Question

我受到给定JSON结构的约束：

{
  "metadata": {
    "eventName": "FooEvent",
    "field1": "bla"
  },
  "event": { ... }
}

如何使用多态反序列化和嵌套类型信息属性对其进行反序列化？我在metadata.eventName中使用@JsonTypeInfo嵌套属性，如下所示：

@JsonTypeInfo(
    use = Id.NAME,
    include = As.EXISTING_PROPERTY,
    visible = true,
    property = "metadata.eventName"
)
@JsonSubTypes({
    @Type(name="fooEvent", value = FooEvent.class)
    @Type(name="barEvent", value = BarEvent.class)
})
public class EventPayload<T> {
     private Metadata metadata;
     private T event;
}

鉴于配置杰克逊抱怨该物业无法找到：

com.fasterxml.jackson.databind.JsonMappingException: Unexpected token (END_OBJECT), expected FIELD_NAME: missing property 'metadata.eventName' that is to contain type id  (for class EventPayload)
 at [Source: {
  "metadata": {
     "eventName": "FooEvent",
     "field1": "bla"
  },
  "content": { ... }
}; line: 16, column: 1]

Answer 1

您在这里面临两个问题：

正如您所见，杰克逊不能轻易地通过注释使用嵌套JSON对象中的属性来推断要反序列化的类型。
@JsonTypeInfo和@JsonSubTypes用于继承，例如class FooEventPayload extends EventPayload。在您的情况下，EventPayload<T>是一个通用类，杰克逊需要通过T Look here for instance

TypeReference

假设你想要一个泛型类，我建议首先序列化到树中，查看树以获取指定类型的属性，然后将树转换为此类型的对象。您可以跳过@JsonTypeInfo和@JsonSubTypes。 E.g。

// user object mapper to parse JSON into a tree (node is the root) 
ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.readTree(jsonString);

// use get as many times as needed by depth
// to get the value that defines the type to deserialise to
String type = node.get("metadata").get("eventName").textValue();

// convert JsonNode variable to the required type
if (type.equals("fooEvent")) {
    EventPayload<FooEvent> event = 
        mapper.convertValue(node, new TypeReference<EventPayload<FooEvent>>(){});
} else if (type.equals("barEvent")) {
    EventPayload<BarEvent> event =
        mapper.convertValue(node, new TypeReference<EventPayload<BarEvent>>(){});
}

具有嵌套类型信息属性的Jackson多态反序列化

1 个答案: