Why is my activity crashing when hitting the home button?

Question

At this point I'm quite frustrated. I've been researching this for a few days and cannot even isolate anything beyond a cursor problem. I am extending ListActivity and using startManagingCursor(newcursor) in the OnCreate method. Here is the code (the database is already filled) that runs and crashes upon hitting the home button:

private void loadAlbums() {
    try {
        newcursor = mDbHelper.getAlbumTitlesCursor();
        dbalbumadapter = new AlbumListCursorAdapter(this, newcursor);
        setListAdapter(dbalbumadapter);
        }
    catch (SQLiteException s) {
        newcursor = null;
        fetchalbums = new FetchAlbumsTask().execute();
        }
    current_view = ALBUMTITLE_VIEW;
}

Here is the error log:

02-03 13:41:42.379: WARN/dalvikvm(340): threadid=3: thread exiting with uncaught exception (group=0x4001b188)
02-03 13:41:42.389: ERROR/AndroidRuntime(340): Uncaught handler: thread main exiting due to uncaught exception

02-03 13:41:42.590: ERROR/AndroidRuntime(340): java.lang.RuntimeException: Unable to stop activity {com.skip.ngRCv2/com.skip.ngRCv2.ngRC}: java.lang.RuntimeException: Unable to stop activity {com.skip.ngRCv2/com.skip.ngRCv2.MusicLibraryActivity}: java.lang.NullPointerException
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:3227)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.handleStopActivity(ActivityThread.java:3272)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.access$2500(ActivityThread.java:119)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1880)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.os.Handler.dispatchMessage(Handler.java:99)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.os.Looper.loop(Looper.java:123)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.main(ActivityThread.java:4363)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at java.lang.reflect.Method.invokeNative(Native Method)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at java.lang.reflect.Method.invoke(Method.java:521)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at dalvik.system.NativeStart.main(Native Method) 02-03 13:41:42.590: ERROR/AndroidRuntime(340): 
Caused by: java.lang.RuntimeException: Unable to stop activity {com.skip.ngRCv2/com.skip.ngRCv2.MusicLibraryActivity}: java.lang.NullPointerException
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:3227)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.performStopActivity(ActivityThread.java:3174)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:176)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.LocalActivityManager.dispatchStop(LocalActivityManager.java:572)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityGroup.onStop(ActivityGroup.java:79)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.Instrumentation.callActivityOnStop(Instrumentation.java:1169)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.Activity.performStop(Activity.java:3797)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:3224)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     ... 11 more
02-03 13:41:42.590: ERROR/AndroidRuntime(340): 
Caused by: java.lang.NullPointerException
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.Activity.performStop(Activity.java:3808)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     at android.app.ActivityThread.performStopActivityInner(ActivityThread.java:3224)
02-03 13:41:42.590: ERROR/AndroidRuntime(340):     ... 18 more

There is a NullPointerException at Activity.performStop(Activity.java:3808), which seems to come from this method in Activity.java even though I can't cross-verify that with the line number:

final void performStop() {
     if (!mStopped) {
         if (mWindow != null) {
             mWindow.closeAllPanels();
         }

         mCalled = false;
         mInstrumentation.callActivityOnStop(this);
         if (!mCalled) {
             throw new SuperNotCalledException(
                 "Activity " + mComponent.toShortString() +
                 " did not call through to super.onStop()");
         }

         synchronized (mManagedCursors) {
             final int N = mManagedCursors.size();
             for (int i=; i<N; i++) {
                 ManagedCursor mc = mManagedCursors.get(i);
                 if (!mc.mReleased) {
                     mc.mCursor.deactivate();
                     mc.mReleased = true;
                 }
             }
         }

         mStopped = true;
     }
     mResumed = false;
 }

I have tried closing and deactivating my cursor in the OnStop method as well as setting the list adapter to null. I was under the impression that none of this was necessary when letting the activity manage the cursor as I indicated. I am passing the cursor to my custom adapter but none of the examples I've found do any managing inside the adapter.

If you can help me at least narrow down what is throwing this exception, I would be grateful!

Answer 1

If this is the right rev, line 3808 corresponds to:

[[ I know its not the official android source, but the line number lines up]]

http://code.google.com/p/pdn-slatedroid/source/browse/trunk/eclair/frameworks/base/core/java/android/app/Activity.java?r=12#3808

mc.mCursor.deactivate();

As far as I can tell, mc cannot ever be null. It seems that mCursor (which is final) can be null only if a null cursor was passed into startManagingCursor(). I captured my notes before for others to confirm this.

Can you log your cursor is just before you pass it into startManagingCursor()? I'd be curious what the last occurrence of that message before the crash says.

You have this line when there's a sql exception:

newcursor = null;

I wonder if that's getting into the startManagingCursor() call.

---

Notes:

• startManagingCursor() creates a ManagedCursor:

http://code.google.com/p/pdn-slatedroid/source/browse/trunk/eclair/frameworks/base/core/java/android/app/Activity.java?r=12#1549

public void startManagingCursor(Cursor c) {
    synchronized (mManagedCursors) {
        mManagedCursors.add(new ManagedCursor(c));
    }
}

• ManagedCursor sets final field mCursor:

http://code.google.com/p/pdn-slatedroid/source/browse/trunk/eclair/frameworks/base/core/java/android/app/Activity.java?r=12#647

private static final class ManagedCursor {
    ManagedCursor(Cursor cursor) {
        mCursor = cursor;
        mReleased = false;
        mUpdated = false;
    }

    private final Cursor mCursor;
    private boolean mReleased;
    private boolean mUpdated;
}

• All the internal calls to startManagingCursor() are guarded, e.g.:

http://code.google.com/p/pdn-slatedroid/source/browse/trunk/eclair/frameworks/base/core/java/android/app/Activity.java?r=12#1465

if (c != null) {
    startManagingCursor(c);
}
return c;

Answer 2

Start your app in a debugger, set an exception breakpoint on NPE and then see where it is bailing out.

Should give you an idea what is going on.