How to instantiate an empty element with JAXB

Question

I use JAXB to create XML messages. The XML I need to create is (for the sake of simplicity):

Answer 1

An empty tag for a String object is essentially the empty string.

If you call the following, you will get what you are looking for:

request.setHeader("")

I should also note that in XML the following two declarations of a header are idential. Both of these have no child text nodes. These are essentially the same and will be treated the same by all XML parsers:

<header></header>

<header/>

Answer 2

As @Tom Hawtin - tackline said

<header/> and <header></header> is same. Parsers will give you "".

You have to to put nillable on your header annotation

@XmlElement(nillable=true, required=true)
public String getHeader() {
  return header;
}

I hope this code will generate following XML for null value.

import javax.xml.bind.*;
import javax.xml.bind.annotation.*;

@XmlRootElement
public class Request {

    public static void main(String[] args) throws JAXBException {
        final Request request = new Request();
        final JAXBContext context = JAXBContext.newInstance(Request.class);
        final Marshaller marshaller = context.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT,
                               Boolean.TRUE);
        marshaller.marshal(request, System.out);
        System.out.flush();
    }

    @XmlElement(nillable=true, required=true)
    private String header;
}

prints

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

Answer 3

In XML, <header/> and <header></header> are the same thing. If you really want the former, then use a prettifier. javax.xml.transform.TransformerFactory.newTransformer() will probably do that for you.

Answer 4

I wanted the same thing, effectively <header/> rather than <header></header> during the xml serialization process.

Since a null value - rather than an empty string - will produce the correct result, I modified my setter method to set the value explicitly to null:

public void setHeader(String header) {
    this.header = "".equals(header) ? null : header;
}