如何在JAXB中使用hashmap属性？

import java.util.HashMap;
import java.util.Map;

public class MyClass {
    private Map<String, String> attributes = new HashMap<>();

    public Map<String, String> getAttributes() {
        return attributes;
    }

    public void setAttributes(Map<String, String> attributes) {
        this.attributes = attributes;
    }
}

import java.util.HashMap;
import java.util.Map;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class MyClass {
    @XmlJavaTypeAdapter(HashMapAdapter.class)
    private Map<String, String> attributes = new HashMap<>();

    public Map<String, String> getAttributes() {
        return attributes;
    }

    public void setAttributes(Map<String, String> attributes) {
        this.attributes = attributes;
    }
}

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class HashMapAdapter extends XmlAdapter<HashMapAdapter.MyMapType[], Map<String, String>> {
    public static class MyMapType {
        public String key;
        public String value;
    }

    @Override
    public MyMapType[] marshal(Map<String, String> map) throws Exception {
        MyMapType[] items = new MyMapType[map.size()];
        int i = 0;
        for (Entry<String, String> entry : map.entrySet()) {
            MyMapType item = new MyMapType();
            item.key = entry.getKey();
            item.value = entry.getValue();
            items[i] = item;
            i++;
        }
        return items;
    }

    @Override
    public Map<String, String> unmarshal(MyMapType[] items) throws Exception {
        Map<String, String> map = new HashMap<>();
        for (MyMapType item : items) {
            map.put(item.key, item.value);
        }
        return map;
    }
}