001/*
002 * Attribute.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;
020
021import java.lang.annotation.RetentionPolicy;
022import java.lang.annotation.Retention;
023
024/**
025 * The <code>Attribute</code> annotation represents a serializable XML
026 * attribute within an XML element. An object annotated with this is
027 * typically a primitive or enumerated type. Conversion from the 
028 * attribute to primitive type is done with a <code>Transform</code>
029 * object. If a suitable transform can be found then this will convert
030 * the attribute string value to an object instance, which can be 
031 * assigned to the annotated field, or passed to the annotated method.
032 * 
033 * @author Niall Gallagher
034 * 
035 * @see org.simpleframework.xml.transform.Transformer
036 */ 
037@Retention(RetentionPolicy.RUNTIME)
038public @interface Attribute {
039
040   /**
041    * This represents the name of the XML attribute. Annotated fields
042    * or methods can optionally provide the name of the XML attribute
043    * they represent. If a name is not provided then the field or 
044    * method name is used in its place. A name can be specified if 
045    * the field or method name is not suitable for the XML attribute.
046    * 
047    * @return the name of the XML attribute this represents
048    */
049   String name() default "";
050   
051   /**
052    * This is used to provide a default value for the attribute if
053    * the annotated field or method is null. This ensures the the
054    * serialization process writes the attribute with a value even
055    * if the value is null, and allows deserialization to determine
056    * whether the value within the object was null or not.
057    * 
058    * @return this returns the default attribute value to use
059    */
060   String empty() default "";
061
062   /**
063    * Determines whether the attribute is required within an XML
064    * element. Any field marked as not required will not have its
065    * value set when the object is deserialized. If an object is to
066    * be serialized only a null attribute will not appear as XML.
067    * 
068    * @return true if the attribute is required, false otherwise
069    */
070   boolean required() default true;   
071}