Hi,

Thanks for that.  I've followed your advice.

Regards

Steve

Juha Laiho <Juha.Laiho@iki.fi> said:

Ok, and after writing an example of this for myself, some streamlining.

- the business logic class will throw a "BusinessException" (subclass
 of exception), or a subclass of BusinessException, if more fine-grained
 exception handling is needed
- the servlet calling the business logic class is prepared to catch
 BusinessException, and rethrow it wrapped in a ServletException
- web.xml contains an error-page declaration to catch BusinessException,
 and the designated error page (JSP or servlet) will pick up the
 exception information from request attributes, and show the desired
 error message to the end user

... so, subclassing the ServletException is not needed, and error-page
declaration can be used to trap also "business" exceptions.

Code fragments showing the essentials:
web.xml error-page declaration:
   <error-page>
       <exception-type>errordemo.business.BusinessException</exception-type>
       <location>/errorHandler.jsp</location>
   </error-page>

original exception throwing from "business logic class" (real code
would naturally try to do some real work as well):
   public void doSomething() throws BusinessException {
       throw new BusinessException("Description / detailed error message");
   }

catch/rethrow in servlet class:
       try {
           business.doSomething();
       } catch (BusinessException be) {
           throw new ServletException(be);
       }

accessing the error message from the original exception in the error handler:
         <li>
           <strong>Error message:</strong>
           <%= request.getAttribute("javax.servlet.error.message") %>
         </li>

accessing the original exception class (here just to get the class name,
but similar access allows "anything" to be done with the exception):
         <li>
           <strong>Exception type was:</strong>
           <%= ((Class) (request.getAttribute("javax.servlet.error.exception_type"))).getName() %>
         </li>

Relevant documentation is the Servlet Specification; see SRV.9.9 (Error
Handling, pp. 75-77) from the specification at
http://www.jcp.org/aboutJava/communityprocess/final/jsr154/index.html .
It describes how the error handling process works, and also contains
descriptions for other available javax.servlet.error.* attributes.