Handling exceptions in servlets

Question

I have a small application using JSPs and servlets for demonstration purposes and currently have code like this to handle exceptions thrown by DAOs and for validation of parameters from requests:

    // Get ID from request. 
    int id = 0;  
    try {
        id = Integer.parseInt(request.getParameter("id"));
    }
    catch (NumberFormatException e) {                         
        messages.addMessage(e.getMessage());
        request.setAttribute("messages", messages.getMessages());            
        response.sendRedirect("/jsp/exceptions/error500.jsp");
    }

    // Check person exists.
    PersonDAOImpl personDAO = new PersonDAOImpl();  
    Person person = null;
    try {
        person = personDAO.get(id);
    } 
    catch (DAOException e) {          
        messages.addMessage(e.getMessage());
        request.setAttribute("messages", messages.getMessages());            
        response.sendRedirect("/jsp/exceptions/error500.jsp");
    }

So when an exception is thrown it can be caught in the servlet and displayed on a general error page. The messages object is simply an instance of a Messages utility class which stores a number of messages in an arraylist.

But what I am puzzled about is how best to remove the clutter caused in the servlets by exception handling code like this:

        messages.addMessage(e.getMessage());
        request.setAttribute("messages", messages.getMessages());            
        response.sendRedirect("/jsp/exceptions/error500.jsp");

Any ideas?

Simple validation messages are currently treated differently. If messages need to be displayed to a user e.g. if a numeric parameter to a servlet is out of range, the messages are written to a Messages object. This object is then written to the JSP where the messages can be displayed using JSTL tags. But a generic exception could be used here.

My main aim is to try to keep this simple without going 'overboard' because this application is only a demonstration., e.g. it doesn't matter if stacktrace text is displayed on a page or not.

Answer 1

One option is to create an abstract parent servlet class that you can extend, then expose the logic for handling exceptional behavior in a protected method or variables, which you can access from your public servlets.

You can also add something like this to you web.xml:

<error-page>
    <exception-type>java.lang.Exception</exception-type >
    <location>/ErrorHandler</location>
</error-page>

We use that in production for small projects with JSF and will do the work for Servlet and JSP. More info here.

What I like to do is hide the exceptions in production, but in development we just display it so it kinda speeds up development.

Just remmember that for scaling well with SOAP and REST you might have to do some extra work, but I think for your scenario it should be fine.

Answer 2

I now have:

int id = 0;  
    try {
        id = Integer.parseInt(request.getParameter("id"));
    }
    catch (NumberFormatException e) {              
        throw new ServletException(e);           
    }

And <%@page isErrorPage="true" %> in my JSPs and everything appears fine.

Thanks to the respondents.