java - Examples of forcing freeing of native memory direct ByteBuffer has allocated, using sun.misc.Unsafe?

Question

JDK provides abillity to allocate so-called direct ByteBuffers, where memory is allocate outside of Java heap. This can be beneficial since this memory is not touched by garbage collector, and as such does not contribute to GC overhead: this is a very useful for property for long-living things like caches.

However, there is one critical problem with existing implementation: underlying memory is only allocated asynchronously when the owning ByteBuffer is garbage-collected; there is no way to force early deallocation. This can be problematic since GC cycle itself is not influenced by handling of ByteBuffers, and given that ByteBuffers are likely to reside in Old Generation memory area, it is possible that GC is called hours after ByteBuffer is no longer in use.

But in theory it should be possible to use sun.misc.Unsafe methods (freeMemory, allocateMemory) directly: this is what JDK itself uses for allocating/deallocating native memory. Looking at code, one potential concern I see is possibility of double-freeing of memory -- so I would want to make sure that state would be properly cleaned.

Can anyone point me to code that does this? Ideally would want to use this instead of JNA.

NOTE: I saw this question which is sort of related.

Looks like the answers pointed out are good way to go: here is code example from Elastic Search that uses the idea. Thanks everyon!

Answer 1

There is a much simpler way to clean memory.

public static void clean(ByteBuffer bb) {
    if(bb == null) return;
    Cleaner cleaner = ((DirectBuffer) bb).cleaner();
    if (cleaner != null) cleaner.clean();
}

Using it can make a big difference if you are discarding direct or memory mapped ByteBuffer fairly quickly.

One of the reasons for using the Cleaner to do this is that you can have multiple copies of the underlying memory resources e.g. with slice(). and the Cleaner has a resource count of these.