Java APIs

i have build a mulit client/server application.. but i have a problem that i can�t figure out how to deal with..

when the user login with his username and pass i want to check if he is already is online because i want to build a application that only accept that one user is online on one client..

someone have any idea or tips how i should do?

i have one idea, that i should add one more colum in my database that is boolean that changes so the user only can log in to the system if the "online" boolean is false... what do you guys think about that solution??

but i guess this solution demand that the user log out proper so i can set the boolean to false again..

i hope you guys can help me find a solution
sry for my bad English..