JAXB编组场，它的类型是一个泛型类型参数

Question

我希望下面的测试与Sun的JAXB RI 2.2.1.1工作，而是它失败，一个NullPointerException在构建的JAXBContext：

public class GenericFieldMarshallTest { 

    public static class CustomType { 
    } 

    public static class CustomTypeAdapter extends XmlAdapter<String, CustomType> { 
     @Override 
     public String marshal(CustomType v) throws Exception { 
      return "CustomType"; 
     } 
     @Override 
     public CustomType unmarshal(String v) throws Exception { 
      return new CustomType(); 
     } 
    } 

    @XmlJavaTypeAdapter(type = CustomType.class, value = CustomTypeAdapter.class) 
    public static class RootElement<ValueType> { 
     @XmlValue public ValueType value; 
    } 

    @XmlRootElement(name = "root") 
    public static class CustomRootElement extends RootElement<CustomType> { 
     public CustomRootElement() { 
      value = new CustomType(); 
     } 
    } 

    @Test 
    public void test() throws Exception { 
     JAXBContext context = JAXBContext.newInstance(CustomRootElement.class, 
       CustomType.class, RootElement.class); 
     StringWriter w = new StringWriter(); 
     context.createMarshaller().marshal(new CustomRootElement(), w); 
     assertThat(w.toString(), equalTo("<root>CustomType</root>")); 
    } 

} 

异常我得到的是：

java.lang.NullPointerException 
     at com.sun.xml.bind.v2.runtime.reflect.TransducedAccessor.get(TransducedAccessor.java:165) 
     at com.sun.xml.bind.v2.runtime.property.ValueProperty.<init>(ValueProperty.java:77) 
     at com.sun.xml.bind.v2.runtime.property.PropertyFactory.create(PropertyFactory.java:106) 
     at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.<init>(ClassBeanInfoImpl.java:179) 
     at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515) 
     at com.sun.xml.bind.v2.runtime.ClassBeanInfoImpl.<init>(ClassBeanInfoImpl.java:166) 
     at com.sun.xml.bind.v2.runtime.JAXBContextImpl.getOrCreate(JAXBContextImpl.java:515) 
     at com.sun.xml.bind.v2.runtime.JAXBContextImpl.<init>(JAXBContextImpl.java:330) 
     at com.sun.xml.bind.v2.runtime.JAXBContextImpl$JAXBContextBuilder.build(JAXBContextImpl.java:1140) 
     at com.sun.xml.bind.v2.ContextFactory.createContext(ContextFactory.java:154) 
     at com.sun.xml.bind.v2.ContextFactory.createContext(ContextFactory.java:121) 
     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:363) 
     at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:574) 
     at javax.xml.bind.JAXBContext.newInstance(JAXBContext.java:522) 

的原因似乎是，JAXB不知道如何封送我的字段的声明类型（？我认为这是消除在运行时对象），即使在运行时我只将该字段设置为JAXB知道的类型。

如何编组一个类型为通用的字段？

（用@XmlAttribute替换@XmlValue并不能修复异常，也不会修改字段的声明类型为Object，当然如果该字段被声明为String，那么一切正常，但String不能从CustomType。@XmlJavaTypeAdapter的放置也没有区别;在我的实际代码中，它在package-info.java中设置在包级别上。）

Answer 1

首先：您的XmlAdapter是错误的。通用类型是相反的。

然后你似乎必须将@XmlJavaTypeAdapter放在CustomRootElement上。

此外JAXBContext需要告诉所有涉及的类。创建jaxb.in​​dex或ObjectFactory并通过将包名称提供给newInstance方法或列出所有类来创建上下文。

完整的代码（因为我用一个main()而不是JUnit测试方法稍作修改）：

public static class CustomType { 
} 

public static class CustomTypeAdapter extends 
     XmlAdapter<String, CustomType> { 

    @Override 
    public String marshal(CustomType v) throws Exception { 
     return "CustomType"; 
    } 

    @Override 
    public CustomType unmarshal(String v) throws Exception { 
     return new CustomType(); 
    } 

} 

public static class RootElement<V> { 
    public V value; 
} 

@XmlJavaTypeAdapter(CustomTypeAdapter.class) 
@XmlRootElement(name = "root") 
public static class CustomRootElement extends RootElement<CustomType> { 
    public CustomRootElement() { 
     value = new CustomType(); 
    } 
} 

public static void main(String[] args) throws Exception { 
    JAXBContext context = JAXBContext.newInstance(CustomRootElement.class, 
      CustomType.class, RootElement.class); 
    StringWriter w = new StringWriter(); 
    CustomRootElement cre = new CustomRootElement(); 
    cre.value = new CustomType(); 

    Marshaller marshaller = context.createMarshaller(); 
    marshaller.setProperty("jaxb.formatted.output", Boolean.TRUE); 
    marshaller.marshal(cre, w); 

    System.err.println(w.toString()); 

    // just to see whether unmarshalling works too 
    CustomRootElement c = (CustomRootElement) context.createUnmarshaller() 
      .unmarshal(new StringReader(w.toString())); 
    System.err.println(c.value); 
} 

现在编组一个CustomRootElement的结果不是你在测试什么期望（并且也不是我期望的），但你可以解开它并得到你以前编组的东西。所以，（联合国）编组都工作，但XML看起来并不漂亮：

<?xml version="1.0" encoding="UTF-8" standalone="yes"?> 
<root> 
    <value xsi:type="customType" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/> 
</root> 

我也把现场的CustomType和它的工作为好。所以如果你不需要这个好的XML，这个解决方案就足够了。 我希望我没有忘记我所做的任何改变。如果我这样做，只需留下评论，我会相应地编辑。