Servlet Name

[Shravan Durvasula]

Hi all:

Why is it that the below servlet throws a null pointer exception when the init(ServletConfig config) is overridden instead of init() ?

  
public class HelloServlet extends HttpServlet
{
      public void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
      {
        PrintWriter out = response.getWriter();        
        out.write(getServletName());
        out.close();
      }
 
      public void init()
      {
        // Initialization
      }
}

Thanks,
-SkD

[Gabriel Forro]

Hi,

the HttpServlet has a private member (type of ServletConfig). This private member is initialized in the init(ServletConfig) method.
The getServletName() calls the ServletConfig's getServletName() function, so it uses the private member (which is null, if it is not initialized).

If You override this function, make sure do it as it follows:

public void init(ServletConfig sc) {
    // your code here
 
    // to make sure the private member is initalized
    base.init(sc);

[Ture Hefner]

Right. If you override the init(ServletConfig sc) method in a Servlet, you must still call the overridden method in order to initialize the instance varible that keeps a handle to the ServletConfig. However, you use the "super" keyword to do this, not "base":

// base.init(sc); // nope
super.init(sc); // yessiree

See http://java.sun.com/docs/books/tutorial/java/javaOO/methodbody.html

[Gabriel Forro]

Of course )

I am ashamed
(I have to use .net at work, where base == super...I am sorry again)

[Shravan Durvasula]

Thanks Gabriel and Ture ... that helped.