java - Serializing with JAXB and the Any

Question

I have a schema that defines the following type:

<xsd:complexType name="Payload">
   <xsd:sequence>
      <xsd:any namespace="##any" minOccurs="0" maxOccurs="unbounded" processContents="lax"/>
   </xsd:sequence>
</xsd:complexType>

And that creates an object like so:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Payload", propOrder = {
    "any"
})
public class Payload {

    @XmlAnyElement(lax = true)
    protected List<Object> any;
}

Now I try adding another generated JAXB object to that Payload doing something like this:

Class payloadClass = ...;
JAXBContext context = JAXBContext.newInstance( WrapperRequest.class, payloadClass);
...
marshaller.marshal( wrappedRequest );

But I get a terrible exception that looks like it'll never work so I decide to serialize the payload object to XML first then add that as a string in the payload.

StringWriter writer = new StringWriter();
JAXBContext context = JAXBContext.newInstance( sdoRequest.getClass() );
Marshaller marshaller = context.createMarshaller();
marshaller.marshal(new JAXBElement(new QName("uri", sdoRequest.getClass().getSimpleName()), sdoRequest.getClass(), sdoRequest), writer);
payload.getAny().add( writer.toString() );

And this blows up with an exception saying "java.lang.String" does not contain an @XmlRootElement.

So how will the use of xs:any ever work with JAXB? Nothing seems to want to work because JAXB turns the Payload into Object, and it won't serialize just anything in Object. This is all inside Axis2 as well so it's been very challenging to get to this point.

Answer 1

Below I will demonstrate JAXB (JSR-222) and any with an example:

Payload

The any property is annotated with @XmlAnyElement(lax=true). This means that for that property if an element is associated with a class via @XmlRootElement or @XmlElementDecl then an instance of the corresponding object will be used to populate the property if not the element will be set as an instance of org.w3c.dom.Element.

package forum13941747;

import java.util.List;
import javax.xml.bind.annotation.*;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "Payload", propOrder = {
    "any"
})
public class Payload {

    @XmlAnyElement(lax = true)
    protected List<Object> any;

}

Foo

Below is an example of a class annotated with @XmlRootElement.

package forum13941747;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Foo {

}

Bar

Below is an example of a class without the @XmlRootElement annotation. In this use case we will leverage the @XmlElementDecl annotation on a factory class (usually called ObjectFactory) annotated with @XmlRegistry.

package forum13941747;

public class Bar {

}

ObjectFactory

Below is an example of specifying an @XmlElementDecl annotation for the Bar class.

package forum13941747;

import javax.xml.bind.JAXBElement;
import javax.xml.bind.annotation.*;
import javax.xml.namespace.QName;

@XmlRegistry
public class ObjectFactory {

    @XmlElementDecl(name="bar")
    public JAXBElement<Bar> createBar(Bar bar) {
        return new JAXBElement<Bar>(new QName("bar"), Bar.class, bar);
    }

}

input.xml

Below is the input document we'll use for this example. There are 3 elements that correspond to the any property. The first corresponds to the @XmlRootElement annotation on the Foo class. The second corresponds to the @XmlElementDecl annotation for the Bar class and the third does not correspond to any of the domain classes.

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

Demo

In the demo code below we will unmarshal the input document, then output the classes of the objects in the resulting any property and then marshal the payload object back to XML.

package forum13941747;

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

public class Demo {

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

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/forum13941747/input.xml");
        Payload payload = (Payload) unmarshaller.unmarshal(xml);

        for(Object o : payload.any) {
            System.out.println(o.getClass());
        }

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(payload, System.out);
    }

}

Output

Below is the output from running the demo code. Note the classes corresponding to the objects in the any property. The foo element became an instance of the Foo class. The bar element became an instance of JAXBElement that holds an instance of Bar. The other element became an instance of org.w3c.dom.Element.

class forum13941747.Foo
class javax.xml.bind.JAXBElement
class com.sun.org.apache.xerces.internal.dom.ElementNSImpl
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<payload>
    <foo/>
    <bar/>
    <other/>
</payload>