volley5--Request<T>类的介绍

源码：

 /*
  * Copyright (C) 2011 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
 
 package com.android.volley;
 
 import android.net.TrafficStats;
 import android.net.Uri;
 import android.os.Handler;
 import android.os.Looper;
 import android.text.TextUtils;
 import com.android.volley.VolleyLog.MarkerLog;
 
 import java.io.UnsupportedEncodingException;
 import java.net.URLEncoder;
 import java.util.Collections;
 import java.util.Map;
 
 /**
  * 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 = -;
         int GET = ;
         int POST = ;
         int PUT = ;
         int DELETE = ;
         int HEAD = ;
         int OPTIONS = ;
         int TRACE = ;
         int PATCH = ;
     }
 
     /** 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;
 
     /** The unique identifier of the request */
     private String mIdentifier;
 
     /** Default tag for {@link TrafficStats}. */
     private final int mDefaultTrafficStatsTag;
 
     /** Listener interface for errors. */
     private 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;
 
     /** 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;
         mIdentifier = createIdentifier(method, 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 this request's {@link com.android.volley.Response.ErrorListener}.
      */
     public Response.ErrorListener getErrorListener() {
         return mErrorListener;
     }
 
     /**
      * @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 ;
     }
 
     /**
      * 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());
         }
     }
 
     /**
      * 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);
             onFinish();
         }
         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());
         }
     }
 
     /**
      * clear listeners when finished
      */
     protected void onFinish() {
         mErrorListener = null;
     }
 
     /**
      * 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;
     }
 
     /**
      * Returns the identifier of the request.
      */
     public String getIdentifier() {
         return mIdentifier;
     }
 
     /**
      * 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 mMethod + ":" + mUrl;
     }
 
     /**
      * 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() > ) {
             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;
     }
 
     /**
      * Returns the content type of the POST or PUT body.
      */
     public String getBodyContentType() {
         return "application/x-www-form-urlencoded; charset=" + getParamsEncoding();
     }
 
     /**
      * Returns the raw POST or PUT body to be sent.
      *
      * <p>By default, the body consists of the request parameters in
      * application/x-www-form-urlencoded format. When overriding this method, consider overriding
      * {@link #getBodyContentType()} as well to match the new body format.
      *
      * @throws AuthFailureError in the event of auth failure
      */
     public byte[] getBody() throws AuthFailureError {
         Map<String, String> params = getParams();
         if (params != null && params.size() > ) {
             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;
     }
 
     private static long sCounter;
     /**
      *  sha1(Request:method:url:timestamp:counter)
      * @param method http method
      * @param url               http request url
      * @return sha1 hash string
      */
     private static String createIdentifier(final int method, final String url) {
         return InternalUtils.sha1Hash("Request:" + method + ":" + url +
                 ":" + System.currentTimeMillis() + ":" + (sCounter++));
     }
 }

Request

Request<T>中的泛型T，是指解析response以后的结果。在上一篇文章中我们知道，ResponseDelivery会把response分派给对应的request(中文翻译就是，把响应分派给对应的请求)。在我们定义的请求中，需要重写parseNetworkResponse(NetworkResponse response)这个方法，解析请求，解析出来的结果，就是T类型的。

首先是一些属性

 /**
  * Base class for all network requests.
  * 请求基类
  * @param <T> The type of parsed response this request expects.
  * T为响应类型
  */
 public abstract class Request<T> implements Comparable<Request<T>> {  
 
     /**
      * Default encoding for POST or PUT parameters. See {@link #getParamsEncoding()}.
      * 默认编码
      */
     private static final String DEFAULT_PARAMS_ENCODING = "UTF-8";  
 
     /**
      * Supported request methods.
      * 支持的请求方式
      */
     public interface Method {
         int DEPRECATED_GET_OR_POST = -;
         int GET = ;
         int POST = ;
         int PUT = ;
         int DELETE = ;
         int HEAD = ;
         int OPTIONS = ;
         int TRACE = ;
         int PATCH = ;
     }  
 
