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package org.apache.taglibs.standard.tag.common.core;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.JspTagException;
import javax.servlet.jsp.jstl.core.ConditionalTagSupport;
import javax.servlet.jsp.tagext.Tag;

import org.apache.taglibs.standard.resources.Resources;

/**
 * <p>WhenTagSupport is an abstract class that facilitates
 * implementation of <when>-style tags in both the rtexprvalue
 * and expression-evaluating libraries.  It also supports
 * <otherwise>.</p>
 *
 * <p>In particular, this base class does the following:</p>
 * 
 * <ul>
 *  <li> overrides ConditionalTagSupport.doStartTag() to implement the
 *       appropriate semantics of subtags of <choose> </li>
 * </ul>
 *
 * @author Shawn Bayern
 */
public abstract class WhenTagSupport extends ConditionalTagSupport
{
    //*********************************************************************
    // Implementation of exclusive-conditional behavior

    /*
     * Includes its body if condition() evalutes to true AND its parent
     * ChooseTag wants it to do so.  The condition will not even be
     * evaluated if ChooseTag instructs us not to run.
     */
    public int doStartTag() throws JspException {

        Tag parent;

        // make sure we're contained properly
        if (!((parent = getParent()) instanceof ChooseTag))
            throw new JspTagException(
		Resources.getMessage("WHEN_OUTSIDE_CHOOSE"));

        // make sure our parent wants us to continue
        if (!((ChooseTag) parent).gainPermission())
            return SKIP_BODY;                   // we've been reeled in

        // handle conditional behavior
        if (condition()) {
            ((ChooseTag) parent).subtagSucceeded();
            return EVAL_BODY_INCLUDE;
        } else
            return SKIP_BODY;
    }
}