How do I instantiate a JAXBElement<String> object?

Question

When you imported the WSDL, you should have an ObjectFactory class which should have bunch of methods for creating various input parameters.

ObjectFactory factory = new ObjectFactory();
JAXBElement<String> createMessageDescription = factory.createMessageDescription("description");
message.setDescription(createMessageDescription);

ObjectFactory fact = new ObjectFactory();   
JAXBElement<String> str = fact.createCompositeTypeStringValue("vik");    
comp.setStringValue(str);
CompositeType retcomp = service.getDataUsingDataContract(comp);
System.out.println(retcomp.getStringValue().getValue());

Here is how I do it. You will need to get the namespace URL and the element name from your generated code.

new JAXBElement(new QName("http://www.novell.com/role/service","userDN"),
                new String("").getClass(),testDN);

Other alternative:

JAXBElement<String> element = new JAXBElement<>(new QName("Your localPart"),
                                                String.class, "Your message");

Then:

System.out.println(element.getValue()); // Result: Your message

I don't know why you think there's no constructor. See the API.