Volley源码解读

Volley源码解读

Volley的中文翻译为“齐射、并发”，是在2013年的Google大会上发布的一款Android平台网络通信库，具有网络请求的处理、小图片的异步加载和缓存等功能，能够帮助 Android APP 更方便地执行网络操作，而且更快速高效。Volley可是说是把AsyncHttpClient和Universal-Image-Loader的优点集于了一身，既可以像AsyncHttpClient一样非常简单地进行HTTP通信，也可以像Universal-Image-Loader一样轻松加载网络上的图片。除了简单易用之外，Volley在性能方面也进行了大幅度的调整，它的设计目标就是非常适合去进行数据量不大，但通信频繁的网络操作，而对于大数据量的网络操作，比如说下载文件等，Volley的表现就会非常糟糕。

在Google IO的演讲上，其配图是一幅发射火弓箭的图，有点类似流星。这表示，Volley特别适合数据量不大但是通信频繁的场景。见下图：

目录

Volley特点

Volley执行流程

Volley初始化

创建RequestQueue

DiskBasedCache

CacheDispatcher & NetworkDispatcher

Request

加入RequestQueue

Request#finish自己

缓存处理

Request请求失败重试机制

PoolingByteArrayOutputStream & ByteArrayPool

Volley加载图片

Handler

volley gson改造

volley okhttp改造

Volley特点

自动调度网络请求；

Volley直接new 5个线程（默认5个），让多个线程去抢夺网络请求对象（Request），抢到就执行，抢不到就等待，直到有网络请求到来。

可以加载图片；

通过标准的 HTTP cache coherence（高速缓存一致性）缓存磁盘和内存透明的响应；

支持指定请求的优先级；

根据优先级去请求数据

网络请求cancel机制。我们可以取消单个请求，或者指定取消请求队列中的一个区域；

例如Activity finish后结束请求

框架容易被定制，例如，定制重试或者网络请求库；

例如基于Okhttp的Volley

回到目录

Volley执行流程

回到目录

Volley初始化

创建RequestQueue

使用Volley时我们需要先创建一个RequestQueue，如下

RequestQueue queue = Volley.newRequestQueue(context);

Volley.newRequestQueue(context)

最终调用了如下方法

public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);
...
if (stack == null) {
if (Build.VERSION.SDK_INT >= 9) {
stack = new HurlStack();
} else {
// Prior to Gingerbread, HttpUrlConnection was unreliable.
// See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
}
}

Network network = new BasicNetwork(stack);

RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
queue.start();

return queue;
}

HttpStack 是一个接口，主要用于请求网络数据，并返回结果。默认情况下stack是null，

当android版本>=9时使用HurlStack，否则使用HttpClientStack

若用户想要使用其他的网络请求类库，比如okhttp等就可以实现HttpStack接口，并在

HttpResponse performRequest(Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError

中调用okhttp进行网络请求，并把请求的结果封装成一个

HttpResponse返回即可，HttpResponse中包含了状态码，响应头，body信息。

newRequestQueue中创建了一个BasicNetwork对象，BasicNetwork使用HttpStack执行网络请求，成功后返回一个NetworkResponse,NetworkResponse只是一个简单的记录状态码，body，响应头，服务端是否返回304并且缓存过，执行网络请求时间的类。

回到目录

DiskBasedCache

newRequestQueue

中还创建了一个

RequestQueue

，

RequestQueue

中持有一个

DiskBasedCache

对象，

DiskBasedCache

是把服务端返回的数据保持到本地文件中的类，默认大小5M。

缓存文件是一个二进制文件，非文本文件，

缓存文件的开头有个特殊的整型魔数（

CACHE_MAGIC

），用于判断是不是缓存文件。

DiskBasedCache

初始化时会

读取特定文件夹下的所有文件，若文件开头魔数不是

CACHE_MAGIC

则删除。是的话就把读取的数据保存到内存中。代码如下

public synchronized void initialize() {
if (!mRootDirectory.exists()) {

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) {
if (file != null) {
file.delete();
}
} finally {
try {
if (fis != null) {
fis.close();
}
} catch (IOException ignored) { }
}
}
}

缓存文件除了可以保存 int，long 型数据，还可以保存 String 字符串，Map

static String readString(InputStream is) throws IOException {
int n = (int) readLong(is);
byte[] b = streamToBytes(is, n);
return new String(b, "UTF-8");
}
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;
}
static long readLong(InputStream is) throws IOException {
long n = 0;
n |= ((read(is) & 0xFFL) << 0);
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;
}

