public class Request {
	private String id;
	private Iterable<Impression> imp;
        /// get,set...
}

		ObjectMapper mapper = new ObjectMapper();
		mapper.configure(SerializationConfig.Feature.WRITE_NULL_PROPERTIES, false);
		ByteArrayOutputStream stream = new ByteArrayOutputStream();
                Request reauest = new Request();
		m.setId("ID123");
		m.setImp(...);
		try{
			mapper.writeValue(stream, request);
			stream.close();
		} catch (Exception e){
			e.printStackTrace();
		}
//		mapper.setSerializationConfig().setSerializationInclusion(SerializationConfig.Feature.WRITE_NULL_MAP_VALUES, false); 
		String result = stream.toString();
		System.out.println(result);