SerializationPolicy error when performing RPC GWT - gwt

Question

In my Google Web Toolkit project, I got the following error:
Type 'com.example.model.User_$$_javassist_1' was not included in the set of types which can be serialized by this SerializationPolicy or its Class object could not be loaded. For security purposes, this type will not be serialized.: instance = com.example.model.User#1dc2baa
What are the possible causes of this error?
package com.example.model;
import java.io.Serializable;
public class User implements Serializable {
private Integer id;
private String username;
private String fullname;
private String firstname;
private String lastname;
private String password;
private String picture;
private String adresse;
private String email;
private Integer telephone;
private Integer fax;
private String role;
public User() {
}
public User(Integer id, String username, String fullname, String firstname,
String lastname, String password, String picture, String adresse,
String email, Integer telephone, Integer fax, String role) {
this.id = id;
this.username = username;
this.fullname = fullname;
this.firstname = firstname;
this.lastname = lastname;
this.password = password;
this.picture = picture;
this.adresse = adresse;
this.email = email;
this.telephone = telephone;
this.fax = fax;
this.role = role;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getFullname() {
return fullname;
}
public void setFullname(String fullname) {
this.fullname = fullname;
}
public String getFirstname() {
return firstname;
}
public void setFirstname(String firstname) {
this.firstname = firstname;
}
public String getLastname() {
return lastname;
}
public void setLastname(String lastname) {
this.lastname = lastname;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public String getPicture() {
return picture;
}
public void setPicture(String picture) {
this.picture = picture;
}
public String getAdresse() {
return adresse;
}
public void setAdresse(String adresse) {
this.adresse = adresse;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Integer getTelephone() {
return telephone;
}
public void setTelephone(Integer telephone) {
this.telephone = telephone;
}
public Integer getFax() {
return fax;
}
public void setFax(Integer fax) {
this.fax = fax;
}
public String getRole() {
return role;
}
public void setRole(String role) {
this.role = role;
}
}
package com.example.model;
import java.io.Serializable;
import java.util.Date;
public class Projet implements Serializable {
private Integer id;
private User user;
private String name;
private String description;
private String abrevation;
private Date dateDebut;
private Date dateFin;
private Date realDateDeb;
private Date realDateFin;
private String icone;
private Double budget;
private Double tauxTva;
public Projet() {
}
public Projet(Integer id, User user,
String name, String description,
String abrevation, Date dateDebut, Date dateFin, Date realDateDeb,
Date realDateFin, String icone, Double budget, Double tauxTva) {
this.id = id;
this.user = user;
this.name = name;
this.description = description;
this.abrevation = abrevation;
this.dateDebut = dateDebut;
this.dateFin = dateFin;
this.realDateDeb = realDateDeb;
this.realDateFin = realDateFin;
this.icone = icone;
this.budget = budget;
this.tauxTva = tauxTva;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public String getAbrevation() {
return abrevation;
}
public void setAbrevation(String abrevation) {
this.abrevation = abrevation;
}
public Date getDateDebut() {
return dateDebut;
}
public void setDateDebut(Date dateDebut) {
this.dateDebut = dateDebut;
}
public Date getDateFin() {
return dateFin;
}
public void setDateFin(Date dateFin) {
this.dateFin = dateFin;
}
public Date getRealDateDeb() {
return realDateDeb;
}
public void setRealDateDeb(Date realDateDeb) {
this.realDateDeb = realDateDeb;
}
public Date getRealDateFin() {
return realDateFin;
}
public void setRealDateFin(Date realDateFin) {
this.realDateFin = realDateFin;
}
public String getIcone() {
return icone;
}
public void setIcone(String icone) {
this.icone = icone;
}
public Double getBudget() {
return budget;
}
public void setBudget(Double budget) {
this.budget = budget;
}
public Double getTauxTva() {
return tauxTva;
}
public void setTauxTva(Double tauxTva) {
this.tauxTva = tauxTva;
}
}
<hibernate-mapping>
<class name="com.example.model.Projet" table="PROJET" lazy="true">
<id name="id" column="PROJET_ID">
<generator class="native"/>
</id>
<property name="name"/>
<property name="description"/>
<property name="abrevation"/>
<property name="dateDebut"/>
<property name="dateFin"/>
<property name="realDateDeb"/>
<property name="realDateFin"/>
<property name="icone"/>
<property name="budget"/>
<property name="tauxTva"/>
<!--
-->
<many-to-one name="user" class="com.example.model.User" fetch="select">
<column name="USER_ID" not-null="true" />
</many-to-one>
</class>
</hibernate-mapping>
<hibernate-mapping>
<class name="com.example.model.User" table="USER" lazy="true">
<id name="id" column="USER_ID">
<generator class="native"/>
</id>
<property name="username"/>
<property name="fullname"/>
<property name="firstname"/>
<property name="lastname"/>
<property name="password"/>
<property name="picture"/>
<property name="adresse"/>
<property name="email"/>
<property name="telephone"/>
<property name="fax"/>
<property name="role"/>
</class>
</hibernate-mapping>