缓存文件名由cache key字符串的前半段字符串的hashCode拼接上cache key（网络请求url）后

半段字符串的hashCode值组成。代码如下

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;
}

当缓存文件占用空间超过指定值时，Volley只是简单的删除的部分文件，删除代码如下

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 * 0.9f) {
break;
}
}
}

可以看到删除时只是遍历mEntries，如果删除成功并且剩余文件所占大小+新的缓存所需空间

CacheDispatcherNetworkDispatcher

RequestQueue 只是一个普通的类，没有继承任何类，RequestQueue 的 start 方法中创建了一个 CacheDispatcher，和4个（默认4个）NetworkDispatcher，

CacheDispatcher 和 NetworkDispatcher 都是 Thread 的直接子类，这5个 Thread 就是前面提到的抢夺网络请求对象的 Thread。

调用start()时先调用了一下stop()，stop()中把5个线程的mQuit设为 true，然后调用interrupt()让 Thread 的run不再执行。

代码如下

public void start() {
stop();  // Make sure any currently running dispatchers are stopped.
// Create the cache dispatcher and start it.
mCacheDispatcher = new CacheDispatcher(mCacheQueue, mNetworkQueue, mCache, mDelivery);
mCacheDispatcher.start();

// Create network dispatchers (and corresponding threads) up to the pool size.
for (int i = 0; i < mDispatchers.length; i++) {
NetworkDispatcher networkDispatcher = new NetworkDispatcher(mNetworkQueue, mNetwork,
mCache, mDelivery);
mDispatchers[i] = networkDispatcher;
networkDispatcher.start();
}

mCache.initialize();
while (true) {
try {
// Get a request from the cache triage queue, blocking until
// at least one is available.
final Request<?> request = mCacheQueue.take();

...

}
}
}

/**
* Stops the cache and network dispatchers.
*/
public void stop() {
if (mCacheDispatcher != null) {
mCacheDispatcher.quit();
}
for (int i = 0; i < mDispatchers.length; i++) {
if (mDispatchers[i] != null) {
mDispatchers[i].quit();
}
}
}

CacheDispatcher启动后先调用了一下DiskBasedCache的initialize()方法，这个方法要读取文件，比较耗时，Volley把他放到了Cache线程中。

CacheDispatcher和NetworkDispatcher的run方法内部很像，都是在 while (true)循环中从PriorityBlockingQueue中读取Request，PriorityBlockingQueue是一个阻塞队列，当队列里没有Request时take()方法就会阻塞，直到有Request到来，PriorityBlockingQueue是线程安全的

同一个 Request 只能被5个线程中的一个获取到。PriorityBlockingQueue 可以根据线程优先级对队列里的reqest进行排序。

Volley 的初始化到这就完成了，下面开始执行网络请求

回到目录

Request

使用 Volley 进行网络请求时我们要把请求封装成一个 Request 对象，包括url，请求参数，请求成功失败 Listener，

Volley 默认给我们提供了 ImageRequest（获取图片），

JsonObjectRequest、JsonArrayRequest（获取json），StringRequest（获取 String）。

例如：

Request<String>request=new StringRequest(Method.GET, "http://mogujie.com", new  Listener<String>(){
@Override
public void onResponse(String response) {
}

}, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
}
});
Volley.newRequestQueue(context).add(request);

只需要把Request丢进requestQueue中就可以。

回到目录

加入RequestQueue

我们来看一下add方法：

public <T> Request<T> add(Request<T> request) {
// Tag the request as belonging to this queue and add it to the set of current requests.
request.setRequestQueue(this);
synchronized (mCurrentRequests) {
mCurrentRequests.add(request);
}

// Process requests in the order they are added.
request.setSequence(getSequenceNumber());
request.addMarker("add-to-queue");

// If the request is uncacheable, skip the cache queue and go straight to the network.
if (!request.shouldCache()) {
mNetworkQueue.add(request);
return request;
}

