Looking up how much direct buffer memory is available to Java?

Question

How can one look up how much direct memory is currently allocated (and may be allocated) by Java? As the evaluator of http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4879883 mentions, Java maintains these buffers outside the normal Java heap.

Answer 1

There are probably other ways to do this with native code (or profiling, etc.), but something like this might work in Java:

Class c = java.nio.Bits.class;
Field maxMemory = c.getDeclaredField("maxMemory");
maxMemory.setAccessible(true);
Field reservedMemory = c.getDeclaredField("reservedMemory");
reservedMemory.setAccessible(true);
synchronized (c) {
    Long maxMemoryValue = (Long)maxMemory.get(null);
    Long reservedMemoryValue = (Long)reservedMemory.get(null);
}