At the time of testing you may required to convert XML string to POJO and vice versa.
After so many searches, only the below way worked for me for XML marshaling and unmarshaling without root element. 

//marshal
JAXBContext jaxbContext = JAXBContext.newInstance(Source.class);//JAXB library
Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
ObjectFactory factory=new ObjectFactory();//Generated class from XSD/WSDL file
JAXBElement<Source> jaxbElement=factory.createSource(sourceObj);        
jaxbMarshaller.marshal(jaxbElement, System.out);

//unmarshal
JAXBContext jaxbContext = JAXBContext.newInstance(Source.class);
XMLStreamReader xmlStreamReader=XMLInputFactory.newInstance().createXMLStreamReader(new FileReader("./src/test/resources/source.xml"));
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
JAXBElement<Source> jAXBElement = (JAXBElement<Source>) jaxbUnmarshaller.unmarshal(xmlStreamReader, Source.class);
Source sourceObj = jAXBElement.getValue();