// Insert request into stage if there's already a request with the same cache key in flight.
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
if (mWaitingRequests.containsKey(cacheKey)) {
// There is already a request in flight. Queue up.
Queue<Request<?>> stagedRequests = mWaitingRequests.get(cacheKey);
if (stagedRequests == null) {
stagedRequests = new LinkedList<Request<?>>();
}
stagedRequests.add(request);
mWaitingRequests.put(cacheKey, stagedRequests);
if (VolleyLog.DEBUG) {
VolleyLog.v("Request for cacheKey=%s is in flight, putting on hold.", cacheKey);
}
} else {
// Insert 'null' queue for this cacheKey, indicating there is now a request in
// flight.
mWaitingRequests.put(cacheKey, null);
mCacheQueue.add(request);
}
return request;
}
}

add方法中首先设置了request所在的请求队列，为了在用户取消请求时能够把request从requestQueue中移除掉。

接下来设置了一下request的序列号，序列号按添加顺序依次从0开始编号，同一个队列中任何两个request的序列号都不相同。序列号可以影响request的优先级。

request.addMarker(“”)用于调试（打印日志等）。

通过查看源码我们可以看到request默认是需要缓存的。

/** Whether or not responses to this request should be cached. */
private boolean mShouldCache = true;

若request不需要缓存则直接把request丢到mNetworkQueue，然后4个 NetworkDispatcher 就可以”抢夺”request了，谁”抢”到谁就执行网络请求

如需要缓存则先判断一下mWaitingRequests中有没有正在请求的相同的request（根据request的url判断），没有的话就把该request丢到mCacheQueue中，

这样 CacheDispatcher 执行完之前的请求后就可以执行该request了，若已经有相同的request正在执行，则只需保存一下该request，

等相同的request执行完后直接使用其结果就可。

CacheDispatcher中获取到request后先判断一下request后有没有取消，有的话就finish掉自己，然后等待下一个request的到来

if (request.isCanceled()) {
request.finish("cache-discard-canceled");
continue;
}

接下来会从缓存中取缓存，没有或者缓存已经过期，就把request丢掉mNetworkQueue中，让mNetworkQueue去“抢夺”request

// Attempt to retrieve this item from cache.
Cache.Entry entry = mCache.get(request.getCacheKey());
if (entry == null) {
request.addMarker("cache-miss");
// Cache miss; send off to the network dispatcher.
mNetworkQueue.put(request);
continue;
}
// If it is completely expired, just send it to the network.
if (entry.isExpired()) {
request.addMarker("cache-hit-expired");
request.setCacheEntry(entry);
mNetworkQueue.put(request);
continue;
}

若取到缓存且没过期，则解析缓存。

“`java

Response<?> response = request.parseNetworkResponse(

new NetworkResponse(entry.data, entry.responseHeaders));

“`

若不需要刷新则把request和response丢到ui线程中，回调request中的请求成功或失败listener，同时finish自己

若还需要刷新的话还需要把request丢到mNetworkQueue中，让NetworkDispatcher去获取数据。

NetworkDispatcher在获取到数据后执行了如下代码：

// TODO: Only update cache metadata instead of entire record for 304s.
if (request.shouldCache() && response.cacheEntry != null) {
mCache.put(request.getCacheKey(), response.cacheEntry);
request.addMarker("network-cache-written");
}

CacheDispatcher 才是处理缓存相关的，为什么 NetworkDispatcher 中还需要进行以上的判断呢？

回到目录

Request#finish自己

前面我们提到过 CacheDispatcher 把相同的request放到了队列中，当获取到数据后调用了request的finish方法，该方法又调用了

mRequestQueue的finish方法。

void finish(final String tag) {
if (mRequestQueue != null) {
mRequestQueue.finish(this);
}
...
}

request的finish方法如下：

<T> void finish(Request<T> request) {

...
if (request.shouldCache()) {
synchronized (mWaitingRequests) {
String cacheKey = request.getCacheKey();
Queue<Request<?>> waitingRequests = mWaitingRequests.remove(cacheKey);
if (waitingRequests != null) {
if (VolleyLog.DEBUG) {
VolleyLog.v("Releasing %d waiting requests for cacheKey=%s.",
waitingRequests.size(), cacheKey);
}
// Process all queued up requests. They won't be considered as in flight, but
// that's not a problem as the cache has been primed by 'request'.
mCacheQueue.addAll(waitingRequests);
}
}
}
}
}

finish中取出了相同的request所在的队列，然后把请求丢到了mCacheQueue中，丢到mCacheQueue后就会导致 CacheDispatcher 去执行网络请求，

这时由于上次的请求已经缓存了，所以可以直接使用上传的数据了，到此为止request如何finish自己就介绍完了。

回到目录

缓存处理

HttpHeaderParser.parseCacheHeaders

处理字符串（分割等）得到响应头，并把响应头，body包装到Cache.Entry中返回。

回到目录

Request请求失败重试机制

Volley的请求重试机制是在Request中设置的，这样的好处是每一个Request都可以有自己的重试机制，代码如下

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

mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
}

Request#setRetryPolicy中只是记录了一下RetryPolicy

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

DefaultRetryPolicy实现了RetryPolicy接口，根据接口我们可以看到DefaultRetryPolicy可以提供当前超时时间，当前重试次数等

