@XmlAccessorType(XmlAccessType.FIELD)
public class ProductFilter {

    @XmlElement(name = "productType")
    private ProductType productType;
    @XmlElement(name = "productId")
    private Integer productId;

    public ProductFilter() {
    }

    public ProductFilter(ProductType productType) {
        this.productType = productType;
    }

    public ProductFilter(ProductType productType, Integer productId) {
        this.productType = productType;
        this.productId = productId;
    }

    @JsonSerialize(using = ProductTypeSerializer.class)
    @XmlAccessorType(XmlAccessType.FIELD)
    @XmlEnum
    public enum ProductType {
        @XmlEnumValue("0")
        A(0),

        @XmlEnumValue("1")
        B(1),

        @XmlEnumValue("2")
        C(2),

        @XmlEnumValue("3")
        D(3);

        public Integer code;

        ProductType(Integer code) {
            this.code = code;
        }

        ProductType() {
        }
    }
}

{"productFilter":{}}

public class ProductTypeSerializer extends JsonSerializer<ProductFilter.ProductType> {

    @Override
    public void serialize(ProductFilter.ProductType value, JsonGenerator generator, SerializerProvider provider) throws IOException, JsonProcessingException {
        generator.writeStartObject();
        generator.writeFieldName("productType");
        generator.writeNumber(value.toString());
        generator.writeEndObject();
    }
}