JAXB Marshaller 没有值为 null 的元素 [英] JAXB Marshaller does not have elements whos value is null

问题描述

当我使用 JAXB Marshaller 编组 java 对象时，编组器不会为 java 对象中的空文件创建空元素.例如，我有以下 java 对象:

When I marshall a java object using JAXB Marshaller, the marshaller does not create empty elements for null files in the java object. For example, I have a following java object:

public class PersonTraining {

    @XmlElement(name = "Val1", required = true)
    protected BigDecimal val1;
    @XmlElement(name = "Val2", required = true, nillable = true)
    protected BigDecimal val2;
    @XmlElement(name = "Val3", required = true, nillable = true)
    @XmlSchemaType(name = "dateTime")
    protected XMLGregorianCalendar val3;
}

当我获取此对象的实例并将其编组为 XML 时，我得到以下信息(这是因为我没有为 Val2 设置值):

When I take an instance of this object, and marshall into an XML, I get the following (This is beacuse I did not set the value for Val2):

<PersonTraining>
      <Val1>1</Val1>
       <Val3>2010-01-01T00:00:00.0-05:00</Val3>
 </PersonTraining>

但是，我曾期望从编组操作中得到以下结果(事实上，我还特别需要元素，以便可以针对 XSD 验证 XML)

However, I had expected hte following result from the marshalling operation (Infact, I specifically need element as well so that the XML can be validated against the XSD)

<PersonTraining>
      <Val1>1</Val1>
      <Val2></Val2>
       <Val3>2010-01-01T00:00:00.0-05:00</Val3>
 </PersonTraining>

请让我知道我需要设置什么选项，以便对象属性中的空值也可以被编组，并作为空/空元素返回.

Please let me know what option I would need to set so that the null value in the object attributes can ALSO be marshalled, and returned as empty/null elements.

这里是编组代码:

StringWriter sw = new StringWriter();
JAXBContext jc = JAXBContext.newInstance("person_training");   
Marshaller m = jc.createMarshaller();
m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
m.marshal(ptl, sw);

推荐答案

默认一个 JAXB (JSR-222) 实现不会为空值封送属性/元素.这将适用于 Java 模型中的以下字段.

By default a JAXB (JSR-222) implementation will not marshal an attribute/element for null values. This will be true for the following field in your Java model.

@XmlElement(name = "Val1", required = true)
protected BigDecimal val1;

您可以通过在 @XmlElement 注释上指定 nillable=true 来覆盖此行为，就像您在此处所做的那样:

You can override this behaviour by specifying nillable=true on the @XmlElement annotation like you have done here:

@XmlElement(name = "Val2", required = true, nillable = true)
protected BigDecimal val2;

这将导致 xsi:nil="true" 属性成为杠杆:

This will cause the xsi:nil="true" attribute to be leverage:

<Val2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>

更多信息:

• http://blog.bdoughan.com/2012/04/binding-to-json-xml-handling-null.html

人员培训

由于您要注释 fields，您应该确保在类或包级别指定 @XmlAccessorType(XmlAccessType.FIELD)(请参阅:http://blog.bdoughan.com/2011/06/using-jaxbs-xmlaccessortype-to.html).

Since you are annotating the fields you should make sure you specify @XmlAccessorType(XmlAccessType.FIELD) at the class or package level (see: http://blog.bdoughan.com/2011/06/using-jaxbs-xmlaccessortype-to.html).

import java.math.BigDecimal;
import javax.xml.bind.annotation.*;
import javax.xml.datatype.XMLGregorianCalendar;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class PersonTraining {

    @XmlElement(name = "Val1", required = true)
    protected BigDecimal val1;
    @XmlElement(name = "Val2", required = true, nillable = true)
    protected BigDecimal val2;
    @XmlElement(name = "Val3", required = true, nillable = true)
    @XmlSchemaType(name = "dateTime")
    protected XMLGregorianCalendar val3;

}

演示代码

演示

import javax.xml.bind.*;

public class Demo {

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

        PersonTraining pt = new PersonTraining();

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(pt, System.out);
    }

}

输出

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<personTraining>
    <Val2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
    <Val3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
</personTraining>

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