轉自：https://blog.csdn.net/iteye_3357/article/details/81862615

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1 可能經常遇到這樣的情況：2 3 在數據庫表中會有這樣的一個字段用來區別記錄的屬性，如：在客戶表中有一個字段表示客戶級別，當這個記錄為A時是一級客戶，為B時是二級客戶。在用hiberante做OR表示時類可能是這樣的：4 5 public class Customer{6 7 private String flag; //表示客戶的級別8 9 ... 10 11 } 12 13 然后，在程序中手動控制flag的值，但是這樣當每個級的客戶有不同的屬性時Customer類將包含所有級別的屬性，這樣不是很好。 14 15 hibernate提供一個Discriminator映射的方法，就是把一個表映射成不同的類，有不同的屬性。 16 17 public class Customer{ 18 19 //包含所有級別的公共屬性 20 21 ... 22 23 } 24 25 26 27 public class CustomerA extends Customer{ 28 29 //只包括一級客戶的特有屬性 30 31 } 32 33 public class CustomerB extends Customer{ 34 35 //只包含二級客戶特有的屬性 36 37 } 38 39 這樣更符合面向對象的原則，然后在hbm.xml中這樣寫： 40 41 <id name="id" type="int"> 42 43 ... 44 45 </id> 46 47 <discriminator column="flag" type="string" /> 48 49 <!-- 公共屬性的映射 --> 50 51 <subclass name="CustomerA" discriminator-value="A"> 52 53 <!-- 一級客戶特有屬性的映射 --> 54 55 </subclass> 56 57 <subclass name="CustomerB" discriminator-value="B"> 58 59 <!-- 二級客戶特有屬性的映射 --> 60 61 </subclass> 62 63 這樣就可以單獨的用CustomerA,CustomerB這樣的實例了，做數據庫修改時就不用關心flag字段的值了，會自動的加A或B。 64 65 如果是使用hibernate Annotation而不是xml來描述映謝關系，代碼如下： 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 這樣就可以了。

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2.

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 }

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3.

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 }

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3.

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 }

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5.

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 }

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6.

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>

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轉載于:https://www.cnblogs.com/sharpest/p/6209674.html

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