jackson2.8.4java对象序列化成json字符串格式化时间

public class User {
private int id;
private Date birthday;
private double money;
private String name;
public User() {
}
public User(int id, String name, Date birthday) {
super();
this.id = id;
this.name = name;
this.birthday = birthday;
}
public User(int id, String name, double money, Date birthday) {
super();
this.id = id;
this.name = name;
this.money = money;
this.birthday = birthday;}
public Date getBirthday() {
return birthday;
}
public int getId() {
return id;
}
public double getMoney() {
return money;
}
public String getName() {
return name;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public void setId(int id) {
this.id = id;
}
public void setMoney(double money) {
this.money = money;
}
public void setName(String name) {
this.name = name;
}
}

ObjectMapper mapper = new ObjectMapper();
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd");
mapper.setDateFormat(format);
User user = new User(1,"JACK",new Date());
String outJson = mapper.writeValueAsString(user);
System.out.println(outJson);