     /**
      * An event log tracing the lifetime of this request; for debugging.
      * 用于跟踪请求的生存时间，用于调试
      * */
     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.
      * 当前请求方式
      */
     private final int mMethod;  
 
     /**
      * URL of this request.
      * 请求地址
      */
     private final String mUrl;  
 
     /**
      * The redirect url to use for 3xx http responses
      * 重定向地址
      */
     private String mRedirectUrl;  
 
     /**
      * The unique identifier of the request
      * 该请求的唯一凭证
      */
     private String mIdentifier;  
 
     /**
      * Default tag for {@link TrafficStats}.
      * 流量统计标签
      */
     private final int mDefaultTrafficStatsTag;  
 
     /**
      * Listener interface for errors.
      * 错误监听器
      */
     private final Response.ErrorListener mErrorListener;  
 
     /**
      * Sequence number of this request, used to enforce FIFO ordering.
      * 请求序号，用于fifo算法
      */
     private Integer mSequence;  
 
     /**
      * The request queue this request is associated with.
      * 请求所在的请求队列
      */
     private RequestQueue mRequestQueue;  
 
     /**
      * Whether or not responses to this request should be cached.
      * 是否使用缓存响应请求
      */
     private boolean mShouldCache = true;  
 
     /**
      * Whether or not this request has been canceled.
      * 该请求是否被取消
      */
     private boolean mCanceled = false;  
 
     /**
      * Whether or not a response has been delivered for this request yet.
      * 该请求是否已经被响应
      */
     private boolean mResponseDelivered = false;  
 
     /**
      * A cheap variant of request tracing used to dump slow requests.
      * 一个简单的变量，跟踪请求，用来抛弃过慢的请求
      * 请求产生时间
      */
     private long mRequestBirthTime = ;  
 
     /** Threshold at which we should log the request (even when debug logging is not enabled). */
     private static final long SLOW_REQUEST_THRESHOLD_MS = ;  
 
     /**
      * The retry policy for this request.
      * 请求重试策略
      */
     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.
      * 缓存记录。当请求可以从缓存中获得响应，但必须从网络上更新时。我们保留这个缓存记录，所以一旦从网络上获得的响应带有Not Modified
      * (没有更新)时，来保证这个缓存没有被回收.
      */
     private Cache.Entry mCacheEntry = null;  
 
     /**
      *  An opaque token tagging this request; used for bulk cancellation.
      *  用于自定义标记，可以理解为用于请求的分类
      */
     private Object mTag;  

接下来看构造方法

/**
     * 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.
     * 根据请求方式，创建新的请求(需要地址，错误监听器等参数)
     */
    public Request(int method, String url, Response.ErrorListener listener) {
        mMethod = method;
        mUrl = url;
        mIdentifier = createIdentifier(method, url);//为请求创建唯一凭证
        mErrorListener = listener;//设定监听器
        setRetryPolicy(new DefaultRetryPolicy());//设置默认重试策略  
 
        mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);//设置流量标志
    } 

首先是请求方式，请求地址的设定，这是作为一个请求必须有的。然后是监听器的设定，注意这里只是这是了ErrorListner，说明errorListener是必须的，但是正确响应，我们有可能不处理。这样设定是合理的，因为出错了，我们必须处理，至于请求成功，我们可以不处理。那么我们想处理成功的请求怎么办呢，这需要在子类中重写构造方法(例如StringRequest)。

然后是创建了一个唯一凭证

/**
     *  sha1(Request:method:url:timestamp:counter)
     * @param method http method
     * @param url               http request url
     * @return sha1 hash string
     * 利用请求方式和地址，进行sha1加密，创建该请求的唯一凭证
     */
    private static String createIdentifier(final int method, final String url) {
        return InternalUtils.sha1Hash("Request:" + method + ":" + url +
                ":" + System.currentTimeMillis() + ":" + (sCounter++));
    } 

