Why do we somtimes encounter a "400 GET not supported" message?

If you don't have a doGet method in your servlet, then that explains it. Someone's calling your servlet with a GET request, and your servlet (via the superclass implementation of doGet) is saying it can't do that.

Have your doGet call doPost and it should work fine.

Aron Tunzi adds:

); INSERT INTO you servlet the following code:

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{	
   doPost(request,response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{	
  //insert your code hier.
}