Is my image caching process going to leak memory?

Question

I have an object that overwrites the Application object. In it, I have a member variable which is a LongSparseArray where the key is some identifier of type long and the value is an object with 2 member variables: a Bitmap and a long which is used as a timestamp.

This is my global image cache. Occasionally, a function is ran that looks at the timestamps and ages things that are over an hour old.

By "age" I mean that it removes that entire entry from the LongSparseArray.

Here is my question:

Suppose I have an Activity with a ListView. Each row in the ListView has an ImageView that is populated with an image from the cache.

Bitmap image = ((MyApp)getApplicationContext()).getImage(id);
holder.imgImage.setImageBitmap(image);

Now, suppose the user clicks some button which takes them to a new Activity. While on this new Activity, the image previously assigned to a row in the ListView in the previous Activity ages.

So, to recap, that Bitmap key/value entry now no longer exists in the global LongSparseArray.

Is that Bitmap really able to be reclaimed by Java? Isn't it still being referred to by the ImageView in the ListView of the previous Activity? Assuming, of course, that Android hasn't reclaimed the memory used by that Activity.

The reason I'm asking about this is my previous aging function would also call .Recycle() on the Bitmap. In this scenario, when the user hit the back button and returned to the previous Activity which was using that Bitmap, the application would crash, presumably because that Bitmap was not only missing from the cache, but also from memory. So I just removed the .Recycle() call.

By the way, once the Bitmap is removed from the cache, and an object with that id shows up on screen again, the application will download the Bitmap again and place it in the cache. If the previous one stayed in memory, you could see how this would present a problem.

Also, does anyone have any ideas for a more effective solution?

What would happen if I set myImageView.setDrawingCacheEnabled(false);?

There are 2 Activities which use this image caching. One is a search screen that displays a list of items (and their images) after the user performs a search. The other is a list of those items the user has then selected to keep.

Answer 1

Issue: Once recycle() method is called on a bitmap, the bitmap should never be used again. If an attempt is made to draw the bitmap, then an exception will be thrown. From docs:

You should use recycle() only when you are sure that the bitmap is no longer being used. If you call recycle() and later attempt to draw the bitmap, you will get the error: "Canvas: trying to use a recycled bitmap".

In this specific case, you have recycled the bitmap, but the ListView item's ImageView has a strong reference to the bitmap. When you return to the Activity, the ListView item attempts to draw the bitmap, hence the exception is thrown.

Bitmap memory management: Prior to Android 2.3.3, the backing pixel data of a bitmap was stored in native memory and bitmap itself in Dalvik memory. Hence to release the native memory, recycle method has to be called.

Here is Bitmap.recycle function definition:

    public void recycle() {
        if (!mRecycled) {
            if (nativeRecycle(mNativeBitmap)) {
                // return value indicates whether native pixel object was actually recycled.
                // false indicates that it is still in use at the native level and these
                // objects should not be collected now. They will be collected later when the
                // Bitmap itself is collected.
                mBuffer = null;
                mNinePatchChunk = null;
            }
            mRecycled = true;
        }
    }

Post Android 3.0, the backing pixel data is also stored in Dalvik memory. When the bitmap is no longer required, we need to ensure we don't hold any strong reference to the bitmap, so that it is garbage collected.

Solution: If you are still supporting Android 2.3.3 and lower version, you still need to use recycle to release the bitmap.

You can use reference counting to track whether the bitmap is currently being referenced by the ListView item, so that even it is aged, you don't call recycle on the bitmap.

ListView adapater's getView method is the place where the bitmap is assigned to the ImageView. Here you increment the reference count. You can attach setRecyclerListener to the ListView to know whenever the listview item is put into recycle bin. This is the place you would decrement the reference count of the bitmap. The aging function need to recycle the bitmap only if the reference count is zero.

You can also consider using LruCache for caching, as mentioned in docs.

setDrawingCacheEnabled: By calling this method with true param, the next call to getDrawingCache will draw the view to a bitmap. The bitmap version of view can be rendered on to the screen. Since it is just a bitmap, we cannot interact with it as done with an actual view. Couple of use cases are:

• When ListView is being scrolled, the bitmap of the displayed items view is captured and rendered. So that the views being scrolled don't undergo measure and layout pass.
• View hierarchy feature in DDMS.

Answer 2

Is that Bitmap really able to be reclaimed by Java? Isn't it still being referred to by the ImageView in the ListView of the previous Activity? Assuming, of course, that Android hasn't reclaimed the memory used by that Activity.

The Bitmap is stilled used in the ListView (a strong reference) so dalvik can't reclaim its memory.

Apparently you can't call recycle on the Bitmap or bad things will happen(app crash, e.g.).

What would happen if I set myImageView.setDrawingCacheEnabled(false);?

If you disable drawing cache, every time your view needs to be redrawn, the onDraw method will be called.I'm not very familiar with ImageView , you can go and read its source for a deep understanding. (Note: the usage of drawing cache is different when hardware accerleration is enabled/disabled, here I just assume you're using software rendering).

For the solution, you can try the following:

1. when the Bitmap cache become stale, you remove it from the cache array(and then you app will try to get a new one, I think).
2. In ListView.getView, you can check whether currently used Bitmap ages. It should be easy because you know the timestamp when you call setImageBitmap the first time and the latest timestamp. If they are not same, you call setImageBitmap again using the new Bitmap and the old one will be reclaimed.

Wish this helps.

Answer 3

Regarding, "Also, does anyone have any ideas for a more effective solution?"

The Picasso library would help solve the problems you are facing http://square.github.io/picasso/

Picasso is "A powerful image downloading and caching library for Android"

"Many common pitfalls of image loading on Android are handled automatically by Picasso:

• Handling ImageView recycling and download cancelation in an adapter.
• Automatic memory and disk caching."