/**
* Retry policy for a request.
*/
public interface RetryPolicy {

/**
* Returns the current timeout (used for logging).
*/
public int getCurrentTimeout();

/**
* Returns the current retry count (used for logging).
*/
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.
*/
public void retry(VolleyError error) throws VolleyError;
}

BasicNetwork中performRequest中请求失败（SocketTimeoutException，ConnectTimeoutException等）时会再次请求一次（默认重试一次）

若还是失败就会抛出VolleyError异常

具体代码如下：

public NetworkResponse performRequest(Request<?> request) throws VolleyError {
...
while (true) {
...
try {
...
httpResponse = mHttpStack.performRequest(request, headers);
...
return new NetworkResponse(statusCode, responseContents, responseHeaders, false,
SystemClock.elapsedRealtime() - requestStart);
} catch (SocketTimeoutException e) {
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) {
...
}
}
}

attemptRetryOnException代码如下：

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

try {
retryPolicy.retry(exception);
} catch (VolleyError e) {
request.addMarker(
String.format("%s-timeout-giveup [timeout=%s]", logPrefix, oldTimeout));
throw e;
}
request.addMarker(String.format("%s-retry [timeout=%s]", logPrefix, oldTimeout));
}

request.getRetryPolicy()得到的是DefaultRetryPolicy类，DefaultRetryPolicy中retry方法

public void retry(VolleyError error) throws VolleyError {
mCurrentRetryCount++;
mCurrentTimeoutMs += (mCurrentTimeoutMs * mBackoffMultiplier);
if (!hasAttemptRemaining()) {
throw error;
}
}

//Returns true if this policy has attempts remaining, false otherwise.
protected boolean hasAttemptRemaining() {
return mCurrentRetryCount <= mMaxNumRetries;
}

request.getRetryPolicy() 得到的是 DefaultRetryPolicy 对象，request重试次数超过规定的次数时

attemptRetryOnException 就会抛出 VolleyError，从而导致 performRequest() 方法中 while

循环终止，同时继续向上抛异常。

回到目录

PoolingByteArrayOutputStream & ByteArrayPool

为了避免读取服务端数据时反复的内存申请，Volley提供了PoolingByteArrayOutputStream和ByteArrayPool。

我们先看一下PoolingByteArrayOutputStream的父类ByteArrayOutputStream

/**
* The byte array containing the bytes written.
*/
protected byte[] buf;

/**
* The number of bytes written.
*/
protected int count;

ByteArrayOutputStream中提供了两个protected 的byte[] buf 和 int count，buf用于write时保存数据，count记录buf已使用的大小，因为都是protected，所有在子类中可以对其进行修改。

我们来看一下ByteArrayOutputStream的write方法，可以看到write中调用了扩展buf大小的expand方法，再来看一下expand的具体实现

private void expand(int i) {
/* Can the buffer handle @i more bytes, if not expand it */
if (count + i <= buf.length) {
return;
}

byte[] newbuf = new byte[(count + i) * 2];
System.arraycopy(buf, 0, newbuf, 0, count);
buf = newbuf;
}

可以看到当已使用的空间+要写入的大小>buf大小时，直接new 了一个两倍大小的空间，并把原来的buf中的数据复制到了新的空间中，最后把新分配的空间赋值给了buf，原来的buf由于没有被任何对象持有，最终会被回收掉。PoolingByteArrayOutputStream就是在重写的expand对buf进行了处理。

@Override
public synchronized void write(byte[] buffer, int offset, int len) {
Arrays.checkOffsetAndCount(buffer.length, offset, len);
if (len == 0) {
return;
}
expand(len);
System.arraycopy(buffer, offset, buf, this.count, len);
this.count += len;
}

/**
* Writes the specified byte {@code oneByte} to the OutputStream. Only the
* low order byte of {@code oneByte} is written.
*
* @param oneByte
*            the byte to be written.
*/
@Override
public synchronized void write(int oneByte) {
if (count == buf.length) {
expand(1);
}
buf[count++] = (byte) oneByte;
}

接下来我们看一下PoolingByteArrayOutputStream是怎么复用内存空间的。

在执行网络请求的BasicNetwork我们看到new 了一个ByteArrayPool

/**
* @param httpStack HTTP stack to be used
*/
public BasicNetwork(HttpStack httpStack) {
// If a pool isn't passed in, then build a small default pool that will give us a lot of
// benefit and not use too much memory.
this(httpStack, new ByteArrayPool(DEFAULT_POOL_SIZE));
}

