Jersey JAXB, как реализовать MessageBodyWriter для List

Я пытаюсь реализовать MessageBodyWriter для List. С этой реализацией у меня есть исключение нулевого указателя в строке; marshaller.marshal (o, entityStream);

@Provider
@Produces(MediaType.APPLICATION_XML) 
public class MyListProvider implements MessageBodyWriter<List<Instrument>> {

    private String myWrapElemName = "datas";
    private Marshaller marshaller;


    public InstrumentModelListProvider(){

        JAXBContext context;
        try {
            context = JAXBContext.newInstance(Data.class);
             marshaller = context.createMarshaller();
             marshaller.setProperty(Marshaller.JAXB_FRAGMENT, true);
             marshaller.setProperty(Marshaller.JAXB_ENCODING, "UTF-8");
             //marshaller.setProperty(JMMarshallerImpl.JAXME_XML_DECLARATION, Boolean.TRUE);
             marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
        } catch (JAXBException e) {
            System.err.println("/////////////"+e.getMessage());
            //e.printStackTrace();
        }

    }
    @Override
    public long getSize(List<Data> as, Class<?> type, Type genericType, Annotation[] annotations,
            MediaType mediaType) {

        return -1;
    }

    @Override
    public boolean isWriteable(Class<?> type, Type genericType, Annotation[] annotations,
            MediaType mediaType) {



        return MediaType.APPLICATION_XML_TYPE.equals(mediaType) 
                && List.class.isAssignableFrom(type) 
                && (((ParameterizedType)genericType).getActualTypeArguments()[0]).equals(Data.class);
    }

    @Override
    public void writeTo(List<Data> list, Class<?> type, Type genericType, Annotation[] annotations,
            MediaType mediaType, MultivaluedMap<String, Object> httpHeaders,
            OutputStream entityStream) throws IOException, WebApplicationException {

        Charset c = Charset.forName("UTF-8");
        String cName = c.name();

        entityStream.write(String.format("<?xml version=\"1.0\" encoding=\"%s\" standalone=\"yes\"?>", cName).getBytes(cName));

        entityStream.write(String.format("<%s>", myWrapElemName).getBytes(cName));

        for (Data o : list){
          try {
              marshaller.marshal(o, entityStream);
          } catch(JAXBException exp) {
             exp.printStackTrace();
              //System.exit(-1);
          }
        }

        entityStream.write(String.format("</%s>", myWrapElemName).getBytes(cName));
    }


}