/*

  * Copyright 2011 The Kuali Foundation

  *

  * Licensed under the Educational Community License, Version 1.0 (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.opensource.org/licenses/ecl1.php

  *

  * 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.kuali.rice.krms.impl.provider.repository;

 import java.util.Collections;

 import java.util.List;

 import org.kuali.rice.krms.api.engine.ExecutionEnvironment;

 import org.kuali.rice.krms.api.repository.proposition.PropositionDefinition;

 import org.kuali.rice.krms.api.repository.type.KrmsTypeDefinition;

 import org.kuali.rice.krms.framework.engine.Proposition;

 import org.kuali.rice.krms.framework.engine.PropositionResult;

 import org.kuali.rice.krms.framework.type.PropositionTypeService;

 import org.kuali.rice.krms.impl.type.KrmsTypeResolver;

 /**

  * TODO... 

  * 

  * @author Kuali Rice Team (rice.collab@kuali.org)

  *

  */

 final class LazyProposition implements Proposition {

         private final PropositionDefinition propositionDefinition;

         private final KrmsTypeResolver resolver;

         private final Object mutex = new Object();

         // volatile for double-checked locking idiom

         private volatile Proposition proposition;

         LazyProposition(PropositionDefinition propositionDefinition, KrmsTypeResolver resolver) {

                 this.propositionDefinition = propositionDefinition;

                 this.resolver = resolver;

                 this.proposition = null;

         }

         @Override

         public PropositionResult evaluate(ExecutionEnvironment environment) {

                 return getProposition().evaluate(environment);

         }

         /**

          * Gets the proposition using a lazy double-checked locking mechanism as documented in Effective Java Item 71.

          */

         private Proposition getProposition() {

                 Proposition localProposition = proposition;

                 if (localProposition == null) {

                         synchronized (mutex) {

                                 localProposition = proposition;

                                 if (localProposition == null) {

                                         proposition = localProposition = constructProposition();

                                 }

                         }

                 }

                 return localProposition;

         }

         private Proposition constructProposition() {

                 PropositionTypeService propositionTypeService = resolver.getPropositionTypeService(propositionDefinition);

                 Proposition proposition = propositionTypeService.loadProposition(propositionDefinition);

                 if (proposition == null) {

                         proposition = new Proposition() {

                                 @Override

                                 public PropositionResult evaluate(ExecutionEnvironment environment) {

                                         return new PropositionResult(true);

                                 }

                             @Override

                             public List<Proposition> getChildren() {

                                 return Collections.emptyList();

                             }

                             @Override

                             public boolean isCompound() {

                                 return false;

                             }

                         };

                 }

                 return proposition;

         }

         @Override

         public List<Proposition> getChildren() {

             return getProposition().getChildren();

         }

         @Override

         public boolean isCompound() {

             return getProposition().isCompound();

         }

 }