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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_customerHibernate: 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：

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_customerHibernate: drop table if exists hibernate_sequencesHibernate: 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 updateHibernate: 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：

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）

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。

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