Volley框架源码分析

Volley框架分析Github链接

Volley框架分析

Volley源码解析

为了学习Volley的网络框架,我在AS中将Volley代码重新撸了一遍,感觉这种照抄代码也是一种挺好的学习方式.再分析Volley源码之前,我们先考虑一下,如果我们自己要设计一个网络请求框架,需要实现哪些事情,有哪些注意事项?

我的总结如下：

需要抽象出request请求类（包括url, params, method等）,抽象出request请求类之后,我们可以对其继承从而实现丰富的扩展功能.

需要抽象出response类.即服务器返回的结果需要抽象出来,方便我们继承扩展.

需要实现并发和异步操作.具体包括：

3-1. 抽象出Http请求类,封装基本操作.

3-2. 将Http请求类在子线程中执行,最好能支撑并发.

3-3. 由于需要并发,所以要用队列控制,并且能随时终止并发.

3-4. 子线程获取结果后,需要支持异步,将请求结果返回给主线程.

最好能实现缓存.当request抽象出来后,那相同的request请求可以直接从本地获取,不需要再通过网络获取.

缓存需要有缓存替换机制,超时更新机制等.

在我总结的这些问题的基础上,我们来学习一下Volley是如何解决并实现这些问题的.

网络请求抽象类

Request类就是Volley抽象出来的网络请求类了.我已经对其进行了中文注解,大家可以直接看一下其实现代码：

/**
* Volley的网络请求基类
*/
@SuppressWarnings("unused")
public abstract class Request<T> implements Comparable<Request<T>> {
/** 默认参数编码是UTF-8. */
private static final String DEFAULT_PARAMS_ENCODING = "UTF-8";

/** Volley支持的Http请求类型,我们一般常用的就是GET和POST. */
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;
}

/** 当前Request的HTTP请求类型. */
private final int mMethod;

/** 请求的url. */
private final String mUrl;

/** 默认的TrafficStats的tag. */
private final int mDefaultTrafficStatsTag;

/** request请求失败时的回调接口. */
private final Response.ErrorListener mErrorListener;

/** request的请求序列号,用于请求队列FIFO时排序查找使用. */
private Integer mSequence;

/** request的投放队列,该队列可采用FIFO方式执行request请求. */
private RequestQueue mRequestQueue;

/** 该request请求是否需要缓存,默认http request请求都是可以缓存的. */
private boolean mShouldCache = true;

/** 该request请求是否被取消的标志. */
private boolean mCanceled = false;

/** 该request是否已经获取请求结果. */
private boolean mResponseDelivered = false;

/** 遇到服务器错误(5xx)时,该request请求是否需要重试. */
private boolean mShouldRetryServerErrors = false;

/** request重试策略. */
private RetryPolicy mRetryPolicy;

/**
* 保存request缓存的结果.
* 因为当一个request可以被缓存,但是又必须要刷新（即需要从网络重新获取时）,我们保存该缓存结果,可以确保该结果
* 不被cache的替换策略清除掉,以防服务器返回“Not Modified”时,我们可以继续使用该缓存结果.
*/
private Cache.Entry mCacheEntry = null;

/**
* 创建一个Http request对象.
*
* @param method HTTP请求方式(GET, POST, PUT, DELETE...).
* @param url HTTP请求的url.
* @param listener 当HTTP访问出错时,用户设置的回调的接口.
*/
public Request(int method, String url, Response.ErrorListener listener) {
mMethod = method;
mUrl = url;
mErrorListener = listener;
mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}

/** 返回HTTP请求方式. */
public int getMethod() {
return mMethod;
}

/** 返回HTTP请求错误时的回调接口. */
public Response.ErrorListener getErrorListener() {
return mErrorListener;
}

/** 返回统计类使用的Tag. */
public int getTrafficStatsTag() {
return mDefaultTrafficStatsTag;
}

/**
* 使用url的host字段的hash值作为统计类的tag.
*/
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;
}

/** 设置重试接口.典型的组合模式,关联关系. */
public Request<?> setRetryPolicy(RetryPolicy retryPolicy) {
mRetryPolicy = retryPolicy;
return this;
}

/** 调试打印当前请求进度使用 */
public void addMarker(String tag) {
Log.e("Volley", tag);
}

/** 用于告知请求队列当前request已经结束. */
void finish(final String tag) {
if (mRequestQueue != null) {
mRequestQueue.finish(this);
}
}

/** 设置当前request的请求队列. */
public Request<?> setRequestQueue(RequestQueue requestQueue) {
mRequestQueue = requestQueue;
return this;
}

/** 设置当前request在当前request队列的系列号. */
public final Request<?> setSequence(int sequence) {
mSequence = sequence;
return this;
}

/** 返回request请求的序列号. */
public final int getSequence() {
if (mSequence == null) {
throw new IllegalStateException("getSequence called before setSequence");
}
return mSequence;
}

/** 返回request的url. */
public String getUrl() {
return mUrl;
}

/** 使用request的url作为volley cache缓存系统存储的key值(默认url可唯一标识一个request). */
public String getCacheKey() {
return getUrl();
}

/** 设置request对应的volley cache缓存系统中的请求结果. */
public Request<?> setCacheEntry(Cache.Entry entry) {
mCacheEntry = entry;
return this;
}

/** 返回request的cache系统的请求结果. */
public Cache.Entry getCacheEntry() {
return mCacheEntry;
}

/** 标识该request已经被取消. */
public void cancel() {
mCanceled = true;
}

/** 返回该request是否被取消标识. */
public boolean isCanceled() {
return mCanceled;
}

/** 返回该request的headers. */
public Map<String, String> getHeaders() throws AuthFailureError {
return Collections.emptyMap();
}

/** 返回该request的请求体中参数,如果是GET请求,则直接返回null. */
protected Map<String, String> getParams() throws AuthFailureError {
return null;
}

/** 返回该request请求参数编码. */
protected String getParamsEncoding() {
return DEFAULT_PARAMS_ENCODING;
}

/** 获取request body content type. */
public String getBodyContentType() {
return "application/x-www-form-urlencoded; charset="
+ getParamsEncoding();
}

/** 返回request请求参数体. */
public byte[] getBody() throws AuthFailureError {
Map<String, String> params = getParams();
if (params != null && params.size() > 0) {
return encodeParameters(params, getParamsEncoding());
}
return null;
}

/** 构造post请求参数体. */
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);
}
}

/** 设置当前request是否需要被缓存. */
public final Request<?> setShouldCache(boolean shouldCache) {
mShouldCache = shouldCache;
return this;
}

/** 返回当前request是否需要被缓存. */
public final boolean shouldCache() {
return mShouldCache;
}

/** 设置request的重试接口. */
public final Request<?> setShouldRetryServerErrors(boolean shouldRetryServerErrors) {
mShouldRetryServerErrors = shouldRetryServerErrors;
return this;
}

/** 返回该request当遇到服务器错误时是否需要重试标志 */
public final boolean shouldRetryServerErrors() {
return mShouldRetryServerErrors;
}

/** request优先级枚举类. */
public enum  Priority {
LOW,
NORMAL,
HIGH,
IMMEDIATE
}

/** 返回当前request的优先级.子类可以重写该方法修改request的优先级. */
public Priority getPriority() {
return Priority.NORMAL;
}

