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.web;
016    
017    import java.util.List;
018    
019    /**
020     * Interface for objects that can hold attributes. Attributes are objects that are stored and
021     * retrieved using unique string names. There may be constraints on when attributes can be accessed
022     * (for example, {@link org.apache.tapestry.web.WebSession}attributes should not be
023     * changed once the response is committed).
024     * 
025     * @author Howard M. Lewis Ship
026     * @since 4.0
027     */
028    public interface AttributeHolder
029    {
030        /**
031         * Returns a list of all known attributes in ascending alphabetical order. May be empty (but
032         * won't be null).
033         * 
034         * @returns Unmodifiable list of string attribute names.
035         */
036    
037        List getAttributeNames();
038    
039        /**
040         * Returns the named object, or null if no attribute has been stored with the given name.
041         */
042    
043        Object getAttribute(String name);
044    
045        /**
046         * Updates the attribute, replacing (or removing) its value. For certain implementations, the
047         * attribute may need to be serializable (for example, a {@link WebSession}
048         *  attribute in a clustered application).
049         * 
050         * @param name
051         *            the name of the attribute to update
052         * @param attribute
053         *            the new value for the attribute, or null to delete the attribute entirely.
054         */
055    
056        void setAttribute(String name, Object attribute);
057    }