概述
Volley是Google推出的一款比较轻巧的网络请求框架，并且可以对请求进行缓存，同时可以实时取消请求，设置请求优先级，内置了ImageRequest，JsonRequest，JsonObjectRequest，JsonArrayRequest，StringRequest等，并且还支持自定义Request，基本上能满足日常的开发，当让Volley原生并不支持文件上传，但是可以通过自定义Request来实现，Volley不仅仅支持网络请求，还支持图片加载，这个主要是通过内置的ImageRequest来实现，Volley的工作原理大致如下：
大致流程就是，当添加一个Request的时候，首先会被添加到CachaQueue中，
正文
工作流程
Volley的缓存跟常规的缓存不太一致，它并不是直接去取缓存，而是构造了一个缓存队列，存放Request，然后根据特有的key值去取缓存，如果缓存存在并且没有过期，请求也没有取消，那么就直接解析缓存数据，发送到主线程，不然就直接加入到网络请求队列，重新请求网络数据，Volley的源码比较多，下面主要是从Request，RequestQueue，Dispatcher,Cache,这几个类分析一下Volley的一些实现细节，毕竟大部分框架，原理都是一两句话都能说清楚，但是有很多细节让自己实现其实还是挺困难的。
Request
继承关系
Request是一个单独的类，实现了Comparable接口，主要是用来对请求进行排序，如果设置了请求的优先级，就会根据优先级来进行排序，如果没有优先级就会按照请求加入的顺序来排序。
成员变量
private final int mMethod;//请求类型，GET，POST
private final String mUrl;//请求的服务器地址
private final Object mLock = new Object();//用来给对象上锁
private Response.ErrorListener mErrorListener;//请求失败的监听
private Integer mSequence;//请求的序列号，按照请求的顺序依次递增
private RequestQueue mRequestQueue;//请求队列
private boolean mShouldCache = true;//是否需要缓存，默认开启缓存
private boolean mCanceled = false;//请求是否取消，默认为false
private boolean mResponseDelivered = false;//解析完的请求是否已经发送
private boolean mShouldRetryServerErrors = false;//遇到服务器异常是否需要重试
private RetryPolicy mRetryPolicy;//重试策略
private Cache.Entry mCacheEntry = null;//缓存的 对象，里面封装了很多跟缓存相关的信息
private Object mTag;//请求的tag
private NetworkRequestCompleteListener mRequestCompleteListener;//网络请求完成回调的结果
//Request的生命周期记录工具
private final MarkerLog mEventLog = MarkerLog.ENABLED ? new MarkerLog() : null;
Method
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;
}
由于方法的辨识度比较高，所以Volley没有采用枚举，而是采用了接口内定义变量，节省开销
Priority
public enum Priority {
LOW,
NORMAL,
HIGH,
IMMEDIATE
}
优先级更注重可读性，所以Volley采用了枚举
构造方法
@Deprecated
public Request(String url, Response.ErrorListener listener) {
this(Method.DEPRECATED_GET_OR_POST, url, listener);
}
public Request(int method, String url, Response.ErrorListener listener) {
mMethod = method;
mUrl = url;
mErrorListener = listener;
setRetryPolicy(new DefaultRetryPolicy());
mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}
传入method，url以及失败的ErrorListener
关键方法
cancel
取消请求
public void cancel() {
synchronized (mLock) {
mCanceled = true;//改变请求标志位
mErrorListener = null;//回调接口置空
}
}
compareTo
设置请求优先级
@Override
public int compareTo(Request<T> other) {
//获取请求优先级
Priority left = this.getPriority();
Priority right = other.getPriority();
//请求优先级默认为Normal
//1.先比较请求优先级，如果相等再比较请求加入的顺序
return left == right ?
