How to prevent JAXB from writing unused namespaces during marshalling

Question

Has someone ever been able to remove unused namespaces during marshal of an object using JAXB? Here is a link of the requested feature: https://github.com/javaee/jaxb-v2/issues/103 (see description)

Is there a property for configuring JAXB for this? Has this been fixed in MOXy?

I am currently traversing the object that needs to be marshalled and extract all classes that need to be bound into Class[] classesToBeBound. Then I create a new JAXBContext.newInstance(classesToBeBound)

The unused namespaces now are not included in the XML.

I know that xml validation is valid even with the unused namespaces but to me this is something a framework should handle.

The following link https://blogs.oracle.com/enterprisetechtips/entry/customizing_jaxb mentions various fixed (see middle of the text somewhere) but when trying to find the solution in those links either the link is broken or no-one really solved it.

Any comments are welcome.

(EDIT) Plain text:

GIVEN

a new instance of JAXBContext and add 2 classes with each a separate namespace. 

WHEN

marshalling a class that has these 2 classes as a property but only 1 of them is not null 

THEN

I expect only the namespace of the property that is not null to be visible in the XML. 

BUT the ACTUAL is

that both namespaces are in the xml. 

So my question was how can I remove or tell JAXB to NOT write the unused namespaces?

To put it in java-code: GIVEN

public class Foo{
  private Bar bar; //namespace something2
  private User user; //namespace user
}

WHEN

JAXBContext c = JAXBContext.newInstance(Foo.class, Bar.class, User.class);
...
Foo foo = new Foo();
foo.setBar(null);
foo.setUser(new User("Bob"));
marshaller.umarshal(foo);

THEN I expect the xml to be

<foo xmlns="something1"  xmlns:user="user">
  <user:name>Bob</user:name>
</foo>

BUT the ACTUAL is (note the something2 namespace)

<foo xmlns="something1" xmlns:user="user" xmlns:bar="something2">
  <user:name>Bob</user:name>
</foo>

Of course this is a simplified example and our specification of a type has around 30 different namespaces.

Answer 1

As far as I know, this is indeed not possible in JAXB - and is actually a well-known issue. As you have noticed it, the list of produced namespaces are the ones that have been registered in your JAXBContext, and not the ones that are effectively used when marshalling :-(

I the past, I used the same workaround as you (identify the various used classes and narrow the JAXBContext to this limited set of classes).

Another typical workaround is a 2-step processing: a first marshalling with JAXB, followed by a XSLT transformation to get rid of let's says "polluting" namespaces.