Java对象转JSON触发JsonIOException，自定义XMLGregorianCalendar TypeAdapter无效

问题背景 #

已为XMLGregorianCalendar接口编写自定义TypeAdapter，但转换包含该类型的Java对象到JSON时仍抛出权限异常，代码及错误栈如下：

问题代码 #

package com;

import java.io.IOException;

import javax.xml.datatype.DatatypeConfigurationException;
import javax.xml.datatype.DatatypeFactory;
import javax.xml.datatype.XMLGregorianCalendar;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

public class DateConversion {

    public static void main(String[] args) throws DatatypeConfigurationException {

        XMLGregorianCalendar date = DatatypeFactory.newInstance().newXMLGregorianCalendar();

        Request req = new Request();
        req.setDate(date);
        TypeAdapter<XMLGregorianCalendar> adapter = new TypeAdapter<XMLGregorianCalendar>() {

            @Override
            public void write(JsonWriter writer, XMLGregorianCalendar value) throws IOException {
                System.out.println("Inside write");
                if(value==null) {
                    writer.nullValue();
                    return;
                }
                writer.value("value");

            }

            @Override
            public XMLGregorianCalendar read(JsonReader reader) throws IOException {
                System.out.println("Inside read");
                return null;
            }
        };
        Gson gsonBuilder = (new GsonBuilder()).registerTypeAdapter(XMLGregorianCalendar.class, adapter).serializeNulls().create();
        System.out.println(gsonBuilder.toJson(req));
    }

}

class Request {
    private XMLGregorianCalendar date;

    public XMLGregorianCalendar getDate() {
        return date;
    }

    public void setDate(XMLGregorianCalendar date) {
        this.date = date;
    }
}

错误栈 #

Exception in thread "main" com.google.gson.JsonIOException: Failed making field 'com.sun.org.apache.xerces.internal.jaxp.datatype.XMLGregorianCalendarImpl#eon' accessible; either change its visibility or write a custom TypeAdapter for its declaring type

错误原因 #

XMLGregorianCalendar是接口，通过DatatypeFactory创建的实例实际是其内部实现类XMLGregorianCalendarImpl。使用registerTypeAdapter仅会为精确匹配的XMLGregorianCalendar类型注册适配器，Gson处理实际对象时会尝试反射访问XMLGregorianCalendarImpl的私有字段，触发权限异常。

解决方法 #

将registerTypeAdapter替换为registerTypeHierarchyAdapter，该方法会为目标类型及其所有子类/实现类注册适配器，确保Gson处理XMLGregorianCalendarImpl时使用自定义适配器而非反射：

修改后的Gson构建代码：

Gson gsonBuilder = new GsonBuilder()
    .registerTypeHierarchyAdapter(XMLGregorianCalendar.class, adapter)
    .serializeNulls()
    .create();

验证效果 #

修改后运行代码，控制台会输出Inside write，并打印出正确的JSON结果：

{"date":"value"}

内容的提问来源于stack exchange，提问作者A Hybris Dev