jdk7之后可以通过jmx监控堆外内存

java.nio:type=BufferPool,name=direct 
java.nio:type=BufferPool,name=mapped 
java.lang.management.BufferPoolMXBean

 
List pools = ManagementFactory.getPlatformMXBeans(BufferPoolMXBean.class);  

java.lang.management
public interface BufferPoolMXBean extends PlatformManagedObject 


DirectBufferPool
MappedBufferPool

SharedSecrets.getJavaNioAccess().getDirectBufferPool()
sun.nio.ch.FileChannelImpl.getMappedBufferPool()