java - What JAXB needs a public no-arg constructor for?

Question

What does JAXB need a public no-arg constructor for, during marshalling?

 Marshaller msh = ctx.createMarshaller();
 msh.marshal(object, System.out);

I'm passing an object, not a class. Why does JAXB need a constructor? To construct what?

Answer 1

A JAXB implementation should not need a no-arg constructor during a marshal operation. JAXB does require one for unmarshalling. Normally the absence of a no-arg constructor causes an error when the JAXBContext is created. The JAXB implementation you are using may be delaying initialization until an actual operation is performed.

In general support for multi-arg constructors is something we should consider in a future version of JAXB. In the EclipseLink implementation of JAXB (MOXy) we have an enhancement request open for this functionality (feel free to add relevant details):

• https://bugs.eclipse.org/328951.

In the current version of JAXB you could use an XmlAdapter to support this use case:

• http://bdoughan.blogspot.com/2010/07/xmladapter-jaxbs-secret-weapon.html