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.io;
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>Write the top value from the evaluation stack to standard output,
028     * without removing it.</p>
029     *
030     * <p><strong>WARNING</strong> - This will probably be
031     * replaced later by a more general purpose input/output mechanism.</p>
032     *
033     * @version $Revision: 155475 $ $Date: 2005-02-26 13:31:11 +0000 (Sat, 26 Feb 2005) $
034     * @author Craig R. McClanahan
035     */
036    
037    public class PeekStep extends BaseStep {
038    
039    
040        // ----------------------------------------------------------= Constructors
041    
042    
043        /**
044         * Construct a default instance of this Step.
045         */
046        public PeekStep() {
047    
048            super();
049    
050        }
051    
052    
053        /**
054         * Construct an instance of this Step with the specified identifier.
055         *
056         * @param id Step identifier
057         */
058        public PeekStep(String id) {
059    
060            super();
061            setId(id);
062    
063        }
064    
065    
066        // --------------------------------------------------------- Public Methods
067    
068    
069        /**
070         * Perform the executable actions related to this Step, in the context of
071         * the specified Context.
072         *
073         * @param context The Context that is tracking our execution state
074         *
075         * @exception StepException if a processing error has occurred
076         */
077        public void execute(Context context) throws StepException {
078    
079            // Retrieve a copy of the top value from the evaluation stack
080            Object value = null;
081            try {
082                value = context.peek();
083            } catch (EmptyStackException e) {
084                throw new StepException("Evaluation stack is empty", e, this);
085            }
086    
087            // Display it to standard output
088            System.out.println(value);
089    
090        }
091    
092    
093    }