org.apache.commons.jelly.expression.Expression Maven / Gradle / Ivy

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Jelly is a Java and XML based scripting engine. Jelly combines the best ideas from JSTL, Velocity, DVSL, Ant and Cocoon all together in a simple yet powerful scripting engine.

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/*
 * Copyright 2002,2004 The Apache Software Foundation.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.jelly.expression;

import java.util.Collections;
import java.util.Iterator;

import org.apache.commons.jelly.JellyContext;

/** Expression represents an arbitrary expression using some pluggable
  * expression language.
  *
  * @author James Strachan
  * @version $Revision: 155420 $
  */
public interface Expression {

    /**
     * @return the textual representation of this expression
     */
    public String getExpressionText();

    /**
     * Evaluates the expression with the given context
     * and returns the result
     */
    public Object evaluate(JellyContext context);

    /**
     * Evaluates the expression with the given context
     * coercing the result to be a String.
     */
    public String evaluateAsString(JellyContext context);

    /**
     * Evaluates the expression with the given context
     * coercing the result to be a boolean.
     */
    public boolean evaluateAsBoolean(JellyContext context);

    /**
     * Evaluates the expression with the given context
     * coercing the result to be an Iterator.
     */
    public Iterator evaluateAsIterator(JellyContext context);

    /**
     * This method evaluates the expression until a value (a non-Expression) object
     * is returned.
     * If the expression returns another expression, then the nested expression is evaluated.
     * 
     * Sometimes when Jelly is used inside Maven the value
     * of an expression can actually be another expression.
     * For example if a properties file is read, the values of variables
     * can actually be expressions themselves.
     * 

     * e.g. ${foo.bar} can lookup "foo.bar" in a Maven context
     * which could actually be another expression.
     * 
     * So using this method, nested expressions can be evaluated to the
     * actual underlying value object.
     */
    public Object evaluateRecurse(JellyContext context);

    /**
     * Singleton instance that represents the expression that evaluates to null.
     */
    Expression NULL = new Expression() {
        public String getExpressionText() {
            return "null";
        }

        public Object evaluate(JellyContext context) {
            return null;
        }

        public String evaluateAsString(JellyContext context) {
            return null;
        }

        public boolean evaluateAsBoolean(JellyContext context) {
            return false;
        }

        public Iterator evaluateAsIterator(JellyContext context) {
            return Collections.emptyList().iterator();
        }

        public Object evaluateRecurse(JellyContext context) {
            return null;
        }
    };
}