XmlJavaTypeAdapter not being detected

The following should help:

FOO AS ROOT OBJECT

When @XmlJavaTypeAdapter is specified at the type level it only applies to fields/properties referencing that class, and not when an instance of that class is a root object in your XML tree. This means that you will have to convert Foo to AdaptedFoo yourself, and create the JAXBContext on AdaptedFoo and not Foo.

Marshal

package forum11966714;

import javax.xml.bind.*;

public class Marshal {
    public static void main(String[] args) {
      Foo foo = new Foo("Adam", 34);

      try {
        JAXBContext jaxbContext = JAXBContext.newInstance(AdaptedFoo.class);
        Marshaller jaxbMarshaller = jaxbContext.createMarshaller();

        // output pretty printed
        jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        jaxbMarshaller.marshal(new AdaptedFoo(foo), System.out);              
      } catch (JAXBException e) {
        e.printStackTrace();
      }   
    }
  }

AdaptedFoo

You will need to add an @XmlRootElement annotation to the AdaptedFoo class. You can remove the same annotation from the Foo class.

package forum11966714;

import javax.xml.bind.annotation.*;

@XmlRootElement
class AdaptedFoo {
    private String name;
    private int age;

    public AdaptedFoo() {
    }

    public AdaptedFoo(Foo foo) {
        this.name = foo.getName();
        this.age = foo.getAge();
    }

    @XmlAttribute
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @XmlAttribute
    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

---

FOO AS NESTED OBJECT

When Foo isn't the root object everything works the way you have it mapped. I have extended your model to demonstrate how this would work.

Bar

package forum11966714;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Bar {

    private Foo foo;

    public Foo getFoo() {
        return foo;
    }

    public void setFoo(Foo foo) {
        this.foo = foo;
    }

}

Demo

Note that the JAXB reference implementation will not let you specify the Foo class when bootstrapping the JAXBContext.

package forum11966714;

import java.io.File;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class Demo {
    public static void main(String[] args) {
        try {
            JAXBContext jaxbContext = JAXBContext.newInstance(Bar.class);

            Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
            File xml = new File("src/forum11966714/input.xml");
            Bar bar = (Bar) jaxbUnmarshaller.unmarshal(xml);

            Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
            jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            jaxbMarshaller.marshal(bar, System.out);
        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }
}

input.xml/Output

<?xml version="1.0" encoding="UTF-8"?>
<bar>
    <foo name="Jane Doe" age="35"/>
</bar>