使用@JsonSubtypes反序列化无值-缺少属性错误

Question

我像这样反序列化jsons：

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

其中负载类型取决于类型。我的班级：

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;
    }

}

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

{
  "type":"d",
}

我得到的错误的堆栈跟踪：

[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

我也遇到过这个问题，无法找到使用Jackson提供的机制（自定义BeandeSerializer、BeandeSerializerModifier等）的优雅解决方案。

它看起来像是处理外部类型ID的方式中的一个bug。我通过以下方式来解决这个问题：

1. 将JSON试验反序列化为JSONNODe；
2. 如果所需属性不存在，则手动插入空节点；
3. 将JsonNode映射到所需的值类型。

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);
}