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.
019     * Represents the named intersection of a
020     * {@link org.apache.tapestry.engine.state.StateObjectPersistenceManager}for
021     * storing the state object between request cycles, and a
022     * {@link org.apache.tapestry.engine.state.StateObjectFactory}to create the
023     * object in the first place.
024     * 
025     * @author Howard M. Lewis Ship
026     * @since 4.0
027     */
028    public interface StateObjectManager
029    {
030    
031        /**
032         * Returns true if the application state object exists (i.e., a value has
033         * been stored).
034         */
035        boolean exists();
036    
037        /**
038         * Gets or creates the application state object.
039         */
040    
041        Object get();
042    
043        /**
044         * Stores (if necessary) the object back into persistent storage. This is
045         * not application to all storage scopes.
046         * 
047         * @param stateObject
048         *            the application state object previously obtained from
049         *            {@link #get()}.
050         */
051    
052        void store(Object stateObject);
053    }