Hibernate系列之ID生成策略

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;
    ...
}

1. 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”）
2. 新建pojo类必须实现 java.io.Serializable 序列化接口
3. 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