this.mSequence - other.mSequence :
right.ordinal() - left.ordinal();
}
finish
通知RequestQueue,这个请求已经结束
void finish(final String tag) {
if (mRequestQueue != null) {
//通知队列移除当前请求
mRequestQueue.finish(this);
}
if (MarkerLog.ENABLED) {
final long threadId = Thread.currentThread().getId();
//判断当前线程是否为主线程，不是的话切换到主线程
if (Looper.myLooper() != Looper.getMainLooper()) {
//通过PostRunnable的方式，保证请求结束的打印时间是有序的
Handler mainThread = new Handler(Looper.getMainLooper());
mainThread.post(new Runnable() {
@Override
public void run() {
mEventLog.add(tag, threadId);
mEventLog.finish(Request.this.toString());
}
});
return;
}
mEventLog.add(tag, threadId);
mEventLog.finish(this.toString());
}
}
getCacheKey
默认作为请求的服务器地址作为key，实际开发过程中需要通过MD5会比较好一点
public String getCacheKey() {
return getUrl();
}
抽象方法
Rquest是一个抽象类，里面还有很多抽象犯法需要子类去实现
//解析网络请求
abstract protected Response<T> parseNetworkResponse(NetworkResponse response);
//传递网络请求结果
abstract protected void deliverResponse(T response);
//请求失败的回调，因为不同的Request需要的返回类型不一样，需要子类实现，但是请求失败确是共同的
//所以Volley做了一些封装
public void deliverError(VolleyError error) {
Response.ErrorListener listener;
synchronized (mLock) {
listener = mErrorListener;
}
if (listener != null) {
listener.onErrorResponse(error);
}
}
RequesQueue
RequesQueue实际上是所有队列的一个管理类，包含正在进行中的队列集合mCurrentRequests，缓存队列mCacheQueue，网络队列mNetworkQueue等
成员变量
//Request添加进去后的序列号
private final AtomicInteger mSequenceGenerator = new AtomicInteger();
//正在进行中的请求集合，采用HashSet实现
private final Set<Request<?>> mCurrentRequests = new HashSet<Request<?>>();
//请求的缓存队列，采用PriorityBlockingQueue实现，可以根据优先级来出队
private final PriorityBlockingQueue<Request<?>> mCacheQueue =
new PriorityBlockingQueue<>();
//请求的网络队列，采用PriorityBlockingQueue实现
private final PriorityBlockingQueue<Request<?>> mNetworkQueue =
new PriorityBlockingQueue<>();
//网络请求分发器的数量，默认为4个
private static final int DEFAULT_NETWORK_THREAD_POOL_SIZE = 4;
//数据的缓存
private final Cache mCache;
//网络请求的实际执行者
private final Network mNetwork;
//网络请求返回结果的分发者
private final ResponseDelivery mDelivery;
//网络请求分发器数组
private final NetworkDispatcher[] mDispatchers;
//缓存分发器线程
private CacheDispatcher mCacheDispatcher;
//网络请求完成的监听器集合
private final List<RequestFinishedListener> mFinishedListeners =new ArrayList<>();
构造方法
public RequestQueue(Cache cache, Network network, int threadPoolSize) {
this(cache, network, threadPoolSize,
new ExecutorDelivery(new Handler(Looper.getMainLooper())));
}
public RequestQueue(Cache cache, Network network) {
this(cache, network, DEFAULT_NETWORK_THREAD_POOL_SIZE);
}
//其它的构造方法最终还是间接调用了这个方法
public RequestQueue(Cache cache, Network network, int threadPoolSize,
ResponseDelivery delivery) {
mCache = cache;
mNetwork = network;
mDispatchers = new NetworkDispatcher[threadPoolSize];
mDelivery = delivery;
}
通过成员变量的注释，比较清晰，就是默认初始化了一些变量
核心方法
start
public void start() {
stop(); //终止正在进行的分发器，包括缓存的分发器以及网络分发器
// 创建缓存分发器
