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;

    ...

}

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