Serialize Jaxb Pojos to multiple or different namespaces

Question

Consider the following code:

Main.java
====
package com.sample;

import com.sample.entity.Customer;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;

public class Main {    

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

       Customer customer = new Customer();
       customer.setId(123);

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


Customer.java
====
package com.sample.entity;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Customer {
    private long id;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }
}

package-info.java
====
@XmlSchema(namespace = "http://www.example.org/package", elementFormDefault = XmlNsForm.QUALIFIED)
package com.sample.entity;

import javax.xml.bind.annotation.XmlSchema;
import javax.xml.bind.annotation.XmlNsForm;

I wish to reuse the Customer POJO and create two different namespace values. So first I wish to print the output

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer xmlns="http://www.example.org/package">
    <id>123</id>
</customer>

like the current code does, then create a second xml with a different main namespace from the same pojo that will look like this

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer xmlns="http://www.another.org/package">
    <id>123</id>
</customer>

or remove the namespace all together

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <id>123</id>
</customer>

Answer 1

This is a good question.

Instead of working on the type-level you could work on the Jaxb node level. There is almost no overhead involved.

The key is creating/accessing JAXBElement (node level elements) out of your business logic classes and customize the XML-Node it represents with an according QName definition.

QName provides several options for customization, ranging from fully qualified namespaces to no namespace at all. The only caveat is that using a namespace without prefix is not possible. Jaxb will use a default namespace if you don't provide one.

Example:

QName elaborateQName = new QName("http://www.another.org/package", "customer", "myNs");
QName simpleQName = new QName("customer");

JAXBElement jx1 = new JAXBElement(elaborateQName, customer.getClass(), customer);
JAXBElement jx2 = new JAXBElement(simpleQName, customer.getClass(), customer);

m.marshal(jx1, System.out);
m.marshal(jx2, System.out);

This will produce the following output:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<myNs:customer xmlns:myNs="http://www.another.org/package">
    <id>123</id>
</myNs:customer>

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer>
    <id>123</id>
</customer>

Using a namespace without prefix is not possible. Trying a QName with empty prefix or the equivalent XMLConstants.DEFAULT_NS_PREFIX as parameter will result in a default prefix (like ns1) being generated:

 // QName defaultNs = new QName("http://www.another.org/package", "customer", "");

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ns2:customer xmlns:ns1="http://www.another.org/package">
    <id>123</id>
</ns2:customer>

Using null as the parameter for prefixyields an Exception:

java.lang.IllegalArgumentException: prefix cannot be "null" when creating a QName
at javax.xml.namespace.QName.<init>(QName.java:251)

Answer 2

How to override the root xmlns attribute with MOXy:

EclipseLink MOXy provides easy customization of JAXB marshalling, including changing the default namespace of your root element. The Object to XML mapping (OXM) for overriding the default namespace is:

<?xml version="1.0"?>
<xml-bindings xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm" version="2.5">
    <xml-schema element-form-default="QUALIFIED" namespace="http://www.another.org/package"/>
</xml-bindings>

To have no default namespace, use <xml-schema element-form-default="UNSET"/> instead.

MOXy Setup:

1) add the library to your project:

<dependency>
    <groupId>org.eclipse.persistence</groupId>
    <artifactId>org.eclipse.persistence.moxy</artifactId>
    <version>2.6.2</version>
</dependency>

2) Add a jaxb.properties to your object model package to enable MOXy's factory (e.g. src/main/resources/com/sample/entity/jaxb.properties):

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

3) Instantiate your context according to your OXM configuration (in this example, the OXM file is at src/main/resources/com/sample/entity/my-oxm.xml):

Map<String, Source> metadata = Collections.singletonMap("com.sample.entity", new StreamSource(Customer.class.getResourceAsStream("my-oxm.xml")));
Map<String, Object> properties = Collections.singletonMap(JAXBContextProperties.OXM_METADATA_SOURCE, metadata);
JAXBContext jaxbContext = JAXBContext.newInstance(new Class[] {customer.getClass()}, properties);

You can then use marshalling on the JAXBContext as normal. Instantiate separate contexts for each different OXM file you want to use.

Additional customizations with MOXy:

Using MOXy allows you to further customize the marshalling of your entity in the future without changing your object model, effectively making your marshalling logic adhere to the open/closed principle even though you have no explicit Marshaller classes. For instance, if you needed to marshal your Customer object as a Person, you could do this by adding another OXM file:

<?xml version="1.0"?>
<xml-bindings xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm" version="2.5">
    <xml-schema element-form-default="QUALIFIED" namespace="http://www.example.com/person"/>
    <java-types>
        <java-type name="com.sample.entity.Customer">
            <xml-root-element name="person"/>
            <java-attributes>
                <xml-attribute java-attribute="id" name="personId"/>
                <xml-element java-attribute="id" xml-path="someOtherId/text()"/>
            </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>