Volley学习笔记（一）

Volley简介

volley是谷歌在2013年I/O大会上提出的一个网络通讯框架，简单易用，可扩展性强，通过查看源码你就会发现他有好多接口设计模式，极大的方便开发者去调用。它内部还实现了图片加载的功能。它主要适合一些数据量不大，但是通讯频繁的网络操作。但是对于一些大数据量，比如文件上传下载，Volley的表现可能会让你失望。

贴一张它的总体设计图：

Volley使用

Volley的使用是很简单的，大体来讲只需要三步：

new 一个全局的RequestQuene 消息队列；

new 一个request对象；

把request对象add到RequestQuene 消息队列中。

这里就不贴代码了。关于它的使用随便搜索就能找一大把，下面主要学习他的源码。

Volley源码

Volley.java

这个类是对外暴漏的一个API，用于创建一个消息队列。

public class Volley {

/** Default on-disk cache directory. */
private static final String DEFAULT_CACHE_DIR = "volley";

/**
* Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
*获取Volley对象
* @param context A {@link Context} to use for creating the cache dir.
* @param stack An {@link HttpStack} to use for the network, or null for default.
* @return A started {@link RequestQueue} instance.
*/
@SuppressLint("NewApi")
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;
/*
* 实例化一个RequestQueue，其中start()主要完成相关工作线程的开启，
* 比如开启缓存线程CacheDispatcher先完成缓存文件的扫描， 还包括开启多个NetworkDispatcher访问网络线程，
* 该多个网络线程将从 同一个 网络阻塞队列中读取消息
*
* 此处可见，start()已经开启，所有我们不用手动的去调用该方法，在start()方法中如果存在工作线程应该首先终止，并重新实例化工作线程并开启
* 在访问网络很频繁，而又重复调用start()，势必会导致性能的消耗；但是如果在访问网络很少时，调用stop()方法，停止多个线程，然后调用start(),反而又可以提高性能，具体可折中选择
*/
}

/**
* Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
*
* @param context A {@link Context} to use for creating the cache dir.
* @return A started {@link RequestQueue} instance.
*/
public static RequestQueue newRequestQueue(Context context) {
return newRequestQueue(context, null);
}

}

代码比较简单，里面有两个newRequestQueue重载方法

public static RequestQueue newRequestQueue(Context context) ｛｝

public static RequestQueue newRequestQueue(Context context, HttpStack stack) ｛｝

第一个的时候内部调用了第二个方法，切给第二个参数传null。看第二个方法，首先初始化了一个文件地址，也就是缓存的地址。

关于userAgent

Volley首先自定义了一个默认的userAgent，在不抛出异常的情况下会使用包名+版本号，通过下面代码可以看到如果SDK<9也就是使用HttpClient的时候，就会使用userAgent。 HttpUrlconnection不需要设置userAgent，他是固定的。Volley设置userAgent是为了自定义Header。

关于HttpClient和HttpURLConnection

Volley里面在SDK大于9就会使用HttpURLConnection，低于9就会使用HttpClient。他俩有什么区别呢？HttpURLConnection在SDK小于9，也就是Android 2.2以前，存在重大的bug，调用close()函数会影响连接池，导致连接复用失败。Android 2.3以后增加了gzip压缩和请求结果缓存。所以2.3以后还是使用HttpURLConnection。

接着下面会实例化两个对象Network、DiskBasedCache并作为参数初始化RequestQueue，最后调用queue.start()方法。Network是用于进行网络请求的一个接口，他的实现类是BasicNetwork；DiskBasedCache则是从缓存查找结果的实现类。后面再看这两个类的具体实现。先看下start（）方法的源码。

RequestQueue.java

这个类是它的核心类之一，实现了把请求放进请求队列的add方法。

/**
* 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.
* RequestQueue类存在2个非常重要的PriorityBlockingQueue类型的成员字段mCacheQueue mNetworkQueue ，该PriorityBlockingQueue为java1.5并发库提供的新类
* 其中有几个重要的方法，比如take()为从队列中取得对象，如果队列不存在对象，将会被阻塞，直到队列中存在有对象，类似于Looper.loop()
*
* 实例化一个request对象，调用RequestQueue.add(request),该request如果不允许被缓存，将会被添加至mNetworkQueue队列中，待多个NetworkDispatcher线程take()取出对象
* 如果该request可以被缓存，该request将会被添加至mCacheQueue队列中，待mCacheDispatcher线程从mCacheQueue.take()取出对象，
* 如果该request在mCache中不存在匹配的缓存时，该request将会被移交添加至mNetworkQueue队列中，待网络访问完成后，将关键头信息添加至mCache缓存中去！
*/
public class RequestQueue {

