Getting cpu usage in java instance, that limits in executing java virtual machine, not os.
Have a look at the java class sun.tools.jconsole.SummaryTab.java in open JDK7 or source in web.(http://www.docjar.com/html/api/sun/tools/jconsole/SummaryTab$Result.java.html)
If you know JConsole, you know that cpu usages graph. that source is below.
So. I made my own class to get java-cpu usage.