Any number of exceptions can occur when a servlet executes. When an exception occurs, the web container generates a default page containing the message
A Servlet Exception Has Occurred
But you can also specify that the container should return a specific error page for a given exception. Review the deployment descriptor file included with the example to learn how to map the exceptions exception.BookNotFound, exception.BooksNotFound, and exception.OrderException returned by the Duke’s Bookstore application to errorpage.html.
See Mapping Errors to Error Screens for instructions on how to specify error pages using NetBeans IDE.