JavaServer Pages, 3rd Edition

Integrating Custom Conditional Actions

The JSTL core library contains one, generic conditional action: <c:if>. This action handles all conditions that can be expressed as Boolean EL expressions, but you often need more than that. Examples from Part II of this book include: testing if a mail address has valid syntax and if the current user belongs to a specific group.

To help developing this type of conditional custom action, JSTL includes a base class called javax.servlet.jsp.jstl.core.ConditionalTagSupport :

public abstract class ConditionalTagSupport
  extends javax.servlet.jsp.tagext.TagSupport

It contains the following public methods:

protected abstract boolean condition(  ) throws JspTagException
public void setVar(String var)
public void setScope(String scope)
public int doStartTag(  ) throws JspException
public void release(  )

The doStartTag( ) implementation calls the condition( ) method and takes care of saving the result if the var and scope attributes are set.

By extending this class and providing an implementation of the condition( ) and setter methods for all attributes you need, you get a conditional action that is consistent with the semantics of the JSTL version.

Example 23-1 shows the tag handler class for <ora:ifUserInRole>, which takes advantage of this JSTL support class.

Example 23-1. Tag handler for a conditional action

package com.ora.jsp.tags; import javax.servlet.http.*; import javax.servlet.jsp.*; import javax.servlet.jsp.jstl.core.*; import org.apache.taglibs.standard.lang.support.*; ...