Updating Cursor in background thread

Question

I have a similar problem to the one described in this discussion: I need to refresh a ListView when the underlying database changes, but the query is expensive so I'm doing it in an AsyncTask.

Here's what I do when the updated Cursor is ready. (This is also how the list is initially populated on startup.)

@Override
protected void onPostExecute(Cursor result) {
    if (activity != null) {
        if (currentCursor != null) {
            // existing cursor is closed by adapter.changeCursor() so
            // we don't need to explicitly close it here 
            stopManagingCursor(currentCursor);
        }
        currentCursor = result;
        startManagingCursor(currentCursor);

        if (adapter == null) {
            adapter = getAdapter(result);
            setListAdapter(adapter);
        } else {
            adapter.changeCursor(result);
        }

        activity.onGotList(result, dbAdapter);
    }
}

Here's the error I get. It doesn't happen every time, which is even more frustrating.

Releasing statement in a finalizer. Please ensure that you explicitly call close() on your cursor: SELECT DISTINCT   t._id AS _id,   t.amount,   t.date,   t.memo,   t.synced,   t.flag,   (children.pa
android.database.sqlite.DatabaseObjectNotClosedException: Application did not close the cursor or database object that was opened here
     at android.database.sqlite.SQLiteCompiledSql.<init>(SQLiteCompiledSql.java:62)
     at android.database.sqlite.SQLiteProgram.<init>(SQLiteProgram.java:100)
     at android.database.sqlite.SQLiteQuery.<init>(SQLiteQuery.java:46)
     at android.database.sqlite.SQLiteDirectCursorDriver.query(SQLiteDirectCursorDriver.java:53)
     at android.database.sqlite.SQLiteDatabase.rawQueryWithFactory(SQLiteDatabase.java:1412)
     at android.database.sqlite.SQLiteDatabase.rawQuery(SQLiteDatabase.java:1382)

So, I am obviously not closing the Cursor correctly. If I call currentCursor.close() instead of relying on the outgoing Cursor being closed by adapter.changeCursor(), then I get warnings about closing the Cursor twice or closing a null Cursor.

What is the correct way to do this?

In the discussion I linked to, Dianne Hackborn suggests using a Loader instead. That is not an option for me since my code has to run on Android 2.1.

`using a Loader instead. That is not an option for me since my code has to run on Android 2.1.` yes, it is an option since there is an compat library v4 — Selvin, Nov 14 '11 at 21:56
@Selvin oh thanks, I'd forgotten about that. I shall investigate. — Graham Borland, Nov 14 '11 at 22:45

score 0 · Answer 1 · answered Nov 14 '11 at 21:38

0

Try to .close() the Cursor when the Activity pause or terminates. In the onPause() or onDestroy() section of the activity.

answered Nov 14 '11 at 21:38

Nik NexAndrX

302
1
6

Please explain how you think that would help. As far as I can tell, the cursor's lifetime is managed by the activity since I'm calling `startManagingCursor()` on it. – Graham Borland Nov 14 '11 at 21:41
In my app work like i describe. In the OnPause() section of the activity i put: super.onPause(); mProductCursor.deactivate(); mProductCursor.close(); – Nik NexAndrX Nov 15 '11 at 17:07

score 0 · Answer 2 · answered Nov 14 '11 at 21:51

0

Basically, it's possible, but very bad practice to access the same database from two different helpers, so if you have an activity performing database queries, you shouldn't also have a thread accessing it, otherwise android will throw up a quiet error in logcat, and then forget about the query...

The best solution I have found is to implement a thread pool of runnables, each one is a database query and they all use the same database helper. Consequently, only one thread is accessing the database at any one time and the database is just opened and closed when the thread pool starts/stops.

An implementation of the thread pool pattern can be found here: http://mindtherobot.com/blog/159/android-guts-intro-to-loopers-and-handlers/

answered Nov 14 '11 at 21:51

Todd Davies

5,484
8
47
71

I only have one database helper. – Graham Borland Nov 14 '11 at 21:51
Hmm, one helper class, or one helper instance running at any one time. Also, make sure you always call close() on the helper once you've finished with it (usually in onDestroy()). – Todd Davies Nov 14 '11 at 21:53
One helper instance which remains open for the lifetime of my application. – Graham Borland Nov 14 '11 at 21:55
And are you sure that you use close() once you've finished with it? Also, it is possible that in an earlier revision of the code, you didn't close the database and so the error is persisting to this version. How about you try clearing the data of your app? – Todd Davies Nov 14 '11 at 21:57

Midpipps · Answer 3 · 2011-11-15T17:01:36.667

0

If you are not changing anything other then redrawing from the list is it necessary to change the cursor at all. Could you get away with just requiring the current adapter.

something along the lines of

adapter.getCursor().requery();

although if you are in a thread other then the main ui thread you may want to call it with

//Did not realize this was deprecated Thanks to Graham Borland for the heads up
runOnUiThread(new Runnable(){
    public void run(){
        adapter.getCursor().requery();
    }
});

Depending on your setup.

New solution still need testing and make sure it is not going to cause issues apparently startManaginCursor and stopManaginCursor are deprecated too so this solution is not worth good either.

stopManagingCursor(adapter.getCursor());
if (!adapter.getCursor().isClosed()) 
    adapter.getCursor().close();
//cursor creation stuff here if needed
startManagingCursor(newCursor);
adapter.changeCursor(newCursor);

edited Nov 15 '11 at 17:01

answered Nov 14 '11 at 21:57

Midpipps

66
4

`requery()` is deprecated. My approach of replacing the existing cursor with a new one is what is apparently recommended instead: http://developer.android.com/reference/android/database/Cursor.html#requery() – Graham Borland Nov 14 '11 at 21:58
Well crunk sorry I guess I missed that well I guess I will be in the same boat as you soon time to go play in some code. Will respond if I find a better way. – Midpipps Nov 14 '11 at 22:01
Here is what I had to do on one of my test applications not sure if it is good yet need to run more test and throw in some error/null checks but it works without cursor close errors. stopManagingCursor(adapter.getCursor()); if (!adapter.getCursor().isClosed()) adapter.getCursor().close(); startManagingCursor(newCursor); albums.changeCursor(newCursor); – Midpipps Nov 14 '11 at 22:23
That last albums should have been an adapter I fail at changing. things. The main point of it is I am managing the cursor directly through the adapter instead of keeping tabs on the cursor. – Midpipps Nov 14 '11 at 22:46

Updating Cursor in background thread

3 Answers3