我的一个 Activity 中的一个按钮调用了一个 AsyncTask，它更新了 ListView 的 SimpleCursorAdapter 的底层 Cursor。每次我单击该按钮时，都会为 AsyncTask 添加一个新线程并且任务完成(进入“等待”状态)。如果我点击按钮 5 次或更多次，5 个 AsyncTasks 最终会以“等待”状态停在那里。这是正常现象还是我某处有内存泄漏？

AsyncTask

private class updateAdapter extends AsyncTask<Void, Void, Void> {

    @Override
    protected Void doInBackground(Void... params) {
        // Open database connection
        if(_db == null || !_db.isOpen()) _db = new DatabaseWrapper(ActivityShowWOD.this).getWritableDatabase();
        Cursor WODcursor;

        // Check if a wod_id is set
        if(_wod_id == -1) {
            // Grab filters from preferences and at the same time build SQLselection string
            SharedPreferences prefs = getSharedPreferences("Preferences", 0);
            String[] filterNames = getResources().getStringArray(R.array.filters_values);
            boolean[] filterValues = new boolean[filterNames.length];
            String SQLselection = "";

            for (int i = 0; i < filterNames.length; i++) {
                filterValues[i] = prefs.getBoolean(filterNames[i], false);

                // Build SQL query
                if(filterValues[i] == true) {
                    SQLselection += filterNames[i] + " = 1 OR " +  filterNames[i] + " = 0";
                } else {
                    SQLselection += filterNames[i] + " = 0";
                }

                // Add an "AND" if there are more filters 
                if(i < filterNames.length - 1) SQLselection += " AND ";
            }

            // Get all WODs matching filter preferences 
            WODcursor = _db.query(DatabaseConstants.TBL_WORKOUTS, 
                                      new String[] { DatabaseConstants.WORKOUTS_ID, DatabaseConstants.WORKOUTS_NAME,
                                                     DatabaseConstants.WORKOUTS_NOTES, DatabaseConstants.WORKOUTS_CFID }, 
                                      SQLselection, null, null, null, null);

            // Move the Cursor to a random position
            Random rand = new Random();
            WODcursor.moveToPosition(rand.nextInt(WODcursor.getCount()));

            // Store wod_id
            _wod_id = WODcursor.getInt(WODcursor.getColumnIndex(DatabaseConstants.WORKOUTS_ID));
        } else {
            // Get the cursor from the wod_id
            WODcursor = _db.query(DatabaseConstants.TBL_WORKOUTS, 
                                 new String[] { DatabaseConstants.WORKOUTS_ID, DatabaseConstants.WORKOUTS_NAME,
                                                DatabaseConstants.WORKOUTS_NOTES, DatabaseConstants.WORKOUTS_CFID }, 
                                 DatabaseConstants.WORKOUTS_ID + " = " + _wod_id, null, null, null, null);

            WODcursor.moveToFirst();
        }

        // Store WOD information into class instance variables and close cursor
        _wod_cfid = WODcursor.getInt(WODcursor.getColumnIndex(DatabaseConstants.WORKOUTS_CFID));
        _wod_name = WODcursor.getString(WODcursor.getColumnIndex(DatabaseConstants.WORKOUTS_NAME));
        _wod_notes = WODcursor.getString(WODcursor.getColumnIndex(DatabaseConstants.WORKOUTS_NOTES));
        WODcursor.close();

        // Return all exercises pertaining to this WOD
        _excCursor = _db.query(DatabaseConstants.TBL_EXERCISES, 
                              new String[] { DatabaseConstants.EXERCISES_ID, DatabaseConstants.EXERCISES_EXERCISE,
                                             DatabaseConstants.EXERCISES_REPS, DatabaseConstants.EXERCISES_NOTES }, 
                              DatabaseConstants.EXERCISES_WOD_ID + " = " + _wod_id, null, null, null, 
                              DatabaseConstants.EXERCISES_ID + " ASC");
        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        _adapter.changeCursor(_excCursor);
        _adapter.notifyDataSetChanged();
        _WODlist.setOnItemClickListener(new WODlistClickListener());
    }

}

我的 onCreate 中调用任务的代码(当 Activity 首次加载时):

upAdapter = new updateAdapter().execute();

在按钮的 onClickListener 中:

            // Reset wod_id
            _wod_id = -1;

            // Update the underlying SimpleCursorAdapter
            upAdapter = new updateAdapter().execute();

其中一个 AsyncTask 的堆栈跟踪(所有 AsyncTask 都相同):

Object.wait(long, int) line: not available [native method]  
Thread.parkFor(long) line: 1535 
LangAccessImpl.parkFor(long) line: 48   
Unsafe.park(boolean, long) line: 317    
LockSupport.park() line: 131    
AbstractQueuedSynchronizer$ConditionObject.await() line: 1996   
LinkedBlockingQueue.take() line: 359    
ThreadPoolExecutor.getTask() line: 1001 
ThreadPoolExecutor.runWorker(ThreadPoolExecutor$Worker) line: 1061  
ThreadPoolExecutor$Worker.run() line: 561   
Thread.run() line: 1096 

最佳答案

AsyncTask 底层使用 ThreadPoolExecutor .这些线程可能不会消失一点，因为太频繁地继续创建和拆除这些线程是一种浪费。一段时间后，如果您创建更多 AsyncTasks，您会发现它会停止创建新线程并重新使用旧线程。

更新以解决一些细节问题:

您会认为如果池中有空闲线程，它就不会创建新线程，但事实并非如此。这个想法是有一定数量的线程可以用来继续处理异步任务。这称为核心池大小。在 Android 的 AsyncTask 案例中，他们似乎已将其设置为 5。如果您查看 ThreadPoolExecutor 的文档，它说:

When a new task is submitted in method execute(Runnable), and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle.

还有一个最大值称为最大池大小。

关于java - AsyncTask 一直在等待？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/8407408/