JAXB - 如何根据 XML 元素的值设置 xsi:type？

发布于 2024-12-11 03:31:54 字数 2279 浏览 0 评论 0原文

我必须生成一个 xml 元素，该元素可以具有任何“原始类型”（xsd:string、xsd:boolean 等）的值。示例：

<field xsi:type="xsd:string" name="aString">String Value</field>
<field xsi:type="xsd:date" name="aDate">2011-10-21</field>
...

所以，我尝试了两种实现：

public class Field {
    @XmlAttribute
    private String name;

    @XmlValue
    Object value;
}

并且...

public class Field<T> {
    @XmlAttribute
    private String name;

    @XmlValue
    T value;
}

我正在测试它：

Marshaller marshaller = JAXBContext.newInstance(Field.class).createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
marshaller.setProperty("com.sun.xml.bind.xmlDeclaration", Boolean.FALSE);

Field field = new Field();
field.name = "name";
field.value = "value";

ByteArrayOutputStream stream = new ByteArrayOutputStream();
marshaller.marshal(field, new PrintWriter(stream));
System.out.println(stream);

但是当我尝试实例化 JAXBContext 时，我收到了这个 NullPointerException 。

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.JAXBContextImpl.<init>(JAXBContextImpl.java:330)
at

这个想法是允许对字段元素进行模式验证（它在模式中定义，必须在每个实例中设置其类型）。那么，即使这是一个 Bug（或不是）...JAXB 如何将正确的 xsi:type 放入此字段实例？我在这里缺少一个概念？

我知道问题可能在于 @XmlValue 的使用，因为存在以下限制（来自 javadoc）：

最多可以使用 @XmlValue 注释对一个字段或属性进行注释。
@XmlValue 可以与以下注释一起使用：XmlList。然而，这是多余的，因为 XmlList 将类型映射到简单的架构类型，该架构类型是通过列表派生的，就像 XmlValue 一样。
如果字段或属性的类型是集合类型，则集合项类型必须映射到简单架构类型。
如果字段或属性的类型不是集合类型，则该类型必须映射到 XML 架构简单类型。

...因为 Object 或通用 T 不一定是 XML Schema 简单类型，因此这种方法似乎不是正确的方法...

提前致谢 .. 。

原文

I have to generate a xml element that can have as value any "primitive type" (xsd:string, xsd:boolean, etc). Examples:

<field xsi:type="xsd:string" name="aString">String Value</field>
<field xsi:type="xsd:date" name="aDate">2011-10-21</field>
...

So, I tried two implementations:

public class Field {
    @XmlAttribute
    private String name;

    @XmlValue
    Object value;
}

and ...

public class Field<T> {
    @XmlAttribute
    private String name;

    @XmlValue
    T value;
}

I'm testing this with:

Marshaller marshaller = JAXBContext.newInstance(Field.class).createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
marshaller.setProperty("com.sun.xml.bind.xmlDeclaration", Boolean.FALSE);

Field field = new Field();
field.name = "name";
field.value = "value";

ByteArrayOutputStream stream = new ByteArrayOutputStream();
marshaller.marshal(field, new PrintWriter(stream));
System.out.println(stream);

But I'm getting this NullPointerException when I try to instantiate the JAXBContext.

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.JAXBContextImpl.<init>(JAXBContextImpl.java:330)
at

The idea is allow schema validation for the field element (It is define in a schema must its type will be set in each instance). So, even is this a Bug (or not) ... how JAXB will put the correct xsi:type to this field instance? I'm missing a concept here?

I know that maybe the problem is the usage of the @XmlValue because of this restrictions (from the javadoc):

At most one field or property can be annotated with the @XmlValue annotation.
@XmlValue can be used with the following annotations: XmlList. However this is redundant since XmlList maps a type to a simple schema type that derives by list just as XmlValue would.
If the type of the field or property is a collection type, then the collection item type must map to a simple schema type.
If the type of the field or property is not a collection type, then the type must map to a XML Schema simple type.

... because an Object or a generic T is not necessarily a XML Schema simple type, this approach seems not to be the correct one ...

Thanks in advance ...

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