我们看一下ByteArrayPool的构造函数

/** The buffer pool, arranged both by last use and by buffer size */
private List<byte[]> mBuffersByLastUse = new LinkedList<byte[]>();
private List<byte[]> mBuffersBySize = new ArrayList<byte[]>(64);

/**
* @param sizeLimit the maximum size of the pool, in bytes
*/
public ByteArrayPool(int sizeLimit) {
mSizeLimit = sizeLimit;
}

可以看到ByteArrayPool只是记录了一下最大的内存池空间，默认是4096 bytes，并创建了两个保存byte[]数组的List。

我们从BasicNetwork中看到读取服务端数据时调用了entityToBytes方法

/** Reads the contents of HttpEntity into a byte[]. */
private byte[] entityToBytes(HttpEntity entity) throws IOException, ServerError {
PoolingByteArrayOutputStream bytes =
new PoolingByteArrayOutputStream(mPool, (int) entity.getContentLength());
byte[] buffer = null;
try {
InputStream in = entity.getContent();
if (in == null) {
throw new ServerError();
}
buffer = mPool.getBuf(1024);
int count;
while ((count = in.read(buffer)) != -1) {
bytes.write(buffer, 0, count);
}
return bytes.toByteArray();
} finally {
try {
// Close the InputStream and release the resources by "consuming the content".
entity.consumeContent();
} catch (IOException e) {
// This can happen if there was an exception above that left the entity in
// an invalid state.
VolleyLog.v("Error occured when calling consumingContent");
}
mPool.returnBuf(buffer);
bytes.close();
}
}

entityToBytes中又new 了一个PoolingByteArrayOutputStream，PoolingByteArrayOutputStream是继承自java.io.ByteArrayOutputStream的。我们看一下PoolingByteArrayOutputStream构造函数

/**
* Constructs a new {@code ByteArrayOutputStream} with a default size of {@code size} bytes. If
* more than {@code size} bytes are written to this instance, the underlying byte array will
* expand.
*
* @param size initial size for the underlying byte array. The value will be pinned to a default
*        minimum size.
*/
public PoolingByteArrayOutputStream(ByteArrayPool pool, int size) {
mPool = pool;
buf = mPool.getBuf(Math.max(size, DEFAULT_SIZE));
}

PoolingByteArrayOutputStream构造函数中调用了mPool.getBuf并赋值给了父类的buf，所以以后调用write时都是把数据写到了mPool.getBuf得到的byte[]数组中，也就是byte[]池中。getBuf代码如下：

/**
* Returns a buffer from the pool if one is available in the requested size, or allocates a new
* one if a pooled one is not available.
*
* @param len the minimum size, in bytes, of the requested buffer. The returned buffer may be
*        larger.
* @return a byte[] buffer is always returned.
*/
public synchronized byte[] getBuf(int len) {
for (int i = 0; i < mBuffersBySize.size(); i++) {
byte[] buf = mBuffersBySize.get(i);
if (buf.length >= len) {
mCurrentSize -= buf.length;
mBuffersBySize.remove(i);
mBuffersByLastUse.remove(buf);
return buf;
}
}
return new byte[len];
}

由于内存池是被多个NetworkDispatcher公用的，所以getBuf前加了synchronized，getBuf就是从byte[]池中找一个满足大小的空间返回，并从List移除掉，若没有足够大的则new一个。再来看一下PoolingByteArrayOutputStream的write方法

@Override
public synchronized void write(byte[] buffer, int offset, int len) {
expand(len);
super.write(buffer, offset, len);
}

@Override
public synchronized void write(int oneByte) {
expand(1);
super.write(oneByte);
}

可以看到write中调用了expand方法，这个方法不是ByteArrayOutputStream中的，而是PoolingByteArrayOutputStream重写的，现在看一下expand方法

