c:catch

We already know how to pass the control to an error page in case of exception in a JSP page, but we can do something better.

If a block could throw an exception, and we know how to recover to the possible error condition, we can shield that code using a c:catch JSTL tag. The name is a bit misleading, because it acts more like a "try" in the try/catch construct.

Let's see it at work:

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<!-- ... -->

About to do a risky thing...
<c:catch>
    <% int x = 10/0; %>
</c:catch>
<p>If you see this, we survived.</p>

The Java scriptlet throws an exception, but this doesn't lead us to the error page, since the code is shielded by a c:catch.

If we want to keep track of the thrown exception we have to declare it as var attribute in the c:catch tag, and check it against null after the end of the c:catch block:

<c:catch var="ex">
Another risky thing ...
<% int x = 10/0; %>
<p> ... this line won't be printed</p>
</c:catch>

<c:if test="${ex != null}">
    <br />Exception caught: ${ex.message}
</c:if>

The c:catch behave like a Java "try" block even in the sense that the code after the exception-generating line is not executed.

More on JSTL and error conditions in chapter nine of Head First Servlet and JSP.