JavaServer Pages, Second Edition

Handling Exceptions

In most cases, the default handling for an exception thrown by JSP elements is sufficient; the container forwards control to an error page where you can display a nice, user-friendly message. But for some types of tag handlers -- for instance a tag handler that uses a pooled resource (such as a connection from a connection pool) -- there must be a fail-safe way to handle exceptions thrown by nested elements, for instance to return the shared resource to the pool. If exceptions aren’t handled correctly, the resource pool “leaks,” and eventually the application runs out of resources and comes to an embarrassing halt. None of the three main interfaces include methods that are called in case of an exception in the element’s body. JSP 1.2 therefore added a new interface to let you deal with exceptions.

The TryCatchFinally interface is a so-called mix-in interface, which means that a tag handler can implement it only in addition to one of the three main tag handler interfaces. It has two methods:

• public void doCatch(Throwable t) throws Trowable

• public void doFinally( )

Example 20-4 shows the tag handler for an action that writes the result of the evaluation of its body to a file, identified by an attribute named fileName.

Example 20-4. A tag handler implementing TryCatchFinally

package com.ora.jsp.tags; import java.io.*; import javax.servlet.*; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; public class FileWriteTag extends BodyTagSupport implements ...