根

package forum13218462;
 
import javax.xml.bind.annotation.*;
 
@XmlRootElement
public class Root {
 
    @XmlAttribute
    String attributeNull;
 
    @XmlAttribute
    String attributeEmpty;
 
    @XmlAttribute(required=true)
    String attributeNullrequired;
 
}

演示

package forum13218462;
 
import javax.xml.bind.*;
 
public class Demo {
 
    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Root.class);
 
        Root root = new Root();
        root.attributeNull = null;
        root.attributeEmpty = "";
        root.attributeNullrequired = null;
 
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT,true);
        marshaller.marshal(root,System.out);
    }
 
}

产量

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<root attributeEmpty=""/>