Bitmap recycle in finalize method not working properly

Question

I am having crashes in my application when I try to load an image with size close to the maximum size of the application heap. The first time I load the image is fine, then I delete references to the Bitmap object and its finalize where I call mBitmap.recycle() //nativeRecycle is called. Apparently memory use decrease, but when I try to load again the image I get the crash of OutOfMemory. The interesting part is that if I call the recycle just in the same class where I delete the references seems like the memory is freed and I can load again the image. So basically i am getting different results if I call directly mBitmap.recycle() than if I put the mBitmap.recycle() in the finalize of the object referencing the Bitmap. Any idea why this is happening?

Answer 1

If you're that close to using the max size of the heap, you may just be getting lucky (or unlucky). Calling recycle() directly vs. calling it from the finalizer may change the timing and behavior of the GC in such a way that you see differences.

Are you certain that your finalizer (and recycle()) are being called before you try re-allocating the bitmap? Finalizers are unreliable, and can be deferred indefinitely. Even if it you currently see the finalizer being called before re-allocation during your testing, there's no guarantee that this will always happen (especially for different heap states or future/different versions of Dalvik).

If you can, try as hard as you can to call recycle() from outside of the finalizer. If you know when you're done, just call the method; otherwise consider using a ReferenceQueue (https://stackoverflow.com/a/10879076/150001).

It's been a while since I looked at Dalvik, but I seem to remember that bitmap buffers could take a few GC cycles to be cleaned up, because the VM needed to be sure that no native code had any remaining pointers into it. So, the buffer may not be freed exactly when you expect it to be.

There's also the possibility that heap fragmentation is causing a problem, though there's a good chance that large buffers are allocated using mmap() and will not contribute to fragmentation (again, depending on the current version of Dalvik).