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

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.tagext.Tag;

import org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager;
import org.apache.taglibs.standard.tag.common.core.NullAttributeException;

/**
 * <p>Contains some static utilities to facilitate common forms of
 * expression evaluation.</p>
 *
 * @author Shawn Bayern
 */

public class ExpressionUtil {

    /** Evaluates an expression if present, but does not allow the expression
     *  to evaluate to 'null', throwing a NullAttributeException if it
     *  does.  The function <b>can</b> return null, however, if the
     *  expression itself is null.
     */
    public static Object evalNotNull(String tagName,
				     String attributeName,
	                             String expression,
				     Class expectedType,
				     Tag tag,
	                             PageContext pageContext)
	        throws JspException {
        if (expression != null) {
            Object r = ExpressionEvaluatorManager.evaluate(
                attributeName, expression, expectedType, tag, pageContext);
            if (r == null)
                throw new NullAttributeException(tagName, attributeName);
	    return r;
        } else
	    return null;
    }
}