九.Spring 实体管理器

我会带着你远行 2022-06-17 14:40 91阅读 0赞

**一.**  
实体管理器：实体与数据库间桥梁,可把实体持久化到数据库，也可把数据库记录打包为实体。  
![这里写图片描述][SouthEast]  
**基础映射：**  
dao层类 UserRepositoryImpl，spring配置文件通过其后缀知道它与接口UserRepository关系。在UserRepositoryImpl中注入一个

@PersistenceContext
        private EntityManager entityManager;

@Transactional
        public void add(User user) {
            entityManager.persist(user);
    
        }

add:entityManager.persist(user);  
update:find(User.class, user.getId())，修改实体属性，返回实体  
addOrUpdate：.merge(user)  
delete:remove  
**高级映射：**  
**一对一**：Person对address，Person的address\_id（外键）对应address的aid(主键）

@OneToOne(cascade={CascadeType.REFRESH,CascadeType.REMOVE},fetch=FetchType.LAZY)

**一对多**：  
depart:did ,name  
depart\_employee:depart\_id ,employ\_id  
employee:eid, name

@OneToMany
        @JoinTable(name = "depart_employee", 
            joinColumns = @JoinColumn(name = "depart_id",referencedColumnName="did"), 
            inverseJoinColumns = @JoinColumn(name = "employee_id",referencedColumnName="eid"))
        private List<Employee> employees;

**另一种方法**：  
在多端加一列：depart\_id  
配置：

class Depart2：
    @OneToMany
        @JoinColumn(name="depart_id",referencedColumnName="id")
        private List<Employee2> employee2s;

**多对多：**  
student:sid,age,gender,height,name  
teacher\_student:teacher\_id,student\_id  
teacher:tid,age,gender,height,name

class Teacher :
     @ManyToMany 
        @JoinTable(name = "teacher_student", 
            joinColumns = @JoinColumn(name = "teacher_id",referencedColumnName="tid"), 
            inverseJoinColumns = @JoinColumn(name = "student_id",referencedColumnName="sid"))
        private List<Student> students;

class Student:
    @ManyToMany(mappedBy = "students") 
        private List<Teacher> teachers;

**实体映射：单一表策略，连接表策略，每个类策略**

**类策略**不浪费空间，效率高：  
book,phone都继承了父类item,子类父类的主键不重复

@Entity
    @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 
    public class Item { 
    /* @Id @GeneratedValue(strategy = GenerationType.AUTO) */
    
      @Id   
      @GeneratedValue(strategy = GenerationType.TABLE, generator = "ItemGenerator")
      @TableGenerator(name = "ItemGenerator",table="pk_generator",
                        pkColumnName="gen_name",
                        valueColumnName="gen_value",
                        pkColumnValue="item_pk",
                        initialValue=0,
                        allocationSize=1)
        private Long id;

[SouthEast]: /images/20220617/3f59c21834394d5a8c669e2afa9668ec.png