In Java there's the method Runtime.getRuntime().availableProcessors() which has the following Javadoc:
Returns the number of processors available to the Java virtual machine. This value may change during a particular invocation of the virtual machine.
How can the value actually change? Under what circumstances would there be less processors available to the JVM than physically installed for example?
The Linux command taskset(1) can be used to force processes to use a specific CPU or specific sets of CPUs; it is pretty easy to modify a running program to force it to one or more processors. For example,
taskset -p `pidof java` --cpu-list 0,5,7,9-11