jpa 多对多关联关系 -- 两方主控

1. user

@Getter
@Setter
@Entity
@Table(name = "t_user")
@NoArgsConstructor
public class User {

    @Id
    private Long id;

    private String username;

    public User(Long id, String username) {
        this.id = id;
        this.username = username;
    }


    /**
     * 所属的角色
     */
    @ManyToMany
    @JoinTable(name = "mt_user_role", joinColumns = @JoinColumn(name = "user_id"), inverseJoinColumns = @JoinColumn(name = "role_id"))
    private Set<Role> roles = new HashSet<>();

}
2. role

@Getter
@Setter
@Entity
@Table(name = "t_role")
@NoArgsConstructor
public class Role {

    @Id
    private Long id;

    private String roleName;

    public Role(Long id, String roleName) {
        this.id = id;
        this.roleName = roleName;
    }

    /**
     * 角色所包含的用户
     */
    @ManyToMany
    @JoinTable(name = "mt_user_role", joinColumns = @JoinColumn(name = "role_id"), inverseJoinColumns = @JoinColumn(name = "user_id"))
    private Set<User> users = new HashSet<>();
}
3. 建立关联关系，无论哪一边均可。
方式一：

User user = this.userRepository.findById(1L).orElse(null);
Role role1 = this.roleRepository.findById(1L).orElse(null);
Role role2 = this.roleRepository.findById(2L).orElse(null);
user.getRoles().add(role1);
user.getRoles().add(role2);
方式二：

User user = this.userRepository.findById(1L).orElse(null);
Role role1 = this.roleRepository.findById(1L).orElse(null);
Role role2 = this.roleRepository.findById(2L).orElse(null);
role1.getUsers().add(user);
role2.getUsers().add(user);
4.删除关联关系 -- 两边均可
方式一：
User user = this.userRepository.findById(1L).orElse(null);
user.setRoles(new HashSet<>());

方式二：

Role role1 = this.roleRepository.findById(1L).orElse(null);
role1.setUsers(new HashSet<>());

5.删除实体  -- 两边均可

Role role2 = this.roleRepository.findById(2L).orElse(null);
this.roleRepository.delete(role2);

6.更新关联关系  -- 两边均可

User user = this.userRepository.findById(1L).orElse(null);
user.getRoles().removeAll(user.getRoles());
Role role3 = this.roleRepository.findById(3L).orElse(null);
user.getRoles().add(role3);
或

User user1 = this.userRepository.findById(1L).orElse(null);
User user2 = this.userRepository.findById(2L).orElse(null);
Role role3 = this.roleRepository.findById(3L).orElse(null);
role3.getUsers().removeAll(role3.getUsers());
role3.getUsers().add(user1);
role3.getUsers().add(user2);