一、概述
hibernate中使用两种方式实现主键生成策略，分别是XML生成id和注解方式(@GeneratedValue),下面逐一进行总结。
二、XML配置方法
这种方式是在XX.hbm.xml文件中对generator进行配置，eg:
<?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 package="com.test.demo">
<class name="Student">
<id name="id">
<generator class="native"></generator>
</id>
<property name="name"></property>
<property name="age"></property>
</class>
</hibernate-mapping>
常用的生成策略有以下几种：
identity：对DB2，Mysql，MS SQL Server等的内置标识字段提供支持，返回的标识符是long，short或者int类型
native：可以是identity类型、sequence类型或者hilo类型，取决于不同的底层数据库
sequence：在Oracle，SAP DB中使用序列(sequence)
uuid:使用一种128位的UUID算法产生的字符类型标识，像IP地址一样全网唯一
三、注解方式生成ID：@GeneratorValue
标准的annotation方式的主键生成策略如下：
- AUTO：可以是identity类型或者是sequence类型或者是table类型，取决于底层的数据库
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
@Entity
public class Person
{
private String name;
private int age;
private int id;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
}
- TABLE:使用表保存id值，即会为应用的表创建一张专门保存id的表
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.TableGenerator;
@Entity
public class Person
{
private String name;
private int age;
private int id;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
@Id
@TableGenerator(name="personID",table="personID_DB",pkColumnName="key_value",pkColumnValue="pk_value",valueColumnName="person",allocationSize=1)
@GeneratedValue(strategy=GenerationType.TABLE,generator="personID")
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
}
- IDENTITY：identity column
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.TableGenerator;
@Entity
public class Person
{
private String name;
private int age;
private int id;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
}
- SEQUENCE：sequence
四、联合主键生成策略
有的时候我们需要将一个实体的2个或多个字段联合起来作为主键，就是说，不能有2个或多个对象的这几个字段值都相同的情况发生。现在我们要将Person字段的id和name字段联合作为主键：
@Entity
public class Person
{
//现在id和name组成联合主键
private int id;
private String name;
private int age;
...
}
- 首先将联合主键的属性提取出来,重新编写一个pojo类（原pojo类中的id，name要删除 并新加入属性“PersonPK”）
- 新建pojo类必须实现 java.io.Serializable 序列化接口
- 新pojo类要重写equals和hashCode方法
public class PersonPK implements Serializable
{
private String name;
private int id;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
@Override
public int hashCode()
{
return this.name.hashCode();
}
@Override
public boolean equals(Object obj)
{
if(obj instanceof PersonPK) {
PersonPK pk = (PersonPK)obj;
if(this.id == pk.getId() && this.name.equals(pk.getName())) {
return true;
}
}
return false;
}
}
联合主键生成策略XML配置方法：
<?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 package="com.test.demo">
<class name="Person">
<composite-id name="personPK" class="com.test.demo.PersonPK">
<key-property name="id"></key-property>
<key-property name="name"></key-property>
</composite-id>
<property name="age" />
</class>
</hibernate-mapping>
联合主键ID生成策略的Annotation版本，共有三种方式，前三步骤一样，另外：
方法1、在新类PersonPK前写@Embeddable,在原Person类的新属性PersonPK的get方法前写@id
@Embeddable
public class PersonPK implements Serializable
{
private static final long serialVersionUID = -7068850328521576106L;
private String name;
private int id;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
@Override
public int hashCode()
{
return this.name.hashCode();
}
@Override
public boolean equals(Object obj)
{
if(obj instanceof PersonPK) {
PersonPK pk = (PersonPK)obj;
if(this.id == pk.getId() && this.name.equals(pk.getName())) {
return true;
}
}
return false;
}
}
Person类中：
@Entity
public class Person
{
private PersonPK personPK;
private int age;
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
@Id
public PersonPK getPersonPK()
{
return personPK;
}
public void setPersonPK(PersonPK personPK)
{
this.personPK = personPK;
}
}
方法2、新类无需添加注解，只需在原类Person新属性PersonPK的get方法前写@EmbeddID即可
@Entity
public class Person
{
private PersonPK personPK;
private int age;
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
@EmbeddedId
public PersonPK getPersonPK()
{
return personPK;
}
public void setPersonPK(PersonPK personPK)
{
this.personPK = personPK;
}
}
方法3、新pojo类无需加注解，原pojo类的id，name属性保留不变，也无需新增“TercherPK”属性。 只在id，name的get方法前都加@Id，并在原pojo类前加@IdClass(PersonPK.class):
原类Person:
@Entity
@IdClass(PersonPK.class)
public class Person
{
private int age;
private String name;
private int id;
@Id
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
@Id
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
public int getAge()
{
return age;
}
public void setAge(int age)
{
this.age = age;
}
}
运行测试程序(针对上述三种方法，测试用例需要稍作修改，这里不在赘述)：
public class PersonTest
{
private static SessionFactory sf=null;
@BeforeClass
public static void beforeClass()
{
sf=new AnnotationConfiguration().configure().buildSessionFactory();
}
@Test
public void test()
{
PersonPK personPK=new PersonPK();
personPK.setId(1);
personPK.setName("xujian");
Person p=new Person();
p.setAge(23);
p.setPersonPK(personPK);
Session session=sf.openSession();
session.beginTransaction();
session.save(p);
//提交事物
session.getTransaction().commit();
session.close();
sf.close();
}
@AfterClass
public static void afterClass()
{
sf.close();
}
}
可以看到：
生成的Person表中id和name组成联合主键
时间： 2024-10-10 02:07:05