Create XML string in java with custom namespace

Here is one approach:

First, here is my class representing the required XML data:

package org.ajames.jaxb.persons;

import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "")
@XmlRootElement(name = "faxml")
public class Faxml {
    
    private Person person;
    
    public static class Person {
        
        private String name;

        public String getName() {
            return name;
        }

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

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }
    
}

I chose to nest the Person class inside the root class - just for this test. There are other ways to arrange these classes, of course, so they are not nested.

Then I define the package-info for org.ajames.jaxb.persons as follows:

@XmlSchema(
        namespace = "",
        elementFormDefault = XmlNsForm.UNSET,
        xmlns = {
            @XmlNs(prefix = "", namespaceURI = "http://www.w3.org/2001/XMLSchema-instance")
        })
package org.ajames.jaxb.persons;

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

To process the data from a Java object to XML, I use the following test data:

Faxml.Person person = new Faxml.Person();
person.setName("ABC");
Faxml faxml = new Faxml();
faxml.setPerson(person);

The JAXB context and marshaller are as follows, assuming we are writing the XML to a Java String, for this test:

JAXBContext jaxbContext = JAXBContext.newInstance(Faxml.class); 
Marshaller marshaller = jaxbContext.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FRAGMENT, Boolean.TRUE);
marshaller.setProperty(Marshaller.JAXB_NO_NAMESPACE_SCHEMA_LOCATION, "test.xsd");
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
        
StringWriter stringWriter = new StringWriter();
marshaller.marshal(faxml, stringWriter);
String xml = stringWriter.toString();
        
System.out.println(xml);

The resulting XML is:

<faxml xsi:noNamespaceSchemaLocation="test.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <person>
        <name>ABC</name>
    </person>
</faxml>