Hibernate的一个问题object references an unsaved transient instance - save the transi5

User.java: 
  @Entity 
  @Table(name="t_user") 
  public class User { 
private int id; 
private String name; 
private Group group; 

@ManyToOne(cascade={CascadeType.ALL} ) //Cascade设置级联 
public Group getGroup() { 
return group; 
} 
public void setGroup(Group group) { 
this.group = group; 
} 
@Id 
@GeneratedValue 
public int getId() { 
return id; 
} 
public void setId(int id) { 
this.id = id; 
} 
public String getName() { 
return name; 
} 
public void setName(String name) { 
this.name = name; 
} 
  } 

Group.java 
  @Entity 
  @Table(name ="t_group")  
  public class Group { 
private int id; 
private String name; 
private Set<User> users = new HashSet<User>(); 
@Id 
@GeneratedValue 
public int getId() { 
return id; 
} 
public void setId(int id) { 
this.id = id; 
} 
public String getName() { 
return name; 
} 
public void setName(String name) { 
this.name = name; 
} 

@OneToMany(mappedBy="group") 
public Set<User> getUsers() { 
return users; 
} 
public void setUsers(Set<User> users) { 
this.users = users; 
} 
  } 

测试程序 HibernateORMappingTest.java: 
  public class HibernateORMappingTest { 

private static SessionFactory sessionFactory; 
@BeforeClass 
public static void beforeClass(){ 
new SchemaExport(new AnnotationConfiguration().configure()).create(false, true); 
sessionFactory= new AnnotationConfiguration().configure().buildSessionFactory(); 
} 
@AfterClass 
public static void afterClass(){ 
sessionFactory.close(); 
} 
@Test 
public void testSaveUser(){ 
User u = new User(); 
u.setName("u1"); 
Group g = new Group(); 
g.setName("g1"); 
u.setGroup(g); 
Session s= sessionFactory.getCurrentSession(); 
s.beginTransaction(); 
//s.save(g); 加上这一句，就不报错了，可是视频教程里没这一句，只是设置了级联Cascade却能通过 
s.save(u); 
s.getTransaction().commit(); 
} 
  @Test 
public void testSchemaExport(){ 
new SchemaExport(new AnnotationConfiguration().configure()).create(false, true); 
} 
public static void main(String[] args){ 
beforeClass(); 
} 
}