hibernate继承关系的映射（single_table，class_per_table，joined）

转自：http://blog.csdn.net/centre10/article/details/6092942

hibernate对象之间存在继承关系，该如何映射呢？有3中方式，反映在数据库表上：

Single table：Student和Teacher类的父类是Person类，用一张数据库表反应3个类的所有信息。这样设计的缺点是：表的信息反映了所有类的属性，造成大量的数据冗余。

Table per class:为每个类设计一张表，其缺点是：

Joined：用一张表存共有的字段，而其子类对应的表存其特有字段。优缺点：查询能提高效率，然而需要作表连接，当添加新的子类时，需要添加新的表。

Annotation：

     ⑴sigle_table：

package com.xie.hibernate.modal;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.JoinColumn;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import com.sun.xml.internal.bind.v2.runtime.Name;
@Entity
//指定继承映射的策略
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
//在数据库表中加上一个字段，用以区分是哪个子类,name表示表的字段名字，discriminatorType
//表示name字段的数据类型
@DiscriminatorColumn(name="dis",discriminatorType=DiscriminatorType.STRING)
//指定这个类存到数据库中用什么区分，value表明dis的值
@DiscriminatorValue(value="person")
public class Person {
      private int id;
      private String name;
      
      @Id
      @GeneratedValue
 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;
 }     
}

package com.xie.hibernate.modal;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@DiscriminatorValue("student")
public class Student extends Person {
 private int score;

 public int getScore() {
  return score;
 }

 public void setScore(int score) {
  this.score = score;
 } 
}

package com.xie.hibernate.modal;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@DiscriminatorValue("teacher")
public class Teacher extends Person{
 private String title;

 public String getTitle() {
  return title;
 }

 public void setTitle(String title) {
  this.title = title;
 } 
}

      ⑵table_per_class：

 

package com.xie.hibernate.modal;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.JoinColumn;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.TableGenerator;

import com.sun.xml.internal.bind.v2.runtime.Name;
@Entity
//指定继承映射的策略
@Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)
@TableGenerator(
  name="k_gen",
        table="key_table",
        pkColumnName="p_key",
        pkColumnValue="p_value",
        initialValue=1,
        allocationSize=1
)
public class Person {
      private int id;
      private String name;
      
      @Id
      @GeneratedValue(generator="k_gen",strategy=GenerationType.TABLE)
 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;
 }     
}

 

package com.xie.hibernate.modal;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
public class Teacher extends Person{
 private String title;

 public String getTitle() {
  return title;
 }

 public void setTitle(String title) {
  this.title = title;
 } 
}

 

      ⑶joined：

package com.xie.hibernate.modal;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorColumn;
import javax.persistence.DiscriminatorType;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.JoinColumn;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.TableGenerator;

import com.sun.xml.internal.bind.v2.runtime.Name;
@Entity
//指定继承映射的策略
@Inheritance(strategy=InheritanceType.JOINED)

public class Person {
      private int id;
      private String name;
      
      @Id
      @GeneratedValue
 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;
 }     
}

 

package com.xie.hibernate.modal;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
public class Student extends Person {
 private int score;

 public int getScore() {
  return score;
 }

 public void setScore(int score) {
  this.score = score;
 } 
}

 

package com.xie.hibernate.modal;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
public class Teacher extends Person{
 private String title;

 public String getTitle() {
  return title;
 }

 public void setTitle(String title) {
  this.title = title;
 } 
}