net.sf.hibernate.MappingException: No persister for: pt.impl.MembershipImpl
   at net.sf.hibernate.impl.SessionFactoryImpl.getPersister(SessionFactoryImpl.java:344)
   at net.sf.hibernate.impl.SessionImpl.getClassPersister(SessionImpl.java:2574)
   at net.sf.hibernate.impl.SessionImpl.getPersister(SessionImpl.java:2581)
   at net.sf.hibernate.impl.SessionImpl.saveWithGeneratedIdentifier(SessionImpl.java:725)
   at net.sf.hibernate.impl.SessionImpl.save(SessionImpl.java:717)
   at pt.comseal.arsol.bo.autenticacao.AutenticacaoBO.inserirMembership(AutenticacaoBO.java:32)

<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
    <class
        name="pt.impl.MembershipImpl"
        table="MEMBERSHIP"
        dynamic-update="false"
        dynamic-insert="false"
    >

        <id
            name="id"
            column="id"
            type="long"
            unsaved-value="null"
        >
            <generator class="pt.util.db.IdGenerator">
            </generator>
        </id>

        <property
            name="role"
            type="string"
            update="true"
            insert="true"
            column="role"
        />

        <property
            name="type"
            type="string"
            update="true"
            insert="true"
            column="type_"
        />

        <many-to-one
            name="group"
            class="pt.impl.GroupImpl"
            cascade="all"
            outer-join="auto"
            update="true"
            insert="true"
            column="group_"
        />

        <many-to-one
            name="user"
            class="pt.impl.UserImpl"
            cascade="all"
            outer-join="auto"
            update="true"
            insert="true"
            column="user_"
        />

    </class>
</hibernate-mapping>

package pt.impl;

import pt.*;

/**
* @hibernate.class table="MEMBERSHIP"
*/
public class MembershipImpl implements Membership {

  // persistent properties
  /**
   * @hibernate.id column="id" type="long" unsaved-value="null" generator-class="pt.util.db.IdGenerator"
   */ 
  public Long getId() { return id; }
  public void setId(Long id) { this.id = id; }
 
  /**
   * @hibernate.property type="string" column="role"
   */
  public String getRole() { return role; }
  public void setRole(String role) { this.role = role; }
 
  /**
   * @hibernate.property type="string" column="type_"
   */
  public String getType() { return type; }
  public void setType(String type) { this.type = type; }
 
  /**
   * @hibernate.many-to-one class="pt.impl.GroupImpl"
   *  cascade="all" column="group_"
   */
  public Group getGroup() { return group; }
  public void setGroup(Group group) { this.group = group; }
 
  /**
   * @hibernate.many-to-one class="pt.impl.UserImpl"
   *  cascade="all" column="user_"
   */
  public User getUser() { return user; }
  public void setUser(User user) { this.user = user; }

  // toString
  public String toString() {
    return "Membership[" + id + "|" + user.getName() + "|" + group.getName() + "]";
  }

  // private members
  private Long id = null;
  private String role = null;
  private String type = null;
  private Group group = null;
  private User user = null;
}