使用jackson注释反序列化多级多态子类型

Question

我想使用jackson注释反序列化2级多态子类型。基本上，我想使用json注释和jacksons将以下json转换为xml，如下所示

{
  "mainSet": {
    "name": "bla bla",
    "myItems": [
      {
        "MySubItem": {
          "id": 1,
          "name": "Value1",
          "itemAProperty1": "Some stuff",
          "itemAProperty2": "Another property value",
          "type": "MySubItemA"
        }
      },
      {
        "MySubItem": {
          "id": 2,
          "name": "Value2",
          "itemBProperty1": 1000,
          "itemBProperty2": "B property",
          "type": "MySubItemB"
        }
      }
    ]
  }
}

我想要的最终Xml是

    <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<mainTag schemaVersion="1" xmlns="http://www.utiba.com/xml/ns/brms/v1"  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <name>bla bla</name>
    <MySubItem xsi:type="MySubItemA" id="1" name="value1" itemAProperty1="Some stuff" itemAProperty2="Another property value"/>
    <MySubItem xsi:type="MySubItemB" id="2" name="value2" itemAProperty1=1000 itemAProperty2="B Property"/>
</mainTag>

我有以下几门课——主课

public abstract class MyItem {

private int id;
private String name;
 //getter and setter
}

它具有抽象子类

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(name = "MySubItem")
@XmlSeeAlso({
    MySubItemA.class,
    MySubItemB.class,
    MySubItemC.class
})
    public abstract class MySubItem extends MyItem {

    private String itemAProperty1;
    private String itemAProperty2;
     //getter and setter
    }

MySubItem有具体的子类，比如MySubItemA、MySubItemB和MySubItemC

最后，我们创建一个客户端类，它包含抽象类的对象列表

 import java.util.ArrayList;
    import java.util.List;

    public class MainSet{
     @XmlElements({
        @XmlElement(name = "MysubItem", type = MySubItem.class),
    })
    private List<MyItem> myItems;

      public List<MyItem> getMyItems() {
        return this.myItems;
    }
    }

我尝试为我的项目创建 Mixin 类作为

@JsonTypeInfo(use=Id.MINIMAL_CLASS, include=As.PROPERTY, property="itemType")
@JsonSubTypes({
   @Type(MySubItem.class)
})

和for和MySubItem

@JsonTypeInfo(use=Id.MINIMAL_CLASS, include=As.PROPERTY, property="type")
@JsonSubTypes({
   @Type(MySubItemA.class)
   @Type(MySubItemB.class)
   @Type(MySubItemC.class)
})

我得到的错误是：

Error parsing JSON from client: com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of MySubItem, problem: abstract types either need to be mapped to concrete types, have custom deserializer, or be instantiated with additional type information
 at [Source: java.io.StringReader@26bd8952; line: 1, column: 498] (through reference chain: bla bla class ["mainSet"]->com.bla.bla["myItems"])

问题:为myItems列表创建mixin类，具有2级抽象子类层次结构

Answer 1

您的子类型注释告诉JAXB要做什么，但不告诉Jackson，这是试图从JSON反序列化您的请求的原因。除了＜code＞TypeInfo＜/code＞之外，尝试将其添加到抽象类中：

@JsonSubTypes({
    MySubItemA.class,
    MySubItemB.class,
    MySubItemC.class
})

Answer 2

Jackson api不支持多级多态类型层次结构。

你可以看看:https://github.com/FasterXML/jackson-databind/issues/374

所以你需要做的是：

请为创建一个反序列化器(为MySubItem.class，比如MySubItemDeserializerMixin.class ),并像我们为其他Mixin类所做的那样将其配置到jsonMapper。

mapper.addMixInAnnotations(MySubItem.class, MySubItemDeserializerMixin.class);

MySubItemDeserializerMixin.java将会是这样的:

@JsonDeserialize(using = MySubItemDeserializer.class)
public abstract class MySubItemDeserializerMixin{
}

您还需要为MySubItemDeserializerMixin.java中指定的MySubItem创建一个反序列化器(MySubItemDeserializer)。

现在您需要创建MySubItemMixin。java，看起来像这样：

@JsonTypeInfo(use=Id.MINIMAL_CLASS, include=As.PROPERTY, property="type")
@JsonSubTypes({
@Type(MySubItemA.class)
@Type(MySubItemB.class)
@Type(MySubItemC.class)
})

在MySubItemDeserializer中，您可以执行以下操作：

@Override
public MySubItem deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {

    JsonNode node = jsonParser.getCodec().readTree(jsonParser);

    ObjectMapper jsonMapper = new ObjectMapper();

    // Omit null values from the JSON.
    jsonMapper.setSerializationInclusion(Include.NON_NULL);

    // Treat received empty JSON strings as null Java values.
    // Note: doesn't seem to work - using custom deserializer through module
    // below instead.
    jsonMapper.enable(DeserializationFeature.ACCEPT_EMPTY_STRING_AS_NULL_OBJECT);

    jsonMapper.addMixInAnnotations(MySubItem.class, MySubItemMixin.class);

    // Enable interpretation of JAXB annotations on our beans (e.g. for 
    // resolving ID/IDREF references).
    jsonMapper.registerModule(new JaxbAnnotationModule());

    MySubItem condition = jsonMapper.readValue(node.toString(), MySubItem.class); 

  }

希望这能解决你的顾虑。

谢谢

纳库尔·瓦希思