Develop Reference

Best way to cache in memory a single Bitmap shared across instances

To draw my sprites in OpenGL i use a single backing bitmap across multiple objects, the Bitmap is reused if size is big enough or recreated if too small. Currently i am using a static Bitmap object but i think this is causing memory leaks even if i am not sure about that.
So, let's say i need a single Bitmap shared between multiple objects, what would be the best approach?
1) Use a single Bitmap as a static reference as i do
2) Use a static weakreference (even if its not suggested in the android dev page here http://developer.android.com/training/displaying-bitmaps/cache-bitmap.html)
3) Use a singleton and then a Bitmap inside it (but this would be like 1)
4) Use an LRU cache and just creating a new bitmap every time i need it to be bigger

1,3 & 4 are essentially all the same. You create a static reference to either your Bitmap directly or to something that holds a reference. The same happens when you use the Application class to "anchor" that bitmap. That class is kept by Android alive and is in this context the same as a static reference.
Whether this is a memory leak or not depends on your definition. Leaked objects are those that are kept safe from the garbage collector by unintentional references to them. So it's certainly not a leak while you want that reference to keep your bitmap.
The problem that arises with cached data that is independent of the life of some Activity, Fragment or in more general terms "task" is that the data will keep memory occupied even if the user is never coming back to your app. The app process is kept alive until Android decides it needs the memory. That time between your last legit use of the bitmap and Android finally killing your app and thereby cleaning the memory can be seen as leak.
If we had magic powers, we could simply clean up the cache once we know that is going to happen. There are some realistic options though:
Using Android's callbacks: understanding onTrimMemory( int level )
time limits on references: e.g. https://github.com/jhalterman/expiringmap
2) is not an option. If you're trying to use WeakReference as cache, you haven't understood what that class is intended for and I honestly don't understand why it is even mentioned in the documentation (weakly referenced objects should be garbage collected as fast as possible once nobody has a strong reference anymore).
SoftReference is intended for "caching" but using it as actual cache is not only broken on Android. It's broken by design because you give the garbage collector the responsibility to maintain a cache for you without telling it how to prioritize objects or how much memory it should keep guaranteed under what conditions. The result is that the GC will clean up the wrong thing or simply everything. SoftReference can be used to in addition to a proper cache that knows how to clean up.
In addition to all of that: be aware that a single Bitmap may not be enough. If you had a look at Tasks and Back Stack you may have noticed that 1 app process can have 2 or more independent tasks in parallel. That means there could be whatever Activity uses the bitmap in different stages. If you don't want to overwrite your cache bitmap between those all the time, you may have to have 1 bitmap per task.
I don't know how to do it per task, but you can easily use a retained fragment to tie the life of your bitmap to that of an activity (ignoring screen rotation etc): http://www.androiddesignpatterns.com/2013/04/retaining-objects-across-config-changes.html / example with bitmap cache https://github.com/google/iosched/blob/master/android/src/main/java/com/google/samples/apps/iosched/util/BitmapCache.java

I think best way is to create Singleton with hard reference plus some methods for recycling and loading, I've created a lot different way Bitmap and graphics loaders and this is probably most efficient and easiest for access way
public class BitmapLoader{
public static BitmapLoader bl;
private static Bitmap b=null;
public static BitmapLoader getInstance(){
if(bl==null)
bl = new BitmapLoader();
return bl;
}
public Bitmap getBitmap(Context c){
if(b==null && c!=null)
b=loadBitmapUsingContectIfNeededOrWhateverYouWant(c);
return b;
}
public void recycleBitmap(){ //for e.g. onDestroy() Activity or other
if(b!=null)
b.recycle();
}
}
keeping loaded Bitmap in LruCache is also good idea

How to clean memory after exiting activity

As title says, I have about eight acitivies with layout full of high-res images. On weaker android devicies with low RAM memory it opens each activity alone, but when I try open another, it crashes. But when I restart app and open that activity, it works. What should I do to clean apps memory from these images from first activity to be able to open another activity? Does onDestroy() clean it?

If it like resource images in xml layout, you don't need to clean up them, Android will do it for you. But if you use some big bitmaps objects.
Bitmpap bmp; // not null
bmp.recycle();
bmp = null;
final boolean bmpIsRecycled = bmp.isRecycled()
// Returns true if this bitmap has been recycled.
Free the native object associated with this bitmap, and clear the reference to the pixel data. This will not free the pixel data synchronously; it simply allows it to be garbage collected if there are no other references. The bitmap is marked as “dead”, meaning it will throw an exception if getPixels() or setPixels() is called, and will draw nothing. This operation cannot be reversed, so it should only be called if you are sure there are no further uses for the bitmap. This is an advanced call, and normally need not be called, since the normal GC process will free up this memory when there are no more references to this bitmap.
And actually when your app crashes, what error log do you have? Maybe it's not related with memory leak?

how to deal with unused bitmap object?

My application needs creating a bitmap object for a certain view every 1 min
private static Bitmap mBitmap = null;
public static Bitmap getBitmap()
{
//create new bitmap object
return mBitmap;
}
My question is, do I need to destroy mBitmap before creating new bitmap ?

