EclipseLink MOXy stores instances of DOM Element for fields of type Object annotated with @XmlElement

Question

We are using MOXy JAXB in our project.

Model class:

@XmlRootElement(name = "field")
@XmlType(propOrder = {"id","value"})
public class FieldData{
    @XmlAttribute
    private String id;
    @XmlAttribute
    private Object value;

    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }

    public Object getValue() {
        return value;
    }
    public void setValue(Object value) {
        this.value = value;
    }
}

For my use case, I want value to be of type Object itself as I may get any primitive data type value here. I will take them as strings initially. Once I get the object, I do the type conversion and save it into the same field. The above use case is working fine. But when I change @XmlAttribute to @XmlElement it is not working. I see that the value is unmarshalled as an instance of ElementNSImpl. Is there any work around for this?

Answer 1

Here is a brain dump on what you are seeing:

---

Demo Code

I will use the same demo code with the different mappings described below:

import java.io.File;
import javax.xml.bind.*;

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("input.xml");
        Foo foo = (Foo) unmarshaller.unmarshal(xml);

        System.out.println(foo.getBar().getClass());
    }

}

---

Use Case #1 - Object Property Mapped With @XmlAttribute

Java Model

Foo

We use the @XmlAttribute annotation to map a property to an @XmlAttribute. Note: This isn't a valid configuration when using the JAXB reference implementation.

import javax.xml.bind.annotation.*;

@XmlRootElement
public class Foo {

    private Object bar;

    @XmlAttribute
    public Object getBar() {
        return bar;
    }

    public void setBar(Object bar) {
        this.bar = bar;
    }

}

XML #1

input.xml

In the XML document below the bar attribute contains numeric digits.

<?xml version="1.0" encoding="UTF-8"?>
<foo bar="123"/>

output

As there is no typing information (in the XML or in the Java class), MOXy brings the value in as a String. A String is the most concrete type that can represent all possible values on an XML attribute.

class java.lang.String

XML #2

input.xml

In the XML document below the bar attribute contains alphabet characters.

<?xml version="1.0" encoding="UTF-8"?>
<foo bar="Hello World"/>

output

A String is the most concrete type that can represent all possible values on an XML attribute.

class java.lang.String

---

Use Case #2 Object Property Mapped With @XmlElement

Java Model

Foo

In this version of the Foo class we will not annotate the bar property, this is the same as annotating it with @XmlElement.

import javax.xml.bind.annotation.*;

@XmlRootElement
public class Foo {

    private Object bar;

    public Object getBar() {
        return bar;
    }

    public void setBar(Object bar) {
        this.bar = bar;
    }

}

XML #1 (Simple Element)

input.xml

<?xml version="1.0" encoding="UTF-8"?>
<foo>
    <bar>Hello World</bar>
</foo>

Output

class com.sun.org.apache.xerces.internal.dom.ElementNSImpl

XML #2 (Complex Element)

input.xml

Instead of containing just test, now the bar element contains XML attributes and child elements.

<?xml version="1.0" encoding="UTF-8"?>
<foo>
    <bar a="1">
       <b>2</b>
       <c>3</c>
    </bar>
</foo>

Output

Now we start to see why JAXB treats the value as a DOM, the element could be arbitrarily complex so a DOM element becomes a structure that can hold any possible value.

class com.sun.org.apache.xerces.internal.dom.ElementNSImpl

XML #3 (Typed Element)

input.xml

In the XML document below

<?xml version="1.0" encoding="UTF-8"?>
<foo xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <bar xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="xsd:string">Hello World</bar>
</foo>

Output

class java.lang.String

---

Use Case #3 Object Property Mapped With @XmlElement(type=String)

Java Model

Foo

In this version of the Foo class we will annotate the bar property with @XmlElement(type=String.class). As far as Java is concerned the property is still of typeObject, but JAXB will treat the property as if it's typeString`.

import javax.xml.bind.annotation.*;

@XmlRootElement
public class Foo {

    private Object bar;

    @XmlElement(type=String)
    public Object getBar() {
        return bar;
    }

    public void setBar(Object bar) {
        this.bar = bar;
    }

}

XML #1 (Simple Element)

Now we see that the value of the bar element is treated as a String.

input.xml

<?xml version="1.0" encoding="UTF-8"?>
<foo>
    <bar>Hello World</bar>
</foo>

Output

class java.lang.String