Java Server Pages

Using a JSP Error Page for All Runtime Errors

Even if an application is developed with different types of components, it should deal with runtime errors in a consistent way. Recall from Chapter 7, how the page directive can specify a JSP page to be used when code in the JSP page throws an exception. The error page gets access to the exception through an implicit variable named exception, and can display a user-friendly message as well as log details about the problem. The way the error page is invoked and how the implicit variable gets its value can easily be mimicked by a servlet. Example 14.4 shows how to do this in the controller servlet used in previous examples in this chapter.

Example 14-4. Invoking a JSP Error Page from a Servlet

public void doPost(HttpServletRequest request, 
    HttpServletResponse response) {
 
    ...
    try {
        action.perform(this, request, response);
    }
    catch (Throwable t) {
        request.setAttribute("javax.servlet.jsp.jspException", t);
        
        RequestDispatcher rd = 
            getServletContext( ).getRequestDispatcher("/error.jsp");
        rd.forward(request, response);
    }
}

The servlet calls the action’s perform( ) method within a try block. If any type of exception occurs while executing an action, the servlet catches it, sets the javax.servlet.jsp.jspException request attribute to the exception object, and forwards the request to the error JSP page. The javax.servlet.jsp.jspException attribute name is reserved for the exception object in the JSP specification, so the JSP container picks up the ...