具有嵌套在对象中的类型属性的Jackson多态反序列化

Question

我正试图找到一种方法来使用jackson的多态反序列化功能，它将基于嵌套在标头/控件对象中的属性反序列化我的对象：

JSON 1-类别1：

{
 "id":"someId",
 "header":{
           "category":"CATEGORY1",
           "somOtherProperty":"someValue"
          }
 "nextField":"nextValue",
 ...
}

JSON 2-类别2

{
 "id":"someId",
 "header":{
           "category":"CATEGORY2",
           "somOtherProperty":"someValue"
          }
 "nextField":"nextValue",
 ...
}

父类（类似这样的注释）

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "category")
@JsonSubTypes({
        @Type(value = Category1Class.class, name = "CATEGORY1"), 
        @Type(value = Category2Class.class, name = "CATEGORY2") })
public class ParentClass{
    private Header header;
    private String nextField;
    ...
}

public class Header{
    private String category;
    private String somOtherProperty;
    ...
}

子类

@JsonTypeName("CATEGORY1")
public class Category1Class extends ParentClass{
    ...
}

@JsonTypeName("CATEGORY2")
public class Category2Class extends ParentClass{
    ...
}

jackson中是否有现成的功能可以让我进行这种反序列化，或者我遗漏了什么？

Answer 1

如果查看Jackson Api，则属性类型反序列化器（PropertyTypeDeserializer）是负责使用属性识别子类型的类。如果查看该类，有一个名为反序列化TypedFromObject的方法，该方法将使用JsonTypeIdResolver识别子类。我们可以扩展这个类并重写方法反序列化类型（deserializeTypedFromObject）和属性（property）。

package com.dilipkumarg.tutorials.dynamicsubtype;

import java.io.IOException;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
import com.fasterxml.jackson.databind.jsontype.TypeIdResolver;
import com.fasterxml.jackson.databind.jsontype.impl.AsPropertyTypeDeserializer;
import com.fasterxml.jackson.databind.node.TreeTraversingParser;
import com.fasterxml.jackson.databind.type.SimpleType;

public class CustomTypeDeserializer extends AsPropertyTypeDeserializer {
public CustomTypeDeserializer(
        final JavaType bt, final TypeIdResolver idRes,
        final String typePropertyName, final boolean typeIdVisible, final Class<?> defaultImpl) {
    super(bt, idRes, typePropertyName, typeIdVisible, defaultImpl);
}

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

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

@Override
public Object deserializeTypedFromObject(
        final JsonParser jp, final DeserializationContext ctxt) throws IOException {
    JsonNode node = jp.readValueAsTree();
    Class<?> subType = findSubType(node);
    JavaType type = SimpleType.construct(subType);

    JsonParser jsonParser = new TreeTraversingParser(node, jp.getCodec());
    if (jsonParser.getCurrentToken() == null) {
        jsonParser.nextToken();
    }
    /* 16-Dec-2010, tatu: Since nominal type we get here has no (generic) type parameters,
    *   we actually now need to explicitly narrow from base type (which may have parameterization)
    *   using raw type.
    *
    *   One complication, though; can not change 'type class' (simple type to container); otherwise
    *   we may try to narrow a SimpleType (Object.class) into MapType (Map.class), losing actual
    *   type in process (getting SimpleType of Map.class which will not work as expected)
    */
    if (_baseType != null && _baseType.getClass() == type.getClass()) {
        type = _baseType.narrowBy(type.getRawClass());
    }
    JsonDeserializer<Object> deser = ctxt.findContextualValueDeserializer(type, _property);
    return deser.deserialize(jsonParser, ctxt);
}

protected Class<?> findSubType(JsonNode node) {
    Class<? extends ParentClass> subType = null;
    String cat = node.get("header").get("category").asText();
    if (cat.equals("CATEGORY1")) {
        subType = Category1Class.class;
    } else if (cat.equals("CATEGORY2")) {
        subType = Category2Class.class;
    }
    return subType;
}
}

在扩展类中，我们使用idResolver绕过了子类型标识，而是使用头字段的类别字段动态标识。
我们需要TypeResolverBuilder来创建新的CustomTypeDeserializer实例。

package com.dilipkumarg.tutorials.dynamicsubtype;

import java.util.Collection;

import com.fasterxml.jackson.databind.DeserializationConfig;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.jsontype.NamedType;
import com.fasterxml.jackson.databind.jsontype.TypeDeserializer;
import com.fasterxml.jackson.databind.jsontype.impl.StdTypeResolverBuilder;


public class CustomTypeResolver extends StdTypeResolverBuilder {
    @Override
    public TypeDeserializer buildTypeDeserializer(final DeserializationConfig config, final JavaType baseType, final Collection<NamedType> subtypes) {
        return new CustomTypeDeserializer(baseType, null,
            _typeProperty, _typeIdVisible, _defaultImpl);
    }
}

现在我们有了一个用于子类型识别的CustomTypeResolver，但是当Jackon找到“家长类”时，它将如何知道查看这个类？
我们可以通过两种方式来做到这一点：

>

使用JsonTypeResolver注释。这是推荐的方法，因为我们不需要配置任何东西。

在包含类型解析器之后，我们的新父类将是：

package com.dilipkumarg.tutorials.dynamicsubtype;

import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.databind.annotation.JsonTypeResolver;

@JsonTypeInfo(use = JsonTypeInfo.Id.NONE)
@JsonTypeResolver(CustomTypeResolver.class)
public class ParentClass {
    private Header header;
    private String nextField;
    ...
}

请参见此处