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 }