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
//启动缓存分发器
mCacheDispatcher.start();
// 根据定义的Dispatcher数组，创建网络分发器
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
//启动网络分发器
networkDispatcher.start();
}
}
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (final NetworkDispatcher mDispatcher : mDispatchers) {
if (mDispatcher != null) {
mDispatcher.quit();
}
}
}
add
public <T> Request<T> add(Request<T> request) {
//将RequestQueue赋值给Request
request.setRequestQueue(this);
//同步添加到正在进行中的请求集合中去
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}
//给请求设置序列号
request.setSequence(getSequenceNumber());
//添加Marker标记位
request.addMarker("add-to-queue");
if (!request.shouldCache()) {
//如果请求队列不需要缓存，那么直接加入到网络对垒中
mNetworkQueue.add(request);
return request;
}
//添加进缓存队列
mCacheQueue.add(request);
return request;
}
cancel
public void cancelAll(final Object tag) {
if (tag == null) {
throw new IllegalArgumentException("Cannot cancelAll with a null tag");
}
cancelAll(new RequestFilter() {
@Override
public boolean apply(Request<?> request) {
//通过tag来匹配需要取消的请求
return request.getTag() == tag;
}
});
}
//通过RequestFilter来过滤需要取消的请求
public void cancelAll(RequestFilter filter) {
synchronized (mCurrentRequests) {
for (Request<?> request : mCurrentRequests) {
if (filter.apply(request)) {
request.cancel();
}
}
}
}
finish
<T> void finish(Request<T> request) {
// 从正在进行的请求中移除
synchronized (mCurrentRequests) {
mCurrentRequests.remove(request);
}
synchronized (mFinishedListeners) {
//移除回调接口
for (RequestFinishedListener<T> listener : mFinishedListeners) {
listener.onRequestFinished(request);
}
}
}
Dispatcher
Volley提供了两个分发器，一个是CacheDispatcher，一个是NetworkDispatcher，实际上就是两个线程，然后进行了死循环，不断地从缓存队列跟网络队列中进行取Request来进行分发。
CacheDispatcher
继承关系
成员变量
//Debug模式的标志
private static final boolean DEBUG = VolleyLog.DEBUG;
//缓存队列，采用BlockingQueue实现生产者消费者模式
private final BlockingQueue<Request<?>> mCacheQueue;
//网络队列，采用BlockingQueue实现生产者消费者模式
private final BlockingQueue<Request<?>> mNetworkQueue;
//缓存类
private final Cache mCache;
//网络请求结果分发类
private final ResponseDelivery mDelivery;
//CacheDispatcher是否退出的标志
private volatile boolean mQuit = false;
//等待管理队列管理器
private final WaitingRequestManager mWaitingRequestManager;
构造方法
public CacheDispatcher(
BlockingQueue<Request<?>> cacheQueue, BlockingQueue<Request<?>> networkQueue,
Cache cache, ResponseDelivery delivery) {
mCacheQueue = cacheQueue;
mNetworkQueue = networkQueue;
mCache = cache;
mDelivery = delivery;
mWaitingRequestManager = new WaitingRequestManager(this);
}
CacheDispatcher持有cacheQueue，networkQueue，cache，delivery这几个类
run
@Override
public void run() {
if (DEBUG) VolleyLog.v("start new dispatcher");
//设置线程优先级
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//缓存初始化,待会在缓存中具体分析
mCache.initialize();
while (true) {
try {
//死循环
processRequest();
} catch (InterruptedException e) {
// We may have been interrupted because it was time to quit.
