001/*
002 * ElementUnion.java March 2011
003 *
004 * Copyright (C) 2011, 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;
020
021import java.lang.annotation.Retention;
022import java.lang.annotation.RetentionPolicy;
023
024/**
025 * The <code>ElementUnion</code> annotation is used to describe fields 
026 * and methods that can dynamically match a schema class. Each union
027 * can have a number of different XML class schemas matched based on
028 * an XML element name or the instance type. This provides a means
029 * of expressing a logical OR. By annotating a field or method as a
030 * union it can take multiple forms. For example.
031 * <pre>
032 * 
033 *    &#64;ElementUnion({
034 *       &#64;Element(name="circle", type=Circle.class),
035 *       &#64;Element(name="square", type=Square.class)       
036 *    })
037 *    private Shape shape;
038 *    
039 * </pre>
040 * For the above definition the <code>Shape</code> field can take
041 * be any of the declared types. On deserialization the name of the
042 * element will determine the type that is instantiated and the XML
043 * structure to be consumed. For serialization the instance type will
044 * determine the name of the element the object will serialized as. 
045 * This provides a useful means of consume more complicated sources. 
046 * 
047 * @author Niall Gallagher
048 * 
049 * @see org.simpleframework.xml.Element
050 */
051@Retention(RetentionPolicy.RUNTIME)
052public @interface ElementUnion {
053   
054   /**
055    * This provides the <code>Element</code> annotations that have 
056    * been defined for this union. Each element describes the XML
057    * class schema to use and the name of the XML element. This 
058    * allows the serialization process to determine which elements
059    * map to the defined types. Also, the types define how the XML
060    * is generated for a given instance.     
061    * 
062    * @return the elements defined for the union declaration
063    */
064   Element[] value();
065}