不能,只能执行一次。为什么?
public final AsyncTask<Params, Progress, Result> execute(Params... params) {
return executeOnExecutor(sDefaultExecutor, params);
}
当我们调用execute()来执行任务时,其实是调用如下方法。
public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,Params... params)
下面贴出一些代码:
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;
}
从注释中,我们就可以知道,一个任务不能被执行多次,当该任务的状态是 Running、Finished时,会抛出异常。
那么为什么要抛出异常呢?
我们可以看到,当我们使用异步任务中维护的线程池进行任务的执行时,其实是执行了一个FutureTask,而FutureTask是封装了一个Callable的。
当我们调用线程池执行任务时,会从任务队列中取得FutureTask任务,取出任务之后调用FutureTask.run()方法,而在FutureTask.run()方法中则会调用Callable.call()方法来真正的执行这一个Callable任务,并且是可以回调将运行结果获取到的。获取到的运行结果就是doInBackground()方法的返回值。
在我们执行一个FutureTask时,我们可以看到下面的代码,是FutureTask的run方法,在run方法执行时,会判断callable任务是否为空,如果为空,调用setException(ex)返回一个异常,如果不为空,则执行,并在result返回结果,然后将状态设置为 ran。最后调用 set(result)设置运行的结果就是callable的结果。
重点:
在setException(ex)、set(result)中都调用了一个方法,finishCompletion()。最后再这个方法中将callable置为了null。
一切都清楚了,在线程池分配线程来执行任务FutureTask时,会调用它的run方法来启动它,在run方法内部,会调用FutureTask的真正任务实体Callable的call方法。最后执行完之后,如果callable为null,抛出异常,如果不为null,执行完之后,置为null。
因此,一个任务只能执行一次。
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
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);
}
}
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t);
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}
done();
callable = null; // to reduce footprint
}