• see: Jackson - ObjectMapper - Custom Null Serializer

1 - Problem

public class Main {
    public static void main(String[] args) throws JsonProcessingException {
        CsvMapper mapper = new CsvMapper();
        CsvSchema schema = mapper.schemaFor(Pojo.class).withUseHeader(true);
        ObjectWriter writer = mapper.writer(schema);

        Pojo pojo = new Pojo();
        pojo.setFirstName("John");
        pojo.setLastName(null);

        String csv = writer.writeValueAsString(pojo);
        System.out.println(csv);
    }

    @Data
    public static class Pojo {
        private String firstName;
        private String lastName;
    }
}

2 - Solutions

Solution 1 - Create Custom Null Serializer

public class CustomNullSerializer extends JsonSerializer<Object> {
    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
        jgen.writeString("null");
    }
}

CsvMapper mapper = new CsvMapper();
DefaultSerializerProvider.Impl sp = new DefaultSerializerProvider.Impl();
sp.setNullValueSerializer(new CustomNullSerializer());
mapper.setSerializerProvider(sp);

Solution 2 - Configure CsvSchema

CsvSchema schema = mapper.schemaFor(Pojo.class).withNullValue("null");