From the Java docs,
The contents of direct buffers may reside outside of the normal garbage-collected heap, and so their impact upon the memory footprint of an application might not be obvious
Also from the Java docs,
MappedByteBuffer: A direct byte buffer whose content is a memory-mapped region of a file.
and
A mapped byte buffer and the file mapping that it represents remain valid until the buffer itself is garbage-collected.
I believe that off the heap memory allocations cannot be garbage-collected by the GC. In this case, these statements make me curious about memory management of a MappedByteBuffer. What happens if the direct ByteBuffer backing a MappedByteBuffer sits outside the normal heap?