JPA教程

JPA - 高级映射

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JPA是一个随Java规范发布的库.因此，它支持实体持久性的所有面向对象的概念.到目前为止，我们已经完成了对象关系映射的基础知识.本章将指导您完成对象和关系实体之间的高级映射.

继承策略

继承是面向对象语言的核心概念，因此我们可以使用实体之间的继承关系或策略. JPA支持三种类型的继承策略，例如SINGLE_TABLE，JOINED_TABLE和TABLE_PER_CONCRETE_CLASS.

让我们考虑一下Staff，TeachingStaff，NonTeachingStaff类及其关系的示例如下:

继承策略

在上图中，Staff是一个实体，TeachingStaff和NonTeachingStaff是工作人员的子实体.在这里，我们将在所有三种继承策略中讨论上述示例.

单表策略

单表策略采用所有类字段(均为超级和子类)并将它们映射到一个称为SINGLE_TABLE策略的表中.这里鉴别器值在区分一个表中三个实体的值时起着关键作用.

让我们考虑上面的例子，TeachingStaff和NonTeachingStaff是类Staff的子类.提醒继承的概念(是一种通过子类继承超类属性的机制)，因此sid，sname是属于TeachingStaff和NonTeachingStaff的字段.创建一个JPA项目.该项目的所有模块如下:

创建实体

创建一个名为'com.it1352.eclipselink.entity'的包 'src'包下.在给定的包下创建一个名为 Staff.java 的新java类. Staff实体类如下所示:

package com.it1352.eclipselink.entity; 
import java.io.Serializable;

import javax.persistence.DiscriminatorColumn;
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.Table;

@Entity
@Table
@Inheritance( strategy = InheritanceType.SINGLE_TABLE )
@DiscriminatorColumn( name = "type" )

public class Staff implements Serializable {
   @Id
   @GeneratedValue( strategy = GenerationType.AUTO )
   
   private int sid;
   private String sname;
   
   public Staff( int sid, String sname ) {
      super( );
      this.sid = sid;
      this.sname = sname;
   }
   
   public Staff( ) {
      super( );
   }
   
   public int getSid( ) {
      return sid;
   }
   
   public void setSid( int sid ) {
      this.sid = sid;
   }
   
   public String getSname( ) {
      return sname;
   }
   
   public void setSname( String sname ) {
      this.sname = sname;
   }
}

在上面的代码 @DescriminatorColumn 中指定字段名称(类型)并且它的值显示剩余的(Teaching和NonTeachingStaff)字段.

创建名为 TeachingStaff.java 在 com.it1352.eclipselink.entity 包下. TeachingStaff Entity类如下所示:

 
 package com.it1352.eclipselink.entity; 
 import javax.persistence.DiscriminatorValue; 
 import javax.persistence.Entity; 
 @Entity 
 @DiscriminatorValue(value ="TS")
公共类TeachingStaff扩展员工{
私人字符串资格; 
 private String subjectexpertise; 
 public TeachingStaff(int sid，String sname，
 String qualification，String subjectexpertise){
 super(sid，sname); 
 this.qualification = qualified; 
 this.subjectexpertise = subjectexpertise; 
} 
 public TeachingStaff(){
 super(); 
} 
 public String getQualification(){
返回资格; 
} 
 public void setQualification(String qualification){
 this.qualification = qualification; 
} 
 public String getSubjectexpertise(){
 return subjectexpertise; 
} 
 public void setSubjectexpertise(String subjectexpertise){
 this.subjectexpertise = subjectexpertise; 
} 
}

创建名为 NonTeachingStaff.java 的Staff类的子类(类) com.it1352.eclipselink.entity 包. NonTeachingStaff Entity类如下所示:

package com.it1352.eclipselink.entity; 
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue( value="TS" )
public class TeachingStaff extends Staff {

   private String qualification;
   private String subjectexpertise;

   public TeachingStaff( int sid, String sname, 
   
   String qualification,String subjectexpertise ) {
      super( sid, sname );
      this.qualification = qualification;
      this.subjectexpertise = subjectexpertise;
   }

   public TeachingStaff( ) {
      super( );
   }

   public String getQualification( ){
      return qualification;
   }

   public void setQualification( String qualification ){
      this.qualification = qualification;
   }

   public String getSubjectexpertise( ) {
      return subjectexpertise;
   }

   public void setSubjectexpertise( String subjectexpertise ){
      this.subjectexpertise = subjectexpertise;
   }
}

Persistence.xml

Persistence.xml文件包含配置信息数据库和实体类的注册信息. xml文件如下所示:

<?xml version="1.0" encoding="UTF-8"?>

<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation="http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">

