自定义杰克逊解组行为 [英] Customize jackson unmarshalling behavior

问题描述

我正在使用Jackson fastxml来解组JSON。在我的对象中有两种属性：输入属性和计算属性。在输入JSON中，我只获得输入值。

I am using Jackson fasterxml for unmarshalling JSON. In my object there are two kinds of properties:Input properties and Calculated properties. In the input JSON, I get only input values.

计算出的值实际上取决于输入值。我必须在引用对象之前填充这些值。所以我只是检查杰克逊是否有任何钩子，以便我可以在那里进行计算。例如，JAXB提供afterUnmarshal方法来自定义解组行为：

The calculated values are actually dependent on input values. I have to populate these values before the object gets referred. So I am just checking if there are any hooks provided by Jackson so that I can do my calculations there. For example JAXB provides afterUnmarshal method to customize the unmarshaling behavior:

void afterUnmarshal(Unmarshaller u, Object parent)

但我找不到有关定制杰克逊的类似信息。杰克逊是否提供了任何这样的框架钩子来定制解组行为？

But I could not find similar information about customizing Jackson. Are any such framework hooks provided by Jackson to customize the unmarshaling behavior?

推荐答案

我宁愿建议你的模型对象保持不变使用构造函数创建者。也就是说，所有JSON值都传递给构造函数，该构造函数将初始化其他计算属性。

I'd rather recommend to keep your model objects immutable by using constructor creators. That is, all the JSON values are passed to a constructor which would initialize the other calculated properties.

无论如何，如果要在反序列化后自定义对象（无需编写你可以以最终调用新构造实例的特殊方法的方式修改反序列化器。这是一个适用于实现特殊接口的所有类的示例（可以考虑使用注释来标记post构造方法）。

Anyway, if you want to customize an object after deserialization (without writing a deserializer for every type) you can modify the deserializer in a way that at the end it calls a special method(s) of a newly constructed instance. Here is an example which would work for all the classes that implements a special interface (one can consider using an annotation to mark the post construct methods).

public class JacksonPostConstruct {

    public static interface PostConstructor {
        void postConstruct();
    }

    public static class Bean implements PostConstructor {
        private final String field;

        @JsonCreator
        public Bean(@JsonProperty("field") String field) {
            this.field = field;
        }

        public void postConstruct() {
            System.out.println("Post construct: " + toString());
        }

        @Override
        public String toString() {
            return "Bean{" +
                    "field='" + field + '\'' +
                    '}';
        }
    }

    private static class PostConstructDeserializer extends DelegatingDeserializer {
        private final JsonDeserializer<?> deserializer;

        public PostConstructDeserializer(JsonDeserializer<?> deserializer) {
            super(deserializer);
            this.deserializer = deserializer;
        }

        @Override
        protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegatee) {
            return deserializer;
        }

        @Override
        public Object deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
            Object result = _delegatee.deserialize(jp, ctxt);
            if (result instanceof PostConstructor) {
                ((PostConstructor) result).postConstruct();
            }
            return result;
        }
    }

    public static void main(String[] args) throws IOException {
        ObjectMapper mapper = new ObjectMapper();
        SimpleModule module = new SimpleModule();
        module.setDeserializerModifier(new BeanDeserializerModifier() {
            @Override
            public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,
                                                          BeanDescription beanDesc,
                                                          final JsonDeserializer<?> deserializer) {
               return new PostConstructDeserializer(deserializer);
            }
        });
        mapper.registerModule(module);
        String json = "{\"field\":\"value\"}";
        System.out.println(mapper.readValue(json, Bean.class));
    }

}

输出：

Post construct: Bean{field='value'}
Bean{field='value'}

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