JAXB和多態性

Question

我無法獲得由Java的JAXB實現編組的以下類結構。我有一個抽象標識符類，它有一個值。

對我來說，重要的是價值應該歸屬於一個屬性。

@XmlSeeAlso({ 
    AbstractIDInt.class 
}) 
@XmlRootElement(name = "AbstractID") 
@XmlAccessorType(XmlAccessType.PROPERTY) 
public abstract class AbstractID { 

    @XmlAttribute 
    abstract Object getValue(); 
} 

而且，這裏是一個擴展了抽象的ID類的類：如果你試圖馬歇爾一些類型AbstractID的

@XmlRootElement(name = "AbstractIDInt") 
@XmlAccessorType(XmlAccessType.NONE) 
public class AbstractIDInt extends AbstractID { 
    Integer value; 

    Integer getValue() { 
     return value; 
    } 

    public void setvalue (Integer value) { 
     this.value = value; 
    } 
} 

目前Java的JAXB實現只是拋出一個異常。

這是違反JAXB標準中的某些東西還是Java的JAXB實現中斷？

Answer 1

JDK中包含的JAXB的實現可能存在一個錯誤。使用JDK 1.6.0_29我拿到那你很可能看到了異常：

Exception in thread "main" java.lang.NullPointerException 
    at com.sun.xml.internal.bind.v2.runtime.reflect.TransducedAccessor.get(TransducedAccessor.java:154) 
    at com.sun.xml.internal.bind.v2.runtime.property.AttributeProperty.<init>(AttributeProperty.java:56) 
    at com.sun.xml.internal.bind.v2.runtime.property.PropertyFactory.create(PropertyFactory.java:93) 
    at com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.<init>(ClassBeanInfoImpl.java:145) 
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:479) 
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:305) 
    at com.sun.xml.internal.bind.v2.runtime.JAXBContextImpl$JAXBContextBuilder.build(JAXBContextImpl.java:1100) 
    at com.sun.xml.internal.bind.v2.ContextFactory.createContext(ContextFactory.java:143) 
    at com.sun.xml.internal.bind.v2.ContextFactory.createContext(ContextFactory.java:110) 
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) 
    at java.lang.reflect.Method.invoke(Method.java:597) 
    at javax.xml.bind.ContextFinder.newInstance(ContextFinder.java:202) 
    at javax.xml.bind.ContextFinder.find(ContextFinder.java:376) 
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:574) 
    at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:522) 
    at forum9382200.Demo.main(Demo.java:9) 

與EclipseLink JAXB (MOXy)運行相同的演示代碼，我得到下面的輸出（我是莫西鉛）：

<?xml version="1.0" encoding="UTF-8"?> 
<AbstractIDInt value="1"/> 

以下是演示代碼，我用：

package forum9382200; 

import javax.xml.bind.JAXBContext; 
import javax.xml.bind.Marshaller; 

public class Demo { 

    public static void main(String[] args) throws Exception { 
     JAXBContext jc = JAXBContext.newInstance(AbstractID.class); 

     AbstractIDInt abstractIDInt = new AbstractIDInt(); 
     abstractIDInt.setvalue(1); 

     Marshaller marshaller = jc.createMarshaller(); 
     marshaller.marshal(abstractIDInt, System.out); 
    } 

} 

對於更多Infomration：

• http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html

---

UPDATE

如果您需要使用JAXB RI包含在Java SE，你可以嘗試以下方法：

AbstractID

馬克父類爲@XmlTransient：

package forum9382200; 

import javax.xml.bind.annotation.*; 

@XmlSeeAlso({ 
    AbstractIDInt.class 
}) 
@XmlTransient 
public abstract class AbstractID { 

    abstract Object getValue(); 
} 

AbstractIDInt

註釋value屬性的子類：

package forum9382200; 

import javax.xml.bind.annotation.*; 

@XmlRootElement(name = "AbstractIDInt") 
@XmlAccessorType(XmlAccessType.NONE) 
public class AbstractIDInt extends AbstractID { 
    Integer value; 

    @XmlAttribute 
    Integer getValue() { 
     return value; 
    } 

    public void setvalue (Integer value) { 
     this.value = value; 
    } 
} 

更多信息

• http://blog.bdoughan.com/2011/06/ignoring-inheritance-with-xmltransient.html