/**
* Ensures there is enough space in the buffer for the given number of additional bytes.
*/
private void expand(int i) {
/* Can the buffer handle @i more bytes, if not expand it */
if (count + i <= buf.length) {
return;
}
byte[] newbuf = mPool.getBuf((count + i) * 2);
System.arraycopy(buf, 0, newbuf, 0, count);
mPool.returnBuf(buf);
buf = newbuf;
}

expand中没有调用父类的expand方法，其与父类expand方法的却别就是由每次的new byte[]变成了从byte[]池中获取。

在entityToBytes方法中我们看到用完之后又调用了mPool.returnBuf(buffer);把byte[]归还给了byte[]池，代码如下：

/**
* Returns a buffer to the pool, throwing away old buffers if the pool would exceed its allotted
* size.
*
* @param buf the buffer to return to the pool.
*/
public synchronized void returnBuf(byte[] buf) {
if (buf == null || buf.length > mSizeLimit) {
return;
}
mBuffersByLastUse.add(buf);
int pos = Collections.binarySearch(mBuffersBySize, buf, BUF_COMPARATOR);
if (pos < 0) {
pos = -pos - 1;
}
mBuffersBySize.add(pos, buf);
mCurrentSize += buf.length;
trim();
}

/**
* Removes buffers from the pool until it is under its size limit.
*/
private synchronized void trim() {
while (mCurrentSize > mSizeLimit) {
byte[] buf = mBuffersByLastUse.remove(0);
mBuffersBySize.remove(buf);
mCurrentSize -= buf.length;
}
}

回到目录

Volley加载图片

Volley除了可以获取json还可以加载图片，用法如下：

ImageRequest imageRequest = new ImageRequest(url,
new Response.Listener<Bitmap>() {
@Override
public void onResponse(Bitmap response) {
imageView.setImageBitmap(response);
}
}, 0, 0, Config.RGB_565, new Response.ErrorListener() {
@Override
public void onErrorResponse(VolleyError error) {
imageView.setImageResource(R.drawable.default_image);
}
});

ImageRequest的构造函数接收6个参数，第一个参数就是图片的URL地址。第二个参数是图片请求成功的回调，这里我们把返回的Bitmap参数设置到ImageView中。第三第四个参数分别用于指定允许图片最大的宽度和高度，如果指定的网络图片的宽度或高度大于这里的最大值，则会对图片进行压缩，指定成0的话就表示不管图片有多大，都不会进行压缩。第五个参数用于指定图片的颜色属性，Bitmap.Config下的几个常量都可以在这里使用，其中ARGB_8888可以展示最好的颜色属性，每个图片像素占据4个字节的大小，而RGB_565则表示每个图片像素占据2个字节大小。第六个参数是图片请求失败的回调，这里我们当请求失败时在ImageView中显示一张默认图片。

ImageRequest默认采用GET方式获取图片，采用ScaleType.CENTER_INSIDE方式缩放图片

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;
}

/**
* For API compatibility with the pre-ScaleType variant of the constructor. Equivalent to
* the normal constructor with {@code ScaleType.CENTER_INSIDE}.
*/
@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);
}

我们来看一下ImageRequest具体执行逻辑

@Override
protected Response<Bitmap> parseNetworkResponse(NetworkResponse response) {
// Serialize all decode on a global lock to reduce concurrent heap usage.
synchronized (sDecodeLock) {
try {
return doParse(response);
} catch (OutOfMemoryError e) {
VolleyLog.e("Caught OOM for %d byte image, url=%s", response.data.length, getUrl());
return Response.error(new ParseError(e));
}
}
}

/**
* The real guts of parseNetworkResponse. Broken out for readability.
*/
private Response<Bitmap> doParse(NetworkResponse response) {
byte[] data = response.data;
BitmapFactory.Options decodeOptions = new BitmapFactory.Options();
Bitmap bitmap = null;
if (mMaxWidth == 0 && mMaxHeight == 0) {
decodeOptions.inPreferredConfig = mDecodeConfig;
bitmap = BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
} else {
// If we have to resize this image, first get the natural bounds.
decodeOptions.inJustDecodeBounds = true;
BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
int actualWidth = decodeOptions.outWidth;
int actualHeight = decodeOptions.outHeight;

// Then compute the dimensions we would ideally like to decode to.
int desiredWidth = getResizedDimension(mMaxWidth, mMaxHeight,
actualWidth, actualHeight, mScaleType);
int desiredHeight = getResizedDimension(mMaxHeight, mMaxWidth,
actualHeight, actualWidth, mScaleType);

// Decode to the nearest power of two scaling factor.
decodeOptions.inJustDecodeBounds = false;
// TODO(ficus): Do we need this or is it okay since API 8 doesn't support it?
// decodeOptions.inPreferQualityOverSpeed = PREFER_QUALITY_OVER_SPEED;
decodeOptions.inSampleSize =
findBestSampleSize(actualWidth, actualHeight, desiredWidth, desiredHeight);
Bitmap tempBitmap =
BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);

// If necessary, scale down to the maximal acceptable size.
if (tempBitmap != null && (tempBitmap.getWidth() > desiredWidth ||
tempBitmap.getHeight() > desiredHeight)) {
bitmap = Bitmap.createScaledBitmap(tempBitmap,
desiredWidth, desiredHeight, true);
tempBitmap.recycle();
} else {
bitmap = tempBitmap;
}
}

if (bitmap == null) {
return Response.error(new ParseError(response));
} else {
return Response.success(bitmap, HttpHeaderParser.parseCacheHeaders(response));
}
}

doParse中对图片进行了缩放处理,可以看到当mMaxWidth == 0 && mMaxHeight == 0时，没有做过多的处理，这种情况适合于不打的图片，否则容易导致内存溢出。else中对图片进行了缩放处理，首先创建一个Options 对象，并设置

decodeOptions.inJustDecodeBounds = true

，然后使用

BitmapFactory.decodeFile(pathName, decodeOptions);

时就可以只获取图片的大小，而不需要把图片完全读到内存中，

BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);

可以直接从内存中获取图片的宽高，之后就可以使用

int actualWidth = decodeOptions.outWidth;

int actualHeight = decodeOptions.outHeight;

来获取图片宽高了。

获取到图片宽高后需要通过

findBestSampleSize

找到一个合适的缩放比例并赋值给

decodeOptions.inSampleSize

，缩放比例一定是2的n次幂。即使不是2的幂最终也会按2的n次幂处理

static int findBestSampleSize(
int actualWidth, int actualHeight, int desiredWidth, int desiredHeight) {
double wr = (double) actualWidth / desiredWidth;
double hr = (double) actualHeight / desiredHeight;
double ratio = Math.min(wr, hr);
float n = 1.0f;
while ((n * 2) <= ratio) {
n *= 2;
}

return (int) n;
}

java

/**

* If set to a value > 1, requests the decoder to subsample the original

* image, returning a smaller image to save memory. The sample size is

* the number of pixels in either dimension that correspond to a single

* pixel in the decoded bitmap. For example, inSampleSize == 4 returns

* an image that is 1/4 the width/height of the original, and 1/16 the

* number of pixels. Any value <= 1 is treated the same as 1. Note: the

* decoder uses a final value based on powers of 2, any other value will

* be rounded down to the nearest power of 2.

*/

public int inSampleSize;

获取到缩放比例后就可以通过一下代码获取到图片了

“`java

decodeOptions.inJustDecodeBounds = false;

decodeOptions.inSampleSize =

findBestSampleSize(actualWidth, actualHeight, desiredWidth, desiredHeight);

Bitmap tempBitmap =BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);

“`

回到目录

Handler

Volley进行网络请求是在非ui线程中进行的，回调是怎么跑到ui线程中执行的呢？

Volley在创建RequestQueue时new 了一个

ExecutorDelivery(new Handler(Looper.getMainLooper()))

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

以前使用handler时我们都是直接new Handler(),没有跟任何参数，但不跟参数的Handler默认使用的是该线程独有的Looper，默认情况下ui线程是有Looper的而其他线程是没有Looper的，在非UI线程中直接new Handler（）会出错，我们可以通过

Looper.getMainLooper()

得到ui线程的Looper，这样任何线程都可以使用ui线程的Looper了，而且可以在任何线程中创建Handler，并且使用handler发送消息时就会跑到ui线程执行了。

也就是说如果我们想在任何线程中都可以创建Hadler，并且handler发送的消息要在ui线程执行的话，就可以采用这种方式创建Handler

new Handler(Looper.getMainLooper());

回到目录

volley gson改造

在实际开发中我们会遇到对文字表情的混排处理，一种做法是服务端直接返回转意后的字符串（比如 ☺ 用 \:wx 代替），然后客户端每次都要在ui线程中解析字符串转换成Spanned，若是在ListView中，滚动就会非常卡顿。

我们可以自定义一个XJsonRequest<T>并继承自Request<T>，同时为XJsonRequest增加一个注册gson类型转换的方法，并把泛型参数中图文混排字段设置为Spanned，然后在

Response<String> parseNetworkResponse(NetworkResponse response)

中把图文混拍json转换成Spanned即可，由于

Response<String> parseNetworkResponse(NetworkResponse response)

