Java HTML/XML - 如何做xml序列化空处理

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;

public class Main {
  public static void main(String[] args) throws Exception {
    JAXBContext jc = JAXBContext.newInstance(Note.class);
    Marshaller marshaller = jc.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
    marshaller.marshal(new Note(), System.out);
  }

  @XmlRootElement
  @XmlType(propOrder = { "to", "from", "header", "body", "assignee" })
  public static class Note {
    private String to;
    private String from;
    private String header;
    private String body;
    private Assignee assignee;

    @XmlElement(nillable = true)
    public String getTo() {
      return to;
    }

    public void setTo(String to) {
      this.to = to;
    }

    @XmlElement(nillable = true)
    public String getFrom() {
      return from;
    }

    public void setFrom(String from) {
      this.from = from;
    }

    @XmlElement(nillable = true)
    public String getHeader() {
      return header;
    }

    public void setHeader(String header) {
      this.header = header;
    }

    @XmlElement(nillable = true)
    public String getBody() {
      return body;
    }

    public void setBody(String body) {
      this.body = body;
    }

    public Assignee getAssignee() {
      return assignee;
    }

    public void setAssignee(Assignee assignee) {
      this.assignee = assignee;
    }

  }

  @XmlJavaTypeAdapter(AssigneeAdapter.class)
  public static class Assignee {

    private String name;
    private String surname;

    @XmlElement(nillable = true)
    public String getName() {
      return name;
    }

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

    @XmlElement(nillable = true)
    public String getSurname() {
      return surname;
    }

    public void setSurname(String surname) {
      this.surname = surname;
    }

    public boolean isNull() {
      return null == name && null == surname;

    }
  }

  public static class AssigneeAdapter extends XmlAdapter<Assignee, Assignee> {

    @Override
    public Assignee unmarshal(Assignee v) throws Exception {
      if (v.isNull()) {
        return null;
      }
      return v;
    }

    @Override
    public Assignee marshal(Assignee v) throws Exception {
      if (null == v) {
        return new Assignee();
      }
      return v;
    }

  }

}