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[Java教程]hibernate(四)ID生成策略


一、ID生成策略配置

1、ID生成方式在

<??><!DOCTYPE hibernate-mapping PUBLIC    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.orlion.hibernate.model">  <class name="Student" table="student"><!-- 如果表名与类名相同可以不用写table属性 -->    <!-- 用id映射主键 -->    <id name="id"> <!-- 列名相同可以不指定column属性 -->      <generator class="native"></generator><!-- 在这里设置id生成方式 -->    </id>        <property name="name" column="name"></property>    <property name="age"></property>  </class></hibernate-mapping>

2、ID用注解配置:

在ID注解下添加注解@generatedValue,只有四种方式,默认的策略是auto

配置方式:@genetatedValue(strategy=GenerationType.AUTO)

(1)AUTO - 相当于采用

(2)TABLE - 使用表保存id值

 

(3)IDENTITY - identity column

(4)SEQUENCE - sequence

在类名上添加注解@SequenceGenerator(name="teacherSEQ" , sequenceName="teacherSEQ_DB");定义sequence生成器,name属性指定的是生成器名字,sequenceName指定的是sequence名字

在getId()方法上添加注解@genetatoedValue(strategy=GenerationType.SEQUENCE , generator="teacherSEQ")表示采用sequence生成策略,采用生成器是teacherSEQ

 

二、ID主要生成策略

hibernate中id生成主要方式有:

(1)native:根据不同的数据库采用不同的ID生成方式,例如:在SQL Server中采用identity; 在MySQL中采用auto_increment; 在ORACLE中就会采用sequence。

(2)uuid:原理是使用128位的uuid算法产生主键,从而能够确保网络环境下的一致性。使用此生成策略时,实体类的主键是String类型的,映射成表中字段为varchar。适用所有数据库。

(3)identity:这种策略在采用SQL Server时,相当于SQL Server的identity关键字。

(4)sequence:在 Oracle中使用序列(sequence)。返回的标识符是long, short或者 int类型的。

 

三、联合主键

1、

单独建一个类作为主键类,以Student类为例,假设student有两个主键id和name,那么可以建一个主键类StudentPK,主键类中包含两个属性即student的两个主键,而且要实现Serializable接口,覆盖equals和hashCode方法:

package cn.orlion.hibernate.model;// 实现Serializable接口public class StudentPK implements java.io.Serializable{  private int id;    private String name;  public int getId() {    return id;  }  public void setId(int id) {    this.id = id;  }  public String getName() {    return name;  }  public void setName(String name) {    this.name = name;  }  // 覆盖equals方法  public boolean equals(Object o){        if (o instanceof StudentPK) {      StudentPK pk = (StudentPK)o;      return this.id == pk.getId() && this.name.equals(((StudentPK) o).getName());    }        return false;  }  // 覆盖hashCode方法  public int hashCode(){    return this.name.hashCode();  }}

 

这时Student应该去掉int和name属性然后加上StudentPK属性,如下:

package cn.orlion.hibernate.model;public class Student {    private StudentPK spk;  //  private int id;//  //  private String name;    public StudentPK getSpk() {    return spk;  }  public void setSpk(StudentPK spk) {    this.spk = spk;  }  private int age;//  public int getId() {//    return id;//  }////  public void setId(int id) {//    this.id = 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;  }}

然后修改配置文件为:

<??><!DOCTYPE hibernate-mapping PUBLIC    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.orlion.hibernate.model">  <class name="Student" table="student"><!-- 如果表名与类名相同可以不用写table属性 -->
  
     <!-- 在这里修改主键配置 --> <composite-id name="spk" class="cn.orlion.hibernate.model.StudentPK"> <key-property name="id"></key-property> <key-property name="name"></key-property> </composite-id> <property name="age"></property> </class></hibernate-mapping>

 

这样往数据库中插入一条数据就应该下new一个StudentPK,然后再new一个Student对象:

