When I use `jhsdb jsnap` to get PerfCounter from core images, I encountered NPE:
-------------
Exception in thread "main" java.lang.NullPointerException
        at sun.jvm.hotspot.tools.JSnap.run(JSnap.java:45)
        at sun.jvm.hotspot.tools.Tool.startInternal(Tool.java:260)
        at sun.jvm.hotspot.tools.Tool.start(Tool.java:223)
        at sun.jvm.hotspot.tools.Tool.execute(Tool.java:118)
        at sun.jvm.hotspot.tools.JSnap.main(JSnap.java:67)
        at sun.jvm.hotspot.SALauncher.runJSNAP(SALauncher.java:352)
        at sun.jvm.hotspot.SALauncher.main(SALauncher.java:404)
-------------

PerfMemory::destroy() clears all members which are used in JSnap.
Thus NPE is occurred.

http://mail.openjdk.java.net/pipermail/serviceability-dev/2016-January/018810.html