001    /*
002     * Copyright 1999-2001,2004 The Apache Software Foundation.
003     * 
004     * Licensed under the Apache License, Version 2.0 (the "License");
005     * you may not use this file except in compliance with the License.
006     * You may obtain a copy of the License at
007     * 
008     *      http://www.apache.org/licenses/LICENSE-2.0
009     * 
010     * Unless required by applicable law or agreed to in writing, software
011     * distributed under the License is distributed on an "AS IS" BASIS,
012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013     * See the License for the specific language governing permissions and
014     * limitations under the License.
015     */ 
016    
017    package org.apache.commons.workflow.core;
018    
019    
020    import java.util.EmptyStackException;
021    import org.apache.commons.workflow.Context;
022    import org.apache.commons.workflow.StepException;
023    import org.apache.commons.workflow.base.BaseStep;
024    
025    
026    /**
027     * <p>Swap the positions of the top two values on the evaluation stack.</p>
028     *
029     * @version $Revision: 155475 $ $Date: 2005-02-26 13:31:11 +0000 (Sat, 26 Feb 2005) $
030     * @author Craig R. McClanahan
031     */
032    
033    public class SwapStep extends BaseStep {
034    
035    
036        // ----------------------------------------------------------= Constructors
037    
038    
039        /**
040         * Construct a default instance of this Step.
041         */
042        public SwapStep() {
043    
044            super();
045    
046        }
047    
048    
049        /**
050         * Construct an instance of this Step with the specified identifier.
051         *
052         * @param id Step identifier
053         */
054        public SwapStep(String id) {
055    
056            super();
057            setId(id);
058    
059        }
060    
061    
062        // --------------------------------------------------------- Public Methods
063    
064    
065        /**
066         * Perform the executable actions related to this Step, in the context of
067         * the specified Context.
068         *
069         * @param context The Context that is tracking our execution state
070         *
071         * @exception StepException if a processing error has occurred
072         */
073        public void execute(Context context) throws StepException {
074    
075            // Retrieve the top value from the evaluation stack
076            Object oldTop = null;
077            try {
078                oldTop = context.pop();
079            } catch (EmptyStackException e) {
080                throw new StepException("Evaluation stack is empty", e, this);
081            }
082    
083            // Retrieve the next value from the evaluation stack
084            Object oldBottom = null;
085            try {
086                oldBottom = context.pop();
087            } catch (EmptyStackException e) {
088                throw new StepException("Evaluation stack is empty", e, this);
089            }
090    
091            // Push the items back onto the evaluation stack in reverse order
092            context.push(oldTop);
093            context.push(oldBottom);
094    
095        }
096    
097    
098    }