Error in spring mvc: class <> nor any of its super class is known to this context

My Response object like below:

public class ValidationResponse {
    @XmlElement(name = "success")
    private boolean success = true;
    @XmlElement(name = "message")
    private Object message;
}

Users class:

public class Users {
    @XmlElement(name = "user")
    @JsonProperty("users")
    private List<User> userList;
}

When I try to produce “application/xml”, I got exception as below:
com.sun.istack.internal.SAXException2: class com.abcd.dao.domain.user.Users nor any of its super class is known to this context.
javax.xml.bind.JAXBException: class com.abcd.dao.domain.user.Users nor any of its super class is known to this context.

How to handle the “Object” type in the response class in spring mvc: ?
Please help me to fix this.

By adding @XmlSeeAlso its working as expected:

@XmlSeeAlso({Users.class, User.class})
    public class ValidationResponse {
}

Thanks