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.engine.state;
016    
017    /**
018     * Interface for an object that manages a single application state object. Represents the named
019     * intersection of a {@link org.apache.tapestry.engine.state.StateObjectPersistenceManager}for
020     * storing the state object between request cycles, and a
021     * {@link org.apache.tapestry.engine.state.StateObjectFactory}to create the object in the first
022     * place.
023     * 
024     * @author Howard M. Lewis Ship
025     * @since 4.0
026     */
027    public interface StateObjectManager
028    {
029        /**
030         * Returns true if the application state object exists (i.e., a value has been stored).
031         */
032        public boolean exists();
033    
034        /**
035         * Gets or creates the application state object.
036         */
037    
038        public Object get();
039    
040        /**
041         * Stores (if necessary) the object back into persistent storage. This is not application to all
042         * storage scopes.
043         * 
044         * @param stateObject
045         *            the application state object previously obtained from {@link #get()}.
046         */
047    
048        public void store(Object stateObject);
049    }