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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?

Is my image caching process going to leak memory?

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.

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.

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:
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).
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.

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."

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.