1 - Create Custom NullSerializer

Override the JsonSerializer serialize method as below.

public class NullSerializer extends JsonSerializer<Object> {
  public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
    jgen.writeString("anything you want here");
  }
}

2 - Using Custom Null Serializer

then you can set NullSerializer as default for custom object mapper:

public class CustomJacksonObjectMapper extends ObjectMapper {

public CustomJacksonObjectMapper() {
    super();
    DefaultSerializerProvider.Impl sp = new DefaultSerializerProvider.Impl();
    sp.setNullValueSerializer(new NullSerializer());
    this.setSerializerProvider(sp);
  }
}

or specify it for some property using @JsonSerialize annotation, e.g:

public class MyClass {
  @JsonSerialize(nullsUsing = NullSerializer.class)
  private String property;
}

or at class level

@JsonSerialize(nullsUsing = NullSerializer.class)
public class MyClass {
  private String property;
}