How to make a only one spectified type object running at the same time ?

I want to run specified function with java program in unix,AIX environment.But i want only one specified type object can run at this time when i make command "java xxx &" several times at this time.I had used thread but it can not work. I think there are two processes for system so it can not work. Which class or way in java can i use for this problem ? or I use thread in the wrong way.
please help me . thank you everybody !!!
ps:the command "java xxx &" mentioned above,xxx is name of java program ,
& means command run in background.

RMI is a good option you can think of. There is one registry and only one object can be bound to it with a specified name. But if your application is large, RMI calls can really slow down your application.
Another option is to use Sockets. When you load the application check if you can start a server socket on a particular port. If you can, then start the program. If not, another application is already running. So exit your program. The socket you have created will be used only to bind to a particular port. In any system, there can be only one socket bound to a port and this guarantees the single instance running. When your application closes, close the socket too.

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