java.lang.IllegalStateException: attempt to re-ope

问题:

I'm trying to figure out why occasionally I'm getting the IllegalStateException. I can't find any good examples that show how to load a list using a thread to query a SQLite database. I've included my code below. Most of the time it works correctly, but occasionally, I'm getting the IllegalStateException.

I have also gotten a similar exception in another activity of mine that is an instance of ExpandableListActivity. That exception states "trying to requery an already closed cursor".

Can somebody tell me the correct way to do this so that it doesn't cause any errors? I would prefer to use the cursors instead of copying all of the data into memory. If I can't figure this out then I will have to load it all into memory.

I think the issue has something to do with startManagingCursor(Cursor) and the fact that the database connection is closed in onDestroy(). plz help
-- tale

public class MyListActivity extends ListActivity {
    private MyCursorAdapter adapter;
    private SQLiteDatabase db = null;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        try {
            new RetrieveCursorTask(this).execute((Void[]) null);
        } catch (Exception e) {
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        // Null out the cursor. 
        if (adapter != null) {
            adapter.changeCursor(null);
            adapter = null;
        }

        if (db != null && db.isOpen()) {
            db.close();
        }
    }

    private class RetrieveCursorTask extends AsyncTask<Void, Void, Cursor> {
        private Context ctx;

        public RetrieveCursorTask(Context ctx) {
            this.ctx = ctx;
        }

        @Override
        protected Cursor doInBackground(Void... params) {
            Cursor cursor = null;
            DbHelper helper = new DbHelper(ctx);

            try {
                db = helper.getReadableDatabase();
                cursor = db.query("users",
                    new String[] {
                        DbHelper.ID_COLUMN,
                        DbHelper.UID_COLUMN
                    }, 
                    null, null, null, null, null);
                startManagingCursor(cursor);
            } catch (Exception e) {
            }
            return cursor;
        }

        @Override
        protected void onPostExecute(Cursor cursor) {
            super.onPostExecute(cursor);

            if (cursor != null) {
                try {
                    adapter = new MyCursorAdapter(ctx, cursor);
                } catch (Exception e) {
                }
                setListAdapter(adapter);
            } 
        }
    }

    private class MyCursorAdapter extends CursorAdapter {
        private Context ctx;

        public MyCursorAdapter(Context context, Cursor c) {
            super(context, c);
            this.ctx = context;
        }

        @Override
        public void bindView(View view, Context context, Cursor cursor) {
            // ...
        }

        @Override
        public View newView(Context context, Cursor cursor, ViewGroup parent) {
            // ...
        }
    }
}

回答1:

Look into AsyncQueryHandler if you want to query DB the way you want.

Your task RetrieveCursorTask is running on separate thread so when your activity gets destroyed your AsyncTask might still be running in background but as you have closed your cursor in main activity onDestroy it might be requeried again after your AsyncTask returns.

回答2:

Sounds like you need to syncronize the block where you set your adapter in onPostExecute. The problem is since AsyncTask is running on a separate thread, the order in which the cursor is set and subsequently requested isn't guaranteed. Try this..

    @Override
    protected void onPostExecute(Cursor cursor) {
        super.onPostExecute(cursor);

        synchronized(anyObject) {
        if (cursor != null) {
            try {
                adapter = new MyCursorAdapter(ctx, cursor);
            } catch (Exception e) {
            }
            setListAdapter(adapter);
        } 
        }
    }