Examples of forcing freeing of native memory direc

问题:

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!

回答1:

Using sun.misc.Unsafe is hardly possible because base address of the allocated native memory is a local variable of java.nio.DirectByteBuffer constructor.

Actually you can force freeing of native memory with the following code:

import sun.misc.Cleaner;

import java.lang.reflect.Field;
import java.nio.ByteBuffer;

...

public static void main(String[] args) throws Exception {
    ByteBuffer direct = ByteBuffer.allocateDirect(1024);
    Field cleanerField = direct.getClass().getDeclaredField("cleaner");
    cleanerField.setAccessible(true);
    Cleaner cleaner = (Cleaner) cleanerField.get(direct);
    cleaner.clean();
}

回答2:

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.

回答3:

Basically what you want is following the same semantics as using IO streams. Just like you need to close a stream once, you need to free the memory once. So you could write your own wrapper around the native calls making early freeing of memory possible