Can we decide the jaxb class at runtime during unmarshalling?

Is there any way to decide at runtime which java class I would like to Unmarshall the XML into?

I tried in this way Unmarshall code -

public Object unmarshallXml(String xmlReq, String className)
{
    String myClass = className+".class";
    Object instances = null;
       try {
        JAXBContext jc = JAXBContext.newInstance( myClass );
           Unmarshaller u = jc.createUnmarshaller();
           StringBuffer xmlStr = new StringBuffer( xmlReq );
           StringReader strReader = new StringReader( xmlStr.toString() );
           StreamSource strSource = new StreamSource(strReader);
           Object o = u.unmarshal( strSource );

    } catch (JAXBException e) {

        e.printStackTrace();
    }
    return instances;
}

but got this error -

" javax.xml.bind.JAXBException: "LookupInstances.class" doesnt contain ObjectFactory.class or jaxb.index"