unmarshaller.unmarshal转换结果是JAXBElement问题

布满荆棘的人生 2021-06-22 15:38 500阅读 0赞

### unmarshaller.unmarshal转换结果是JAXBElement问题 ###

*  问题描述
 *  问题解决
 *   *   *   *   *  解释一下：
                 *  回去一看

# 问题描述 #

根据xml字符串转换成对象，返回的类型不是传入的Class类对象，而是JAXBElement对象。  
示例：

public <T> T fromXml(String xml, Class<T> targetClass, String encoding) { 
            try { 
                JAXBContext jaxbContext = createContext(targetClass);
                Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
                XMLStreamReader read = XMLInputFactory.newInstance().createXMLStreamReader(
                        new ByteArrayInputStream(xml.getBytes(encoding)));
                T object = (T) unmarshaller.unmarshal(read);
                return object;  //此时返回的不是targetClass实例，而是JAXBElement对象，JAXBElement对象中的value属性确是targetClass实例
            } catch (Exception e) { 
            	...
            }
        }

# 问题解决 #

https://stackoverflow.com/questions/10243679/when-does-jaxb-unmarshaller-unmarshal-returns-a-jaxbelementmyschemaobject-or-a

*  It depends on the presence of XmlRootElement annotation on the class of your root element.  
    If you generate your JAXB classes from an XSD, the following rules are applied:

if the type of the root element is an anonymous type -> XmlRootElement annotation is added to the generated class  
if the type of the root element is a top level type -> XmlRootElement annotation is omitted from the generated class  
For that reason I often choose anonymous types for root elements.

You can customize the class name of this anonymous type with a customization file. E.g. create a bindings.xjc file like this:

<jxb:bindings version=“1.0”  
xmlns:jxb=“http://java.sun.com/xml/ns/jaxb”  
xmlns:xs=“http://www.w3.org/2001/XMLSchema”>  
<jxb:bindings schemaLocation=“yourXsd.xsd” node="/xs:schema">  
<jxb:bindings node="//xs:element\[@name=‘yourRootElement’\]">  
<jxb:class name=“YourRootElementType”/>  
</jxb:bindings>  
</jxb:bindings>  
</jxb:bindings>

##### 解释一下： #####

和XmlRootElement 注解有关，如果你的xml的bean类是从xsd生成的，那么就会出现这些情况：

*  如果根元素的类型是匿名类型-> XmlRootElement注解将自动添加到生成的类中
 *  如果根元素的类型是顶级类型->生成的类中会省略XmlRootElement注解

##### 回去一看 #####

当前转化的类的确没有XmlRootElement注解，加上注解后，问题解决。