I develop a website on Java. But I want to use Drupal for the rest of the system. So I want that my Drupal users can be logged in at the same time as my Java website.

In the webserver, default port is for Drupal, and 8080 is for my Java website. When a user logged into Drupal, I want to redirect him to Java system (or put a link to go there). How can I make him logged in Java (or another web platform) by only logging to Drupal website? What should I know in order to make that happen? What keywords do I need to research?

Thanks for help.