java - 使用 Jackson 反序列化一个属性，该属性可以是对象列表或对象

Question

我的库正在调用一个 API，它可以返回以下 JSON 结构之一 -

{
  "key_is_same" : {
     "inner" : "val"
  } 
}

-或-

 {
  "key_is_same" : [{
     "inner" : "val1"
    },
    {
     "inner" : "val2"
    }
  ]
}

jakson 中是否有任何注释可以处理此问题并将其反序列化为相应的类型

Answer 1

您似乎正在寻找 ACCEPT_SINGLE_VALUE_AS_ARRAY反序列化功能。

Feature that determines whether it is acceptable to coerce non-array (in JSON) values to work with Java collection (arrays, java.util.Collection) types. If enabled, collection deserializers will try to handle non-array values as if they had "implicit" surrounding JSON array. This feature is meant to be used for compatibility/interoperability reasons, to work with packages (such as XML-to-JSON converters) that leave out JSON array in cases where there is just a single element in array.

Feature is disabled by default.

它可以在ObjectMapper中启用:

ObjectMapper mapper = new ObjectMapper();
mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);

或通过 @JsonFormat注释:

@JsonFormat(with = Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
private List<Foo> oneOrMany;

<小时/>

出于说明目的，请考虑以下 JSON 文档:

{
  "oneOrMany": [
    {
      "value": "one"
    },
    {
      "value": "two"
    }
  ]
}

{
  "oneOrMany": {
    "value": "one"
  }
}

它可以反序列化为以下类:

@Data
public class Foo {
    private List<Bar> oneOrMany;
}

@Data
public class Bar {
    private String value;
}

只需确保在您的 ObjectMapper 中启用该功能，或者您的字段使用 @JsonFormat(with = Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY) 进行注释。

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如果您正在寻找等效的序列化功能，请参阅 WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED .