001/*
002 * Strategy.java July 2006
003 *
004 * Copyright (C) 2006, Niall Gallagher <niallg@users.sf.net>
005 *
006 * Licensed under the Apache License, Version 2.0 (the "License");
007 * you may not use this file except in compliance with the License.
008 * You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or 
015 * implied. See the License for the specific language governing 
016 * permissions and limitations under the License.
017 */
018
019package org.simpleframework.xml.strategy;
020
021import java.util.Map;
022
023import org.simpleframework.xml.stream.InputNode;
024import org.simpleframework.xml.stream.NodeMap;
025import org.simpleframework.xml.stream.OutputNode;
026
027/**
028 * The <code>Strategy</code> interface represents a strategy that can be 
029 * used to resolve and load the <code>Class</code> objects that compose 
030 * a serializable object. A strategy implementation will make use of the
031 * provided attribute node map to extract details that can be used to
032 * determine what type of object must be used. 
033 * <pre>
034 * 
035 *    &lt;xml version="1.0"&gt;
036 *    &lt;example class="some.example.Demo"&gt;
037 *       &lt;integer&gt;2&lt;/integer&gt;
038 *    &lt;/example&gt;
039 *    
040 * </pre> 
041 * The above example shows how the default strategy augments elements
042 * with "class" attributes that describe the type that should be used
043 * to instantiate a field when an object is deserialized. So looking at
044 * the above example the root element would be a "some.example.Demo".
045 * <p>
046 * Custom <code>Strategy</code> implementations give the persister a
047 * chance to intercept the class loading and type resolution for XML
048 * documents. It also opens up the possibility for class versioning.
049 * To establish contextual information a <code>Map</code> object can be
050 * used. The map object is a transient object that is created and used
051 * for the duration of a single operation of the persister.
052 * 
053 * @author Niall Gallagher
054 *
055 * @see org.simpleframework.xml.core.Persister
056 */
057public interface Strategy {
058
059   /**
060    * This is used to resolve and load a class for the given element.
061    * The class should be of the same type or a subclass of the class
062    * specified. It can be resolved using the details within the
063    * provided XML node map, if the details used do not represent any
064    * serializable values they should be removed so as not to disrupt
065    * the deserialization process. For example the default strategy
066    * removes all "class" attributes from the given node map.
067    * 
068    * @param type this is the type of the root element expected
069    * @param node this is the node map used to resolve an override
070    * @param map this is used to maintain contextual information
071    * 
072    * @return the value that should be used to describe the instance
073    * 
074    * @throws Exception thrown if the class cannot be resolved
075    */
076   Value read(Type type, NodeMap<InputNode> node, Map map) throws Exception;
077
078   /**
079    * This is used to attach attribute values to the given node
080    * map during the serialization process. This method allows
081    * the strategy to augment the XML document so that it can be
082    * deserialized using a similar strategy. For example the 
083    * default strategy adds a "class" attribute to the node map.
084    *  
085    * @param type this is the declared class for the field used
086    * @param value this is the instance variable being serialized
087    * @param node this is the node map used to represent the value
088    * @param map this is used to maintain contextual information
089    * 
090    * @return this returns true if serialization is complete    
091    * 
092    * @throws Exception thrown if the details cannot be set
093    */
094   boolean write(Type type, Object value, NodeMap<OutputNode> node, Map map) throws Exception;
095
096}