由上面的方法可以看出，这个凭证和当前时间有关，因此是独一无二的

接着是设置重试策略，这个类等下再介绍，接下来是流量标志的设置，所谓流量标志，是用于调试日志记录的，不是重点

/**
     * @return The hashcode of the URL's host component, or 0 if there is none.
     * 返回url的host(主机地址)部分的hashcode，如果host不存在，返回0
     */
    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 ;
    }  

我们再回过头来看这个重试策略。

volley为重试策略专门定义了一个类，这样我们就可以根据需要实现自己的重试策略了，至于源码内部，为我们提供了一个默认的重试策略DefaultRetryPolicy()

要介绍重试策略，我们先看重试策略的基类RetryPolicy

/**
 * Retry policy for a request.
 * 请求重试策略类
 */
public interface RetryPolicy {  
 
    /**
     * Returns the current timeout (used for logging).
     * 获得当前时间，用于日志
     */
    public int getCurrentTimeout();  
 
    /**
     * Returns the current retry count (used for logging).
     * 返回当前重试次数，用于日志
     */
    public int getCurrentRetryCount();  
 
    /**
     * Prepares for the next retry by applying a backoff to the timeout.
     * @param error The error code of the last attempt.
     * @throws VolleyError In the event that the retry could not be performed (for example if we
     * ran out of attempts), the passed in error is thrown.
     * 重试实现
     */
    public void retry(VolleyError error) throws VolleyError;
}  

重要的是retry()这个方法，我们来看DefaultRetryPolicy里面这个方法的具体实现

/**
     * 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++;//当前重试次数
        mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier);//当前超出时间
        if (!hasAttemptRemaining()) {//是否已经到达最大重试次数
            throw error;
        }
    }  
 
    /**
     * Returns true if this policy has attempts remaining, false otherwise.
     * 是否还重试
     */
    protected boolean hasAttemptRemaining() {
        return mCurrentRetryCount <= mMaxNumRetries;//最大重试次数
    }  

可以看到，在默认的重试策略中，只是简单地统计了重试的次数，然后，在超出最大次数以后，抛出异常。

就这么简单，那么究竟volley是怎么实现重试的呢？

实际上，当从队列中取出一个request去进行网络请求的时候，我们是写在一个死循环里面的(在以后的代码可以看到，这样不贴出来以免类过多造成困扰)。

一旦请求失败，就会调用上面的retry()方法，但是没有跳出循环。直到请求成功获得response，才return。如果一直请求失败，根据上面的重试策略，最后会抛出VolleyError异常,这个异常不处理，而是通过throws向外抛，从而结束死循环。

从程序设计的角度来说，通过抛出异常结束死循环，显得不是那么的优雅(通常我们用设置标记的方法结束循环)，但是在volley中使用了这个方式，原因是对于这个异常，要交给程序员自己处理，虽然这样使异常传递的过程变得复杂，但是增加了程序的灵活性。

最终的异常，我们会在Request<T>的parseNetworkError()和deliverError()方法里面处理，parseNetworkError()用于解析Volleyerror，deliverError()方法回调了上面一开始就提到的ErrorListener

  /**
     * 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
     * 解析网络错误
     */
    public VolleyError parseNetworkError(VolleyError volleyError) {
        return volleyError;
    }  
 
    /**
     * Delivers error message to the ErrorListener that the Request was
     * initialized with.
     *
     * @param error Error details
     * 分发网络错误
     */
    public void deliverError(VolleyError error) {
        if (mErrorListener != null) {
            mErrorListener.onErrorResponse(error);
        }
    }

其实除了上面两个处理错误的方法，还有两个方法用于处理成功响应,是必须要继承的

/**
     * 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
     * 解析响应
     */
    public abstract Response<T> parseNetworkResponse(NetworkResponse response);
<pre name="code" class="java">/**
     * 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)}
     * 分发响应
     */
    public abstract void deliverResponse(T response);  

parseNetworkResponse(NetworkResponse response)用于将网络response解析为本地response，解析出来的response,会交给deliverResponse(T response)方法。

