7.5 Hibernate：内置生成器 – identity

由数据库自己生成标识符，映射的主键必须设置为自增长，要求数据库底层支持自增长字段类型。

适用于 Sybase，MySQL，MS SQL Server，DB2 和 Hypersonic SQL，不适用 Oracle 因其无自增字段类型。

返回的 ID 类型为 short，int 或 long。

如果使用 MySQL 数据库，则主键字段必须设置为 auto_increment 。

特点：只能用在支持自动增长的字段数据库中使用。

使用 MySQL 演示：

1 使用 XML

1.1 持久化类定义：

package hibernate;import java.util.Date;public class Person {    private Integer id;    private String account;    private String name;    private Date birth;    public Person() {}    public Person(String account, String name, Date birth) {        this.account = account;        this.name = name;        this.birth = birth;    }    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    public String getAccount() {        return account;    }    public void setAccount(String account) {        this.account = account;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Date getBirth() {        return birth;    }    public void setBirth(Date birth) {        this.birth = birth;    }    @Override    public String toString() {        return "Person [id=" + id + ", account=" + account + ", name=" + name + ", birth=" + birth + "]";    }}

1.2 定义映射：

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping>  <class name="hibernate.Person" table="PERSON">    <id name="id" type="int">      <column name="ID" />      <generator class="identity" />    </id>    <property name="account" type="java.lang.String">      <column name="ACCOUNT" />    </property>    <property name="name" type="java.lang.String">      <column name="NAME" />    </property>    <property name="birth" type="java.util.Date">      <column name="BIRTH" />    </property>  </class></hibernate-mapping>

1.3 单元测试：

package hibernate;import java.util.Date;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;import org.junit.Test;public class HibernateTest {    @Test    public void test() {        Configuration configuration = new Configuration().configure("hibernate.cfg.xml");        SessionFactory sessionFactory = configuration.buildSessionFactory();        Session session = sessionFactory.openSession();        Transaction transaction = session.beginTransaction();        Date date = new Date(System.currentTimeMillis());        Person person1 = new Person("admin1", "Nick", date);        session.save(person1);        Person person2 = new Person("admin2", "King", date);        session.save(person2);        transaction.commit();        session.close();        sessionFactory.close();    }}

单元测试通过，查询数据库，Hibernate 自动生成的表结构定义如下：

可见主键 ID 为自增列。数据库中新插入两条记录，主键从 1 开始，每次递增 1 。

2 使用注解（annotation）
使用注解定义持久化类：

package hibernate;import java.util.Date;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.Table;import org.hibernate.annotations.GenericGenerator;@Entity@Tablepublic class Person {    @Id    @GeneratedValue(generator = "assignedGenerator")    @GenericGenerator(name = "assignedGenerator", strategy = "identity")    private Integer id;    private String account;    private String name;    private Date birth;    public Person() {}    public Person(String account, String name, Date birth) {        this.account = account;        this.name = name;        this.birth = birth;    }    public Integer getId() {        return id;    }    public void setId(Integer id) {        this.id = id;    }    public String getAccount() {        return account;    }    public void setAccount(String account) {        this.account = account;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Date getBirth() {        return birth;    }    public void setBirth(Date birth) {        this.birth = birth;    }    @Override    public String toString() {        return "Person [id=" + id + ", account=" + account + ", name=" + name + ", birth=" + birth + "]";    }}

运行【1.3 单元测试】，测试结果相同。