AsyncTask一直在等待吗？

Question

我的一个活动中的一个按钮调用AsyncTask，该AsyncTask更新ListView的SimpleCursorAdapter的基础Cursor。每次单击按钮时，都会为AsyncTask添加一个新线程并完成任务（进入“等待”状态）。如果我单击按钮5次或更多次，则5个AsyncTasks最终以“等待”状态坐在那里。这是正常现象还是在某处内存泄漏？

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());
    }

}

还有我的onCreate中调用任务的代码（首次加载活动时）：

upAdapter = new updateAdapter().execute();

并在按钮onClickListener中：

            // Reset wod_id
            _wod_id = -1;

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

AsyncTask之一的堆栈跟踪（所有这些都相同）：

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

Answer 1

AsyncTask引擎盖下使用一个ThreadPoolExecutor。这些线程可能不会消失一点，因为过于频繁地创建和拆除这些线程是一种浪费。过一会儿，如果您创建更多AsyncTasks线程，您会发现它将停止创建新线程，并将重新使用旧线程。

更新以解决一些细节：

您可能会认为，如果池中有空闲线程，则不会创建新线程，但这并非完全正确。这个想法是，有一定数量的线程对于保持处理异步任务很有用。这称为核心池大小。在Android的AsyncTask情况下，他们似乎已将其设置为5。如果您查看ThreadPoolExecutor它的文档说：

当在方法execute（Runnable）中提交新任务，并且正在运行的线程少于corePoolSize线程时，即使其他工作线程处于空闲状态，也会创建一个新线程来处理请求。

还有一个最大值适当地称为最大池大小。