I have three struts applications, in run time I have to switch from one application to another application based on single login page. I have taught cookies are one of the solution. But while I'm googling I unable to get such a good material.
The HTTPSession won't work between different applications, since the J2EE specification mandates that different applications not share the same HTTPSession even if they're on the same server.
I'd suggest you just set a cookie with some sort of token indicating who the user is in your login action. Then have your other application check for the presence of this cookie. If it's not there, make them log in. If it is there, use the information in the cookie to get their information from a database, set it in the HTTPSession and let them continue.
Below is a link that shows you how to set and retrieve cookies. The sample code it gives is in a Servlet, but the same code could be used in a Struts Action as well.