
前言
在上一节AsyncTask深入解读(上)中已经介绍了AsyncTask的基本知识,本节将从源码出发,深层次分析AsyncTask的工作原理。
1 AsyncTask对象创建
使用AsyncTask时,一般根据具体的任务继承AsyncTask类,例如:
private class MyAsyncTask extends AsyncTask<String, Integer, String> {
protected String doInBackground(String... args1) {
Log.i(TAG, "doInBackground in:" + args1[0]);
int times = 10;
for (int i = 0; i < times; i++) {
publishProgress(i);//提交之后,会执行onProcessUpdate方法
}
Log.i(TAG, "doInBackground out");
return "over";
}
/**
* 在调用cancel方法后会执行到这里
*/
protected void onCancelled() {
Log.i(TAG, "onCancelled");
}
/**
* 在doInbackground之后执行
*/
protected void onPostExecute(String args3) {
Log.i(TAG, "onPostExecute:" + args3);
}
/**
* 在doInBackground之前执行
*/
@Override
protected void onPreExecute() {
Log.i(TAG, "onPreExecute");
}
/**
* @param args2
*/
@Override
protected void onProgressUpdate(Integer... args2) {
Log.i(TAG, "onProgressUpdate:" + args2[0]);
}
}
MyAsyncTask对象的创建必须在主线程中,使用方式为:
new MyAsyncTask().execute("AsyncTask Test");
2 execute方法
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
execute
方法调用了executeOnExecutor
方法并传递参数sDefaultExecutor和params。
再看executeOnExecutor
方法:
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
Params... params) {
//判断当前状态
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)");
}
}
//将状态置为运行态
mStatus = Status.RUNNING;
//主线程中最先调用onPreExecute方法,进行准备工作
onPreExecute();
//将参数传给mWorker
mWorker.mParams = params;
//调用线程池,执行任务
exec.execute(mFuture);
return this;
}
executeOnExecutor
方法首先判断状态,若处于可执行态,则将状态置为RUNNING。然后调用了onPreExecute
方法,交给用户进行执行任务前的准备工作。核心部分在于 exec.execute(mFuture)
。exec即sDefaultExecutor。
3 sDefaultExecutor串行线程池
查看sDefaultExecutor定义:
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
sDefaultExecutor是一个串行线程池,作用在于任务的排队执行。其源码如下:
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);
}
}
}
从SerialExecutor的源码可以看出,mFuture是插入到mTasks任务队列的对象。当mTasks中没有正在活动的AsyncTask任务,则调用scheduleNext
方法执行下一个任务。若一个AsyncTask任务执行完毕,则继续执行下一个AsyncTask任务,直至所有任务执行完毕。通过分析可以发现真正去执行后台任务的是线程池THREAD_POOL_EXECUTOR。
4 线程池THREAD_POOL_EXECUTOR
THREAD_POOL_EXECUTOR定义如下:
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;
}
5 mFuture并发执行对象
线程池THREAD_POOL_EXECUTOR中执行的Runnable对象为mFuture。
mFuture的定义:
private final FutureTask<Result> mFuture;
mFuture为线程池执行的真正任务,mFuture的执行过程过程是怎样的呢?
再看AsyncTask的构造:
public AsyncTask(@Nullable Looper callbackLooper) {
mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
? getMainHandler()
: new Handler(callbackLooper);
//创建mworker对象
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;
}
};
//创建mFuture对象
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);
}
}
};
}
通过AsyncTask的构造中可以看到mWorker为FutureTask的构造函数参数,则FutureTask对象中持有mWorker的引用。
FutureTask的构造函数:
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;
}
FutureTask的run
方法:
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);
}
}
FutureTask的run
方法中调用了 result = c.call()
; 即调用了mWorker的call
方法。
6 mWorker对象
mWorker的call
方法:
mWorker = new WorkerRunnable<Params, Result>() {
//call方法
public Result call() throws Exception {
mTaskInvoked.set(true);
Result result = null;
try {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
//noinspection unchecked
//执行任务
result = doInBackground(mParams);
Binder.flushPendingCommands();
} catch (Throwable tr) {
mCancelled.set(true);
throw tr;
} finally {
postResult(result);
}
return result;
}
};
通过代码可以发现,最终在线程池中执行的是mWorker的call
方法,call
方法中调用了doInBackground
方法,因此可以看出doInBackground
方法是在线程池中调用的。
当任务执行完毕后则调用postResult
方法:
//任务执行完后调用方法
private Result postResult(Result result) {
@SuppressWarnings("unchecked")
Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(this, result));
message.sendToTarget();
return result;
}
postResult
方法发送MESSAGE_POST_RESULT消息和result。
7 InternalHandler接收处理消息
InternalHandler代码:
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;
}
}
}
InternalHandler接收到MESSAGE_POST_RESULT时调用result.mTask.finish(result.mData[0])
;
8 finish方法:
private void finish(Result result) {
if (isCancelled()) {
onCancelled(result);
} else {
onPostExecute(result);
}
mStatus = Status.FINISHED;
}
finish
方法中若任务没有取消则调用onPostExecute
方法发送结果,若任务取消则调用onCancelled
方法。finish方法是在主线程中执行的。
9 onProgressUpdate
通过上述流程已经顺序找到了onPreExecute
、doInBackground
、onPostExecute
方法,那么onProgressUpdate
是如何执行的呢?
首先查看 publishProgress
方法:
protected final void publishProgress(Progress... values) {
if (!isCancelled()) {
getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}
}
MyAsyncTask示例中在doInBackground
中调用publishProgress
方法,publishProgress
方法发送MESSAGE_POST_PROGRESS消息和进度values,InternalHandler在接收到MESSAGE_POST_PROGRESS消息中调用onProgressUpdate
方法。因此onProgressUpdate
也是在主线程中调用。
10 总结
通过上述一步步的源码分析过程,已经掌握了AsyncTask任务的执行过程。AsyncTask中有两个线程池串行线程池sDefaultExecutor和线程池THREAD_POOL_EXECUTOR。sDefaultExecutor用于任务的排队,THREAD_POOL_EXECUTOR真正的执行任务。线程的切换使用Handler(InternalHandler)实现。
11 问题回答
1)为什么AsyncTask在主线程创建执行?
因为AsyncTask需要在主线程创建InternalHandler,以便onProgressUpdate
, onPostExecute
, onCancelled
可以正常更新UI。
2)为什么AsyncTask不适合特别耗时任务?
AsyncTask实际上是一个线程池。如果有线程长时间占用,且没有空闲,则其他线程只能处于等待状态,会造成阻塞。
3)AsyncTask内存泄漏问题
如果AsyncTask被声明为Activity的非静态的内部类,那么AsyncTask会保留一个对创建了AsyncTask的Activity的引用。如果Activity已经被销毁,AsyncTask的后台线程还在执行,它将继续在内存里保留这个引用,导致Activity无法被回收,引起内存泄露。
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