001    // Copyright 2005 The Apache Software Foundation
002    //
003    // Licensed under the Apache License, Version 2.0 (the "License");
004    // you may not use this file except in compliance with the License.
005    // You may obtain a copy of the License at
006    //
007    //     http://www.apache.org/licenses/LICENSE-2.0
008    //
009    // Unless required by applicable law or agreed to in writing, software
010    // distributed under the License is distributed on an "AS IS" BASIS,
011    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012    // See the License for the specific language governing permissions and
013    // limitations under the License.
014    
015    package org.apache.tapestry.binding;
016    
017    import org.apache.hivemind.ApplicationRuntimeException;
018    import org.apache.hivemind.Location;
019    import org.apache.hivemind.util.Defense;
020    import org.apache.tapestry.coerce.ValueConverter;
021    import org.apache.tapestry.engine.state.ApplicationStateManager;
022    
023    /**
024     * Binding used to efficiently query whether an application state object (visit, global and friends)
025     * exists without actually creating it (or creating a session).
026     * 
027     * @author Howard M. Lewis Ship
028     * @since 4.0
029     */
030    public class StateBinding extends AbstractBinding
031    {
032        private final ApplicationStateManager _applicationStateManager;
033    
034        private final String _objectName;
035    
036        public StateBinding(String description, ValueConverter valueConverter, Location location,
037                ApplicationStateManager applicationStateManager, String objectName)
038        {
039            super(description, valueConverter, location);
040    
041            Defense.notNull(applicationStateManager, "applicationStateManager");
042            Defense.notNull(objectName, "objectName");
043    
044            _applicationStateManager = applicationStateManager;
045            _objectName = objectName;
046        }
047    
048        /**
049         * Returns false; these ASO objects can be created at any time.
050         */
051        public boolean isInvariant()
052        {
053            return false;
054        }
055    
056        public Object getObject()
057        {
058            try
059            {
060                boolean exists = _applicationStateManager.exists(_objectName);
061    
062                return exists ? Boolean.TRUE : Boolean.FALSE;
063            }
064            catch (Exception ex)
065            {
066                throw new ApplicationRuntimeException(ex.getMessage(), getLocation(), ex);
067            }
068        }
069    }