什么@JsonTypeInfo.ID为property ="type.id"选择反序列化,JsonTypeInfo.Id.CUSTOM？

Question

所以我的JSON看起来像这样:

{
    "ActivityDisplayModel" : {
        "name" : "lunch with friends",
        "startTime" : "12:00:00",
        "type" : {
            "id" : "MEAL",
            "description" : "Meal"
        },
        "complete" : false
    }
}

我试图找到方法让@JsonTypeInfo我不要生气,因为在type对象中有类型参数.当字段type是String而不是对象本身之前,我已经有了这个工作,但是为了以后的处理,我需要它作为一个对象.我知道下面的内容不起作用,我猜这是一种使用JsonTypeInfo.Id.CUSTOM的方法,但是在浏览了互联网之后,没有完整的JSON示例.此外,如果使用objectMapper设置可以做到这一点,我会全神贯注.

/** 
 * My ActivityDisplayModel Abstract Class
 */
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type.id")
@JsonSubTypes({
@JsonSubTypes.Type(value = MealDisplayModel.class, name = "MEAL"),
@JsonSubTypes.Type(value = EntertainmentDisplayModel.class, name = "ENTERTAINMENT")
})
public abstract class ActivityDisplayModel { 
    ...

以上基本上是我想要做的,但当然我得到一个例外:

Could not read JSON: Could not resolve type id '{' into a subtype of  [simple type, class ... .ActivityDisplayModel]

对于这样一个简单的问题,只需要在JSON中更深层次地看一个级别,谁会想到它会如此麻烦？

Answer 1

我不知道，你可以指定内部属性做到这一点：type.id。在我看来，您应该将 JSON 更改为更简单的版本。如果您不能强制您的 JSON 供应商更改 JSON 架构，您必须手动进行。假设您的 JSON 如下所示：

{
    "activityDisplayModel": {
        "name": "lunch with friends",
        "type": {
            "id": "MEAL",
            "description": "Meal"
        },
        "complete": false
    }
}

下面的 POJO 类适合上面的 JSON：

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = MealDisplayModel.class, name = "MEAL"),
    @JsonSubTypes.Type(value = EntertainmentDisplayModel.class, name = "ENTERTAINMENT")
})
abstract class ActivityDisplayModel {

    protected String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return name;
    }
}

class MealDisplayModel extends ActivityDisplayModel {

    private boolean complete;

    public boolean isComplete() {
        return complete;
    }

    public void setComplete(boolean complete) {
        this.complete = complete;
    }

    @Override
    public String toString() {
        return "MealDisplayModel [complete=" + complete + ", toString()=" + super.toString() + "]";
    }
}

@JsonIgnoreProperties("complete")
class EntertainmentDisplayModel extends ActivityDisplayModel {

    @Override
    public String toString() {
        return "EntertainmentDisplayModel [toString()=" + super.toString() + "]";
    }
}

class Root {

    private ActivityDisplayModel activityDisplayModel;

    public ActivityDisplayModel getActivityDisplayModel() {
        return activityDisplayModel;
    }

    public void setActivityDisplayModel(ActivityDisplayModel activityDisplayModel) {
        this.activityDisplayModel = activityDisplayModel;
    }

    @Override
    public String toString() {
        return activityDisplayModel.toString();
    }
}

下面的脚本显示了如何解析上面的 JSON：

ObjectMapper mapper = new ObjectMapper();
// Updated JSON in memory
ObjectNode rootNode = (ObjectNode)mapper.readTree(json);
ObjectNode activityDisplayModelNode = (ObjectNode)rootNode.path("activityDisplayModel");
JsonNode typeNode = activityDisplayModelNode.path("type");
activityDisplayModelNode.set("type", typeNode.path("id"));

System.out.println("Result: " + mapper.convertValue(rootNode, Root.class));

以上脚本打印：

Result: MealDisplayModel [complete=false, toString()=lunch with friends]

另见：

1. 杰克逊树模型示例。
2. 在 Jackson 中将 Java 对象转换为 JsonNode。

Answer 2

我知道距最初的问题已经3年了，但是点嵌套属性仍然不受支持，也许这会帮助到某个人。我最终创建了一个类，NestedTypeResolver因此我们可以按预期使用点语法。只需将其添加@JsonTypeResolver(NestedTypeResolver.class)到带有嵌套标识符的任何类中，发布者的原始尝试就会起作用：

/** 
 * My ActivityDisplayModel Abstract Class
 */
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type.id")
  @JsonSubTypes({
  @JsonSubTypes.Type(value = MealDisplayModel.class, name = "MEAL"),
  @JsonSubTypes.Type(value = EntertainmentDisplayModel.class, name = "ENTERTAINMENT")
})
@JsonTypeResolver(NestedTypeResolver.class)
public abstract class ActivityDisplayModel {

NestedTypeResolver：

/**
 * Allows using nested "dot" dyntax for type discriminators. To use, annotate class with @JsonTypeResolver(NestedTypeResolver.class)
 */
public class NestedTypeResolver extends StdTypeResolverBuilder {
    @Override
    public TypeDeserializer buildTypeDeserializer(DeserializationConfig config, JavaType baseType,
            Collection<NamedType> subtypes) {
            //Copied this code from parent class, StdTypeResolverBuilder with same method name
            TypeIdResolver idRes = idResolver(config, baseType, subtypes, false, true);
            return new NestedTypeDeserializer(baseType, idRes, _typeProperty, _typeIdVisible,
                null, _includeAs);
    }
}

所有繁重的工作都在此处NestedTypeDeserializer中完成：

/**
 * Heavy work to support {@link NestedTypeResolver}
 */
public class NestedTypeDeserializer extends AsPropertyTypeDeserializer {

    private static final Logger LOGGER = LoggerFactory.getLogger(NestedTypeDeserializer.class);

    public NestedTypeDeserializer(JavaType bt,
            TypeIdResolver idRes, String typePropertyName, boolean typeIdVisible,
            JavaType defaultImpl) {
        super(bt, idRes, typePropertyName, typeIdVisible, defaultImpl);
    }

    public NestedTypeDeserializer(JavaType bt, TypeIdResolver idRes, String typePropertyName, boolean typeIdVisible,
            JavaType defaultImpl, JsonTypeInfo.As inclusion) {
        super(bt, idRes, typePropertyName, typeIdVisible, defaultImpl, inclusion);
    }

    public NestedTypeDeserializer(AsPropertyTypeDeserializer src, BeanProperty property) {
        super(src, property);
    }

    @Override
    public TypeDeserializer forProperty(BeanProperty prop) {
        return (prop == _property) ? this : new NestedTypeDeserializer(this, prop);
    }

    @Override
    public Object deserializeTypedFromObject(JsonParser p, DeserializationContext ctxt) throws IOException {
        JsonNode originalNode = p.readValueAsTree();
        JsonNode node = originalNode;
        //_typePropertyName is the dot separated value of "property" in @JsonTypeInfo
        LOGGER.debug("Searching for type discriminator [{}]...", _typePropertyName);
        for (String property : _typePropertyName.split("\\.")) { //traverse down any nested properties
            JsonNode nestedProp = node.get(property);
            if (nestedProp == null) {
                ctxt.reportWrongTokenException(p, JsonToken.FIELD_NAME,
                        "missing property '" + _typePropertyName + "' that is to contain type id  (for class "
                                + baseTypeName() + ")");
                return null;
            }
            node = nestedProp;
        }
        LOGGER.debug("Found [{}] with value [{}]", _typePropertyName, node.asText());
        JsonDeserializer<Object> deser = _findDeserializer(ctxt, "" + node.asText());
        //Since JsonParser is a forward-only operation and finding the "type" discriminator advanced the pointer, we need to reset it
        //Got clues from https://www.dilipkumarg.com/dynamic-polymorphic-type-handling-jackson/
        JsonParser jsonParser = new TreeTraversingParser(originalNode, p.getCodec());
        if (jsonParser.getCurrentToken() == null) {
            jsonParser.nextToken();
        }
        return deser.deserialize(jsonParser, ctxt);
    }
}

免责声明：我们已经在Jackson 2.8.10中使用了一个月，并且没有任何问题，但是我们必须深入研究Jackson源代码杂草才能完成此工作，因此是YMMV。希望杰克逊有一天会允许开箱即用，因此我们不需要这些解决方法。