if (mQuit) {
return;
}
}
}
}
processRequest
private void processRequest() throws InterruptedException {
//从缓存队列中取队列
final Request<?> request = mCacheQueue.take();
//给取出的Requet打上标记
request.addMarker("cache-queue-take");
//如果请求已取消，结束掉这个请求
if (request.isCanceled()) {
request.finish("cache-discard-canceled");
return;
}
//拿到缓存的entry
Cache.Entry entry = mCache.get(request.getCacheKey());
//缓存数据为空，就将Request添加进mNetworkQueue
if (entry == null) {
request.addMarker("cache-miss");
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
// 缓存过期，直接加入到网络队列
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
if (!mWaitingRequestManager.maybeAddToWaitingRequests(request)) {
mNetworkQueue.put(request);
}
return;
}
//缓存有效，直接解析发送给主线程
request.addMarker("cache-hit");
Response<?> response = request.parseNetworkResponse(
new NetworkResponse(entry.data, entry.responseHeaders));
request.addMarker("cache-hit-parsed");
}
quit
退出线程
public void quit() {
mQuit = true;
//中断线程
interrupt();
}
NetworkDispatcher
继承关系
成员变量
//网络请求队列
private final BlockingQueue<Request<?>> mQueue;
//网络请求的实际操作类
private final Network mNetwork;
//缓存类
private final Cache mCache;
//请求响应的结果发送者
private final ResponseDelivery mDelivery;
//线程是否发送的标志
private volatile boolean mQuit = false;
构造方法
public NetworkDispatcher(BlockingQueue<Request<?>> queue,
Network network, Cache cache, ResponseDelivery delivery) {
mQueue = queue;
mNetwork = network;
mCache = cache;
mDelivery = delivery;
}
对比CacheDispatcher，发现少了缓存队列，不过也很好理解，因为既然都到了网络这边了，说明缓存肯定GG了，所以只需要在获取到网络请求结果之后，放入缓存中就行了。
run
run方法其实跟CacheDispatcher是一样的，只是processRequest有些区别
private void processRequest() throws InterruptedException {
long startTimeMs = SystemClock.elapsedRealtime();
//从队列中取出一个队列
Request<?> request = mQueue.take();
try {
request.addMarker("network-queue-take");
//请求取消，直接finished
if (request.isCanceled()) {
request.finish("network-discard-cancelled");
request.notifyListenerResponseNotUsable();
return;
}
addTrafficStatsTag(request);
// 进行网络请求
NetworkResponse networkResponse = mNetwork.performRequest(request);
request.addMarker("network-http-complete");
// If the server returned 304 AND we delivered a response already,
// we're done -- don't deliver a second identical response
if (networkResponse.notModified && request.hasHadResponseDelivered()) {
request.finish("not-modified");
request.notifyListenerResponseNotUsable();
return;
}
// 解析网络请求数据
Response<?> response = request.parseNetworkResponse(networkResponse);
request.addMarker("network-parse-complete");
//如果请求结果需要缓存，那么缓存请求的结果
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}
// 将解析好的数据发送给主线程
request.markDelivered();
mDelivery.postResponse(request, response);
request.notifyListenerResponseReceived(response);
} catch (VolleyError volleyError) {
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
parseAndDeliverNetworkError(request, volleyError);
request.notifyListenerResponseNotUsable();
} catch (Exception e) {
VolleyLog.e(e, "Unhandled exception %s", e.toString());
VolleyError volleyError = new VolleyError(e);
volleyError.setNetworkTimeMs(SystemClock.elapsedRealtime() - startTimeMs);
mDelivery.postError(request, volleyError);
request.notifyListenerResponseNotUsable();
}
}
quit
public void quit() {
mQuit = true;
//中断线程
interrupt();
}
Cache
Volley的缓存主要是磁盘缓存，首先Volley提供了一个Cache接口，然后DiskBasedCache实现了这个接口，下面说一下这两个类
Cache
public interface Cache {
Entry get(String key);
void put(String key, Entry entry);
void initialize();
void invalidate(String key, boolean fullExpire);
void remove(String key);
void clear();
class Entry {
public byte[] data;
public String etag;
public long serverDate;
public long lastModified;
public long ttl;
public long softTtl;
public Map<String, String> responseHeaders = Collections.emptyMap();
public List<Header> allResponseHeaders;
public boolean isExpired() {
return this.ttl < System.currentTimeMillis();
}
public boolean refreshNeeded() {
return this.softTtl < System.currentTimeMillis();
}
}
很常规的接口，只不过缓存的value不是请求的结果，而是封装了请求的数据的一个Entry，可以对缓存做一些判断。
DiskBaseCache
成员变量
//当前缓存的容量
private long mTotalSize = 0;
//缓存的路径
private final File mRootDirectory;
//分配的最大缓存容量
private final int mMaxCacheSizeInBytes;
//默认的最大缓存容量
private static final int DEFAULT_DISK_USAGE_BYTES = 5 * 1024 * 1024;
//缓存的负载因子，到达这个点之后会自动进行缓存清理
private static final float HYSTERESIS_FACTOR = 0.9f;
//底层采用LinkedHashMap实现Lru算法，按照使用的顺序进行排序
private final Map<String, CacheHeader> mEntries =
new LinkedHashMap<String, CacheHeader>(16, .75f, true);
构造方法
public DiskBasedCache(File rootDirectory, int maxCacheSizeInBytes) {
mRootDirectory = rootDirectory;
mMaxCacheSizeInBytes = maxCacheSizeInBytes;
}
public DiskBasedCache(File rootDirectory) {
this(rootDirectory, DEFAULT_DISK_USAGE_BYTES);
}
通过缓存的大小跟路径初始化DiskBasedCache
put
*/
@Override
public synchronized void put(String key, Entry entry) {
//检查容量是否合理，不合理就进行删除
pruneIfNeeded(entry.data.length);
//获取缓存的文件
File file = getFileForKey(key);
try {
BufferedOutputStream fos = new BufferedOutputStream(createOutputStream(file));
CacheHeader e = new CacheHeader(key, entry);
boolean success = e.writeHeader(fos);
if (!success) {
fos.close();
VolleyLog.d("Failed to write header for %s", file.getAbsolutePath());
throw new IOException();
}
fos.write(entry.data);
fos.close();
//缓存数据
putEntry(key, e);
return;
} catch (IOException e) {
}
boolean deleted = file.delete();
if (!deleted) {
VolleyLog.d("Could not clean up file %s", file.getAbsolutePath());
}
}
pruneIfNeed
private void pruneIfNeeded(int neededSpace) {
//如果现有容量+即将存储的容量小于最大容量，返回
if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes) {
return;
}
long before = mTotalSize;
int prunedFiles = 0;
long startTime = SystemClock.elapsedRealtime();
Iterator<Map.Entry<String, CacheHeader>> iterator = mEntries.entrySet().iterator();
//遍历LinkedHashMap,删除链表头部的数据
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;
} else {
VolleyLog.d("Could not delete cache entry for key=%s, filename=%s",
e.key, getFilenameForKey(e.key));
}
iterator.remove();
prunedFiles++;
if ((mTotalSize + neededSpace) < mMaxCacheSizeInBytes * HYSTERESIS_FACTOR) {
break;
}
}
}
get
@Override
public synchronized Entry get(String key) {
//通过key获取缓存的entry
CacheHeader entry = mEntries.get(key);
//如果entry为null的话直接返回
if (entry == null) {
return null;
}
//通过key获取到file文件
File file = getFileForKey(key);
try {
CountingInputStream cis = new CountingInputStream(
new BufferedInputStream(createInputStream(file)), file.length());
try {
CacheHeader entryOnDisk = CacheHeader.readHeader(cis);
if (!TextUtils.equals(key, entryOnDisk.key)) {
// File was shared by two keys and now holds data for a different entry!
VolleyLog.d("%s: key=%s, found=%s",
file.getAbsolutePath(), key, entryOnDisk.key);
// Remove key whose contents on disk have been replaced.
removeEntry(key);
return null;
}
byte[] data = streamToBytes(cis, cis.bytesRemaining());
//将解析好的数据返回
return entry.toCacheEntry(data);
} finally {
// Any IOException thrown here is handled by the below catch block by design.
//noinspection ThrowFromFinallyBlock
cis.close();
}
} catch (IOException e) {
VolleyLog.d("%s: %s", file.getAbsolutePath(), e.toString());
remove(key);
return null;
}
}
RetryPolicy
成员变量
private int mCurrentTimeoutMs;//超时时间
private int mCurrentRetryCount;//已重试次数
private final int mMaxNumRetries;//最大重试次数
private final float mBackoffMultiplier;//失败后重连的间隔因子
public static final int DEFAULT_TIMEOUT_MS = 2500;//默认超时时间
public static final int DEFAULT_MAX_RETRIES = 1;//默认重试次数
public static final float DEFAULT_BACKOFF_MULT = 1f;//默认的失败之后重连的间隔因子为1
构造方法
public DefaultRetryPolicy() {
this(DEFAULT_TIMEOUT_MS, DEFAULT_MAX_RETRIES, DEFAULT_BACKOFF_MULT);
}
public DefaultRetryPolicy(int initialTimeoutMs, int maxNumRetries, float backoffMultiplier) {
mCurrentTimeoutMs = initialTimeoutMs;
mMaxNumRetries = maxNumRetries;
mBackoffMultiplier = backoffMultiplier;
}
传入超时时间，最大重试次数，重试间隔
retry
@Override
public void retry(VolleyError error) throws VolleyError {
mCurrentRetryCount++;
//计算重试时间
mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier);
if (!hasAttemptRemaining()) {
//如果到达最大次数，还是失败就抛异常
throw error;
}
}
Image
Volley不仅支持网络请求，还可以用来加载图片，主要相关的两个核心类是ImageLoader跟ImageRequest
ImageLoader
成员变量
private final RequestQueue mRequestQueue;//请求队列
private int mBatchResponseDelayMs = 100;//请求响应结果发送延时
private final ImageCache mCache;//图片缓存
//用HashMap来保存延时的请求
private final HashMap<String, BatchedImageRequest> mInFlightRequests =
new HashMap<String, BatchedImageRequest>();
//HashMap来保存延时的请求响应结果
private final HashMap<String, BatchedImageRequest> mBatchedResponses =
new HashMap<String, BatchedImageRequest>();
//切换线程的Handler
private final Handler mHandler = new Handler(Looper.getMainLooper());
构造方法
public ImageLoader(RequestQueue queue, ImageCache imageCache) {
mRequestQueue = queue;
mCache = imageCache;
}
get
public ImageContainer get(String requestUrl, final ImageListener listener) {
return get(requestUrl, listener, 0, 0);
}
间接调用
public ImageContainer get(String requestUrl, ImageListener imageListener,
int maxWidth, int maxHeight) {
return get(requestUrl, imageListener, maxWidth, maxHeight, ScaleType.CENTER_INSIDE);
}
继续调用
public ImageContainer get(String requestUrl, ImageListener imageListener,
int maxWidth, int maxHeight, ScaleType scaleType) {
// 检测是否在主线程
throwIfNotOnMainThread();
//通过转换得到缓存的key
final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight, scaleType);
// 从缓存中查找对应的bitmap
Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
if (cachedBitmap != null) {
//找到直接返回
ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
imageListener.onResponse(container, true);
return container;
}
// 缓存失败，初始化ImageContainer
ImageContainer imageContainer =
new ImageContainer(null, requestUrl, cacheKey, imageListener);
//回调
imageListener.onResponse(imageContainer, true);
// 判断当前的请求是否在mBatchedResponses中
BatchedImageRequest request = mInFlightRequests.get(cacheKey);
if (request != null) {
// If it is, add this request to the list of listeners.
request.addContainer(imageContainer);
return imageContainer;
}
// 传达
Request<Bitmap> newRequest = makeImageRequest(requestUrl, maxWidth, maxHeight, scaleType,cacheKey);
mRequestQueue.add(newRequest);
mInFlightRequests.put(cacheKey, new BatchedImageRequest(newRequest, imageContainer));
return imageContainer;
}
get方法返回的是一个ImageContainer，里面包好了很多跟Image相关的信息，类似Cache,mCacheKey，mRequestUrl，mListener。
ImageRequest
继承关系
ImageRequest继承自Request，然后定义的泛型是Bitmap
成员变量
//超时时间
public static final int DEFAULT_IMAGE_TIMEOUT_MS = 1000;
