There are many ways. The right one for you depends on what you're doing and where the applications are located. If you search this forum you'll find that this question is asked and answered a lot here.
One way to communicate is through the ServletContext of different web applications.Firstly you will have to get the ServletContext of different web applications.You can get the ServletContext of another web application by using this.getServletContext().getContext(uripath),and use this ServletContext which is of another web application to share objects and this way you will be able to communicate through them. There may be more and better ways to do this,I am just specifying one.
Joined: Mar 22, 2005
Just to throw a few other methods into the mix: mail, JMS, database, files, web service, HTTP -which probably would be the most natural for a web application-, ...
As was mentioned before, you'll find this discussed plenty of time if you search through old posts.