Jackson全面解析--注解全讲解十二（动态添加字段@Json

@JsonAppend

有时候我们需要在不改变类的属性字段的情况下，添加字段，Jackson提供了@JsonAppend注解来实现这个功能，使用方式如下

 @Getter
    @Setter
    @AllArgsConstructor(staticName = "of")
    @NoArgsConstructor
    @JsonAppend(attrs = {@JsonAppend.Attr(value = "age"),@JsonAppend.Attr(value = "height")},
                props = {@JsonAppend.Prop(value =TestWriter.class ,type = String.class,name = "version")})
    class JsonPropertyPojo{


        private String sex;

        private String name;

        private String unknown;

    }
@NoArgsConstructor
    class TestWriter extends VirtualBeanPropertyWriter{


        private TestWriter(BeanPropertyDefinition propDef, Annotations contextAnnotations, JavaType declaredType) {
            super(propDef, contextAnnotations, declaredType);
        }


        @Override
        protected Object value(Object bean, JsonGenerator gen, SerializerProvider prov) throws Exception {

            Field nameField = ReflectionUtils.findField(bean.getClass(),"name");
            ReflectionUtils.makeAccessible(nameField);

           if( nameField.get(bean).toString().length()>2){

               return "1.2";


           }

           return "1.0";

        }

        @Override
        public VirtualBeanPropertyWriter withConfig(MapperConfig<?> config, AnnotatedClass declaringClass, BeanPropertyDefinition propDef, JavaType type) {
            return new TestWriter(propDef, declaringClass.getAnnotations(), type);
        }
    }
测试代码如下
  @Test
    public void JsonAppendTest() throws Exception{

        CombineJacksonAnnotation.JsonPropertyPojo pojo = CombineJacksonAnnotation.JsonPropertyPojo.of("男","小小刘","some");
        System.out.println(om.writerFor(CombineJacksonAnnotation.JsonPropertyPojo.class).withAttribute("age","10").withAttribute("height","12").writeValueAsString(pojo));

    }
序列化结果如下
{
  "sex" : "男",
  "name" : "小小刘",
  "unknown" : "some",
  "age" : "10",
  "height" : "12",
  "version" : "1.2"
}

通过上面的例子可以看到，@JsonAppend提供了两种方式来动态的添加虚拟字段
1 attrs
此种方式需要在序列化时候手动的添加Attribute，如下
om.writerFor(CombineJacksonAnnotation.JsonPropertyPojo.class).withAttribute("age","10").withAttribute("height","12")
2 props
此种方式比较灵活，但是要实现一个VirtualBeanPropertyWriter类即可，如果真的有这种需求，推荐使用第二种方式来实现