/** 返回重试的时间,用于日志记录. */
public final int getTimeoutMs() {
return mRetryPolicy.getCurrentTimeout();
}

/** 返回重试接口. */
public RetryPolicy getRetryPolicy() {
return mRetryPolicy;
}

/** 用于标识已经将response传给该request. */
public void markDelivered() {
mResponseDelivered = true;
}

/** 返回该request是否有response delivered. */
public boolean hasHadResponseDelivered() {
return mResponseDelivered;
}

/** 子类必须重写该方法,用来解析http请求的结果. */
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

/** 子类可以重写该方法,从而获取更精准的出错信息. */
protected VolleyError parseNetworkError(VolleyError volleyError) {
return volleyError;
}

/** 子类必须重写该方法用于将网络结果返回给用户设置的回调接口. */
abstract protected void deliverResponse(T response);

/** 将网络错误传递给回调接口. */
public void deliverError(VolleyError error) {
if (mErrorListener != null) {
mErrorListener.onErrorResponse(error);
}
}

/** 先判断执行顺序,再判断request优先级. */
@Override
public int compareTo(@NonNull Request<T> another) {
Priority left = this.getPriority();
Priority right = another.getPriority();

return left == right ? this.mSequence - another.mSequence :
right.ordinal() - left.ordinal();
}

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

代码虽然很长,但是都是对request很好的抽象,建议大家结合HTTP协议阅读一下该源码.

Request中的泛型T用来对结果进行泛型表示,当定义出request基类之后,我们可以很轻松的对其进行继承,从而扩展出我们想要的request请求.

例如Volley提供的StringRequest,源码如下:

/** 一个返回结果的String的request实现类 */
@SuppressWarnings("unused")
public class StringRequest extends Request<String>{
private final Response.Listener<String> mListener;

/** 根据给定的METHOD设置对应的request. */
public StringRequest(int method, String url, Response.Listener<String> listener,
Response.ErrorListener errorListener) {
super(method, url, errorListener);
mListener = listener;
}

/** 默认为GET请求的request. */
public StringRequest(String url, Response.Listener<String> listener,
Response.ErrorListener errorListener) {
this(Method.GET, url, listener, errorListener);
}

/** 将HTTP请求结果转换为String. */
@Override
protected Response<String> parseNetworkResponse(NetworkResponse response) {
String parsed;

try {
parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
} catch (UnsupportedEncodingException e) {
parsed = new String(response.data);
}
return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
}

/** 将解析的String结果传递给用户的回调接口. */
@Override
protected void deliverResponse(String response) {
mListener.onResponse(response);
}
}

有了这个StringRequest类示例,我们也可以参考其实现很方便的对Request类进行扩展.再对request进行扩展时,我们通常只需要实现两个方法即可：

deliverResponse：这个方法很简单,就是将网络解析的结果传递给用户设置的回调接口.

parseNetworkResponse : 这个方法比较关键,我们主要也是来重写该方法.如果我需要返回JsonObject,那么我就需要将参数NetworkResponse在该方法中转换成JsonObject.

getParams : 这个方法是如果有POST参数时,需要重写该方法.

介绍完Request抽象,那我们继续来看一下Response抽象.

网络请求结果抽象类

Response.java

Response是Volley抽象出来对网络请求结果进行封装的类.具体注释源码如下：

/** 网络请求结果的封装类.其中泛型T为网络解析结果. */
public class Response<T> {
/** request请求成功回调接口, 用于用户自行处理网络请求返回的结果. */
public interface Listener<T> {
void onResponse(T response);
}

/** request请求失败回调接口,用于用户自行处理网络请求失败的情况. */
public interface ErrorListener {
void onErrorResponse(VolleyError error);
}

/** 构造一个request请求成功的response对象. */
public static <T> Response<T> success(T result, Cache.Entry cacheEntry) {
return new Response<T>(result, cacheEntry);
}

/** 构造一个request请求失败的response对象. */
public static <T> Response<T> error(VolleyError error) {
return new Response<T>(error);
}

/** request的网络请求解析结果. */
public final T result;

/** request的缓存内容. */
public final Cache.Entry cacheEntry;

/** 请求错误内容. */
public final VolleyError error;

/** 当前结果是否为中间请求结果. */
public boolean intermediate = false;

/** 返回当前request请求结果是否成功. */
public boolean isSuccess() {
return error == null;
}

private Response(T result, Cache.Entry cacheEntry) {
this.result = result;
this.cacheEntry = cacheEntry;
this.error = null;
}

private Response(VolleyError error) {
this.result = null;
this.cacheEntry = null;
this.error = error;
}
}

其实,Response只是对request请求结果的进一步封装.真正的HTTP Request请求结果的抽象其实是NetworkResponse类.

NetworkResponse.java

NetworkResponse类是真正的HTTP网络请求结果类,其注释源码如下：

/** HTTP网络请求结果抽象类. */
public class NetworkResponse {
/** HTTP响应状态码. */
public final int statusCode;

/** HTTP响应信息. */
public final byte[] data;

/** 服务器状态码304代表未修改 */
public final boolean notModified;

/** HTTP请求的往返延迟. */
public final long networkTimeMs;

/** HTTP响应头信息. */
public final Map<String, String> headers;

public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified, long networkTimeMs) {
this.statusCode = statusCode;
this.data = data;
this.headers = headers;
this.notModified = notModified;
this.networkTimeMs = networkTimeMs;
}

public NetworkResponse(int statusCode, byte[] data, Map<String, String> headers,
boolean notModified) {
this(statusCode, data, headers, notModified, 0);
}

public NetworkResponse(byte[] data) {
this(HttpURLConnection.HTTP_OK, data, Collections.<String, String>emptyMap(), false, 0);
}

public NetworkResponse(byte[] data, Map<String, String> headers) {
this(HttpURLConnection.HTTP_OK, data, headers, false, 0);
}
}

网络请求的并发和异步

在讲解网络请求的并发和异步之前,我们先来看一下,Volley是如何封装网络请求的.

HurlStack.java

这个类封装了HttpURLConnection类的构造操作,我自己实现网络请求时,也会封装这些重复的HttpURLConnection构造代码.注释代码如下:

/** 封装HttpURLConnection类,简化网络请求代码. */
public class HurlStack implements HttpStack {
private static final String HEADER_CONTENT_TYPE = "Content-Type";

private final SSLSocketFactory mSslSocketFactory;

/** 默认创建一个HTTP请求类. */
public HurlStack() {
this(null);
}

/** 创建一个HTTPS请求类. */
public HurlStack(SSLSocketFactory sslSocketFactory) {
mSslSocketFactory = sslSocketFactory;
}

/** HTTP or HTTPS请求真正执行的地方 */
@Override
public HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
HashMap<String, String> map = new HashMap<String, String>();
map.putAll(request.getHeaders());
map.putAll(additionalHeaders);

// 构造HttpURLConnection,封装一些固定参数.
String url = request.getUrl();
URL parsedUrl = new URL(url);
HttpURLConnection connection = openConnection(parsedUrl, request);
// 构造http请求的header.
for (String headerName: map.keySet()) {
connection.addRequestProperty(headerName, map.get(headerName));
}
// 构造http请求的body.
setConnectionParametersForRequest(connection, request);

// Initialize HttpResponse with data from the HttpURLConnection
ProtocolVersion protocolVersion = new ProtocolVersion("HTTP", 1, 1);
int responseCode = connection.getResponseCode();
if (responseCode == -1) {
throw new IOException("Could not retrieve response code from HttpUrlConnection.");
}

// 使用apache提供的BasicHttpResponse来封装请求.
StatusLine responseStatus = new BasicStatusLine(protocolVersion,
connection.getResponseCode(), connection.getResponseMessage());
BasicHttpResponse response = new BasicHttpResponse(responseStatus);
if (hasResponseBody(request.getMethod(), responseStatus.getStatusCode())) {
response.setEntity(entityFromConnection(connection));
}
for (Map.Entry<String, List<String>> header : connection.getHeaderFields().entrySet()) {
if (header.getKey() != null) {
Header h = new BasicHeader(header.getKey(), header.getValue().get(0));
response.addHeader(h);
}
}

return response;
}

/** 封装HttpURLConnection类的构造函数. */
private HttpURLConnection openConnection(URL url, Request<?> request) throws IOException {
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
connection.setInstanceFollowRedirects(HttpURLConnection.getFollowRedirects());

int timeoutMs = request.getTimeoutMs();
connection.setConnectTimeout(timeoutMs);
connection.setReadTimeout(timeoutMs);
connection.setUseCaches(false);
connection.setDoInput(true);

if ("https".equals(url.getProtocol()) && mSslSocketFactory != null) {
((HttpsURLConnection)connection).setSSLSocketFactory(mSslSocketFactory);
}

return connection;
}

/* package */ static void setConnectionParametersForRequest(HttpURLConnection connection,
Request<?> request)
throws IOException, AuthFailureError {
switch (request.getMethod()) {
case Request.Method.GET:
connection.setRequestMethod("GET");
break;
case Request.Method.POST:
connection.setRequestMethod("POST");
addBodyIfExists(connection, request);
break;
}
}

/** 添加POST请求参数到HttpURLConnection中. */
private static void addBodyIfExists(HttpURLConnection connection, Request<?> request)
throws AuthFailureError, IOException {
byte[] body = request.getBody();
if (body != null) {
connection.setDoOutput(true);
connection.addRequestProperty(HEADER_CONTENT_TYPE, request.getBodyContentType());
DataOutputStream out = new DataOutputStream(connection.getOutputStream());
out.write(body);
out.flush();
}
}

/** 判断当前request请求结果是否有响应体. */
private boolean hasResponseBody(int requestMethod, int responseCode) {
return requestMethod != Request.Method.HEAD
&& !(HttpStatus.SC_CONTINUE <= responseCode && responseCode <= HttpStatus.SC_OK)
&& responseCode != HttpStatus.SC_NO_CONTENT
&& responseCode != HttpStatus.SC_NOT_MODIFIED;
}

/** 保存Http Body. */
private HttpEntity entityFromConnection(HttpURLConnection connection) {
BasicHttpEntity entity = new BasicHttpEntity();
InputStream inputStream;
try {
inputStream = connection.getInputStream();
} catch (IOException ioe) {
inputStream = connection.getErrorStream();
}
entity.setContent(inputStream);
entity.setContentLength(connection.getContentLength());
entity.setContentEncoding(connection.getContentEncoding());
entity.setContentType(connection.getContentType());

return entity;
}
}

当用户new出HurlStack对象,调用它的performRequest方法,即可以发出HTTP请求,并获取HTTP请求结果.

但是,Android主线程中是不允许进行耗时操作的,所以Volley实现了并发访问HurlStack的performRequest的方法.

至于HurlStack的并发访问,就需要看NetworkDispatcher的实现.

NetworkDispatcher.java

NetworkDispatcher是一个线程,用来调度处理网络请求.启动后会不断从网络请求队列中取请求处理,队列为空则等待,请求处理结束则将结果传递给ResponseDelivery去执行后续处理,并判断结果是否要进行缓存.

NetworkDispatcher的执行流程图如下:



NetworkDispatcher中文注释代码如下：

/** 调度网络请求线程. */
public class NetworkDispatcher extends Thread{
/** 网络请求队列. */
private final BlockingQueue<Request<?>> mQueue;

/** 封装了HurlStack的网络类,其performRequest方法是单个request请求真正执行的地方. */
private final Network mNetwork;

/** 缓存类,存储请求结果的缓存. */
private final Cache mCache;

/** 请求结果传递类. */
private final ResponseDelivery mDelivery;

/** 暂停线程的标志位,替换Thread自身的stop方法. */
private volatile boolean mQuit = false;

/** 构造网络请求调度线程类. */
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache, ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}

/** 强制停止当前调度线程. */
public void quit() {
mQuit = true;
interrupt();
}

@Override
public void run() {
android.os.Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
while (true) {
long startTimeMs = SystemClock.elapsedRealtime();
Request<?> request;
try {
// 使用BlockingQueue实现了生产者-消费者模型.
// 消费者是该调度线程.
// 生产者是request网络请求.
request = mQueue.take();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
continue;
}

try {
if (request.isCanceled()) {
continue;
}

addTrafficStatsTag(request);

// 真正执行网络请求的地方.
NetworkResponse networkResponse = mNetwork.performRequest(request);

// If the server returned 304 AND we delivered a response already,
// we're done -- don't deliver a second identical response.
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
continue;
}

// 在当前线程中解析网络结果.
// 不同的Request实现的parseNetworkResponse是不同的(例如StringRequest和JsonRequest).
Response<?> response = request.parseNetworkResponse(networkResponse);

//
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
}

// 将网络请求结果进行传递.
// ResponseDelivery调用顺序如下:
// ResponseDelivery.postResponse==>ResponseDeliveryRunnable[Runnable]->run
// ==>Request->deliverResponse==>用户设置的Listener回调接口
request.markDelivered();
mDelivery.postResponse(request, response);
} catch (VolleyError volleyError) {
volleyError.printStackTrace();
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
} catch (Exception e) {
VolleyError volleyError = new VolleyError(e);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
mDelivery.postError(request, volleyError);
}
}
}

private void parseAndDeliverNetworkError(Request<?> request, VolleyError error) {
error = request.parseNetworkError(error);
mDelivery.postError(request, error);
}

private void addTrafficStatsTag(Request<?> request) {
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
TrafficStats.setThreadStatsTag(request.getTrafficStatsTag());
}
}
}

这里还有一点需要说明,NetworkDispatcher真正执行Http request请求时,并不是直接使用HurlStack类的performRequest方法,而是又对其进行了一个封装,封装成了Network类.

