----------------------多对多manytomany
要点：
多对多必须有mappedBy，让一个端维护关系
多对多会创建一张关系表，维护关系就是维护第三张表。
所以增加关系，则在集合中添加实体。删除关系，则从集合中删除实体。mappedBy使一方丧失关系维护@Entity
public class User {@Id
@GeneratedValue
private long id;@Column(name="name")
private String name;@ManyToMany(mappedBy="users")
@Cascade(value=CascadeType.SAVE_UPDATE)
private Set<Role> roles = new HashSet<Role>();
}@Entity
public class Role {
@Id
@GeneratedValue
private long id;
@Column
private String name;
@ManyToMany
private Set<User> users = new HashSet<User>();
public String getName() {
return name;
} --------------------------------保存mappedBy端
User user = new User();
user.setName("zhangsan");
User user2 = new User();
user2.setName("lisi"); Role r1= new Role();
r1.setName("ADMIN");
Role r2= new Role();
r2.setName("EDITOR"); user.getRoles().add(r1);
user.getRoles().add(r2);
user2.getRoles().add(r1); r1.getUsers().add(user);
r1.getUsers().add(user2);
r2.getUsers().add(user); session.save(user);
session.save(user2);结果：
进行了级联保存。
如果使用
CascadeType.PERSIST注解
@ManyToMany(cascade=CascadeType.PERSIST,mappedBy="users") 则需要调用session.persist方法进行保存。注意每个方法上都有支持的级联注解的说明！
Hibernate:
insert
into
User
(name, id)
values
(?, ?)
Hibernate:
insert
into
Role
(name, id)
values
(?, ?)
Hibernate:
insert
into
Role
(name, id)
values
(?, ?)
Hibernate:
insert
into
User
(name, id)
values
(?, ?)
Hibernate:
insert
into
Role_User
(roles_id, users_id)
values
(?, ?)
Hibernate:
insert
into
Role_User
(roles_id, users_id)
values
(?, ?)
Hibernate:
insert
into
Role_User
(roles_id, users_id)
values
(?, ?)删除用户139的ADMIN权限
User u = session.get(User.class,139L);
Set<Role> roles = u.getRoles();
Iterator<Role> ites = roles.iterator();
while(ites.hasNext()) {
Role r = ites.next();
if(r.getName().equals("ADMIN")){
r.getUsers().remove(u);//执行成功，删除关系，而不删除用户
//u.getRoles().remove(r);//执行失败，关系它不维护
}
}
