session.merge(object);

object = session.merge(object);
session.update(object);

object = session.merge(object);

@Entity
@Table(name = "prepod")
public class Prepod {

	private Long id;
	private String name;
	@Column
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	List<Student> students = new ArrayList<Student>();

	@ManyToMany(fetch=FetchType.EAGER)
	public List<Student> getStudents() {
		return students;
	}

	public void setStudents(List<Student> students) {
		this.students = students;
	}

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id")
	public Long getId() {
		return id;
	}

	public void setId(Long i) {
		id = i;
	}
}

@Entity
@Table(name = "Student")
public class Student {

	private Long id;
	private String name;
	private Long age;
	private List<Prepod> prepods = new ArrayList<Prepod>();

	@ManyToMany(mappedBy = "students",fetch=FetchType.EAGER)
	public List<Prepod> getPrepods() {
		return prepods;
	}

	public void setPrepods(List<Prepod> prepods) {
		this.prepods = prepods;
	}

	public Student() {
		name = null;
	}

	public Student(Student s) {
		name = s.getName();
	}

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "id")
	public Long getId() {
		return id;
	}

	@Column(name = "name")
	public String getName() {
		return name;
	}

	@Column(name = "age")
	public Long getAge() {
		return age;
	}

	public void setId(Long i) {
		id = i;
	}

	public void setName(String s) {
		name = s;
	}

	public void setAge(Long age) {
		this.age = age;
	}
}

Session session = HibernateUtil.getSessionFactory().openSession();
		session.beginTransaction();
		Prepod prepod = (Prepod) session.get(Prepod.class, 1l);
		Student student = (Student) session.get(Student.class, 1l);
		

		
		session.getTransaction().commit();
		session.flush();
		session.close();
		
		List<Student> students = new ArrayList<Student>();
		students.add(student);

		List<Prepod> prepods = new ArrayList<Prepod>();
		prepods.add(prepod);

		prepod.setStudents(students);
		student.setPrepods(prepods);
	
	
	    session = HibernateUtil.getSessionFactory().openSession();
		session.beginTransaction();
	
		session.merge(student);
		session.merge(prepod);
	
		
		session.getTransaction().commit();
		session.close();

 Hibernate: select prepod0_.id as id1_1_1_, prepod0_.name as name2_1_1_, students1_.prepods_id as prepods1_1_3_, student2_.id as students2_2_3_, student2_.id as id1_0_0_, student2_.age as age2_0_0_, student2_.name as name3_0_0_ from prepod prepod0_ left outer join prepod_Student students1_ on prepod0_.id=students1_.prepods_id left outer join Student student2_ on students1_.students_id=student2_.id where prepod0_.id=?
Hibernate: select student0_.id as id1_0_1_, student0_.age as age2_0_1_, student0_.name as name3_0_1_, prepods1_.students_id as students2_0_3_, prepod2_.id as prepods1_2_3_, prepod2_.id as id1_1_0_, prepod2_.name as name2_1_0_ from Student student0_ left outer join prepod_Student prepods1_ on student0_.id=prepods1_.students_id left outer join prepod prepod2_ on prepods1_.prepods_id=prepod2_.id where student0_.id=?
Hibernate: select student0_.id as id1_0_0_, student0_.age as age2_0_0_, student0_.name as name3_0_0_ from Student student0_ where student0_.id=?
Hibernate: select prepods0_.students_id as students2_0_1_, prepods0_.prepods_id as prepods1_2_1_, prepod1_.id as id1_1_0_, prepod1_.name as name2_1_0_ from prepod_Student prepods0_ inner join prepod prepod1_ on prepods0_.prepods_id=prepod1_.id where prepods0_.students_id=?
Hibernate: select prepod0_.id as id1_1_1_, prepod0_.name as name2_1_1_, students1_.prepods_id as prepods1_1_3_, student2_.id as students2_2_3_, student2_.id as id1_0_0_, student2_.age as age2_0_0_, student2_.name as name3_0_0_ from prepod prepod0_ left outer join prepod_Student students1_ on prepod0_.id=students1_.prepods_id left outer join Student student2_ on students1_.students_id=student2_.id where prepod0_.id=?
Hibernate: insert into prepod_Student (prepods_id, students_id) values (?, ?)

object = session.merge(object);
session.update(object);

session.merge(object);

object = session.merge(object);
session.update(object);

object = session.merge(object);
session.update(object);

object1 = session.merge(object);
session.update(object1);