导语：最近项目中使用AsyncTask遇到一个坑，特此机会随便看了一下Asynctask的源码。
项目中之前对页面数据做本地缓存，读写缓存时是用了AsyncTask做了异步操作：
public void getCache(String key, final Class<Rsp> type, final OnCacheListener<Rsp> onCacheListener) {
if (!TextUtils.isEmpty(key)) {
new AsyncTask<String, Void, Rsp>() {
@Override
protected Rsp doInBackground(String... strings) {
//读缓存...
return result;
}
@Override
protected void onPostExecute(Rsp result) {
super.onPostExecute(result);
if (result != null && onCacheListener != null) {
onCacheListener.onCache(result);
}
}
}.execute(key);
}
}
可是最近发现首页加载比较慢，跟踪发现首页业务层读缓存有将近500ms的耗时，和之前测得的的40ms左右相差了有10倍多，明显不合理。打印日志一看，发现读缓存时AsyncTask.doInBackground()方法要要等待很久才会执行。这时我才想起，AsyncTask.execute()方法虽然是4个线程池的，但是其内部却实现的按调用execute()顺序依次回调onPostExecute()（其实这理解有偏差，下面会有说明）。首页刚启动时，由于请求的接口（接口请求底层也是用AsyncTask实现异步操作）比较多，而读缓存的AsyncTask调用比较晚，自然会被其他地方调用的AsyncTask阻塞了。
这个问题比较好解决，读缓存时改成调用AsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, key)方法，或改用Handler方式实现异步操作就能解决。虽然问题解决了，但AsyncTask这么基础的东西，自己使用时还是遇到了坑（与其说是坑，不如说是自己了解得不够透彻，使用不合理），这样让我不得不翻看AsyncTask的源码，一看究竟。
源码分析(基于Android SDK 28)
AsyncTask最基础的使用方式是new AsyncTask().execute(),我们先看看execute()方法。
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
if (mStatus != Status.PENDING) {
switch (mStatus) {
case RUNNING:
throw new IllegalStateException("Cannot execute task:"
+ " the task is already running.");
case FINISHED:
throw new IllegalStateException("Cannot execute task:"
+ " the task has already been executed "
+ "(a task can be executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();
mWorker.mParams = params;
exec.execute(mFuture);
return this;
}
可以看到execute()内部也是调用executeOnExecutor()方法，传递一个默认的sDefaultExecutor。executeOnExecutor()方法中首先做了mStatus状态的判断，也就是为什么一个AsyncTask实例多次调用execute()方法会报IllegalStateException。接着是调用我们熟悉的onPreExecute()方法，这个方法运行在主线程，一般用于执行doInBackground()前在这个方法里做一些UI处理，比如show一个Dialog。最后会将params赋值给mWorker.mParams，调动exec.execute(mFuture)。
那mWorker和mFuture到底是什么呢？我们先看看它们是在哪里初始化的？
可以看到mWorker和mFuture都是在AsyncTask的构造函数中初始化的：
public AsyncTask() {
this((Looper) null);
}
public AsyncTask(@Nullable Looper callbackLooper) {
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
try {
postResultIfNotInvoked(get());
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occurred while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
postResultIfNotInvoked(null);
}
}
};
}
private static Handler getMainHandler() {
synchronized (AsyncTask.class) {
if (sHandler == null) {
sHandler = new InternalHandler(Looper.getMainLooper());
}
return sHandler;
}
}
AsyncTask构造函数，首先调用getMainHandler()初始化了一个Handler(InternalHandler)成员变量。mWorker的call()方法中，我们看到我们熟悉doInBackground()，调用doInBackground得到结果result，最后在finally中调用并调用postResult()，我们看看postResult()代码：
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
private Handler getHandler() {
return mHandler;
}
private static class InternalHandler extends Handler {
public InternalHandler(Looper looper) {
super(looper);
}
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult<?> result = (AsyncTaskResult<?>) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
}
}
}
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
我们可以看到postResult()中通过mHandler把Result发送到InternalHandler的handleMessage()，然后调用finish()方法。在finish()方法中，如果没有isCancelled()取消，则回调到我们熟悉的onPostExecute()中，并将状态置为FINISHED。
因此，可以看到其实AsyncTask内部线程通信也是用Handler实现的。
接着看mFuture，mFuture是FutureTask实例，实现了Runnable接口。用mWorker作为参数实现了初始化（这点很关键，后面会提到）
