Question

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

JSON 1 - CATEGORY1：

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

JSON 2 - CATEGORY2

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

父类（注释类似于此）

@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;
    ...
}

儿童班

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

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

杰克逊是否有开箱即用的功能可以让我进行这种反序列化或者我错过了什么？

Answer 1

如果你看杰克逊Api AsPropertyTypeDeserializer是使用属性负责子类型识别的类。如果您查看该类，则会有一个名为deserializeTypedFromObject的方法，该方法将使用JsonTypeIdResolver标识子类。我们可以扩展此类并覆盖方法deserializeTypedFromObject和forProperty。

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;
}
}

在扩展课程中，我们使用idResolver绕过子类型识别，而不是使用category字段的header字段动态识别。
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);
    }
}

现在我们有一个CustomTypeResolver用于子类型识别，但Jackon在找到'ParentClass'时会怎么看这个类？我们可以通过两种方式来实现：

使用自定义配置扩展JackonAnnotationInterceptor并在创建ObjectMapper时对其进行配置。
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，我们新的ParentClass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;
    ...
}

请参阅here

具有嵌套在对象中的type属性的Jackson多态反序列化

1 个答案: