Only a handful of source code lines is required to make a JAXB Marshaller object write a document tree as an XML file. First you obtain a Marshaller from a JAXBContext. Then, you might set a number of properties, such as the one that's used below, which requests nice formatting of the XML text. Other properties concern the inclusion of a schema location as an attribute in the top-level element, or the encoding in the XML prolog. The first argument must be an object that is either a root element, as defined by your schema, or a JAXBElement<?>.

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

void writeDocument( Object document, String pathname )
    throws JAXBException, IOException {
    Class<T> clazz = document.getValue().getClass();
    JAXBContext context =
        JAXBContext.newInstance( clazz.getPackage().getName() );
    Marshaller m = context.createMarshaller();
    m.setProperty( Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE );
    m.marshal( document, new FileOutputStream( pathname ) );
}

Sometimes marshalling needs to be done not only for one or two root documents but for objects of many different schema types. You could, of course, add xsd:element definitions for all of them to the top level xsd:schema element, but this is cumbersome. A generic solution is presented below. The method wraps an arbitrary element of some type T into a JAXBElement<T>.

<T> JAXBElement<T> wrap( String ns, String tag, T o ){
    QName qtag = new QName( ns, tag );
    Class<?> clazz = o.getClass();
    @SuppressWarnings( "unchecked" )
    JAXBElement<T> jbe = new JAXBElement( qtag, clazz, o );
    return jbe;
}