content: /*
   * ElementUnion.java March 2011
   *
   * Copyright (C) 2011, Niall Gallagher <niallg@users.sf.net>
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or 
   * implied. See the License for the specific language governing 
   * permissions and limitations under the License.
   */
  
  package com.aliyun.odps.simpleframework.xml;
  
  import java.lang.annotation.Retention;
  import java.lang.annotation.RetentionPolicy;
  
  /**
   * The <code>ElementUnion</code> annotation is used to describe fields 
   * and methods that can dynamically match a schema class. Each union
   * can have a number of different XML class schemas matched based on
   * an XML element name or the instance type. This provides a means
   * of expressing a logical OR. By annotating a field or method as a
   * union it can take multiple forms. For example.
   * <pre>
   * 
   *    &#64;ElementUnion({
   *       &#64;Element(name="circle", type=Circle.class),
   *       &#64;Element(name="square", type=Square.class)       
   *    })
   *    private Shape shape;
   *    
   * </pre>
   * For the above definition the <code>Shape</code> field can take
   * be any of the declared types. On deserialization the name of the
   * element will determine the type that is instantiated and the XML
   * structure to be consumed. For serialization the instance type will
   * determine the name of the element the object will serialized as. 
   * This provides a useful means of consume more complicated sources. 
   * 
   * @author Niall Gallagher
   * 
   * @see Element
   */
  @Retention(RetentionPolicy.RUNTIME)
  public @interface ElementUnion {
     
     /**
      * This provides the <code>Element</code> annotations that have 
      * been defined for this union. Each element describes the XML
      * class schema to use and the name of the XML element. This 
      * allows the serialization process to determine which elements
      * map to the defined types. Also, the types define how the XML
      * is generated for a given instance.     
      * 
      * @return the elements defined for the union declaration
      */
     Element[] value();
  }
  