Restrict multiple instances of an application in java

Question

I want to prevent multiple instances of application being launched in java. I know 2 methods for this:

1. locking file
2. locking socket

But

Answer 1

My vote goes to locking on a port (i think this is what you mean by socket). I don't know the exact reason for this. But in fact i come across only this as a solution in most practical projects. Though i will be happy to hear the alternative ways.