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 = -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;

    /** 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 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());
        }
    }

    /**
     * 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() > 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;
    }

    /**
     * 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() > 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;
    }

    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<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 = -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. 
     * 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 = 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. 
     * 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 0;  
    }  

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。