JAXB jersey how to implement MessageBodyWriter for a list

Question

I am trying to implement MessageBodyWriter for a List. with this implementation, I have a null pointer exception on the line; 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)); } } 

Answer 1

 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)); InstrumentModelListProvider(); 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)); }