hibernate id 联合主键生成策略

如果数据库中用多个字段而不仅仅是一个字段作为主键，也就是联合主键，这个时候就可以使用hibernate提供的联合主键生成策略。 

具体如下： 

可以使用一个组件作为一个实体类的标识符。你的组件类必须满足以下要求： 

它必须实现 java.io.Serializable 接口 

它必须重新实现 equals() 和 hashCode() 方法，始终和组合关键字在数据库中的概念保持一致 

注意：在 Hibernate3 中，第二个要求并非是 Hibernate 强制必须的。但最好这样做。 

不能使用一个 IdentifierGenerator 产生组合关键字。一个应用程序必须分配它自己的标识符。 

具体做法如下步骤： 

1.写一个包含表中联合主键的所有字段的类，作为主键类，实例如下： 

 package com.seed.lee.model;

 /**
 * 这个类作为Person类的（组合主键）主键类
 *
 * @author Administrator
 *
 */
 public class PersonUionPKID implements java.io.Serializable {

     private String firstName;

     private String secondName;

     public String getFirstName() {
         return firstName;
     }

     public void setFirstName(String firstName) {
         this.firstName = firstName;
     }

     public String getSecondName() {
         return secondName;
     }

     public void setSecondName(String secondName) {
         this.secondName = secondName;
     }

     @Override
     public boolean equals(Object obj) {
         if(obj instanceof PersonUionPKID){
             PersonUionPKID pk=(PersonUionPKID)obj;
             if(this.firstName.equals(pk.firstName)&&this.secondName.equals(pk.secondName)){
                 return true;
             }
         }
         return false;
     }

     @Override
     public int hashCode() {
         return super.hashCode();
     }

 }

这里要特别注意，如hibernate API文档所述，主键类必须实现java.io.Serializable接口，而且推荐用自己的方法override    equals（） 和hashCode（）方法，保证主键的唯一性. 





2.再写一个类与表字段相当的类，这个类里面不一定要包含表主键的所有字段，实例如下： 

 package com.seed.lee.model;

 public class Person {

     PersonUionPKID uionPKID = new PersonUionPKID();

     private int age;

     private String sex;

     private String job;

     public PersonUionPKID getUionPKID() {
         return uionPKID;
     }

     public void setUionPKID(PersonUionPKID uionPKID) {
         this.uionPKID = uionPKID;
     }

     public int getAge() {
         return age;
     }

     public void setAge(int age) {
         this.age = age;
     }

     public String getSex() {
         return sex;
     }

     public void setSex(String sex) {
         this.sex = sex;
     }

     public String getJob() {
         return job;
     }

     public void setJob(String job) {
         this.job = job;
     }

 }

3.配置hibernate配置文件，实例如下： 

 "1.0"?>

         "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
         "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

 package="com.seed.lee.model">

     <class name="Person" table="person">

         "uionPKID" class="com.seed.lee.model.PersonUionPKID">
             "secondName" />
             "firstName" />

         "age" column="age" />
         "sex" length="2" />
         "job" length="50" />
     class>

在使用annotation的时候，有三种方法来实现联合组建： 
1.将组件类注解为@Embeddable,并将组件的属性注解为@Id 

 package com.seed.lee.model;

 /**
 * 这个类作为Person类的（组合主键）主键类
 *
 * @author Administrator
 *
 */
 @Embeddable
 public class PersonUionPKID implements java.io.Serializable {

     private String firstName;

     private String secondName;

     public String getFirstName() {
         return firstName;
     }

     public void setFirstName(String firstName) {
         this.firstName = firstName;
     }

     public String getSecondName() {
         return secondName;
     }

     public void setSecondName(String secondName) {
         this.secondName = secondName;
     }

     @Override
     public boolean equals(Object obj) {
         if(obj instanceof PersonUionPKID){
             PersonUionPKID pk=(PersonUionPKID)obj;
             if(this.firstName.equals(pk.firstName)&&this.secondName.equals(pk.secondName)){
                 return true;
             }
         }
         return false;
     }

     @Override
     public int hashCode() {
         return super.hashCode();
     }

 }

 package com.seed.lee.model;

 public class Person {

     PersonUionPKID uionPKID = new PersonUionPKID();

     private int age;

     private String sex;

     private String job;

      @Id
     public PersonUionPKID getUionPKID() {
         return uionPKID;
     }


     public void setUionPKID(PersonUionPKID uionPKID) {
         this.uionPKID = uionPKID;
     }

     public int getAge() {
         return age;
     }

     public void setAge(int age) {
         this.age = age;
     }

     public String getSex() {
         return sex;
     }

     public void setSex(String sex) {
         this.sex = sex;
     }

     public String getJob() {
         return job;
     }

     public void setJob(String job) {
         this.job = job;
     }

 }

2. 将组件的属性注解为@EmbeddedId 

 package com.seed.lee.model;

 public class Person {

     PersonUionPKID uionPKID = new PersonUionPKID();

     private int age;

     private String sex;

     private String job;

     @EmbeddedId
     public PersonUionPKID getUionPKID() {
         return uionPKID;
     }


     public void setUionPKID(PersonUionPKID uionPKID) {
         this.uionPKID = uionPKID;
     }

     public int getAge() {
         return age;
     }

     public void setAge(int age) {
         this.age = age;
     }

     public String getSex() {
         return sex;
     }

     public void setSex(String sex) {
         this.sex = sex;
     }

     public String getJob() {
         return job;
     }

     public void setJob(String job) {
         this.job = job;
     }

 }

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 

 package com.seed.lee.model;

 @@IdClass(PersonUionPKID.class)
 public class Person {

     private String firstName;

     private String secondName;

     private int age;

     private String sex;

     private String job;

    @Id
     public String getFirstName() {
         return firstName;
     }

     public void setFirstName(String firstName) {
         this.firstName = firstName;
     }

     @Id
     public String getSecondName() {
         return secondName;
     }

     public void setSecondName(String secondName) {
         this.secondName = secondName;
     }

     public int getAge() {
         return age;
     }

     public void setAge(int age) {
         this.age = age;
     }

     public String getSex() {
         return sex;
     }

     public void setSex(String sex) {
         this.sex = sex;
     }

     public String getJob() {
         return job;
     }

     public void setJob(String job) {
         this.job = job;
     }

 }