How to marshal an object via JAXB without any information about it?

Question

I have an object value, which is of some type, either @XmlRootElement-annotated, or not. I want to marshal it into XML:

String value1 = "test";
assertEquals("<foo>test</foo>", toXml("foo", value1));
// ...
@XmlRootElement
class Bar {
  public String bar = "test";
}
assertEquals("<foo><bar>test</bar></foo>", toXml("foo", new Bar()));

Can I do it with JAXB existing facilities, or I should create some custom analyzer?

Answer 1

You could leverage JAXBIntrospector to do the following:

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBElement;
import javax.xml.bind.JAXBIntrospector;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.namespace.QName;

public class Demo {


    public static void main(String[] args) throws Exception {
        Object value = "Hello World";
        //Object value = new Bar();

        JAXBContext jc = JAXBContext.newInstance(String.class, Bar.class);
        JAXBIntrospector introspector = jc.createJAXBIntrospector();
        Marshaller marshaller = jc.createMarshaller();
        if(null == introspector.getElementName(value)) {
            JAXBElement jaxbElement = new JAXBElement(new QName("ROOT"), Object.class, value);
            marshaller.marshal(jaxbElement, System.out);
        } else {
            marshaller.marshal(value, System.out);
        }
    }

    @XmlRootElement
    public static class Bar {

    }

}

With the above code when the JAXBElement is marshalled it will be qualified with an xsi:type attribute corresponding to the appropriate schema type:

<ROOT 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xmlns:xs="http://www.w3.org/2001/XMLSchema" xsi:type="xs:string">Hello World</ROOT>

To eliminate the qualification you can simply change the line that creates the JAXBElement to:

JAXBElement jaxbElement = new JAXBElement(new QName("ROOT"), value.getClass(), value);

This will result in the following XML:

<ROOT>Hello World</ROOT>

Answer 2

Here is how to marshal value1, which is a String. You can pass yourObject.getClass() to the JAXBElement constructor, and value1:

try {
    JAXBContext jc = JAXBContext.newInstance();
    Marshaller m = jc.createMarshaller();
    String value1 = "test";
    JAXBElement jx = new JAXBElement(new QName("foo"), value1.getClass(), value1);
    m.marshal(jx, System.out);
} catch (JAXBException ex) {
    ex.printStackTrace();
}

This works without using @XmlRootElement. The result of the code above was:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><foo>test</foo>

On the other part, this will not work with a Bar object: javax.xml.bind.JAXBException: myPackage.Bar is not known to this context. However, you can get the value from inside Bar, and create the JAXBElement with that, not the object itself.