public class FutureTask<V> implements RunnableFuture<V> {
}
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
找到了调用doInBackground()的代码，那doInBackground()是怎么运行在子线程中的呢？带着疑惑，我接着exec.execute(mFuture)内部实现。
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
我们调用new AsyncTask().execute()时，其实是调用executeOnExecutor()，传递一个默认的sDefaultExecutor。而sDefaultExecutor其实是SerialExecutor对象。可以看到SerialExecutor内部维持了一个任务队列，在execute()方法中首先new一个Runnable放入mTasks队尾，然后判断当前mActive是否为空，如果不为空在调用scheduleNext()方法。在scheduleNext()方法中首先取出任务队列mTasks中的队首任务，如果不为null则赋值给mActive对象并传入THREAD_POOL_EXECUTOR进行执行。
THREAD_POOL_EXECUTOR我们也很熟悉，就是我刚开始提到了4个线程池。这里可以看到execute()其实是和THREAD_POOL_EXECUTOR共用一个线程池的。
接着我们看看THREAD_POOL_EXECUTOR的实现：
public static final Executor THREAD_POOL_EXECUTOR;
static {
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
sPoolWorkQueue, sThreadFactory);
threadPoolExecutor.allowCoreThreadTimeOut(true);
THREAD_POOL_EXECUTOR = threadPoolExecutor;
}
private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
// We want at least 2 threads and at most 4 threads in the core pool,
// preferring to have 1 less than the CPU count to avoid saturating
// the CPU with background work
private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
private static final int KEEP_ALIVE_SECONDS = 30;
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
private static final BlockingQueue<Runnable> sPoolWorkQueue =
new LinkedBlockingQueue<Runnable>(128);
可以看到THREAD_POOL_EXECUTOR其实就是一个线程池。数量跟CPU的核数有关，线程数最少2个，最多4个，线程存活时间为30秒。这里纠正了刚开始提到的4个线程池，其实线程数量跟CPU核数有关。
我们接着看看THREAD_POOL_EXECUTOR.execute()内部实现(代码量有点多，做了省略)：
public void execute(Runnable command) {
//省略...
addWorker(command, true)
//省略...
}
private boolean addWorker(Runnable firstTask, boolean core) {
//省略...
w = new Worker(firstTask);
//省略...
return workerStarted;
}
Worker(Runnable firstTask) {
setState(-1);
this.firstTask = firstTask;
this.thread = getThreadFactory().newThread(this);
}
可以看到，THREAD_POOL_EXECUTOR.execute()内部会调用sThreadFactory.newThread()创建新线程运行mTask任务队列里的Runnable，也就是运行了mFuture.run()。
看到这里可能有点乱了，我们重新梳理一下。
我们上面有提到doInBackground是在mWorker的call方法中调用的，所以我们找mWorker.call()的调用地方。
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
由于在executeOnExecutor()方法中，mWorker是以参数传递到mFuture中的，所以看一下FutureTask源码是否有调用mWorker.call()。
public class FutureTask<V> implements RunnableFuture<V> {
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
}
果然，在mFuture的run方法找到了result = c.call()，也就是调用了mWork的call()方法。
接着mFuture作为参数传递给了SerialExecutor.execute()，而在SerialExecutorSerialExecutor.execute()方法中通过THREAD_POOL_EXECUTOR创建子线程最终调用mFuture.run()。也就是解释了为什么doInBackground是运行在子线程中的了。 r.run()执行完后finally继续调用scheduleNext()，保证了mTasks任务按execute()顺序执行。
private static class SerialExecutor implements Executor {
final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
Runnable mActive;
public synchronized void execute(final Runnable r) {
mTasks.offer(new Runnable() {
public void run() {
try {
r.run();
} finally {
scheduleNext();
}
}
});
if (mActive == null) {
scheduleNext();
}
}
protected synchronized void scheduleNext() {
if ((mActive = mTasks.poll()) != null) {
THREAD_POOL_EXECUTOR.execute(mActive);
}
}
}
总结：
1.AsyncTask内部也是用Handler实现线程间通信
2.AsyncTask.execute()和AsyncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, key)内部其实用的是同一个线程池。
3.THREAD_POOL_EXECUTOR线程池线程数量跟CPU核数有关，最少2个，最多4个，线程存活时间30秒。
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