1 可能经常遇到这样的情况：2 3        在数据库表中会有这样的一个字段用来区别记录的属性，如：在客户表中有一个字段表示客户级别，当这个记录为A时是一级客户，为B时是二级客户。在用hiberante做OR表示时类可能是这样的：4 5        public class Customer{6 7           private String flag;   //表示客户的级别8 9           ...
10
11  } 12 13  然后，在程序中手动控制flag的值，但是这样当每个级的客户有不同的属性时Customer类将包含所有级别的属性，这样不是很好。 14 15  hibernate提供一个Discriminator映射的方法，就是把一个表映射成不同的类，有不同的属性。 16 17  public class Customer{ 18 19  //包含所有级别的公共属性 20 21  ... 22 23  } 24 25 26 27  public class CustomerA extends Customer{ 28 29  //只包括一级客户的特有属性 30 31  } 32 33  public class CustomerB extends Customer{ 34 35  //只包含二级客户特有的属性 36 37  } 38 39 这样更符合面向对象的原则，然后在hbm.xml中这样写： 40 41 <id name="id" type="int"> 42 43  ... 44 45 </id> 46 47 <discriminator column="flag" type="string" /> 48 49 <!-- 公共属性的映射 --> 50 51 <subclass name="CustomerA" discriminator-value="A"> 52 53 <!-- 一级客户特有属性的映射 --> 54 55 </subclass> 56 57 <subclass name="CustomerB" discriminator-value="B"> 58 59 <!-- 二级客户特有属性的映射 --> 60 61 </subclass> 62 63 这样就可以单独的用CustomerA,CustomerB这样的实例了，做数据库修改时就不用关心flag字段的值了，会自动的加A或B。 64 65 如果是使用hibernate Annotation而不是xml来描述映谢关系，代码如下： 66 67 @Entity 68 69 @Table(name = "customer") 70 71 @Inheritance(strategy = InheritanceType.SINGLE_TABLE) 72 73 @DiscriminatorColumn(name = "flag", discriminatorType = DiscriminatorType.STRING) 74 75 public class Customer{ 76 77 } 78 79 @Entity 80 81 @DiscriminatorValue(value = "A") 82 83 public class CustomerA extends Customer{ 84 85 } 86 87 @Entity 88 89 @DiscriminatorValue(value = "B") 90 91 public class CustomerB extends Customer{ 92 93 } 94 95 这样就可以了。

 1 public class Singer {2     private String region;3     private Long id;4     private Set<Song> songs = new HashSet<Song>();5 6     public String getRegion() {7         return region; 8  } 9 10 public void setRegion(String region) { 11 this.region = region; 12  } 13 14 public Long getId() { 15 return id; 16  } 17 18 public void setId(Long id) { 19 this.id = id; 20  } 21 22 public Set<Song> getSongs() { 23 return songs; 24  } 25 26 public void setSongs(Set<Song> songs) { 27 this.songs = songs; 28  } 29 30 public void addSong(Song song){ 31  songs.add(song); 32 song.setSinger(this); 33  } 34 }

 1 public class SingleSinger extends Singer {2     private String name;3     private char sex;4     5     public String getName() {6         return name;7  } 8 9 public void setName(String name) { 10 this.name = name; 11  } 12 13 public char getSex() { 14 return sex; 15  } 16 17 public void setSex(char sex) { 18 this.sex = sex; 19  } 20 }

 1 package edu.jlu.fuliang.domain;2 3 import java.util.HashSet;4 import java.util.Set;5 6 public class CompositeSinger extends Singer {7     private Set<SingleSinger> singleSingers = new HashSet<SingleSinger>();8 9     public Set<SingleSinger> getSingleSingers() { 10 return singleSingers; 11  } 12 13 public void setSingleSingers(Set<SingleSinger> singleSingers) { 14 this.singleSingers = singleSingers; 15  } 16 public void addSinger(SingleSinger singleSinger){ 17  singleSingers.add(singleSinger); 18  } 19 }

 1 public class Band extends CompositeSinger {2     private String name;3     4     public String getName() {5         return name;6     }7 8     public void setName(String name) { 9 this.name = name; 10  } 11 }

 1 <?xml version="1.0" encoding="utf-8"?>2 <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"3 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">4 5 <hibernate-mapping package="edu.jlu.fuliang.domain">6     <class name="Singer" table="SINGER">7       <id name="id" type="java.lang.Long">8          <column name="SINGER_ID"/>9          <generator class="identity"/>
10       </id>
11
12      <discriminator column="TYPE" type="string"/>
13
14      <property name="region" type="java.lang.String">
15          <column name="REGION"/>
16      </property>
17
18       <set name="songs" table="SONG" inverse="true" cascade="save-update">
19          <key column="SINGER_ID"/>
20          <one-to-many class="Song"/>
21       </set>
22
23       <subclass name="SingleSinger" discriminator-value="1">
24          <property name="name" type="java.lang.String">
25               <column name="NAME"/>
26          </property>
27          <property name="sex" type="java.lang.Character">
28              <column name="SEX"/>
29          </property>
30       </subclass>
31
32       <subclass name="CompositeSinger" discriminator-value="2">
33          <set name="singleSingers" table="SINGER" cascade="save-update" lazy="false">
34              <key column="COMPSITESINGER_ID"/>
35              <one-to-many class="SingleSinger"/>
36          </set>
37       </subclass>
38
39       <subclass name="Band" discriminator-value="3">
40           <set name="singleSingers" table="SINGER" cascade="save-update">
41              <key column="BAND_ID"/>
42              <one-to-many class="SingleSinger"/>
43          </set>
44       </subclass>
45     </class>
46 </hibernate-mapping>