Question

我正在尝试实现2个异步任务，其中一个应该等待另一个完成。

我为它做了一个测试用例，看起来像这样：

public class MainActivity extends Activity{

    public static boolean SYNCING = false;
    public static Object LOCK = new Object();

    public static void setSync(){
        Log.i(TAG, "setting Sync=true");
        synchronized (LOCK) {
            SYNCING = true;
        }
    }

    public static void waitForSync(){
        Log.i(TAG, "waiting for Sync");
        synchronized (LOCK) {
            try{
                while (SYNCING){
                    LOCK.wait();
                }
            } catch (InterruptedException ignore){}
        }
    }

    public static void setSyncFinish(){
        Log.i(TAG, "setSyncFinish to notify all");
        synchronized (LOCK) {
            SYNCING = false;
            LOCK.notifyAll();
        }
    }

    public class WaitingOnLoaderTask extends AsyncTask<Void, Void, Void>{

        @Override
        protected Void doInBackground(Void... params) {
            Log.i(TAG, "doInBackground of WaitingForLoaderTask");

            try {
                Thread.sleep(500);
            } catch (InterruptedException e) {

            }

            waitForSync();
            Log.i(TAG, "finishedWaiting!");
            return null;
        }

    }

    public class LoaderTask extends AsyncTask<Void, Void, Void>{

        long mRuntime;
        int mTimes;

        public LoaderTask(long runtime, int times){
            mRuntime = runtime;
            mTimes = times;
        }

        @Override
        protected Void doInBackground(Void... params) {
            while(mTimes >= 0){
                Log.i(TAG, "sleeping for " + mTimes + " times for " + mRuntime + " milliseconds");
                mTimes--;
                try {
                    Thread.sleep(mRuntime);
                } catch (InterruptedException e) {
                    Log.e("MainActivity", "Interrupted", e);
                }
            }

            setSyncFinish();

            return null;
        }

    }
}

但我无法让WaitingOnLoaderTask正确等待。如果我在onCreate

中运行它

new WaitingOnLoaderTask().execute((Void[])null);
new LoaderTask(1000, 5).execute((Void[])null);

我得到像这样的logOutput（发生死锁）：

03-08 18:17:23.309: I/myUniqueLogTag(4432): setting Sync=true
03-08 18:17:23.317: I/myUniqueLogTag(4432): doInBackground of WaitingForLoaderTask
03-08 18:17:23.817: I/myUniqueLogTag(4432): waiting for Sync

和

new LoaderTask(1000, 5).execute((Void[])null);
new WaitingOnLoaderTask().execute((Void[])null);

我得到这样的日志：

03-08 18:18:53.082: I/myUniqueLogTag(4621): sleeping for 5 times for 1000 milliseconds
03-08 18:18:54.082: I/myUniqueLogTag(4621): sleeping for 4 times for 1000 milliseconds
03-08 18:18:55.090: I/myUniqueLogTag(4621): sleeping for 3 times for 1000 milliseconds
03-08 18:18:56.082: I/myUniqueLogTag(4621): sleeping for 2 times for 1000 milliseconds
03-08 18:18:57.090: I/myUniqueLogTag(4621): sleeping for 1 times for 1000 milliseconds
03-08 18:18:58.106: I/myUniqueLogTag(4621): sleeping for 0 times for 1000 milliseconds
03-08 18:18:59.113: I/myUniqueLogTag(4621): doInBackground of WaitingForLoaderTask
03-08 18:18:59.113: I/myUniqueLogTag(4621): setSyncFinish to notify all
03-08 18:18:59.621: I/myUniqueLogTag(4621): waiting for Sync
03-08 18:18:59.621: I/myUniqueLogTag(4621): finishedWaiting!

所以我猜一个人阻止对方跑。他们是否在同一个线程中实际计算了doInBackground，并且LOCK.wait()阻止此线程进一步执行？

有没有办法解决它（并与Android 2.2保持兼容）？

注意：第二个时间基本上是我想要的，但在其他任务实现中，我不能保证WaitingOnLoaderTask仅在另一个完成时执行。

Answer 1

您最好的选择是使用Geobits建议。

您可以使用onPostExecute（）函数调用下一个Async任务，以便它执行它：

class WaitingOnLoaderTask extends AsyncTask<Void,Void,Void>
{
    protected void onPreExecute(Void)
    {
        super.onPreExecute();
    }

    protected void doInBackground(Void... params)
    {
        //Commands for first AsyncTask
    }

    protected void onPostExecute(Void)
    {
        //Call AsyncTask 2
    }
}

这样它将等到任务1完成。

Answer 2

您的代码在Android 3.2及更低版本上按预期工作。在更高版本的Android版本上，AsyncTasks没有并行执行，请尝试使其在Android 4 +上运行：

LoaderTask loaderTask = new LoaderTask(1000, 5);
WaitingOnLoaderTask waitingTask = new WaitingOnLoaderTask();
if (Build.VERSION.SDK_INT >= 14) {
    loaderTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
    waitingTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
} else {
    loaderTask.execute();
    waitingTask.execute();
}

注意：您应该将AsyncTasks标记为static，以避免在配置更改（例如轮换）时泄漏活动。

如果没有deadLock，AsyncTask如何等待另一个

2 个答案: