java – 类型为泛型类型参数的JAXB编组字段

2019年8月1日 210次阅读

我希望以下测试可以与Sun的JAXB RI 2.2.1.1一起使用,但是在构造JAXBContext时它会因NullPointerException而失败：

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>"));
    }

}

我得到的例外是：

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)

原因似乎是JAXB不知道如何编组我的字段的声明类型(我认为它在运行时被删除为Object？),即使在运行时我只将字段设置为JAXB知道的类型.

如何编组类型为通用的字段？

(用@XmlAttribute替换@XmlValue不会修复异常,也不会将字段的声明类型更改为Object,但当然,如果字段声明为String,则一切正常,但String不能从CustomType分配. @XmlJavaTypeAdapter也没有区别;在我的实际代码中,它是在package-info.java中的包级别设置的.)

最佳答案 首先：您的XmlAdapter是错误的.通用类型是相反的方式.

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

此外,需要告知JAXBContext所涉及的所有类.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(稍微修改为我使用main()而不是JUnit测试方法)：

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CustomRootElement的结果不是您在测试中所期望的结果(也不是我预期的结果),但是您可以解组它并获得您之前编组的内容.因此(un)编组工作但XML看起来不那么好：

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

我还在CustomType中放置了一个字段,它也可以工作.所以,如果你不需要漂亮的XML,那么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,只需发表评论,我会相应地进行编辑.

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