这样写不起作用
Shell after = JSON.parseObject(json, Shell.class, (FieldTypeResolver) (object, fieldName) -> {
 if (fieldName.equals("parent")) {
  return Child.class;
 }
 return null;
});

package com.alibaba.json.bvt.issue_3400;

import com.alibaba.fastjson.JSON;
import com.alibaba.fastjson.serializer.SerializerFeature;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.NoArgsConstructor;
import org.junit.Test;

public class Issue3486 {
    @Test
    public void test() {
        Shell shell = new Shell(new Child(13).setName("zs"));
        String json = JSON.toJSONString(shell, SerializerFeature.WriteClassName);
        System.out.printf("json: %s\n", json); // 输出 {"parent":{"age":13,"name":"zs"}}
        Shell after = JSON.parseObject(json, Shell.class);
        System.out.printf("after json: %s\n", JSON.toJSONString(after, SerializerFeature.WriteClassName)); // 这里输出，缺少子类age字段，因为示例化成了父类Parent
    }
}

@Data
@AllArgsConstructor
@NoArgsConstructor
class Shell { // 壳
    private Parent parent;
}

@Getter
class Parent { // 父类
    private String name;

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

@Data
@EqualsAndHashCode(callSuper = false)
@AllArgsConstructor
@NoArgsConstructor
class Child extends Parent { // 子类
    private Integer age;
}

输出

json: {"@type":"com.alibaba.json.bvt.issue_3400.Shell","parent":{"@type":"com.alibaba.json.bvt.issue_3400.Child","age":13,"name":"zs"}}
after json: {"@type":"com.alibaba.json.bvt.issue_3400.Shell","parent":{"@type":"com.alibaba.json.bvt.issue_3400.Child","age":13,"name":"zs"}}