/*

  * Copyright 1999-2001,2004 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.workflow.core;

 import java.util.EmptyStackException;

 import org.apache.commons.workflow.Activity;

 import org.apache.commons.workflow.Context;

 import org.apache.commons.workflow.StepException;

 import org.apache.commons.workflow.base.BaseStep;

 /**

  * <p>Pop the top value from the evaluation stack, which must be an

  * <code>Activity</code>, and initiate a "subroutine call" to execute

  * this Activity before resuming the current one.</p>

  *

  * <p><strong>NOTE</strong> - The means by which the Activity on the top

  * of the stack was acquired is NOT mandated by this Step implementation.</p>

  *

  * @version $Revision: 155475 $ $Date: 2005-02-26 13:31:11 +0000 (Sat, 26 Feb 2005) $

  * @author Craig R. McClanahan

  */

 public class CallStep extends BaseStep {

     // ----------------------------------------------------------= Constructors

     /**

      * Construct a default instance of this Step.

      */

     public CallStep() {

         super();

     }

     /**

      * Construct an instance of this Step with the specified identifier.

      *

      * @param id Step identifier

      */

     public CallStep(String id) {

         super();

         setId(id);

     }

     // --------------------------------------------------------- Public Methods

     /**

      * Perform the executable actions related to this Step, in the context of

      * the specified Context.

      *

      * @param context The Context that is tracking our execution state

      *

      * @exception StepException if a processing error has occurred

      */

     public void execute(Context context) throws StepException {

         // Pop the evaluation stack

         Object value = null;

         try {

             value = context.pop();

         } catch (EmptyStackException e) {

             throw new StepException("Evaluation stack is empty", e, this);

         }

         // Call the requested Activity 

         if (!(value instanceof Activity))

             throw new StepException("Top of stack is not an Activity", this);

         context.call((Activity) value);

     }

 }