in Project.java  defined:
@ManyToMany(
            targetEntity=Employee.class,
            cascade={CascadeType.PERSIST, CascadeType.MERGE}
     )
//     @JoinTable(
//            name="project_employee_rel",
//            joinColumns={@JoinColumn(name="PROJECT", nullable = false)},
//            inverseJoinColumns = {@JoinColumn( name="EMPLOYEE")}
//     )
     private Collection<Employee> employees = new HashSet<Employee>(0);// * - *


in Employee.java :
@ManyToMany(
          cascade={CascadeType.PERSIST, CascadeType.MERGE},
           mappedBy="employees",
           targetEntity=Project.class
    )
    private Collection<Project> projects = new HashSet<Project>(0);

**
     * setter method for attribute associatedProjects. Column name is 'PROJECT' in JoinTable "project_employee_rel".
     *
     * @param associatedProjects Associated Projects
     */
    public void setAssociatedProjects(Collection<Project> associatedProjects) {
       //in fact only need this line
        this.associatedProjects = associatedProjects;
       
      // the following code is to update relationships of this employee through project-bean
      // 1. delete  old relationships   2. add new relationships 
        Object[] assProjectsArray = associatedProjects.toArray();
        // 1. delete all old relationships with projects of this Employees
        ProjectDAO dao = new ProjectDAO();
         
      List projectList = dao.findAllProjects();
      for (Iterator it = projectList.iterator(); it.hasNext();) {
         Project pro = (Project) it.next();
         Collection<Employee> involvedEmployeesNew = new HashSet<Employee>(0);
         //pro.getInvolvedEmployees() to get old involvedEmployees
         Object[] em = pro.getInvolvedEmployees().toArray();
         for (int i=0; i< em.length; i++)
         {   
            if (!em.toString().equals(this.getNickname()))
            {
               involvedEmployeesNew.add((Employee) em[i]);
            }
         }
         pro.setInvolvedEmployees(involvedEmployeesNew);
      }   
      
      // 2. add all "new" relationships of this Employees through Project.involvedEmployees.add(thisEmployee)
      for(int i=0; i<assProjectsArray.length; i++)
        {
           Project pr = (Project) assProjectsArray[i];
           Collection<Employee> involvedEmployees = pr.getInvolvedEmployees();
           involvedEmployees.add(this);
           pr.setInvolvedEmployees(involvedEmployees);
        }
    }