Network.java

Network.java的源码如下：

/** 网络接口,处理网络请求 */
public interface Network {
NetworkResponse performRequest(Request<?> request) throws VolleyError;
}

可以看到,Network有一个子类需要实现的方法,和HurlStack的具体执行HTTP请求的方法的名称是一样的.那为什么Volley要多此一举对HurlStack进行进一步封装呢？

这是因为Volley向下兼容到Android2.3之下的版本,而Android2.3以下的版本构造Http请求时推荐使用的是HttpClient类,所以这里Volley做了一个适配器模式的封装.也就是说,HurlStack类只需要负责对HttpURLConnection进行封装,HttpClientStack只需要对HttpClient类进行封装.

封装更多的处理操作.包括：缓存新鲜度验证、超时重试等.

至于Network接口的具体实现类是BasicNetwork类,其注释源码如下：

/** Volley默认的网络接口实现类. */
public class BasicNetwork implements Network {
/** 网络请求真正实现类. */
private final HttpStack mHttpStack;

public BasicNetwork(HttpStack httpStack) {
mHttpStack = httpStack;
}

@Override
public NetworkResponse performRequest(Request<?> request) throws VolleyError {
long requestStart = SystemClock.elapsedRealtime();
while (true) {
HttpResponse httpResponse = null;
byte[] responseContents = null;
Map<String, String> responseHeaders = Collections.emptyMap();
try {
// 构造Cache的HTTP headers,主要是添加If-None-Match和If-Modified-Since两个字段
// 当客户端发送的是一个条件验证请求时,服务器可能返回304状态码.
// If-Modified-Since：代表服务器上次修改是的日期值.
// If-None-Match：服务器上次返回的ETag响应头的值.
Map<String, String> headers = new HashMap<String, String>();
addCacheHeaders(headers, request.getCacheEntry());

// 调用HurlStack的performRequest方法执行网络请求, 并将请求结果存入httpResponse变量中
httpResponse = mHttpStack.performRequest(request, headers);

StatusLine statusLine = httpResponse.getStatusLine();
int statusCode = statusLine.getStatusCode();
responseHeaders = convertHeaders(httpResponse.getAllHeaders());

// 当服务端返回304状态码时,直接将Volley缓存中结果返回
if (statusCode == HttpStatus.SC_NOT_MODIFIED) {
Cache.Entry entry = request.getCacheEntry();
if (entry == null) {
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, null,
responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}

// A HTTP 304 response dose not have all header filed. We
// have to use the header fields from the cache entry plus
// the new ones from the response.
entry.responseHeaders.putAll(responseHeaders);
return new NetworkResponse(HttpStatus.SC_NOT_MODIFIED, entry.data,
entry.responseHeaders, true,
SystemClock.elapsedRealtime() - requestStart);
}

// Some responses such as 204s do not have content. We mush check
if (httpResponse.getEntity() != null) {
responseContents = entityToBytes(httpResponse.getEntity());
} else {
responseContents = new byte[0];
}

if (statusCode < 200 || statusCode > 299) {
throw new IOException();
}

return new NetworkResponse(statusCode, responseContents, responseHeaders, false,
SystemClock.elapsedRealtime() - requestStart);
} catch (SocketTimeoutException e) {
// 捕获各种异常,进行重试操作.
attemptRetryOnException("socket", request, new TimeoutError());
} catch (ConnectTimeoutException E) {
attemptRetryOnException("connection", request, new TimeoutError());
} catch (MalformedURLException e) {
throw new RuntimeException("Bad URL " + request.getUrl(), e);
} catch (IOException e) {
int statusCode;
if (httpResponse != null) {
statusCode = httpResponse.getStatusLine().getStatusCode();
} else {
throw new NoConnctionError(e);
}
NetworkResponse networkResponse;
if (responseContents != null) {
networkResponse = new NetworkResponse(statusCode, responseContents,
responseHeaders, false, SystemClock.elapsedRealtime() - requestStart);
if (statusCode == HttpStatus.SC_UNAUTHORIZED ||
statusCode == HttpStatus.SC_FORBIDDEN) {
attemptRetryOnException("auth",
request, new AuthFailureError(networkResponse));
} else if (statusCode >= 400 && statusCode <= 499) {
throw new ClientError(networkResponse);
} else if (statusCode >= 500 && statusCode <= 599) {
if (request.shouldRetryServerErrors()) {
attemptRetryOnException("server",
request, new ServerError(networkResponse));
} else {
throw new ServerError(networkResponse);
}
} else {
// 3xx?
throw new ServerError(networkResponse);
}
} else {
attemptRetryOnException("network", request, new NetworkError());
}
}
}
}

private void addCacheHeaders(Map<String, String> headers, Cache.Entry entry) {
if (entry == null) {
return;
}

if (entry.etag != null) {
headers.put("If-None-Match", entry.etag);
}

if (entry.lastModified > 0) {
Date refTime = new Date(entry.lastModified);
headers.put("If-modified-Since", DateUtils.formatDate(refTime));
}
}

private static Map<String, String> convertHeaders(Header[] headers) {
Map<String, String> result = new TreeMap<String, String>(String.CASE_INSENSITIVE_ORDER);
for (Header header : headers) {
result.put(header.getName(), header.getValue());
}
return result;
}

/**
* 将服务器返回的InputStream输入流转换成byte数组.
* 这个函数让我实现的话,我会使用StringBuffer来替换ByteArrayOutputStream来实现字符串拼接.
*/
private byte[] entityToBytes(HttpEntity entity) throws IOException, ServerError {
ByteArrayOutputStream bytes = new ByteArrayOutputStream();

byte[] buffer = new byte[1024];

try {
InputStream in = entity.getContent();
if (in == null) {
throw new ServerError();
}
int count;
while ((count = in.read(buffer)) != -1) {
bytes.write(buffer, 0, count);
}
return bytes.toByteArray();
} finally {
try {
entity.consumeContent();
} catch (IOException e){
e.printStackTrace();
}
bytes.close();
}
}

private void attemptRetryOnException(String logPrefix, Request<?> request,
VolleyError exception) throws VolleyError{
RetryPolicy retryPolicy = request.getRetryPolicy();
int oldTimeout = request.getTimeoutMs();

retryPolicy.retry(exception);
Log.e("Volley", String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout));
}
}

RequestQueue.java

RequestQueue是Volley框架的核心类,用户在使用Volley时,就是将一个Request加入到RequestQueue来完成请求操作的.所以,RequestQueue既是request的存储仓库,也是NetworkDispatcher的调度核心.

由于RequestQueue其中还包括Volley的缓存机制,我们稍后会对缓存机制进行讲解,所以这里只看跟NetworkDispatcher调度相关的源码.