是在非ui线程中执行的，所已不会导致ANR。

回到目录

volley okhttp改造

public class OkHttpStack implements HttpStack {
private final OkHttpClient mClient;

public OkHttpStack(OkHttpClient client) {
this.mClient = client;
}

@Override
public HttpResponse performRequest(com.android.volley.Request<?> request, Map<String, String> additionalHeaders)
throws IOException, AuthFailureError {
OkHttpClient client = mClient.clone();
int timeoutMs = request.getTimeoutMs();
client.setConnectTimeout(timeoutMs, TimeUnit.MILLISECONDS);
client.setReadTimeout(timeoutMs, TimeUnit.MILLISECONDS);
client.setWriteTimeout(timeoutMs, TimeUnit.MILLISECONDS);

com.squareup.okhttp.Request.Builder okHttpRequestBuilder = new com.squareup.okhttp.Request.Builder();
okHttpRequestBuilder.url(request.getUrl());

Map<String, String> headers = request.getHeaders();

for (final String name : headers.keySet()) {
if (name!=null&& headers.get(name)!=null) {
okHttpRequestBuilder.addHeader(name, headers.get(name));
}

}

for (final String name : additionalHeaders.keySet()) {
if (name!=null&& headers.get(name)!=null)
okHttpRequestBuilder.addHeader(name, additionalHeaders.get(name));
}

setConnectionParametersForRequest(okHttpRequestBuilder, request);

com.squareup.okhttp.Request okHttpRequest = okHttpRequestBuilder.build();
Response okHttpResponse = client.newCall(okHttpRequest).execute();

StatusLine responseStatus = new BasicStatusLine(parseProtocol(okHttpResponse.protocol()), okHttpResponse.code(),
okHttpResponse.message());

BasicHttpResponse response = new BasicHttpResponse(responseStatus);
response.setEntity(entityFromOkHttpResponse(okHttpResponse));

Headers responseHeaders = okHttpResponse.headers();

for (int i = 0, len = responseHeaders.size(); i < len; i++) {
final String name = responseHeaders.name(i), value = responseHeaders.value(i);

if (name != null) {
response.addHeader(new BasicHeader(name, value));
}
}

return response;
}

private static HttpEntity entityFromOkHttpResponse(Response r) throws IOException {
BasicHttpEntity entity = new BasicHttpEntity();
ResponseBody body = r.body();

entity.setContent(body.byteStream());
entity.setContentLength(body.contentLength());
entity.setContentEncoding(r.header("Content-Encoding"));

if (body.contentType() != null) {
entity.setContentType(body.contentType().type());
}
return entity;
}

private static void setConnectionParametersForRequest(com.squareup.okhttp.Request.Builder builder,
com.android.volley.Request<?> request) throws IOException, AuthFailureError {
switch (request.getMethod()) {
case com.android.volley.Request.Method.DEPRECATED_GET_OR_POST:
// Ensure backwards compatibility.
// Volley assumes a request with a null body is a GET.
byte[] postBody = request.getPostBody();

if (postBody != null) {
builder.post(RequestBody.create(MediaType.parse(request.getPostBodyContentType()), postBody));
}
break;

case com.android.volley.Request.Method.GET:
builder.get();
break;

case com.android.volley.Request.Method.DELETE:
builder.delete();
break;

case com.android.volley.Request.Method.POST:
builder.post(createRequestBody(request));
break;

case com.android.volley.Request.Method.PUT:
builder.put(createRequestBody(request));
break;

// case com.android.volley.Request.Method.HEAD:
// builder.head();
// break;
//
// case com.android.volley.Request.Method.OPTIONS:
// builder.method("OPTIONS", null);
// break;
//
// case com.android.volley.Request.Method.TRACE:
// builder.method("TRACE", null);
// break;
//
// case com.android.volley.Request.Method.PATCH:
// builder.patch(createRequestBody(request));
// break;

default:
throw new IllegalStateException("Unknown method type.");
}
}

private static ProtocolVersion parseProtocol(final Protocol p) {
switch (p) {
case HTTP_1_0:
return new ProtocolVersion("HTTP", 1, 0);
case HTTP_1_1:
return new ProtocolVersion("HTTP", 1, 1);
case SPDY_3:
return new ProtocolVersion("SPDY", 3, 1);
case HTTP_2:
return new ProtocolVersion("HTTP", 2, 0);
}

throw new IllegalAccessError("Unkwown protocol");
}

private static RequestBody createRequestBody(com.android.volley.Request<?> r) throws AuthFailureError {
final byte[] body = r.getBody();
if (body == null)
return null;

return RequestBody.create(MediaType.parse(r.getBodyContentType()), body);
}
}

END

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