XmlAdapter怎么用

[weixin_45433596]

各位大佬，我在学习XmlAdapter怎么用。
下面的代码只是把HashMap绑定成一个List。
帮我看看下面代码哪里不对?为什么它输出的是空的mypeople?
输出如下

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<MyRootElement>
    <mypeople/>
</MyRootElement>

代码如下

package com.test;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlAccessType;
import javax.xml.bind.annotation.XmlAccessorType;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlValue;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

@XmlRootElement(name="MyRootElement")
@XmlAccessorType(XmlAccessType.FIELD)
public class XmlAdapter2 {
	//@XmlElement
	//@XmlElementWrapper(name="PeopleWrapper")
	@XmlJavaTypeAdapter(MyMapAdapter.class)
	private Map<Integer, String> mypeople;
	
	
	public static void main(String[] args) throws Exception {
		XmlAdapter2 root = new XmlAdapter2();
		
		root.getPeople().put(1, "One");
		root.getPeople().put(2, "Two");
		root.getPeople().put(3, "Three");
		
		System.out.println(root.getPeople().size()); 
		
		JAXBContext context = JAXBContext.newInstance(XmlAdapter2.class);
        Marshaller m = context.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
		m.marshal(root, System.out);
	}


	public Map<Integer, String> getPeople() {
		if (mypeople == null) {
			mypeople  = new HashMap<>();
		}
		return mypeople;
	}
	
}

class People {	
	@XmlAttribute
	private Integer id;
	@XmlValue
	private String name;
	public Integer getId() {
		return id;
	}
	public void setId(Integer id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
}

class PeopleList {
	private List<People> peopleList;

	public List<People> getPeopleList() {
		if (peopleList == null) {
			peopleList = new ArrayList<>();
		}
		System.out.println(peopleList.size());
		return peopleList;
	}

}
class MyMapAdapter extends javax.xml.bind.annotation.adapters.XmlAdapter<PeopleList, Map<Integer, String>> {

	@Override
	public PeopleList marshal(Map<Integer, String> peopleMap) throws Exception {
		
		PeopleList peopleList = new PeopleList();
		for (Integer key: peopleMap.keySet()) {
			People p = new People();
			p.setId(key);
			p.setName(peopleMap.get(key));
			peopleList.getPeopleList().add(p);
		}
		System.out.println(peopleList.getPeopleList().size());
		return peopleList;
	}

	@Override
	public Map<Integer, String> unmarshal(PeopleList peopleList) throws Exception {
		Map<Integer, String> map = new HashMap<>();
		for (People p: peopleList.getPeopleList()) {
			map.put(p.getId(), p.getName());
		}
		return map;
	}
	
}