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

import javax.servlet.jsp.JspException;

import org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager;
import org.apache.taglibs.standard.tag.common.fmt.ParamSupport;

/**
 * <p>A handler for <param> that accepts message arguments as
 * Strings and evaluates them as expressions at runtime.</p>
 *
 * @author Jan Luehe
 */

public class ParamTag extends ParamSupport {

    //*********************************************************************
    // 'Private' state (implementation details)

    private String value_;                    // stores EL-based property


    //*********************************************************************
    // Constructor

    /**
     * Constructs a new ParamTag.  As with TagSupport, subclasses
     * should not provide other constructors and are expected to call
     * the superclass constructor
     */
    public ParamTag() {
        super();
        init();
    }


    //*********************************************************************
    // Tag logic

    // evaluates expression and chains to parent
    public int doStartTag() throws JspException {

        // evaluate any expressions we were passed, once per invocation
        evaluateExpressions();

	// chain to the parent implementation
	return super.doStartTag();
    }

    // Releases any resources we may have (or inherit)
    public void release() {
        super.release();
        init();
    }


    //*********************************************************************
    // Accessor methods

    // for EL-based attribute
    public void setValue(String value_) {
        this.value_ = value_;
	this.valueSpecified = true;
    }


    //*********************************************************************
    // Private (utility) methods

    // (re)initializes state (during release() or construction)
    private void init() {
        // null implies "no expression"
	value_ = null;
    }

    // Evaluates expressions as necessary
    private void evaluateExpressions() throws JspException {
        /* 
         * Note: we don't check for type mismatches here; we assume
         * the expression evaluator will return the expected type
         * (by virtue of knowledge we give it about what that type is).
         * A ClassCastException here is truly unexpected, so we let it
         * propagate up.
         */

	if (value_ != null) {
	    value = ExpressionEvaluatorManager.evaluate(
	        "value", value_, Object.class, this, pageContext);
	}
    }
}