使用@JsonSubTypes反序列化没有值 - 缺少属性错误

Question

我像这样反序列化jsons:

{
  "type":"a",
  "payload" : {...}
}

有效负载类型取决于类型.我的课:

public class Sth<T extends Payload> {

    @JsonProperty("type")
    private String type;
    @Valid
    private T payload;

    @JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        include = JsonTypeInfo.As.EXTERNAL_PROPERTY,
        property = "type",
        visible = true,
        defaultImpl = NoClass.class)
    @JsonSubTypes({
        @JsonSubTypes.Type(value = APayload.class, name = "a"),
        @JsonSubTypes.Type(value = BPayload.class, name = "b"),
        @JsonSubTypes.Type(value = CPayload.class, name = "c")})
    public void setPayload(T payload) {
    this.payload = payload;
    }

    public void setType(String type) {
    this.type = type;
    }

}

我也输入了没有负载的"d".如果我尝试反序列化:

{
  "type":"d",
  "payload" : null
}

它工作但它没有负载有效:

{
  "type":"d",
}

如何让它与上一个例子一起工作？

我得到错误的Stacktrace:

[error] Caused by: com.fasterxml.jackson.databind.JsonMappingException: Missing property 'payload' for external type id 'type
[error]  at [Source: N/A; line: -1, column: -1]
[error]     at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164)
[error]     at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:700)
[error]     at com.fasterxml.jackson.databind.deser.impl.ExternalTypeHandler.complete(ExternalTypeHandler.java:160)
[error]     at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeWithExternalTypeId(BeanDeserializer.java:690)
[error]     at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeWithExternalTypeId(BeanDeserializer.java:639)
[error]     at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:266)
[error]     at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:124)
[error]     at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:2965)
[error]     at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1587)
[error]     at com.fasterxml.jackson.databind.ObjectMapper.treeToValue(ObjectMapper.java:1931)
[error]     at play.libs.Json.fromJson(Json.java:47)

Answer 1

有明确的反序列化功能：

new ObjectMapper().configure(
DeserializationFeature.FAIL_ON_MISSING_EXTERNAL_TYPE_ID_PROPERTY, 
false);

Answer 2

我也遇到过这个问题，并且无法使用 Jackson 提供的机制（custom BeanDeserializer、BeanDeserializerModifier等）找到一个优雅的解决方案。

它看起来像是处理外部类型 ID 的方式中的一个错误。我通过以下方式解决了这个问题：

1. 将 JSON 字符串反序列化为JsonNode;
2. null如果所需的属性不存在，则手动插入节点；
3. 将 映射JsonNode到我想要的值类型。

我的代码如下所示：

public <T> T decode(String json, Class<T> type) throws IOException {
    JsonNode jsonNode = mapper.readTree(json);

    if (jsonNode.isObject() && (jsonNode.get("payload") == null  || jsonNode.get("payload").size() == 0)) {
        ObjectNode objectNode = (ObjectNode) jsonNode;
        objectNode.putNull("payload");
    }

    return mapper.treeToValue(jsonNode, type);
}