RequestQueue类的注释代码如下：

/** Request请求调度队列. */
@SuppressWarnings("unused")
public class RequestQueue {
/**
* Callback interface for completed requests.
*/
public interface RequestFinishedListener<T> {
void onRequestFinished(Request<T> request);
}

/** 为每一个request申请独立的序列号. */
private AtomicInteger mSequenceGenerator = new AtomicInteger();

/**
* Staging area for requests that already have a duplicate request in flight.
*/
private final Map<String, Queue<Request<?>>> mWaitingRequests =
new HashMap<String, Queue<Request<?>>>();

/** 保存所有被加入到当前队列的request集合. */
private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();

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

/** 存储需要进行网络通信的request的存储队列. */
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<Request<?>>();

/** RequestQueue默认开启的网络线程的数量. */
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;

/**
* Cache interface for retrieving and storing responses.
*/
private final Cache mCache;

/** 封装request网络请求的Network类. */
private final Network mNetwork;

/** 网络请求传输结果实现类. */
private final ResponseDelivery mDelivery;

/** 网络请求线程数组. */
private NetworkDispatcher[] mDispatchers;

/** 缓存线程 */
private CacheDispatcher mCacheDispatcher;

private List<RequestFinishedListener> mFinishedListeners =
new ArrayList<RequestFinishedListener>();

public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}

public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}

/**
* Creates the worker pool.
* @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;
}

/** 开启request的缓存线程和多个网络请求线程 */
public void start() {
// 关闭所有正在运行的缓存线程和网络请求线程.
stop();

// 默认开启DEFAULT_NETWORK_THREAD_POOL_SIZE(4)个线程来执行request网络请求.
for (int i = 0; i < mDispatchers.length; i ++) {
// 将NetworkDispatcher线程与mNetworkQueue这个队列进行绑定.
// NetworkDispatcher会使用生产者-消费者模型从mNetworkQueue获取request请求,并执行.
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}
}

/** 停止所有的缓存线程和网络请求线程. */
private void stop() {
for (NetworkDispatcher dispatcher : mDispatchers) {
if (dispatcher != null) {
dispatcher.quit();
}
}
}

/** 将Request请求加入到调度队列中. */
public <T> Request<?> 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);
}

// 分配request唯一的序列号.
request.setSequence(getSequenceNumber());

// request不允许缓存,则直接将request加入到mNetworkQueue当中
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}
}

/** 提供request请求序列号. */
private int getSequenceNumber() {
return mSequenceGenerator.incrementAndGet();
}
}

RequestQueue在构造函数中,会默认生成4个NetworkDispatcher线程,并且将NetworkDispatcher线程与mNetworkQueue进行绑定,然后start NetworkDispatcher执行网络请求操作.

异步

前面已经详细讲解了一个Request是如何被并发处理的,那现在回到我们的3-4问题,子线程中并发处理的结果如何异步传递给用户设置的Listener回调接口.

从NetworkDispatcher最后传递结果的代码：

request.markDelivered();
mDelivery.postResponse(request, response);

我们就可以看出,异步回调是通过ResponseDelivery类实现的.

ResponseDelivery.java

ResponseDelivery的中文注释源码如下：

/** 网络结果分发接口类. */
public interface ResponseDelivery {
/**
* Parses a response from the network or cache and delivers it.
*/
void postResponse(Request<?> request, Response<?> response);

/**
* Parses a response from the network or cache and delivers it.
*/
void postResponse(Request<?> request, Response<?> response, Runnable runnable);

/**
* Posts an error for the given request.
*/
void postError(Request<?> request, VolleyError error);
}

在RequestQueue中,ResponseDelivery的实现类为ExecutorDelivery类.

ExecutorDelivery

众所周知,Android中实现异步肯定是需要用到Handler、Looper和Message机制的.ExecutorDelivery的实现异步的机制也是居于Handler机制.

我们先来看一下,RequestQueue中ExecutorDelivery是如何被构造的:

ResponseDelivery delivery = new ExecutorDelivery(new Handler(Looper.getMainLooper()));

可以看到,RequestQueue将绑定主线程Looper对象的Handler对象传递给了ExecutorDelivery,这样我们通过handler发送的消息其实都是在主线程进行处理了.

ExecutorDelivery的中文注释源码如下:

/**
* 网络请求结果传递类.(实现异步功能,主线程传递数据给子线程)
*/
@SuppressWarnings("unused")
public class ExecutorDelivery implements ResponseDelivery {
/**
* 构造执行已提交的Runnable任务对象.
*/
private final Executor mResponsePoster;

public ExecutorDelivery(final Handler handler) {
mResponsePoster = new Executor() {
@Override
public void execute(@NonNull Runnable command) {
// 所有的Runnable通过绑定主线程Looper的Handler对象最终在主线程执行.
handler.post(command);
}
};
}

public ExecutorDelivery(Executor executor) {
mResponsePoster = executor;
}

@Override
public void postResponse(Request<?> request, Response<?> response) {
postResponse(request, response, null);
}

@Override
public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
request.markDelivered();
mResponsePoster.execute(
new ResponseDeliveryRunnable(request, response, runnable)
);
}

@Override
public void postError(Request<?> request, VolleyError error) {
Response<?> response = Response.error(error);
mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, null));
}

/** 在主线程执行的Runnable类 */
@SuppressWarnings("unchecked")
private class ResponseDeliveryRunnable implements Runnable {
private final Request mRequest;
private final Response mResponse;
private final Runnable mRunnable;

public ResponseDeliveryRunnable(Request request, Response response, Runnable runnable) {
mRequest = request;
mResponse = response;
mRunnable = runnable;
}

@Override
public void run() {
// 如果request被取消,则不回调用户设置的Listener接口
if (mRequest.isCanceled()) {
mRequest.finish("canceled-at-delivery");
return;
}

// 通过response状态标志,来判断是回调用户设置的Listener接口还是ErrorListener接口
if (mResponse.isSuccess()) {
mRequest.deliverResponse(mResponse.result);
} else {
mRequest.deliverError(mResponse.error);
}

if (mResponse.intermediate) {
mRequest.addMarker("intermediate-response");
} else {
// 通知RequestQueue终止该Request请求
mRequest.finish("done");
}

if (mRunnable != null) {
mRunnable.run();
}
}
}
}

缓存机制

前面讲解了并发和异步的实现,接下来,我们就来看一下Volley的缓存机制.再学习Volley缓存实现方案之前,我们先来感受一下Google I/O大会上Volley官方一张宣传图片：



这张图片非常形象的表达了Volley适合频繁的网络请求.接下来,我们就从Volley的缓存系统入手,介绍一下为什么Volley适合频繁的网络请求.

Cache.java

既然要缓存Request请求,那我们首先就需要抽象出缓存对象.而Cache类就是对缓存对象的抽象描述:

/** 缓存内存的抽象接口 */
@SuppressWarnings("unused")
public interface Cache {
/** 通过key获取请求的缓存实体. */
Entry get(String key);

/** 存入一个请求的缓存实体. */
void put(String key, Entry entry);

void initialize();

void invalidate(String key, boolean fullExpire);

/** 移除指定的缓存实体. */
void remove(String key);

/** 清空缓存. */
void clear();

/** 真正HTTP请求缓存实体类. */
class Entry {
/** HTTP响应体. */
public byte[] data;

/** HTTP响应首部中用于缓存新鲜度验证的ETag. */
public String etag;

/** HTTP响应时间. */
public long serverDate;

/** 缓存内容最后一次修改的时间. */
public long lastModified;

/** Request的缓存过期时间. */
public long ttl;

/** Request的缓存新鲜时间. */
public long softTtl;

/** HTTP响应Headers. */
public Map<String, String> responseHeaders = Collections.emptyMap();

/** 判断缓存内容是否过期. */
public boolean isExpired() {
return this.ttl < System.currentTimeMillis();
}

/** 判断缓存是否新鲜,不新鲜的缓存需要发到服务端做新鲜度的检测. */
public boolean refreshNeeded() {
return this.softTtl < System.currentTimeMillis();
}
}
}

Cache接口定义规定了缓存实体的内容和其需要实现的方法.在RequestQueue中,Cache的实现类是DiskBasedCache类.

DiskBasedCache.java

DiskBasedCache类的主要作用是：实现了基于Disk的对象存储类,并提供替换策略.代码比较简单,中文注释的代码如下:

/** 基于Disk的缓存实现类. */
@SuppressWarnings("ResultOfMethodCallIgnored")
public class DiskBasedCache implements Cache {
/** 默认硬盘最大的缓存空间(5M). */
private static final int DEFAULT_DISK_USAGE_BYTES = 5 * 1024 * 1024;

/** 标记缓存起始的MAGIC_NUMBER. */
private static final int CACHE_MAGIC = 0x20150306;

/**
* High water mark percentage for the cache.
*/
private static final float HYSTERESIS_FACTOR = 0.9f;

/**
* Map of the Key, CacheHeaders pairs.
*/
private final Map<String, CacheHeader> mEntries =
new LinkedHashMap<String, CacheHeader>(16, 0.75f, true);

/** 目前使用的缓存字节数. */
private long mTotalSize = 0;

/** 硬盘缓存目录. */
private final File mRootDirectory;

/** 硬盘缓存最大容量(默认5M). */
private final int mMaxCacheSizeInBytes;

public DiskBasedCache(File rootDirectory) {
this(rootDirectory, DEFAULT_DISK_USAGE_BYTES);
}

public DiskBasedCache(File rootDirectory, int maxCacheSizeInBytes) {
mRootDirectory = rootDirectory;
mMaxCacheSizeInBytes = maxCacheSizeInBytes;
}

/** 清空缓存内容. */
@Override
public synchronized void clear() {
File[] files = mRootDirectory.listFiles();
if (files != null) {
for (File file : files) {
file.delete();
}
}
mEntries.clear();
mTotalSize = 0;
}

/** 从Disk中根据key获取并构造HTTP响应体Cache.Entry. */
@Override
public synchronized Entry get(String key) {
CacheHeader entry = mEntries.get(key);
if (entry == null) {
return null;
}

File file = getFileForKey(key);
CountingInputStream cis = null;
try {
cis = new CountingInputStream(new BufferedInputStream(new FileInputStream(file)));
// 读完CacheHeader部分,并通过CountingInputStream的bytesRead成员记录已经读取的字节数.
CacheHeader.readHeader(cis);
// 读取缓存文件存储的HTTP响应体内容.
byte[] data = streamToBytes(cis, (int)(file.length() - cis.bytesRead));
return entry.toCacheEntry(data);
} catch (IOException e) {
remove(key);
return null;
} finally {
if (cis != null) {
try {
cis.close();
} catch (IOException ignored) {
}
}
}
}

/** 初始化Disk缓存系统.
* 作用是：遍历Disk缓存系统,将缓存文件中的CacheHeader和key存储到Map对象中. */
@Override
public void initialize() {
if (!mRootDirectory.exists() && !mRootDirectory.mkdirs()) {
return;
}

File[] files = mRootDirectory.listFiles();
if (files == null) {
return;
}

for (File file : files) {
BufferedInputStream fis = null;
try {
fis = new BufferedInputStream(new FileInputStream(file));
CacheHeader entry = CacheHeader.readHeader(fis);
entry.size = file.length();
putEntry(entry.key, entry);
}catch (IOException e) {
file.delete();
e.printStackTrace();
}finally {
if (fis != null) {
try {
fis.close();
} catch (IOException ignored) {
}
}
}
}
}

/** 标记指定的cache过期. */
@Override
public synchronized void invalidate(String key, boolean fullExpire) {
Entry entry = get(key);
if (entry != null) {
entry.softTtl = 0;
if (fullExpire) {
entry.ttl = 0;
}
put(key, entry);
}
}

/** 将Cache.Entry存入到指定的缓存文件中. 并在Map中记录<key,CacheHeader>. */
@Override
public synchronized void put(String key, Entry entry) {
pruneIfNeeded(entry.data.length);
File file = getFileForKey(key);
try {
BufferedOutputStream fos = new BufferedOutputStream(new FileOutputStream(file));
CacheHeader e = new CacheHeader(key, entry);
boolean success = e.writeHeader(fos);
if (!success) {
fos.close();
throw new IOException();
}
fos.write(entry.data);
fos.close();
putEntry(key, e);
return;
} catch (IOException e) {
e.printStackTrace();
}
file.delete();
}

/** Disk缓存替换更新机制. */
private void pruneIfNeeded(int neededSpace) {
if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes) {
return;
}

Iterator<Map.Entry<String, CacheHeader>> iterator = mEntries.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<String, CacheHeader> entry = iterator.next();
CacheHeader e = entry.getValue();
boolean deleted = getFileForKey(e.key).delete();
if (deleted) {
mTotalSize -= e.size;
}
iterator.remove();

if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes * HYSTERESIS_FACTOR) {
break;
}
}
}

/** 获取存储当前key对应value的文件句柄. */
private File getFileForKey(String key) {
return new File(mRootDirectory, getFilenameForKey(key));
}

/** 根据key的hash值生成对应的存储文件名称. */
private String getFilenameForKey(String key) {
int firstHalfLength = key.length() / 2;
String localFilename = String.valueOf(key.substring(0, firstHalfLength).hashCode());
localFilename += String.valueOf(key.substring(firstHalfLength).hashCode());
return localFilename;
}

/** 将key和CacheHeader存入到Map对象中.并更新当前占用的总字节数. */
private void putEntry(String key, CacheHeader entry) {
if (!mEntries.containsKey(key)) {
mTotalSize += entry.size;
} else {
CacheHeader oldEntry = mEntries.get(key);
mTotalSize += (entry.size - oldEntry.size);
}

mEntries.put(key, entry);
}

@Override
public synchronized void remove(String key) {
boolean deleted = getFileForKey(key).delete();
removeEntry(key);
if (!deleted) {
Log.e("Volley", "没能删除key=" + key + ", 文件名=" + getFilenameForKey(key) + "缓存.");
}
}

/** 从Map对象中删除key对应的键值对. */
private void removeEntry(String key) {
CacheHeader entry = mEntries.get(key);
if (entry != null) {
mTotalSize -= entry.size;
mEntries.remove(key);
}
}

/** 抽象出来的缓存文件摘要信息.
* 与Cache.Entry类几乎相同,但是只存储了响应体的大小,没保存响应体的内容.
*/
static class CacheHeader {
/** HTTP响应头(header)和响应体(body)的整体大小.也就是Disk缓存系统中对应缓存文件的大小. */
public long size;

public String key;

/** HTTP响应首部中用于缓存新鲜度验证的ETag. */
public String etag;

/** HTTP响应时间. */
public long serverDate;

/** 缓存内容最后一次修改的时间. */
public long lastModified;

/** Request的http缓存过期时间. */
public long ttl;

/** Request的http缓存新鲜时间. */
public long softTtl;

