Using Bitmap Pools in Android


I Amit Shekhar, writing this article to share my knowledge of Bitmap pools in Android.

Image heavy applications have to decode many images, so there will be continuous allocation and deallocation of memory in application. This results in frequent calling of the Garbage Collector (GC). And if you call the GC too many times, your application UI freezes.

If you want to read more about the mechanics of UI freezes and how to avoid them, here’s a good primer.

Glide and Fresco use the Bitmap Pool Concept to load images efficiently.

GET RID OF : GC_FOR_ALLOC freed 1568K, 23% free 37664K/48844K, paused 141ms, total 143ms - (whenever you see this log , your application is lagging)


By using the Bitmap pool to avoid continuous allocation and deallocation of memory in your application, you reduce GC overhead, which results in a smooth-running application.

One way to do this is to use inBitmap (which reuses bitmap memory).

Suppose we have to load a few bitmaps in an Android application.

When we load bitmapOne, it will allocate the memory for bitmapOne.

Then if when we no longer need bitmapOne, do not recycle the bitmap (as recycling involves calling GC). Instead, use this bitmapOne as an inBitmap for bitmapTwo. This way, the same memory can be reused for bitmapTwo.

Bitmap bitmapOne = BitmapFactory.decodeFile(filePathOne);
// lets say , we do not need image bitmapOne now and we have to set // another bitmap in imageView
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeFile(filePathTwo, options);
if (canUseForInBitmap(bitmapOne, options)) { 
//canUseForInBitmap check if the image can be reuse or not by //checking the image size and other factors
options.inMutable = true;
    options.inBitmap = bitmapOne;
options.inJustDecodeBounds = false;
Bitmap bitmapTwo = BitmapFactory.decodeFile(filePathTwo, options);
public static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {

// From Android 4.4 (KitKat) onward we can re-use if the byte size of
        // the new bitmap is smaller than the reusable bitmap candidate
        // allocation byte count.
int width = targetOptions.outWidth / targetOptions.inSampleSize;
int height = targetOptions.outHeight / targetOptions.inSampleSize;
int byteCount = width * height * getBytesPerPixel(candidate.getConfig());

try {
return byteCount <= candidate.getAllocationByteCount();
        } catch (NullPointerException e) {
return byteCount <= candidate.getHeight() * candidate.getRowBytes();
// On earlier versions, the dimensions must match exactly and the inSampleSize must be 1
return candidate.getWidth() == targetOptions.outWidth
&& candidate.getHeight() == targetOptions.outHeight
&& targetOptions.inSampleSize == 1;
private static int getBytesPerPixel(Bitmap.Config config) {
 if (config == null) {
     config = Bitmap.Config.ARGB_8888;
int bytesPerPixel;
 switch (config) {
   case ALPHA_8:
       bytesPerPixel = 1;
   case RGB_565:
   case ARGB_4444:
       bytesPerPixel = 2;
   case ARGB_8888:
       bytesPerPixel = 4;
 return bytesPerPixel;

So, we are reusing the memory of bitmapOne while decoding bitmapTwo.

InBitmap works here.

So inBitmap is important here.

Similarly, while implementing it in the listview, recyclerview — we will be able to get smooth scrolling. Glide and Fresco load the image smoothly by using this bitmap pool concept.

Without Bitmap Pool, there will be flickering and lagging in UI while scrolling.

In this way, we can avoid continuous allocation and deallocation of memory in the application, reduce the GC overhead, and maintain a smooth-running application.

But there’s one problem: there are a few restrictions for using BitMap Pools in versions of Android that are older than Honeycomb. And only a few android versions older than Kitkat will only allow us to use inSampleSize = 1. But any version newer than KitKat will support BitMap with only a few minor issues.

So, all these types of cases are handled in this library Glide Bitmap Pool, which you can use to optimize your Android application.

As all the cases are handled in this library, you just have to decode images using this library and recycle these images using this library. There’s no need to handle version-wise cases.

Here you can find the Glide Bitmap Pool

Amit Shekhar

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