Question

I find that this.getServletName() fails in a constructor but works in a method. Note getServletName() is provided by the parent of the parent. This was observed in Google App Engine. What is the rationale for this.getServletName() behaving this way?

(The failure is a null pointer dereferencing but I note that this is not null at the time so I think the null may be something internal to the JRE. Furthermore and as expected, assignments of the sort this.myprivate = myconstructorarg; do not produce a null dereferencing in constructors.)

public class ResponderServlet extends HttpServlet 
{
        public ResponderServlet() 
        {
            String ss = this.getServletName();  // RUNTIME ERROR
        }

        public void doMethod(HttpServletRequest req, HttpServletResponse resp)
          throws IOException 
        {
            String ss = this.getServletName();  // WORKS WELL
        }
}

Answer 1

The getServletName() method will work after the web container calls the init(config) method. You should likely put your initialization logic there instead of the constructor. E.g.:

public void init(ServletConfig config) {
    super.init(config);
    String ss = this.getServletName();
    // put your logic here instead of constructor
}