/** HTTP的响应headers. */
public Map<String, String> responseHeaders;

private CacheHeader(){}

/**
* Instantiates a new CacheHeader object
* @param key The key that indentifies the cache entry
* @param entry The cache entry
*/
public CacheHeader(String key, Entry entry) {
this.key = key;
this.size = entry.data.length;
this.etag = entry.etag;
this.serverDate = entry.serverDate;
this.lastModified = entry.lastModified;
this.ttl = entry.ttl;
this.softTtl = entry.softTtl;
this.responseHeaders = entry.responseHeaders;
}

/** 从InputStream中构造CacheHeader对象.其实就是实现对象的反序列化. */
public static CacheHeader readHeader(InputStream is) throws IOException {
CacheHeader entry = new CacheHeader();
// 以CACHE_NUMBER作为读取一个对象的开始
int magic = readInt(is);
if (magic != CACHE_MAGIC) {
throw new IOException();
}
entry.key = readString(is);
entry.etag = readString(is);
if (entry.etag.equals("")) {
entry.etag = null;
}
entry.serverDate = readLong(is);
entry.lastModified = readLong(is);
entry.ttl = readLong(is);
entry.softTtl = readLong(is);
entry.responseHeaders = readStringStringMap(is);

return entry;
}

/** 通过传入的data数组构造一个Cache.Entry对象. */
public Entry toCacheEntry(byte[] data) {
Entry e = new Entry();
e.data = data;
e.etag = etag;
e.serverDate = serverDate;
e.lastModified = lastModified;
e.ttl = ttl;
e.softTtl = softTtl;
e.responseHeaders = responseHeaders;
return e;
}

/** 将CacheHeader对象序列化. */
public boolean writeHeader(OutputStream os) {
try {
writeInt(os, CACHE_MAGIC);
writeString(os, key);
writeString(os, etag == null ? "" : etag);
writeLong(os, serverDate);
writeLong(os, lastModified);
writeLong(os, ttl);
writeLong(os, softTtl);
writeStringStringMap(responseHeaders, os);
os.flush();
return true;
} catch (IOException e) {
e.printStackTrace();
return false;
}
}
}

static void writeString(OutputStream os, String s) throws IOException {
byte[] b = s.getBytes("UTF-8");
writeLong(os, b.length);
os.write(b, 0, b.length);
}

/** InputStream中读取字符串的方法是:
*  1. 读取字符串长度n.
*  2. 读取n个字节保存在字符数组中.
*  3. 将字符数组转换成字符串.
*/
private static String readString(InputStream is) throws IOException {
int n = (int)readLong(is);
byte[] b = streamToBytes(is, n);
return new String(b, "UTF-8");
}

private static byte[] streamToBytes(InputStream in, int length) throws IOException {
byte[] bytes = new byte[length];
int count;
int pos = 0;
// 这里调用的是InputStream的read(byte[] b, int off, int len)方法.作用是：
// 从输入流中最多读取len个数据字节到byte数组中,并将读取的第一个字节存储在byte[pos]位置上.
// 由于,每次读取的字节数count可能小于len,所以需要循环读取.
while (pos < length && ((count = in.read(bytes, pos, length - pos)) != -1)) {
pos += count;
}
if (pos != length) {
throw new IOException("Expected " + length + " bytes, read " + pos + " bytes");
}
return bytes;
}

static void writeLong(OutputStream os, long n) throws IOException {
os.write((byte)(n));
os.write((byte)(n >>> 8));
os.write((byte)(n >>> 16));
os.write((byte)(n >>> 24));
os.write((byte)(n >>> 32));
os.write((byte)(n >>> 40));
os.write((byte)(n >>> 48));
os.write((byte)(n >>> 56));
}

private static long readLong(InputStream is) throws IOException {
long n = 0;
n |= ((read(is) & 0xFFL));
n |= ((read(is) & 0xFFL) << 8);
n |= ((read(is) & 0xFFL) << 16);
n |= ((read(is) & 0xFFL) << 24);
n |= ((read(is) & 0xFFL) << 32);
n |= ((read(is) & 0xFFL) << 40);
n |= ((read(is) & 0xFFL) << 48);
n |= ((read(is) & 0xFFL) << 56);
return n;
}

private static void writeInt(OutputStream os, int n) throws IOException {
os.write((n) & 0xff);
os.write((n >> 8) & 0xff);
os.write((n >> 16) & 0xff);
os.write((n >> 24) & 0xff);
}

private static int readInt(InputStream is) throws IOException {
int n = 0;
n |= (read(is));
n |= (read(is) << 8);
n |= (read(is) << 16);
n |= (read(is) << 24);

return n;
}

private static int read(InputStream is) throws IOException {
int b = is.read();
if (b == -1) {
throw new EOFException();
}
return b;
}

static void writeStringStringMap(Map<String, String> map, OutputStream os) throws IOException {
if (map != null) {
writeInt(os, map.size());
for (Map.Entry<String, String> entry : map.entrySet()) {
writeString(os, entry.getKey());
writeString(os, entry.getValue());
}
} else {
writeInt(os, 0);
}
}

/**
* 从输入流中读取Map对象.读取方法如下:
* 1. 读取Map对象的数量size.
* 2. 然后循环读取size次,每次先读一个String作为key,再读一个String作为Value.
*/
private static Map<String, String> readStringStringMap(InputStream is) throws IOException {
int size = readInt(is);
Map<String, String> result = (size == 0) ? Collections.<String, String>emptyMap()
: new HashMap<String, String>(size);
for (int i = 0; i < size; i ++) {
String key = readString(is).intern();
String value = readString(is).intern();
result.put(key, value);
}

return result;
}

/** 继承FilterInputStream,增加记录读取总字节数的功能. */
private static class CountingInputStream extends FilterInputStream{
private int bytesRead = 0;

private CountingInputStream(InputStream in) {
super(in);
}

@Override
public int read() throws IOException {
int result = super.read();
if (result != -1) {
bytesRead ++;
}
return result;
}

@Override
public int read(@NonNull byte[] buffer, int byteOffset, int byteCount) throws IOException {
int result = super.read(buffer, byteOffset, byteCount);
if (result != -1) {
bytesRead += result;
}
return result;
}
}
}

有了DiskBasedCache类,我们就可以看一下Volley是如何对缓存进行存储的了.

回到RequestQueue类中,我们看一下跟缓存相关的代码实现.

CacheDispatcher.java

在RequestQueue的start方法里,有如下代码:

public void start() {
// 关闭所有正在运行的缓存线程和网络请求线程.
stop();
// 开启缓存线程.
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();
}

从上面代码,可以看到,Volley是启动了一个线程来实现缓存功能.我们再学习CacheDispatcherd的实现之前,可以来思考一下,如果让我们来实现CacheDispatcher,我们的思路是什么呢?

我的思路如下：

在当前DiskBasedCache缓存系统中,查找是否已经缓存过该Request.

如果已经缓存过,且没有过期,则直接返回缓存系统中的内容.

如果没有缓存,或者缓存已经过期,则走网络请求,并且网络请求之后的结果记录到DiskBasedCache缓存系统中.