You don't have to manually destroy a Bitmap after you used it, but you can help the garbage collector do its job. There is a method called recycle(), the following paragraph is from its documentation:
Free the native object associated with this bitmap, and clear the
reference to the pixel data. This will not free the pixel data
synchronously; it simply allows it to be garbage collected if there
are no other references. The bitmap is marked as "dead", meaning it
will throw an exception if getPixels() or setPixels() is called, and
will draw nothing. This operation cannot be reversed, so it should
only be called if you are sure there are no further uses for the
bitmap. This is an advanced call, and normally need not be called,
since the normal GC process will free up this memory when there are no
more references to this bitmap.
In earlier Android versions Bitmaps were handled natively by the OS. This was the original reason for the recycle() method. Since the Bitmaps were handled outside of the Java VM the garbage collector could not automatically free the memory of unused Bitmaps, you can find more information about that here but the important part is this:
On Android 2.3.3 (API level 10) and lower, the backing pixel data for
a bitmap is stored in native memory. It is separate from the bitmap
itself, which is stored in the Dalvik heap. The pixel data in native
memory is not released in a predictable manner, potentially causing an
application to briefly exceed its memory limits and crash. As of
Android 3.0 (API level 11), the pixel data is stored on the Dalvik
heap along with the associated bitmap.
So if you want to support Android 2.3.3 (Gingerbread) or below you have to be careful with Bitmaps. You always have to remember to call recycle() otherwise your application may crash in an unpredictable manner.
If you only support Android versions above Android 3.0 then you don't have to worry about freeing Bitmap memory, but if you create a lot of Bitmaps and/or are getting close to OutOfMemoryExceptions then calling recycle() on all not needed Bitmaps can still have
a considerable positive effect.
If you want to learn more about handling Bitmaps then visit this link.
I hope I could help you and if you have any further questions please feel free to ask.

You should never do that, you should either:
Reuse the Bitmap if size is always the same (clearing and reusing)
Caching bitmaps using an LRU cache
If you absolutely need to destroy it every time call Bitmap.recycle() and then create a new one, but this is a very very bad thing to do.

Reference counting android Bitmaps with caches

On Android pre-honeycomb, Bitmaps have freaky memory issues because their data isn't stored in the VM. As a result it isn't tracked or removed by the GC. Instead it is removed when Bitmap.recycle() is called (and that is also done automatically in the Bitmap's finalizer).
This leads to some problems when doing image caching. When a bitmap is due to be evicted, I can't simply call recycle() on it, because I have no idea if anyone else is using it.
My first thought was to do System.gc() just before I load each bitmap. That way, hopefully orphaned Bitmaps will be finalized and the native memory freed. But it doesn't work. Edit: Actually it does sort of work. I had my System.gc() in the wrong place, after moving it, and halving my cache size (to what seems like a ridiculously small 2 MB of uncompressed bitmap data), my app no longer seems to crash (so far)!
My next thought was to implement manual reference counting, by subclassing Bitmap and calling ReferenceCountedBitmap.decrementCount() in all my activities' onDestroy() methods. But I can't because Bitmap is final.
I am now planning a BitmapManager which keeps WeakReference's to the bitmaps, and has methods like:
public void using(Bitmap bm);
public void free(Bitmap bm);
which count the references.
Does anyone have any experience or advice handling this? Before you suggest it, I can't ignore 80% of the market.

Well, I solved this with a bitmap manager, where I save the referencing views. In a map-like structure bitmap -> list of views.
Before I call recycle() on a bitmap, I first set all the references from the views to null (otherwise it will throw bitmap recycled exception).
Manual garbage collection, as you say, doesn't work for bitmaps pre-honeycomb, since they are allocated in the native heap and even with System.gc() you can't make assumptions when this memory will be released.

Calling Bitmap#recycle() from a finalizer causes Android app to terminate silently

The Bitmap memory problem in Android has led me to implement a custom loader and downloader class that will download an image from the web, store a local copy, and create a Bitmap if requested to do so from the local copy. I'm keeping these in a list of SoftReference<T> so that they are kept for a while, then garbage collected, at which point the class's finalize() method is called:
protected void finalize() throws Throwable {
Log.w("IMAGEPACK", "Finalizing " + mBitmap);
if(mBitmap!=null&&!mBitmap.isRecycled()) mBitmap.recycle();
super.finalize();
}
Looking at LogCat I've determined that this piece of code is happening right before the crash. I've also stepped through the code and it's on this line that it fails.
I had a solution that used synchronized reference counting previously, and which seemed pretty reliable, but having had big problems with manually-written reference-counting before I wanted to avoid this. I may have to revert to it, but I would like to know WHY bitmap recycling is failing here.
I am currently testing on a Samsung Galaxy S on 2.3.3.

You actually don't need to call recycle() if you lose the reference to be Bitmap. The Bitmap class is already overriding finalize() and deallocating the memory so what you're doing is redundant.
Also, according to the docs itself:
This is an advanced call, and normally need not be called, since the
normal GC process will free up this memory when there are no more
references to this bitmap.
Recycle exists if you don't need the bitmap anymore, but for whatever reason you need to keep the reference around (perhaps you're overloading the Bitmap object with extra meta-data that you want to keep). If you're just holding a reference to the Bitmap object, then dereferencing it is enough.