Well, with a MappedByteBuffer (or any DirectByteBuffer), if you want to manipulate the data as a Java type (e.g. byte[]) you have to copy the data into the heap. byte[] cannot exist outside of the heap.

Still, I wonder why they're using a MappedByteBuffer in the first place if they're working with the data in the Java heap.

mmap uses the virtual memory manager to map a file into virtual memory as it does for swapping.

The memory you have is therefore allocated by the kernel of your operating system and oblivious to any garbage collector you might have.