接下来,我们来看一下CacheDispatcher的源码,看看它是不是这么操作的：

/** 线程,用来调度可以走缓存的Request请求. */
public class CacheDispatcher extends Thread{
/** 可以走Disk缓存的request请求队列. */
private final BlockingQueue<Request<?>> mCacheQueue;

/** 需要走网络的request请求队列. */
private final BlockingQueue<Request<?>> mNetworkQueue;

/** DiskBasedCache缓存实现类. */
private final Cache mCache;

/** 网络请求结果传递类. */
private final ResponseDelivery mDelivery;

/** 用来停止线程的标志位. */
private volatile boolean mQuit = false;

public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
}

/** 通过标记位机制强行停止CacheDispatcher线程. */
public void quit() {
mQuit = true;
interrupt();
}

@Override
public void run() {
android.os.Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);

// 初始化DiskBasedCache缓存类.
mCache.initialize();

while (true) {
try {
// 从缓存队列中获取request请求.(缓存队列实现了生产者-消费者队列模型)
final Request<?> request = mCacheQueue.take();

// 判断请求是否被取消
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}

// 从缓存系统中获取request请求结果Cache.Entry.
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
// 如果缓存系统中没有该缓存请求,则将request加入到网络请求队列中.
// 由于NetworkQueue跟NetworkDispatcher线程关联,并且也是生产者-消费者队列,
// 所以这里添加request请求就相当于将request执行网络请求.
mNetworkQueue.put(request);
continue;
}

// 判断缓存结果是否过期.
if (entry.isExpired()) {
request.setCacheEntry(entry);
// 过期的缓存需要重新执行request请求.
mNetworkQueue.put(request);
continue;
}

// We have a cache hit; parse its data for delivery back to the request.
Response<?> response = request.parseNetworkResponse(new NetworkResponse(entry.data,
entry.responseHeaders));

// 判断Request请求结果是否新鲜?
if (!entry.refreshNeeded()) {
// 请求结果新鲜,则直接将请求结果分发,进行异步回调用户接口.
mDelivery.postResponse(request, response);
} else {
// 请求结果不新鲜,但是同样还是将缓存结果返回给用户,并且同时执行网络请求,刷新Request网络结果缓存.
request.setCacheEntry(entry);

response.intermediate = true;

mDelivery.postResponse(request, response, new Runnable() {
@Override
public void run() {
try {
mNetworkQueue.put(request);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
});
}
} catch (InterruptedException e) {
e.printStackTrace();
if (mQuit) {
return;
}
}
}
}
}

从源码中可以看出,CacheDispatcher的执行流程和我们设想的基本一致,但是当缓存内容不存在时,如何将网络拉取的最新内容存储在Cache缓存中却没有在CacheDispatcher类中体现.这是因为：

NetworkDispatcher代码中,所有进行网络请求的request默认都会进行缓存存储,所以这里CacheDispatcher就不需要重复操作了.

之前介绍RequestQueue的时候,我们只介绍了不进行缓存的Request请求是如何被调度的,那这里我们继续看一下,默认情况下,Request都是需要进行缓存的,那缓存是如何调度的呢?

来看一下RequestQueue完整的add方法源码:

/** 将Request请求加入到调度队列中. */
public <T> Request<?> 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);
}

// 分配request唯一的序列号.
request.setSequence(getSequenceNumber());

// request不允许缓存,则直接将request加入到mNetworkQueue当中
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)) {
// 表示RequestQueue正在调度过该Request,因为后续相同的Request先入队列,排队等待执行.
Queue<Request<?>> stageRequests = mWaitingRequests.get(cacheKey);
if (stageRequests == null) {
stageRequests = new LinkedList<Request<?>>();
}
stageRequests.add(request);
mWaitingRequests.put(cacheKey, stageRequests);
} else {
// 将Request加入到等待Map中,表示Request正在执行.
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}

add方法之前也介绍过,这里要特殊强调一下mWaitingRequests的妙用.

在应用的网络请求过程中,有时可能由于多线程或者后台Service更新等机制,导致同一个Url的Request被同一时间多次请求.这时,RequestQueue通过mWaitingRequests这个Map很好的控制了这种情况.

通过mWaitingRequests,同一时间相同Url的Request只能有一个再执行.我想大家可能会有疑问(至少我看这部分代码时存在这个疑问):从代码逻辑中,可以看出,相同的Request被加入到Map该url对应的队列中,但是后续什么时候执行呢?add方法中并没有体现.

那既然同一时间相同url的Request只能有一个在执行,那mWaitingRequests中url对应队列的Request当然是在上一个Request执行完毕后才会执行.Request执行完毕后会调用自身的finish方法.

Request的finish调用时机肯定是ExecutorDelivery类将结果回调给用户接口时调用的,具体代码大家可以翻看之前的ExecutorDelivery类源码.

Request的finish方法源码如下:

/** 用于告知请求队列当前request已经结束. */
void finish(final String tag) {
if (mRequestQueue != null) {
mRequestQueue.finish(this);
}
}

可以看到,Request的finish方法其实是通知RequestQueue,调用RequestQueue的finish方法来结束自己.继续跟踪RequestQueue的finish方法:

/** 该方法的调用时机为:参数Request将请求结果回调给用户接口时,会调用该方法告知此Request已经结束. */
<T> void finish(Request<T> request) {
// 从正在执行的Request队列中删除指定的request.
synchronized (mCurrentRequests) {
mCurrentRequests.remove(request);
}

// 观察者模式,通知Observer该request请求结束.
synchronized (mFinishedListeners) {
for (RequestFinishedListener<T> listener : mFinishedListeners) {
listener.onRequestFinished(request);
}
}

if (request.shouldCache()) {
synchronized (mWaitingRequests) {
// 因为当前Request已经正常结束,而且该request是可以缓存的,所以这时需要直接把正在等待的所有相同
// url的Request全部加入到缓存队列中,从缓存系统读取结果后回调用户接口.
String cacheKey = request.getCacheKey();
Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
if (waitingRequests != null) {
mCacheQueue.addAll(waitingRequests);
}
}
}
}

相信上面的注释足够让大家理解mWaitingRequests的妙用了.

Volley框架概览

讲到这里,Volley的整体框架基本就算介绍完全了.相信坚持看到这里的同学,肯定对Volley框架也已经非常熟悉,这时候我们再来看一下Volley框架的整体架构,回顾一下之前所讲的知识:

问答

欢迎大家提出跟Volley架构相关的问题，我会挑选出某个问题进行具体解答.

为什么Volley适合频繁的网络请求，不适合文件上传等大数据请求呢？

答：Volley为什么适合频繁的网络请求，是因为:

1. Volley有四个并发的线程,并有一个阻塞队列来对并发线程进行调度.

2. Volley有自己的Disk缓存系统,相同url的Request再没过期前可以直接从Disk缓存系统中获取结果.

3. Volley的RequestQueue类有一个mWaitingRequest的Map,用来存储相同url的request,key为url,value为request队列。保证同一时间相同url的request只有一个再执行,后续Request再第一个request结束后可直接从缓存系统中获取结果.

为什么不适合文件上传,是因为文件上传这种操作都是唯一的，用不到缓存，而且4个线程的并发似乎也有点少.