[Ubersoldat]

http://monocaffe.blogspot.com/2011/10/javas-ghost-jvms.html

"how does tools like JPS, JConsole or VisualVM know the JVMs running in a system? They simply look in the folder /tmp/hsperfdata_foo where a 32KB data file with the PID as name is created for each JVM."

Not a hidden feature of Java, but of Sun's (Oracle) JVM which I'm not sure if happens in other JVMs but it's kind of useful for doing different stuff.