json - 棘手的(？)JSON，多态反序列化

Question

我想使用 Jackson 2.x 反序列化一段 JSON。

有一个看起来像这样的json:

{
     "response":[
      230,
      {
         "id":1627,
         "from_id":4534,
         "attachments":[
            {
               "type":"audio",
               "audio":{
                  "aid":25918,
                  "owner_id":20000,
               }
            },
            {
               "type":"link",
               "link":{
                  "url":"http://lenta.ru",
                  "title":"bla"
               }
            }
         ]
      }
      ...
   ]
}

所以我想使用多态类型处理来反序列化附件
进入附件列表。
我有以下带有 Mixins 的 POJO:

public class Post {
    private final String id;
    private List<? extends Attachment> attachments;
    // ...
}

@JsonIgnoreProperties(ignoreUnknown = true)
public class PostMixin {
    @JsonProperty("id")
    String id;

    @JsonProperty("attachments")
    List<? extends Attachment> attachments;
    // ...
}

public abstract class Attachment {
    public static enum AttachmentType {
        AUDIO, LINK
    }

    private AttachmentType type;
    public AttachmentType getType() {
        return type;
    }
}

// Not sure here if using these annotations propper way
@JsonTypeInfo(use=JsonTypeInfo.Id.NAME,
include=JsonTypeInfo.As.PROPERTY, property = "type", visible = true)
@JsonSubTypes({
    @JsonSubTypes.Type(name="link", value=LinkAttachment.class),
    @JsonSubTypes.Type(name="audio", value=AudioAttachment.class)
})
@JsonIgnoreProperties(ignoreUnknown = true)
public class AttachmentMixin {
    @JsonProperty("type")
    @JsonDeserialize(using = AttachmentTypeDeserializer.class)
    Attachment.AttachmentType type;

    // parse type into enum
    private static class AttachmentTypeDeserializer extends
JsonDeserializer<Attachment.AttachmentType> {
        @Override
        public Attachment.AttachmentType deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
            return Attachment.AttachmentType.valueOf(jp.getText().toUpperCase());
        }
    }
}

public class LinkAttachment extends Attachment {
    private String url;
    private String title;

    public String getUrl() {
        return url;
    }
    public String getTitle() {
        return title;
    }
}

@JsonIgnoreProperties(ignoreUnknown = true)
public class LinkAttachmentMixin extends AttachmentMixin {
    @JsonProperty("url")
    String url;

    @JsonProperty("title")
    String title;
}

public class AudioAttachment extends Attachment {
    private String id;
    private String ownerId;

    public String getId() {
        return id;
    }
    public String getOwnerId() {
        return ownerId;
    }
}

@JsonIgnoreProperties(ignoreUnknown = true)
public class AudioAttachmentMixin extends AttachmentMixin {
    @JsonProperty("aid")
    private String id;

    @JsonProperty("owner_id")
    private String ownerId;
}

创建了注册 Mixins 的模块:

public class MyModule extends SimpleModule {
    public MyModule() {
        super("MyModule");
    }

    @Override
    public void setupModule(SetupContext context) {
        context.setMixInAnnotations(Post.class, PostMixin.class);
        context.setMixInAnnotations(Attachment.class, AttachmentMixin.class);
        context.setMixInAnnotations(LinkAttachment.class, LinkAttachmentMixin.class);
        context.setMixInAnnotations(AudioAttachment.class,AudioAttachmentMixin.class);
    }
}

初始化 ObjectMapper:

objectMapper = new ObjectMapper();
objectMapper.registerModule(new MyModule());

当我尝试反序列化 JSON 时，出现以下异常:

java.lang.IllegalArgumentException: Class my.package.LinkAttachmentMixin is not assignable to my.package.Attachment
(through reference chain: my.package.Post["attachments"])

是否可以使用 jackson 多态类型反序列化此 JSON
处理支持？
我需要编写自己的反序列化器吗？谁能给个好
sample 开始？
是否有可能仅使用注释来解析这个 JSON？

Answer 1

试着把 @JsonTypeInfo和 @JsonSubTypes您的 Attachment 上的注释类而不是在 mixin 上。您甚至可以通过正确注释其他类来完全消除混入。如果你走那条路，那么你可能必须用 @JsonCreator 注释你的 VO 类/属性和 AttachmentType。 .

Here's an article这描述了类似于您正在尝试做的反序列化。我发现它过去对做这种事情很有用。