Bitmaps in Android

Question

I have a few questions regarding Bitmap objects and memory and their general taxonomy.

1. What is an in-memory or native bitmap?
2. How is Bitmap memory different from Heap memory?

Answer 1

The memory that backs a Bitmap object is allocated using native code (malloc()), rather than the Java new keyword. This means that the memory is managed directly by the OS, rather than by Dalvik.

The only real difference between the native heap and Dalvik's heap is that Dalvik's heap is garbage collected, and the native one isn't.

For these purposes though, here's not much difference. When your Bitmap object gets garbage collected, it's destructor will recycle the associated memory in the native heap.

Source:

• http://osdir.com/ml/AndroidDevelopers/2009-03/msg00023.html
• android:platform/frameworks/base/graphics/java/android/graphics/Bitmap.java
• android:platform/frameworks/base/core/jni/android/graphics/Bitmap.cpp

Answer 2

There is an important subtlety here: though Bitmap pixels are allocated in the native heap, some special tricks in Dalvik cause it to be accounted against the Java heap. This is done for two reasons:

(1) To control the amount of memory an application allocates this. Without the accounting, an application could allocate a huge amount of memory (since the Bitmap object itself is very small yet can hold on to an arbitrarily large amount of native memory), extending beyond the 16MB or 24MB heap limit.

(2) To help determine when to GC. Without the accounting, you could allocate and release references on say 100 Bitmap objects; the GC wouldn't run, because these objects are tiny, but they could in fact represent a large number of megabytes of actual allocations that is now not being GCed in a timely manner. By accounting these allocations against the Java heap, the garbage collector will run as it thinks memory is being used.

Note that in many ways this is an implementation detail; it is very likely that it could change in the future, though this basic behavior would remain in some form since these are both important characteristics for managing bitmap allocations.