//默认的重试次数
public static final int DEFAULT_IMAGE_MAX_RETRIES = 2;
//默认重试延迟因子
public static final float DEFAULT_IMAGE_BACKOFF_MULT = 2f;
private final Object mLock = new Object();//全局对象锁
private Response.Listener<Bitmap> mListener;//回调监听
private final Config mDecodeConfig;//解码的配置信息
private final int mMaxWidth;//ImageView传入的最大宽度
private final int mMaxHeight;//ImageView传入的最大高度
private final ScaleType mScaleType;//缩放类型
private static final Object sDecodeLock = new Object();//解码的同步锁
构造方法
@Deprecated
public ImageRequest(String url, Response.Listener<Bitmap> listener, int maxWidth, int maxHeight,
Config decodeConfig, Response.ErrorListener errorListener) {
this(url, listener, maxWidth, maxHeight,
ScaleType.CENTER_INSIDE, decodeConfig, errorListener);
}
public ImageRequest(String url, Response.Listener<Bitmap> listener, int maxWidth, int maxHeight,
ScaleType scaleType, Config decodeConfig, Response.ErrorListener errorListener) {
super(Method.GET, url, errorListener);
setRetryPolicy(new DefaultRetryPolicy(DEFAULT_IMAGE_TIMEOUT_MS, DEFAULT_IMAGE_MAX_RETRIES,
DEFAULT_IMAGE_BACKOFF_MULT));
mListener = listener;
mDecodeConfig = decodeConfig;
mMaxWidth = maxWidth;
mMaxHeight = maxHeight;
mScaleType = scaleType;
}
构造方法里面都是一些配置信息，没什么好说的
cancel
@Override
public void cancel() {
super.cancel();
synchronized (mLock) {
mListener = null;
}
}
跟前面的一个套路，不解释
doParse
网络请求回来之后，经过传递最终到了doParse方法
private Response<Bitmap> doParse(NetworkResponse response) {
//拿到字节数组
byte[] data = response.data;
BitmapFactory.Options decodeOptions = new BitmapFactory.Options();
Bitmap bitmap = null;
if (mMaxWidth == 0 && mMaxHeight == 0) {
//传入的宽高都为0，不缩放，直接返回原始尺寸
decodeOptions.inPreferredConfig = mDecodeConfig;
bitmap = BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
} else {
// If we have to resize this image, first get the natural bounds.
//先不加载进内存
decodeOptions.inJustDecodeBounds = true;
BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
//获取实际宽高
int actualWidth = decodeOptions.outWidth;
int actualHeight = decodeOptions.outHeight;
// 进行比例缩放，获取时间宽高
int desiredWidth = getResizedDimension(mMaxWidth, mMaxHeight,
actualWidth, actualHeight, mScaleType);
int desiredHeight = getResizedDimension(mMaxHeight, mMaxWidth,
actualHeight, actualWidth, mScaleType);
// 进行缩放
decodeOptions.inJustDecodeBounds = false;
decodeOptions.inSampleSize =
findBestSampleSize(actualWidth, actualHeight, desiredWidth, desiredHeight);
Bitmap tempBitmap =
BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
// 如果有必要的话，把得到的bitmap的最大边进行压缩来适应尺寸
if (tempBitmap != null && (tempBitmap.getWidth() > desiredWidth ||
tempBitmap.getHeight() > desiredHeight)) {
bitmap = Bitmap.createScaledBitmap(tempBitmap,
desiredWidth, desiredHeight, true);
tempBitmap.recycle();
} else {
bitmap = tempBitmap;
}
}
if (bitmap == null) {
//解析失败回调
return Response.error(new ParseError(response));
} else {
//解析成功回调
return Response.success(bitmap, HttpHeaderParser.parseCacheHeaders(response));
}
}
Volley加载图片的大致流程就到了这里，可能会有些奇怪，Volley并没有采用Lrucache在内存中进行缓存，是因为ImageRequest继承自Request，所以就依赖于缓存队列，只有File的缓存，可能这也是为什么提到图片加载大家可能会想到很多的Fresco，Glide,Picasso，但是很少人会想到Volley，提到Volley想到的还是网络请求，没有LRUCache应该是最主要的原因了。
总结
Volley是一款扩展性很强的框架，抽取了Request基类，用户可以自定义任意的Request，底层并没有使用线程池，而是采用了四个网络线程从RequestQueue中取数据，如果是数据量较小的网络请求，使用起来比较灵活，如果网络请求比较耗时，那么Volley的四个线程可能就不够用了，我们可以创建更多的线程，但是线程的开销会很高，而且对线程的利用率不大，这个时候就需要使用线程池了。Volley提供图片加载的功能，但是没有实现内存缓存，所以性能不是很高。Volley原生没有提供图片上传功能，不过由于他的扩展性很好，所以我们可以自己继承Request类来实现这个功能。
