This is not a limitation of Java, it will use as much as it needs of what is made available by the operating system and the cpu.
The limitations are totally dependent on the combination of os and cpu, this thread discusses it some
http://forum.java.sun.com/thread.jsp?forum=37&thread=567484
There are other threads that discuss this, search for them.
MS's MSDN has the detail information for Windows on the various cpu's.