为什么要解析，其实上面已经说过，要将结果解析为T类型。至于这两个方法，其实是在ResponseDelivery响应分发器里面调用的。

看完初始化方法，我们来看结束请求的方法finish()，有时候我们想要主动终止请求，例如停止下载文件，又或者请求已经成功了，我们从队列中去除这个请求

 /**
      * Notifies the request queue that this request has finished (successfully or with error).
      * 提醒请求队列，当前请求已经完成(失败或成功)
      * <p>Also dumps all events from this request's event log; for debugging.</p>
      *
      */
     public void finish(final String tag) {
         if (mRequestQueue != null) {
             mRequestQueue.finish(this);//该请求完成
         }
         if (MarkerLog.ENABLED) {//如果开启调试
             final long threadId = Thread.currentThread().getId();//线程id
             if (Looper.myLooper() != Looper.getMainLooper()) {//请求不是在主线程
                 // If we finish marking off of the main thread, we need to
                 // actually do it on the main thread to ensure correct ordering.
                 //如果我们不是在主线程记录log，我们需要在主线程做这项工作来保证正确的顺序
                 Handler mainThread = new Handler(Looper.getMainLooper());
                 mainThread.post(new Runnable() {
                     @Override
                     public void run() {
                         mEventLog.add(tag, threadId);
                         mEventLog.finish(this.toString());
                     }
                 });
                 return;
             }
             //如果在主线程，直接记录
             mEventLog.add(tag, threadId);
             mEventLog.finish(this.toString());
         } else {//不开启调试
             long requestTime = SystemClock.elapsedRealtime() - mRequestBirthTime;
             if (requestTime >= SLOW_REQUEST_THRESHOLD_MS) {
                 VolleyLog.d("%d ms: %s", requestTime, this.toString());
             }
         }
     }  

上面主要是做了一些日志记录的工作，最重要的是调用了mRequestQueue的finish()方法，来从队列中去除这个请求。

看完上面的介绍以后，大家是否注意到，Request<T>继承了Comparable<Request<T>>接口，为什么要继承这个接口了，我们当然要来看compareTo()方法了

/**
     * 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();
    }  

这个方法比较了两个请求的优先级，如果优先级相等，就按照顺序

实现这个接口的目的，正如上一篇文章提到的，有的请求比较重要，希望早点执行，也就是说让它排在请求队列的前头

通过比较方法，我们就可以设定请求在请求队列中排队顺序的根据，从而让优先级高的排在前面。

OK，Request<T>就基本介绍完了，当然有些属性，例如缓存mCacheEntry，mRedirectUrl重定向地址等我们还没有用到，我们先记住它们，在以后会使用到的。

其实Request<T>类并不复杂，主要就是一些属性的设置，这些属性有的比较难考虑到，例如优先级，重定向地址，自定义标记，重试策略等。

最后，我们通过StringRequest来看一下Request<T>类的具体实现

/**
 * A canned request for retrieving the response body at a given URL as a String.
 */
public class StringRequest extends Request<String> {
    private final Listener<String> mListener;  
 
    /**
     * Creates a new request with the given method.
     *
     * @param method the request {@link Method} to use
     * @param url URL to fetch the string at
     * @param listener Listener to receive the String response
     * @param errorListener Error listener, or null to ignore errors
     */
    public StringRequest(int method, String url, Listener<String> listener,
            ErrorListener errorListener) {
        super(method, url, errorListener);
        mListener = listener;
    }

上面的构造方法中，添加了一个新的接口Listener<String> listener,用于监听成功的response

然后是parseNetworkResponse(NetworkResponse response)，这个方法

@Override
    public 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));
    }

可以看到，将NetworkResponse解析为String类型的了，然后再构造成对应的本地response

@Override
    public void deliverResponse(String response) {
        mListener.onResponse(response);
    } 

至于deliverResponse(String response)，则调用了构造方法里面要求的，新的监听器。

到此为止，对于Request<T>的介绍就结束了，由于Request<T>和其他类的耦合并不是特别重，相信是比较容易理解。

在下一篇文章中，我们会来看RequestQueue队列，看看这个队列的作用到底是什么，我们为什么要创建一个队列来保存request而不是直接每个request开启一个线程去加载网络数据。