/** 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<?>>();

/** The cache triage queue.
* 缓存队列
* */
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<Request<?>>();

/** The queue of requests that are actually going out to the network.
* 网络队列
* */
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();

/** 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;

/** The network dispatchers. */
private NetworkDispatcher[] mDispatchers;

/** The cache dispatcher. */
private CacheDispatcher mCacheDispatcher;

/**
* 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
* @param threadPoolSize Number of network dispatcher threads to create
*/
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}

/**
* 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);
}

/**
* Starts the dispatchers in this queue.
* 如果该request可以被缓存，该request将会被添加至mCacheQueue队列中，待mCacheDispatcher线程从mCacheQueue.take()取出对象，
* 如果该request在mCache中不存在匹配的缓存时，该request将会被移交添加至mNetworkQueue队列中，待网络访问完成后，将关键头信息添加至mCache缓存中去！
*/
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();
}
}

/**
* 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();
}
}
}

/**
* Gets a sequence number.
*/
public int getSequenceNumber() {
return mSequenceGenerator.incrementAndGet();
}

/**
* Gets the {@link Cache} instance being used.
*/
public Cache getCache() {
return mCache;
}

/**
* A simple predicate or filter interface for Requests, for use by
* {@link RequestQueue#cancelAll(RequestFilter)}.
*/
public interface RequestFilter {
public boolean apply(Request<?> request);
}

/**
* Cancels all requests in this queue for which the given filter applies.
* @param filter The filtering function to use
*/
public void cancelAll(RequestFilter filter) {
synchronized (mCurrentRequests) {
for (Request<?> request : mCurrentRequests) {
if (filter.apply(request)) {
request.cancel();
}
}
}
}

/**
* Cancels all requests in this queue with the given tag. Tag must be non-null
* and equality is by identity.
*/
public void cancelAll(final Object tag) {
if (tag == null) {
throw new IllegalArgumentException("Cannot cancelAll with a null tag");
}
cancelAll(new RequestFilter() {
@Override
public boolean apply(Request<?> request) {
return request.getTag() == tag;
}
});
}

/**
* Adds a Request to the dispatch queue.
* 将请求添加到队列中
* @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.
//如果不允许为缓存队列，则为网络队列
//默认缓存
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;
}
}

/**
* Called from {@link Request#finish(String)}, indicating that processing of the given request
* has finished.
*
* <p>Releases waiting requests for <code>request.getCacheKey()</code> if
*      <code>request.shouldCache()</code>.</p>
*/
void finish(Request<?> request) {
// Remove from the set of requests currently being processed.
synchronized (mCurrentRequests) {
mCurrentRequests.remove(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);
}
}
}
}
}

start()方法主要作用是启动了一个CacheDispatcher和四个NetworkDispatcher，即一个缓存分发线程和四个网络请求分发线程，他们两个都是继承自Thread的，四个网络线程是可配置的，可以根据自己的需求更改。

add()方法是把请求放进队列的方法。放进请求后会给request设置一个序列号和标志，然后根据request.shouldCache()判断如果为false则加入mNetworkQueue网络请求队列，默认是不允许缓存的。然后判断是否在等待队列中，则继续排队等待。如果不在则放进mCacheQueue。

看下两个队列吧，两个队列是PriorityBlockingQueue类型的，这个类型是JAVA1.5提供的新类，可以通过调用他的take方法取出里面的对象，如果不存在则队列阻塞，直到有对象。它里面存储的对象必须是实现Comparable接口的，request就实现了Comparable接口，一会再看。还有几个队列的类，一起看一下：

ArrayBlockingQueue：一个由数组支持的有界阻塞队列。使用时需要指定大小。

LinkedBlockingQueue：基于链表实现的阻塞队列，使用时不需要指定大小,，他是无界的。他的排序原则是先进先出。

SynchronousQueue：他也是一个无界的队列。他的特性是必须等待前面的线程取走以后才会添加下一个。newCachedThreadPool()就是基于这样一个队列。

RequestQueue这个类里面还有其他的finish,cancel方法实现了对请求的关闭和取消，这里就不细讲了。

Request.java

这个类是网络请求的抽象类，Volley里面的StringRequest、JsonRequest都是他的子类。

/**
* Base class for all network requests.
*
* @param <T> The type of parsed response this request expects.
*/
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;

