How do I mark fields as required/optional when reading XML with MOXy?

问题

Having a trivial code like this:

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class A {
  @XmlPath("B/text()")
  private String b;

  @XmlPath("C/text()")
  private Integer c;
  ...
}

It works absolutely fine as long as I have apt values in my XML. I'd like to mark the field c as required, so MOXy throw every time I try to read the document where c is either not set, or invalid. What's the easiest solution?

Update:

Setting default values will also do.

回答1:

EclipseLink JAXB (MOXy) or any JAXB implementation will not perform a set operation for missing nodes, so you could do the following:

Option #1 - Default the field value

import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.XmlPath;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class A {
  @XmlPath("B/text()")
  private String b = "fieldDefault";

  @XmlPath("C/text()")
  private Integer c;
}

With the following demo code:

import java.io.StringReader;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Unmarshaller;

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        A a = (A) unmarshaller.unmarshal(new StringReader("<a/>"));

        System.out.println(a.getB());
        System.out.println(a.getC());
    }

}

Will produce the following output:

fieldDefault
null

Option #2 - Specify defaultValue on @XmlElement

You can specify a defaultValue on the @XmlElement annotation, but this will only set the defaultValue when an empty element is unmarshalled.

import javax.xml.bind.annotation.*;
import org.eclipse.persistence.oxm.annotations.XmlPath;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
class A {
  @XmlPath("B/text()")
  @XmlElement(defaultValue="annotationDefault")
  private String b;

  @XmlPath("C/text()")
  @XmlElement(defaultValue="annotationDefault")
  private Integer c;

}

With the following demo code:

import java.io.StringReader;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Unmarshaller;

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        A a = (A) unmarshaller.unmarshal(new StringReader("<a><B/></a>"));

        System.out.println(a.getB());
        System.out.println(a.getC());
    }

}

Will produce the following output: