Java JAXB Marshaller 没有值为 null 的元素
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JAXB Marshaller does not have elements whos value is null
提问by Rahul Agrawal
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:
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：
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;
}
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):
当我获取此对象的一个实例并将其编组为 XML 时，我得到以下信息（这是因为我没有为 Val2 设置值）：
<PersonTraining>
<Val1>1</Val1>
<Val3>2010-01-01T00:00:00.0-05:00</Val3>
</PersonTraining>
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)
但是，我曾期望从编组操作中获得以下结果（事实上，我还特别需要元素，以便可以针对 XSD 验证 XML）
<PersonTraining>
<Val1>1</Val1>
<Val2></Val2>
<Val3>2010-01-01T00:00:00.0-05:00</Val3>
</PersonTraining>
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.
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。
Here is the marshalling code:
这是编组代码：
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);
采纳答案by bdoughan
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.
默认情况下，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。
@XmlElement(name = "Val1", required = true)
protected BigDecimal val1;
You can override this behaviour by specifying nillable=trueon the @XmlElementannotation like you have done here:
您可以通过nillable=true在@XmlElement注释上指定来覆盖此行为，就像您在此处所做的那样：
@XmlElement(name = "Val2", required = true, nillable = true)
protected BigDecimal val2;
This will cause the xsi:nil="true"attribute to be leverage:
这将导致xsi:nil="true"属性成为杠杆：
<Val2 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:nil="true"/>
For more information:
想要查询更多的信息：
Java Model
Java模型
PersonTraining
人员培训
Since you are annotating the fieldsyou 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).
由于您正在注释，fields您应该确保@XmlAccessorType(XmlAccessType.FIELD)在类或包级别指定（请参阅：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;
}
Demo Code
演示代码
Demo
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);
}
}
Output
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>