/** The redirect url to use for 3xx http responses */
private String mRedirectUrl;

/** 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;

/**
* Creates a new request with the given URL and error listener.  Note that
* the normal response listener is not provided here as delivery of responses
* is provided by subclasses, who have a better idea of how to deliver an
* already-parsed response.
*
* @deprecated Use {@link #Request(int, String, com.android.volley.Response.ErrorListener)}.
*/
@Deprecated
public Request(String url, Response.ErrorListener listener) {
this(Method.DEPRECATED_GET_OR_POST, url, listener);
}

/**
* Creates a new request with the given method (one of the values from {@link Method}),
* URL, and error listener.  Note that the normal response listener is not provided here as
* delivery of responses is provided by subclasses, who have a better idea of how to deliver
* an already-parsed response.
*/
public Request(int method, String url, Response.ErrorListener listener) {
mMethod = method;
mUrl = url;
mErrorListener = listener;
setRetryPolicy(new DefaultRetryPolicy());

mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}

/**
* Return the method for this request.  Can be one of the values in {@link Method}.
*/
public int getMethod() {
return mMethod;
}

/**
* Set a tag on this request. Can be used to cancel all requests with this
* tag by {@link RequestQueue#cancelAll(Object)}.
*
* @return This Request object to allow for chaining.
*/
public Request<?> setTag(Object tag) {
mTag = tag;
return this;
}

/**
* Returns this request's tag.
* @see Request#setTag(Object)
*/
public Object getTag() {
return mTag;
}

/**
* @return A tag for use with {@link TrafficStats#setThreadStatsTag(int)}
*/
public int getTrafficStatsTag() {
return mDefaultTrafficStatsTag;
}

/**
* @return The hashcode of the URL's host component, or 0 if there is none.
*/
private static int findDefaultTrafficStatsTag(String url) {
if (!TextUtils.isEmpty(url)) {
Uri uri = Uri.parse(url);
if (uri != null) {
String host = uri.getHost();
if (host != null) {
return host.hashCode();
}
}
}
return 0;
}

/**
* Sets the retry policy for this request.
*
* @return This Request object to allow for chaining.
*/
public Request<?> setRetryPolicy(RetryPolicy retryPolicy) {
mRetryPolicy = retryPolicy;
return this;
}

/**
* Adds an event to this request's event log; for debugging.
*/
public void addMarker(String tag) {
if (MarkerLog.ENABLED) {
mEventLog.add(tag, Thread.currentThread().getId());
} else if (mRequestBirthTime == 0) {
mRequestBirthTime = SystemClock.elapsedRealtime();
}
}

/**
* Notifies the request queue that this request has finished (successfully or with error).
*
* <p>Also dumps all events from this request's event log; for debugging.</p>
*/
void finish(final String tag) {
if (mRequestQueue != null) {
mRequestQueue.finish(this);
}
if (MarkerLog.ENABLED) {
final long threadId = Thread.currentThread().getId();
if (Looper.myLooper() != Looper.getMainLooper()) {
// If we finish marking off of the main thread, we need to
// actually do it on the main thread to ensure correct ordering.
Handler mainThread = new Handler(Looper.getMainLooper());
mainThread.post(new Runnable() {
@Override
public void run() {
mEventLog.add(tag, threadId);
mEventLog.finish(this.toString());
}
});
return;
}

mEventLog.add(tag, threadId);
mEventLog.finish(this.toString());
} else {
long requestTime = SystemClock.elapsedRealtime() - mRequestBirthTime;
if (requestTime >= SLOW_REQUEST_THRESHOLD_MS) {
VolleyLog.d("%d ms: %s", requestTime, this.toString());
}
}
}

/**
* Associates this request with the given queue. The request queue will be notified when this
* request has finished.
*
* @return This Request object to allow for chaining.
*/
public Request<?> setRequestQueue(RequestQueue requestQueue) {
mRequestQueue = requestQueue;
return this;
}

/**
* Sets the sequence number of this request.  Used by {@link RequestQueue}.
*
* @return This Request object to allow for chaining.
*/
public final Request<?> setSequence(int sequence) {
mSequence = sequence;
return this;
}

/**
* Returns the sequence number of this request.
*/
public final int getSequence() {
if (mSequence == null) {
throw new IllegalStateException("getSequence called before setSequence");
}
return mSequence;
}

/**
* Returns the URL of this request.
*/
public String getUrl() {
return (mRedirectUrl != null) ? mRedirectUrl : mUrl;
}