StudentPK spk = new StudentPK();    spk.setId(1);    spk.setName("test1");    Student t = new Student();    t.setSpk(spk);    // t.setName("test1");    t.setAge(1);        Session session = sf.openSession();        session.beginTransaction();    session.save(t);    session.getTransaction().commit();        session.close();

 

2、注解方式配置联合主键

共有三种方式:

(1)将组件类注解为@Embeddable,并将组件的属性注解为@ID

具体过程:

创建组件类:TeacherPK(实现serializable接口,覆盖equals和hashCode方法)

package cn.orlion.hibernate.model;import javax.persistence.Embeddable;@Embeddablepublic class TeacherPK implements java.io.Serializable{  private int id;    private String name;  public int getId() {    return id;  }  public void setId(int id) {    this.id = id;  }  public String getName() {    return name;  }  public void setName(String name) {    this.name = name;  }  // 覆盖equals方法  public boolean equals(Object o){          if (o instanceof StudentPK) {      StudentPK pk = (StudentPK)o;      return this.id == pk.getId() && this.name.equals(((StudentPK) o).getName());    }          return false;  }  // 覆盖hashCode方法  public int hashCode(){    return this.name.hashCode();  }}

然后Teacher类:(注释掉id和name属性,添加TeacherPK属性,并加上注解@Id)

package cn.orlion.hibernate.model;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;@Entitypublic class Teacher{    private TeacherPK tpk;//  private int id;//  //  private String name;  @Id  public TeacherPK getTpk() {    return tpk;  }  public void setTpk(TeacherPK tpk) {    this.tpk = tpk;  }  private String title;//  @Id//  @GeneratedValue(strategy = GenerationType.AUTO)//  public int getId() {//    return id;//  }////  public void setId(int id) {//    this.id = id;//  }////  public String getName() {//    return name;//  }////  public void setName(String name) {//    this.name = name;//  }  public String getTitle() {    return title;  }  public void setTitle(String title) {    this.title = title;  }}

保存teacher对象的时候:

TeacherPK tpk = new TeacherPK();    tpk.setId(1);    tpk.setName("test1");    Teacher t = new Teacher();    t.setTpk(tpk);//    t.setName("test2");    t.setTitle("title1");        Session session = sf.openSession();        session.beginTransaction();    session.save(t);    session.getTransaction().commit();        session.close();

可以看到建表:

(2)在组件属性添加注解@EmbeddedId

具体为:去掉TeacherPK类上的注解@Embeddable,然后在Teacher类中的getTpk()方法上的注解由@Id改为@EmbeddedId

(3)将类注解为@IdClass,并将该实体中所有属于主键的属性都注解为@Id

具体:

修改Teacher类为:(注:id生成策略设为AUTO的时候会在TeacherPK的setId()方法上抛出一个异常IllegalArgumentException,"POST_INSERT_INDICATOR")

package cn.orlion.hibernate.model;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.IdClass;@Entity@IdClass(TeacherPK.class)public class Teacher{  private int id;    private String name;    private String title;  @Id  public int getId() {    return id;  }  public void setId(int id) {    this.id = id;  }  @Id  public String getName() {    return name;  }  public void setName(String name) {    this.name = name;  }  public String getTitle() {    return title;  }  public void setTitle(String title) {    this.title = title;  }}

TeacherPK:

package cn.orlion.hibernate.model;public class TeacherPK implements java.io.Serializable{  private int id;    private String name;  public int getId() {    return id;  }  public void setId(int id) {    this.id = id;  }  public String getName() {    return name;  }  public void setName(String name) {    this.name = name;  }  // 覆盖equals方法  public boolean equals(Object o){          if (o instanceof StudentPK) {      StudentPK pk = (StudentPK)o;      return this.id == pk.getId() && this.name.equals(((StudentPK) o).getName());    }          return false;  }  // 覆盖hashCode方法  public int hashCode(){    return this.name.hashCode();  }}

保存Teacher对象的时候:

Teacher t = new Teacher();    t.setName("test1");    t.setId(1);    t.setTitle("title1");        Session session = sf.openSession();        session.beginTransaction();    session.save(t);    session.getTransaction().commit();        session.close();