I am not sure if this is what you really want (you gave too little information), but javax.jnlp.SingleInstanceService and javax.jnlp.SingleInstanceListener might be what you need. Make your main application class implement this interface, and then whenever a new instance of this app is to be started, and there is a running instance already, the running instance's callback method will be invoked.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com
subject: How to avoid web start application relaunch ?