How to do Servlet - Servlet Communication.. How to call a servlet from a servlet
These are two quite different questions. I hope you were expecting different answers.
To communicate between servlets you typically use the "application context" ( returned by getServletContext() ). Any objects placed as attributes in this context will be visible to all servlets and JSPs in an application.
To call a servlet from another servlet is more complex to answer. It largely depends whet you mean by "call a servlet". You can return an HTTP page to the browser which makes a redirect request to the new servlet, you can use a client API (I usually use HTTPUnit, but there are plenty of others) to make an HTTP call to the other servlet. You can forward the current HTTP request to the other servlet by obtaining a RequestDispatcher, etc..
Can you explain a bit more about what you want to do with this?