/**
* Returns the URL of the request before any redirects have occurred.
*/
public String getOriginUrl() {
return mUrl;
}

/**
* Sets the redirect url to handle 3xx http responses.
*/
public void setRedirectUrl(String redirectUrl) {
mRedirectUrl = redirectUrl;
}

/**
* Returns the cache key for this request.  By default, this is the URL.
*/
public String getCacheKey() {
return getUrl();
}

/**
* Annotates this request with an entry retrieved for it from cache.
* Used for cache coherency support.
*
* @return This Request object to allow for chaining.
*/
public Request<?> setCacheEntry(Cache.Entry entry) {
mCacheEntry = entry;
return this;
}

/**
* Returns the annotated cache entry, or null if there isn't one.
*/
public Cache.Entry getCacheEntry() {
return mCacheEntry;
}

/**
* Mark this request as canceled.  No callback will be delivered.
*/
public void cancel() {
mCanceled = true;
}

/**
* Returns true if this request has been canceled.
*/
public boolean isCanceled() {
return mCanceled;
}

/**
* Returns a list of extra HTTP headers to go along with this request. Can
* throw {@link AuthFailureError} as authentication may be required to
* provide these values.
* @throws AuthFailureError In the event of auth failure
*/
public Map<String, String> getHeaders() throws AuthFailureError {
return Collections.emptyMap();
}

/**
* Returns a Map of POST parameters to be used for this request, or null if
* a simple GET should be used.  Can throw {@link AuthFailureError} as
* authentication may be required to provide these values.
*
* <p>Note that only one of getPostParams() and getPostBody() can return a non-null
* value.</p>
* @throws AuthFailureError In the event of auth failure
*
* @deprecated Use {@link #getParams()} instead.
*/
@Deprecated
protected Map<String, String> getPostParams() throws AuthFailureError {
return getParams();
}

/**
* Returns which encoding should be used when converting POST parameters returned by
* {@link #getPostParams()} into a raw POST body.
*
* <p>This controls both encodings:
* <ol>
*     <li>The string encoding used when converting parameter names and values into bytes prior
*         to URL encoding them.</li>
*     <li>The string encoding used when converting the URL encoded parameters into a raw
*         byte array.</li>
* </ol>
*
* @deprecated Use {@link #getParamsEncoding()} instead.
*/
@Deprecated
protected String getPostParamsEncoding() {
return getParamsEncoding();
}

/**
* @deprecated Use {@link #getBodyContentType()} instead.
*/
@Deprecated
public String getPostBodyContentType() {
return getBodyContentType();
}

/**
* Returns the raw POST body to be sent.
*
* @throws AuthFailureError In the event of auth failure
*
* @deprecated Use {@link #getBody()} instead.
*/
@Deprecated
public byte[] getPostBody() throws AuthFailureError {
// Note: For compatibility with legacy clients of volley, this implementation must remain
// here instead of simply calling the getBody() function because this function must
// call getPostParams() and getPostParamsEncoding() since legacy clients would have
// overridden these two member functions for POST requests.
Map<String, String> postParams = getPostParams();
if (postParams != null && postParams.size() > 0) {
return encodeParameters(postParams, getPostParamsEncoding());
}
return null;
}

/**
* Returns a Map of parameters to be used for a POST or PUT request.  Can throw
* {@link AuthFailureError} as authentication may be required to provide these values.
*
* <p>Note that you can directly override {@link #getBody()} for custom data.</p>
*
* @throws AuthFailureError in the event of auth failure
*/
protected Map<String, String> getParams() throws AuthFailureError {
return null;
}

/**
* Returns which encoding should be used when converting POST or PUT parameters returned by
* {@link #getParams()} into a raw POST or PUT body.
*
* <p>This controls both encodings:
* <ol>
*     <li>The string encoding used when converting parameter names and values into bytes prior
*         to URL encoding them.</li>
*     <li>The string encoding used when converting the URL encoded parameters into a raw
*         byte array.</li>
* </ol>
*/
protected String getParamsEncoding() {
return DEFAULT_PARAMS_ENCODING;
}

public String getBodyContentType() {
return "application/x-www-form-urlencoded; charset=" + getParamsEncoding();
}

/**
* Returns the raw POST or PUT body to be sent.
*
* @throws AuthFailureError in the event of auth failure
*/
public byte[] getBody() throws AuthFailureError {
Map<String, String> params = getParams();
if (params != null && params.size() > 0) {
return encodeParameters(params, getParamsEncoding());
}
return null;
}

