public class EmployeeServiceImpl implements EmployeeService
{
    @PersistenceContext(unitName="EmployeeService")

    EntityManager em;

     public void assignEmployeeToProject(int empId, int projectId)
     {
        Project project = em.find(Project.class, projectId);
        Employee employee = em.find(Employee.class, empId);
        project.getEmployees().add(employee);
        employee.getProjects().add(project);
     }
}