Configuring error pages in the DD

Sure, you want to be friendly when the user doesn’t know the exact resource to ask for when they get to your site or web app, so you specify default/welcome files. But you also want to be friendly whenthings go wrong. We already looked at this in the chapter on Using Custom Tags, so this is just a review.

Declaring a catch-all error page

This applies to everything in your web app—not just JSPs.

<error-page>
  <exception-type>java.lang.Throwable</exception-type>
  <location>/errorPage.jsp</location>
</error-page>

(FYI: you can override this in individual JSPs by adding a page directive with an errorPage attribute.)

Declaring an error page for a more explicit exception

This configures an error page that’s called only when there’s an ArithmeticException. If you have both this declaration and the catch-all above, then any exception other than ArithmeticException will still end up at the “errorPage.jsp”.

<error-page>
  <exception-type>java.lang.ArithmeticException</exception-type>
  <location>/arithmeticError.jsp</location>
</error-page>

Declaring an error page based on an HTTP status code

This configures an error page that’s called only when the status code for the response is “404” (file not found).

<error-page>
  <error-code>404</error-code>
  <location>/notFoundError.jsp</location>
</error-page>

Note

You can’t use <error-code> and <exception-type> together!

You can configure an error page based on the HTTP status code OR based on the exception type thrown, but you CANNOT have both in the same <error-page> tag.

There are no Dumb Questions

Q:

Q: What are you allowed to declare as an exception type in <exception-type>?

A:

A: Anything that’s a Throwable, so that includes java.lang.Error, runtime exceptions, and any checked exception (as long as the checked exception class is on the Container’s classpath, of course).

Q:

Q: Speaking of error handling, can you programmatically generate error codes yourself?

A:

A: Yes, you can. You can invoke the sendError() method on the HttpServletResponse, and it’ll tell the Container to generate that error just as if the Container generated the error on its own. And if you’ve configured an error page to be sent to the client based on that error code, that’s what the client will get. And by the way, “error” codes are also known as “status” codes, so if you see either one, they mean the same thing—HTTP codes for errors.

Q:

Q: How about an example of generating your own error code?

A:

A: OK, here’s an example:

response.sendError(HttpServletResponse.SC_FORBIDDEN);

which is the same as:

response.sendError(403);

If you look in the HttpServletResponse interface, you’ll see a bunch of constants defined for the common HTTP error/status codes. Keep in mind that for the exam, you don’t need to memorize the status codes! It’s enough to simply know that you can generate error codes, that the method is response.sendError(), and that in terms of the error pages you’ve defined in the DD, or any other error-handling you do in your JSPs, there’s no difference between Container-generated and programmer-generated HTTP errors. A 403 is a 403 regardless of WHO sends the error. Oh yeah, there’s also an overloaded two-argument version of sendError() that takes an int and a String message.

Note

You must use the fully-qualified class name in <exception-type>!

Don’t be fooled by something like this:

<exception-type>
     IOException
</exception-type>

You MUST use the fully-qualified class name, and any Throwable is allowed.

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