/**
* Converts <code>params</code> into an application/x-www-form-urlencoded encoded string.
*/
private byte[] encodeParameters(Map<String, String> params, String paramsEncoding) {
StringBuilder encodedParams = new StringBuilder();
try {
for (Map.Entry<String, String> entry : params.entrySet()) {
encodedParams.append(URLEncoder.encode(entry.getKey(), paramsEncoding));
encodedParams.append('=');
encodedParams.append(URLEncoder.encode(entry.getValue(), paramsEncoding));
encodedParams.append('&');
}
return encodedParams.toString().getBytes(paramsEncoding);
} catch (UnsupportedEncodingException uee) {
throw new RuntimeException("Encoding not supported: " + paramsEncoding, uee);
}
}

/**
* Set whether or not responses to this request should be cached.
*
* @return This Request object to allow for chaining.
*/
public final Request<?> setShouldCache(boolean shouldCache) {
mShouldCache = shouldCache;
return this;
}

/**
* Returns true if responses to this request should be cached.
*/
public final boolean shouldCache() {
return mShouldCache;
}

/**
* Priority values.  Requests will be processed from higher priorities to
* lower priorities, in FIFO order.
*/
public enum Priority {
LOW,
NORMAL,
HIGH,
IMMEDIATE
}

/**
* Returns the {@link Priority} of this request; {@link Priority#NORMAL} by default.
*/
public Priority getPriority() {
return Priority.NORMAL;
}

/**
* Returns the socket timeout in milliseconds per retry attempt. (This value can be changed
* per retry attempt if a backoff is specified via backoffTimeout()). If there are no retry
* attempts remaining, this will cause delivery of a {@link TimeoutError} error.
*/
public final int getTimeoutMs() {
return mRetryPolicy.getCurrentTimeout();
}

/**
* Returns the retry policy that should be used  for this request.
*/
public RetryPolicy getRetryPolicy() {
return mRetryPolicy;
}

/**
* Mark this request as having a response delivered on it.  This can be used
* later in the request's lifetime for suppressing identical responses.
*/
public void markDelivered() {
mResponseDelivered = true;
}

/**
* Returns true if this request has had a response delivered for it.
*/
public boolean hasHadResponseDelivered() {
return mResponseDelivered;
}

/**
* 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 can override this method to parse 'networkError' and return a more specific error.
*
* <p>The default implementation just returns the passed 'networkError'.</p>
*
* @param volleyError the error retrieved from the network
* @return an NetworkError augmented with additional information
*/
protected VolleyError parseNetworkError(VolleyError volleyError) {
return volleyError;
}

/**
* 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);

/**
* Delivers error message to the ErrorListener that the Request was
* initialized with.
*
* @param error Error details
*/
public void deliverError(VolleyError error) {
if (mErrorListener != null) {
mErrorListener.onErrorResponse(error);
}
}

/**
* Our comparator sorts from high to low priority, and secondarily by
* sequence number to provide FIFO ordering.
*/
@Override
public int compareTo(Request<T> other) {
Priority left = this.getPriority();
Priority right = other.getPriority();

// High-priority requests are "lesser" so they are sorted to the front.
// Equal priorities are sorted by sequence number to provide FIFO ordering.
return left == right ?
this.mSequence - other.mSequence :
right.ordinal() - left.ordinal();
}

@Override
public String toString() {
String trafficStatsTag = "0x" + Integer.toHexString(getTrafficStatsTag());
return (mCanceled ? "[X] " : "[ ] ") + getUrl() + " " + trafficStatsTag + " "
+ getPriority() + " " + mSequence;
}
}

首先他实现了Comparable接口并实现了compareTo的方法，通过上面的RequestQueue的介绍，实现Comparable是为了设置请求的优先级。优先级高的就会排在前面，优先级相等的情况会按照添加RequestQueue时设置的序列号，按照先进先出排序。

Volley 支持 8 种 Http 请求方式 GET, POST, PUT, DELETE, HEAD, OPTIONS, TRACE, PATCH。Request 类中包含了请求 url，请求请求方式，请求 Header，请求 Body，请求的优先级等信息。

既然是抽象类，有两个子类必须实现的方法：

abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

子类重写此方法，将网络返回的原生字节内容，转换成合适的类型。此方法会在工作线程中被调用。

abstract protected void deliverResponse(T response);

子类重写此方法，将解析成合适类型的内容传递给它们的监听回调。

public byte[] getBody() throws AuthFailureError｛｝

重写此方法，可以构建用于 POST、PUT、PATCH 请求方式的 Body 内容。