Pass values from main window to pop up window

I am doing a program in JSP now. What I want to do is : when user click a button, a pop up window will be displayed and the problem is how to pass values from the main window (the window in which the button is located) to the pop up window ? Usually, I use request.getParameter() to get values from another window, but it didn't work for pop up window. Thanks for your help.

Params in the URL should be picked up by request.getParameter() in your popup. How are you launching the popup?