HomeJava & J2EE Page 5 - The JSP Files (part 7): Bugs, Beans And Banks
You Throw(), I'll Catch - Java
In this week's episode, find out how the JSP Exception objectprovides developers with a graceful way to recover from script errors. Andthen take a quick tour of the JSP directives you need to know in order tointegrate standalone JavaBeans into your JSP scripts.
It's also possible to use the Java "throw" construct to artificially induce an exception in your JSP script. This comes in handy, for example, when validating form field data - if the values entered are not in the expected format, you can throw an exception (with an informative error message) and re-direct the user to an error page.
Here's an example of how this can be used. This is a simple form which asks you to enter a number
<html>
<head>
<basefont face="Arial">
</head>
<body>
<form action="number.jsp">
Enter a number between 1 and 3 <input type=text name=number size=1>
</form>
</body>
</html>
and this is the server-side JSP script which checks it
for errors, and throws an exception if certain conditions are not met.
<html>
<head>
<basefont face="Arial">
</head>
<body>
<%@ page errorPage="error.jsp" %>
<%
String temp = request.getParameter("number");
int number = Integer.parseInt(temp);
if (number != 2)
{
throw new Exception ("How dumb can you get?!") ;
}
else
{
out.println("Hmmm...maybe you're not as dumb as you look!");
}
%>
</body>
</html>
Next up, a brief look at JavaBeans and how they integrate
with the JSP environment.
Java & J2EE 
Overview of Java Web Technologies, Part 2
