@Entity()
@Table(name = "user_roles")
public class DbUserRole  implements java.io.Serializable {

   
   private UserRolePK id;

    // Constructors

    public DbUserRole(String userName, String role)
   {
      super();
      // TODO Auto-generated constructor stub
      //this.id = new UserRolePK(userName, role);
      this.id = new UserRolePK();
      this.setRole(role);
      this.setUserName(userName);
   }


   public DbUserRole(String role)
   {
      super();
      this.id = new UserRolePK(role);
   }


   /** default constructor */
    public DbUserRole() {
    }

    @EmbeddedId()
    public UserRolePK getId(){
      return this.id;
   }
   
    public void setId(UserRolePK pk){
       this.id = pk;
    }
   
    @Column(nullable = false, length = 255, insertable = false, updatable = false)
    public String getUserName(){
       return this.id.getUserName();
    }
   
   public void setUserName(String userName){
      this.id.setRole(userName);
   }
   
   @Column(insertable = false, updatable = false, nullable = false, length = 255)
   public String getRole(){
      return this.id.getRole();
   }
   
   public void setRole(String role){
      this.id.setRole(role);
   }
}

/**
* Realisiert den Composite Key von UserRole.
*
* @author Mathias Peters
* @version $Id: DbUserRole.java,v 1.5 2006/01/03 11:33:35 mathias peters Exp $
*/
@Embeddable
class UserRolePK implements Serializable{
   
   static final long serialVersionUID = 0;
//    Fields   
   private String userName;
   private String role;
   
   public UserRolePK()
   {
      
   }
   
   public UserRolePK(String userName, String role){
      this.userName = userName;
      this.role = role;
   }
   
   public UserRolePK(String role){
      this.role = role;
   }
   
   public String getRole()
   {
      return role;
   }


   public void setRole(String role)
   {
      this.role = role;
   }


   public String getUserName()
   {
      return userName;
   }


   public void setUserName(String userName)
   {
      this.userName = userName;
   }
   
   public boolean equals(Object obj){
      if(this == obj) return true;
      if(!(obj instanceof UserRolePK))
         return false;
      final UserRolePK pkObj = (UserRolePK)obj;
      
      if(!pkObj.getUserName().equals(this.getUserName()))
         return false;
      
      if(!pkObj.getRole().equals(this.getRole()))
         return false;
      return true;
   }
   
   public int hashCode(){
      return (29 * this.getUserName().hashCode()) + (31 * this.getRole().hashCode());
   }
   
}

[color=red]
The Exception is the following:
3274 [main] ERROR org.hibernate.property.BasicPropertyAccessor  - IllegalArgumentException in class: i2.smartsearch.db.DbUserRole, getter method of property: id
Exception in thread "main" org.hibernate.PropertyAccessException: IllegalArgumentException occurred calling getter of i2.smartsearch.db.DbUserRole.id
   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:171)
   at org.hibernate.tuple.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:176)
   at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:3256)
   at org.hibernate.persister.entity.AbstractEntityPersister.isTransient(AbstractEntityPersister.java:2982)
   at org.hibernate.engine.ForeignKeys.isTransient(ForeignKeys.java:181)
   at org.hibernate.event.def.AbstractSaveEventListener.getEntityState(AbstractSaveEventListener.java:459)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:84)
   at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:70)
   at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:501)
   at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:495)
   at org.hibernate.engine.CascadingAction$1.cascade(CascadingAction.java:134)
   at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:213)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:157)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:108)
   at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:290)
   at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:185)
   at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:160)
   at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:108)
   at org.hibernate.engine.Cascade.cascade(Cascade.java:248)
   at org.hibernate.event.def.AbstractFlushingEventListener.cascadeOnFlush(AbstractFlushingEventListener.java:130)
   at org.hibernate.event.def.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:121)
   at org.hibernate.event.def.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:65)
   at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:26)
   at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:905)
   at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:345)
   at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:106)
   at i2.smartsearch.db.DbUserManager.update(DbUserManager.java:54)
   at i2.smartsearch.db.HibernateUtil.main(HibernateUtil.java:254)
Caused by: java.lang.IllegalArgumentException: object is not an instance of declaring class
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
   at java.lang.reflect.Method.invoke(Method.java:585)
   at org.hibernate.property.BasicPropertyAccessor$BasicGetter.get(BasicPropertyAccessor.java:145)
   ... 27 more



[/color]