JavaServer Pages, 3rd Edition

Handling Exceptions

Methods called by a tag handler may throw exceptions. Exception handling in a simple tag handler is very easy compared to how it’s done for a classic tag handler, since all processing takes place in the single doTag( ) method. You must catch exceptions and either deal with them, or rethrow them wrapped in a JspException or the JspTagException subclass. Both exception classes have the same types of constructors:

public JspTagException(  );
public JspTagException(String msg);
public JspTagException(String msg, Throwable rootCause);
public JspTagException(Throwable rootCause);

When you rethrow an exception that you have caught, use one of the two latter constructors and pass on the exception, since the root cause is often needed to figure out what the problem is. Most containers unwrap the exception chain and write stack traces for all of the exceptions in the chain to the application log file. When throwing exceptions, you should avoid the no-argument constructor, since it doesn’t let you say what’s wrong, and use the second one for exceptions generated internally in the tag handler, for instance to report an invalid input type or insufficient privileges.

An exception thrown by a custom action can be caught and handled in the JSP page with the JSTL <c:catch> action, as I described in Chapter 9. If it’s not caught, it’s handled by the JSP container by forwarding to a custom error page, if specified, or to a container default page.

Calling the invoke( ) method of a fragment ...