时间戳

首页 > 解决方案 > Polymorphic deserialization in Jackson without annotations

java - Polymorphic deserialization in Jackson without annotations

问题描述

I have a CloudEvent<T> class that uses polymorphic deserialization using Jackson (2.9.0 - last version) like this:

@Data
@NoArgsConstructor
@AllArgsConstructor
public class CloudEvent<T> {

    @NonNull
    private String eventType;

    @JsonTypeInfo(
            use = JsonTypeInfo.Id.NAME,
            include = JsonTypeInfo.As.EXTERNAL_PROPERTY,
            property = "eventType",
            defaultImpl = Void.class)
    @JsonSubTypes({
            @JsonSubTypes.Type(value = MyEvent1.class, name = "event-1"),
            @JsonSubTypes.Type(value = MyEvent2.class, name = "event-2")
    })
    private T data;
}

And then deserialized with:

String cloudEventJson1 = "{\"eventType\":\"event-1\",\"data\":{\"id\":\"123\",\"details\":\"detail1\"}}";

CloudEvent deserializedEvent1 = objectMapper.readValue(cloudEventJson1, CloudEvent.class);   //without subtypes

All this works fine. But because some limitations, I can not use annotations on the CloudEvent class (provided by external dependency).

So I configured the ObjectMapper like this:

    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerSubtypes(new NamedType(MyEvent1.class, "event-1"));
    objectMapper.registerSubtypes(new NamedType(MyEvent2.class, "event-2"));

    TypeResolverBuilder  typeResolverBuilder = new ObjectMapper.DefaultTypeResolverBuilder(ObjectMapper.DefaultTyping.OBJECT_AND_NON_CONCRETE)
      .init(JsonTypeInfo.Id.NAME, null)   //CLASS works
      .inclusion(JsonTypeInfo.As.EXTERNAL_PROPERTY)
      .typeProperty("eventType")
      .typeIdVisibility(true)
//    .defaultImpl(Void.class);
    objectMapper.setDefaultTyping(typeResolverBuilder);

But deserializing with same method as above does not work. It is reading the eventType but it is not managing to match to the registered subtype. I can not use generics or TypeReferance in the deserialization because I need to use spring-integration for reading the events which only accepts main class; the pattern matching being done manually after deserialization.

Exception:

com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Could not resolve type id 'event-1' as a subtype of [simple type, class java.lang.Object]: known type ids = [] (for POJO property 'data')
 at [Source: (String)"{"eventType":"event-1","data":{"id":"123","details":"detail1"}}"; line: 1, column: 271]

Also this is configuring the ObjectMapper for all input classes. Is it possible to connect this typeResolverBuilder and subtypes to the CloudEvent.class (like the annotation way does it).

标签： javajsonspringjacksondeserialization

解决方案

即使你不能修改你的类，你仍然可以依赖注解。Jackson 支持一种称为mix-ins的功能：您可以将其视为一种在运行时添加更多注释的面向方面的方式，以增加静态定义的注释。

首先定义一个接口如下：

public interface CloudEventMixIn<T> {

     @JsonTypeInfo(
            use = JsonTypeInfo.Id.NAME,
            include = JsonTypeInfo.As.EXTERNAL_PROPERTY,
            property = "eventType",
            defaultImpl = Void.class)
    @JsonSubTypes({
            @JsonSubTypes.Type(value = MyEvent1.class, name = "event-1"),
            @JsonSubTypes.Type(value = MyEvent2.class, name = "event-2")
    })
    public T getData();
}

然后配置ObjectMapper为使用定义的接口作为实际类/接口的混合：

ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(CloudEvent.class, CloudEventMixIn.class);

从addMixIn(Class<?> target, Class<?> mixinSource)方法文档：

用于添加混合注释以用于扩充指定类或接口的方法。来自的所有注释mixinSource都被用来覆盖target（或其超类型）具有的注释。

推荐阅读