JPA--4种主键生成策略

た入场券 2021-08-13 16:39 776阅读 0赞

### 主键生成策略 ###

JPA中的主键生成策略由 **@GeneratedValue的strategy** 属性指定：

strategy的取值有4种：

*  GenerationType.IDENTITY：自增、底层数据库必须支持自动增长方式（MySQL）
 *  GenerationType.SEQUENCE：序列、底层数据库必须支持序列（Oracle）
 *  GenerationType.TABLE：JPA提供的一种机制，通过一张数据库表的形式帮助我们完成主键自增
 *  GenerationType.AUTO：由程序自动控制、程序自动帮我们选择主键生成策略

GenerationType.IDENTITY

package pers.zhang.domain;
    
    import javax.persistence.*;
    
    @Entity
    @Table(name = "cst_customer")
    public class Customer { 
    
        /* @Id：配置主键 @GeneratedValue：配置主键的生成策略 strrategy: GenerationType.IDENTITY：自增、底层数据库必须支持自动增长方式（MySQL） GenerationType.SEQUENCE：序列、底层数据库必须支持序列（Oracle） GenerationType.TABLE：JPA提供的一种机制，通过一张数据库表的形式帮助我们完成主键自增 GenerationType.AUTO：由程序自动控制、程序自动帮我们选择主键生成策略 @Column：配置属性和字段的映射关系 name：表中字段名 */
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        @Column(name = "cust_id")
        private Long custId;//客户主键
    
        @Column(name = "cust_name")
        private String custName;//客户名称
    
        @Column(name = "cust_source")
        private String custSource;//客户来源
    
        @Column(name = "cust_industry")
        private String custIndustry;//客户所属行业
    
        @Column(name = "cust_level")
        private String custLevel;//客户级别
    
        @Column(name = "cust_address")
        private String custAddress;//客户地址
    
        @Column(name = "cust_phone")
        private String custPhone;//客户电话
    
        public Long getCustId() { 
            return custId;
        }
    
        public void setCustId(Long custId) { 
            this.custId = custId;
        }
    
        public String getCustName() { 
            return custName;
        }
    
        public void setCustName(String custName) { 
            this.custName = custName;
        }
    
        public String getCustSource() { 
            return custSource;
        }
    
        public void setCustSource(String custSource) { 
            this.custSource = custSource;
        }
    
        public String getCustIndustry() { 
            return custIndustry;
        }
    
        public void setCustIndustry(String custIndustry) { 
            this.custIndustry = custIndustry;
        }
    
        public String getCustLevel() { 
            return custLevel;
        }
    
        public void setCustLevel(String custLevel) { 
            this.custLevel = custLevel;
        }
    
        public String getCustAddress() { 
            return custAddress;
        }
    
        public void setCustAddress(String custAddress) { 
            this.custAddress = custAddress;
        }
    
        public String getCustPhone() { 
            return custPhone;
        }
    
        public void setCustPhone(String custPhone) { 
            this.custPhone = custPhone;
        }
    
        @Override
        public String toString() { 
            return "Customer{" +
                    "custId=" + custId +
                    ", custName='" + custName + '\'' +
                    ", custSource='" + custSource + '\'' +
                    ", custIndustry='" + custIndustry + '\'' +
                    ", custLevel='" + custLevel + '\'' +
                    ", custAddress='" + custAddress + '\'' +
                    ", custPhone='" + custPhone + '\'' +
                    '}';
        }
    }

测试：

@Test
        public void saveTest(){ 
            //1.加载配置文件创建工厂对象（实体管理类工厂）
            EntityManagerFactory factory = Persistence.createEntityManagerFactory("myJpa");
            //2.通过实体管理类工厂获取实体管理器
            EntityManager entityManager = factory.createEntityManager();
            //3.获取事务对象，开启事务
            EntityTransaction transaction = entityManager.getTransaction();
            transaction.begin();
            //4.进行CRUD
            Customer customer = new Customer();
            customer.setCustName("阿里巴巴");
            customer.setCustIndustry("电商");
            //保存
            entityManager.persist(customer);
            //5.提交事务或回滚事务
            transaction.commit();
            //6.释放资源
            entityManager.close();
            factory.close();
    
        }

运行JUnit测试，控制台打印：

Hibernate: drop table if exists cst_customer
    Hibernate: create table cst_customer (cust_id bigint not null auto_increment, cust_address varchar(255), cust_industry varchar(255), cust_level varchar(255), cust_name varchar(255), cust_phone varchar(255), cust_source varchar(255), primary key (cust_id))
    Hibernate: insert into cst_customer (cust_address, cust_industry, cust_level, cust_name, cust_phone, cust_source) values (?, ?, ?, ?, ?, ?)

JPA为我们自动生成了表并插入了数据：  
![在这里插入图片描述][20191214213537621.png]  
GenerationType.TABLE  
修改strategy的值为GenerationType.TABLE

@Id
       @GeneratedValue(strategy = GenerationType.TABLE)
       @Column(name = "cust_id")
       private Long custId;//客户主键

运行测试方法，控制台打印：

Hibernate: drop table if exists cst_customer
    Hibernate: drop table if exists hibernate_sequences
    Hibernate: create table cst_customer (cust_id bigint not null, cust_address varchar(255), cust_industry varchar(255), cust_level varchar(255), cust_name varchar(255), cust_phone varchar(255), cust_source varchar(255), primary key (cust_id))
    Hibernate: create table hibernate_sequences (sequence_name varchar(255) not null, next_val bigint, primary key (sequence_name))
    Hibernate: select tbl.next_val from hibernate_sequences tbl where tbl.sequence_name=? for update
    Hibernate: insert into hibernate_sequences (sequence_name, next_val)  values (?,?)
    Hibernate: update hibernate_sequences set next_val=?  where next_val=? and sequence_name=?
    Hibernate: insert into cst_customer (cust_address, cust_industry, cust_level, cust_name, cust_phone, cust_source, cust_id) values (?, ?, ?, ?, ?, ?, ?)

生成了cst\_customer表，并插入了数据：

![在这里插入图片描述][20191214213954695.png]  
于GenerationType.IDENTITY不同的是，JPA同时由生成了另一张表hibernate\_sequences用与生成主键。（保存了下一次插入数据时，主键的值）

![在这里插入图片描述][20191214214145467.png]

GenerationType.SEQUENCE  
应用于Oracle，根据Oracle提供的序列生成主键。

GenerationType.AUTO  
与使用GenerationType.TABLE一致，生成一张表用来生成主键。

[20191214213537621.png]: /images/20210813/5d58f332637549ecad726b76f772cb70.png
[20191214213954695.png]: /images/20210813/74ab80e110f242458432201dab551df0.png
[20191214214145467.png]: /images/20210813/58494be62e574d02aaee6b7ca2921217.png