新
(!entry.refreshNeeded())，那么给Request设置cacheEntry，在给使用者回调该结果后，还要再把该Request加入到网络队列。 - 若是缓存不须要刷新，那么直接返回便可。
4.2 NetworkDispatcher
网络线程的处理方式和缓存线程相似，它的构造函数包括：
queue：网络队列network：网络框架cache：缓存管理类delivery：递送类
@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);
}
}
}
private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}
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- 先判断
Request是否被Cancel，若是是，那么直接回调#finish，这和上面相似。 - 调用
Network请求，获得NetworkResponse。 - 判断这个
NetworkResponse是不是304而且已经递送给使用者了，若是是，那么直接调用#finish，退出。 - 经过
Request#parseNetworkResponse将NetworkResponse解析为Response。 - 判断
Request是否须要缓存，若是须要缓存，那么调用缓存管理类mCache保存缓存。 - 经过
mDelivery递送最终的结果。 - 若是在上述的请求中发生了异常，那么会经过
mDelivery发送错误给使用者。
5、返回结果
从上面的两个Thread的处理过程来看，对于从队列中取出的Request，最终的处理方式无非有这两种：一种是调用Request#finish(xxx)，另外一种是经过mDelivery。
5.1 Request#finish(xxx)
//Request.java
void finish(final String tag) {
if (mRequestQueue != null) {
mRequestQueue.finish(this);
}
}
//RequestQueue.java
<T> void finish(Request<T> request) {
// Remove from the set of requests currently being processed.
synchronized (mCurrentRequests) {
mCurrentRequests.remove(request);
}
synchronized (mFinishedListeners) {
for (RequestFinishedListener<T> listener : mFinishedListeners) {
listener.onRequestFinished(request);
}
}
if (request.shouldCache()) {
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
if (waitingRequests != null) {
if (VolleyLog.DEBUG) {
VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
waitingRequests.size(), cacheKey);
}
// Process all queued up requests. They won't be considered as in flight, but // that's not a problem as the cache has been primed by 'request'.
mCacheQueue.addAll(waitingRequests);
}
}
}
}
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这里面涉及到两个集合mCurrentRequests和mWaitingRequests：
Set<Request<?>>：全部被添加的Request都进入这一集合，这样在RequestQueue#cancelAll时，就是取消这个集合当中的队列。Map<String, Queue<Request<?>>>：它的Key是每一个Request的getCacheKey，具备相同key的Request会被放在同一个队列当中。
5.2 ResponseDelivery
public interface ResponseDelivery {
/**
* Parses a response from the network or cache and delivers it.
*/
public void postResponse(Request<?> request, Response<?> response);
/**
* Parses a response from the network or cache and delivers it. The provided
* Runnable will be executed after delivery.
*/
public void postResponse(Request<?> request, Response<?> response, Runnable runnable);
/**
* Posts an error for the given request.
*/
public void postError(Request<?> request, VolleyError error);
}
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它的默认实现是ExecutorDelivery，它负责将请求的结果返回给使用者，它全部的方法，最终都会走到内部的ResponseDeliveryRunnable的run()方法当中，该run()方法运行所在的线程和构建ExecutorDelivery所使用的handler有关：
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; } // 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();
}
}
}
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6、RequestQueue
先看下Volley，它其中有一个静态方法RequestQueue newRequestQueue(Context, HttpStack)，最终会调用RequestQueue#start()。
public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
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 = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();
return queue;
}
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首先看一下RequestQueue的构造函数，它会构造四个NetworkDispatcher。
/**
* 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;
}
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以后再看一下start()方法，它会启动缓存线程，并依次启动网络线程。
/**
* 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();
}
}
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当咱们调用这个RequestQueue的添加方法时：
/**
* 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; } } 复制代码
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相关文章
- 1. Volley——阅读梳理
- 2. Volley原理解析
- 3. Volley学习(一)Android Volley源码解析
- 4. Volley的原理解析
- 5. Volley理解
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