   <persistence-unit name="Eclipselink_JPA" transaction-type="RESOURCE_LOCAL">
   
      <class>com.IT屋.eclipselink.entity.Staff</class>
      <class>com.IT屋.eclipselink.entity.NonTeachingStaff</class>
      <class>com.IT屋.eclipselink.entity.TeachingStaff</class>
      
      <properties>
         <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpadb"/>
         <property name="javax.persistence.jdbc.user" value="root"/>
         <property name="javax.persistence.jdbc.password" value="root"/>
         <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
         <property name="eclipselink.logging.level" value="FINE"/>
         <property name="eclipselink.ddl-generation" value="create-tables"/>
      </properties>
      
   </persistence-unit>
</persistence>

服务类

服务类是业务组件的实现部分.在名为'com.it1352.eclipselink.service'的'src'包下创建一个包.

创建一个名为SaveClient的类给定包下的.java存储Staff，TeachingStaff和NonTeachingStaff类字段. SaveClient类如下所示:

 
 package com.it1352.eclipselink.service; 
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.it1352.eclipselink.entity.NonTeachingStaff;
import com.it1352.eclipselink.entity.TeachingStaff;

public class SaveClient {

   public static void main( String[ ] args ) {
   
      EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
      EntityManager entitymanager = emfactory.createEntityManager( );
      entitymanager.getTransaction( ).begin( );

      //Teaching staff entity 
      TeachingStaff ts1=new TeachingStaff(1,"Gopal","MSc MEd","Maths");
      TeachingStaff ts2=new TeachingStaff(2, "Manisha", "BSc BEd", "English");
      
      //Non-Teaching Staff entity
      NonTeachingStaff nts1=new NonTeachingStaff(3, "Satish", "Accounts");
      NonTeachingStaff nts2=new NonTeachingStaff(4, "Krishna", "Office Admin");

      //storing all entities
      entitymanager.persist(ts1);
      entitymanager.persist(ts2);
      entitymanager.persist(nts1);
      entitymanager.persist(nts2);
      
      entitymanager.getTransaction().commit();
      
      entitymanager.close();
      emfactory.close();
   }
}

编译和执行上述程序后，您将在Eclipse IDE的控制台面板中收到通知.检查MySQL工作台的输出.表格格式的输出如下所示:

Sid类型SnameAreaexpertise资格Subjectexpertise
1TSGopal
MSC MEDMaths
2TSManisha
BSC BEDEnglish
3NSSatishAccounts

4NSKrishnaOffice Admin

最后，您将获得包含所有三个类的字段的单个表，并且与名为的标识符列不同'(字段).

加入表策略

连接表策略是共享引用的列，该列包含连接表的唯一值并进行简单的事务处理.让我们考虑与上面相同的例子.

创建一个JPA项目.所有项目模块如下所示:

创建实体

创建一个名为'com.it1352.eclipselink.entity'的包/b>在'src'包下.在给定的包下创建一个名为 Staff.java 的新java类. Staff实体类如下所示:

package com.it1352.eclipselink.entity; 
import java.io.Serializable;

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.Table;

@Entity
@Table
@Inheritance( strategy = InheritanceType.JOINED )

public class Staff implements Serializable {

   @Id
   @GeneratedValue( strategy = GenerationType.AUTO )
   
   private int sid;
   private String sname;
   
   public Staff( int sid, String sname ) {
      super( );
      this.sid = sid;
      this.sname = sname;
   }
   
   public Staff( ) {
      super( );
   }
   
   public int getSid( ) {
      return sid;
   }
   
   public void setSid( int sid ) {
      this.sid = sid;
   }
   
   public String getSname( ) {
      return sname;
   }
   
   public void setSname( String sname ) {
      this.sname = sname;
   }
}

创建名为 TeachingStaff.java 的Staff类的子类(类) com.it1352.eclipselink.entity 包. TeachingStaff Entity类如下所示:

package com.it1352.eclipselink.entity; 
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@PrimaryKeyJoinColumn(referencedColumnName="sid")

public class TeachingStaff extends Staff {
   private String qualification;
   private String subjectexpertise;

   public TeachingStaff( int sid, String sname, 
   
   String qualification,String subjectexpertise ) {
      super( sid, sname );
      this.qualification = qualification;
      this.subjectexpertise = subjectexpertise;
   }

   public TeachingStaff( ) {
      super( );
   }

   public String getQualification( ){
      return qualification;
   }

   public void setQualification( String qualification ){
      this.qualification = qualification;
   }

   public String getSubjectexpertise( ) {
      return subjectexpertise;
   }

   public void setSubjectexpertise( String subjectexpertise ){
      this.subjectexpertise = subjectexpertise;
   }
}

创建名为 NonTeachingStaff.java 的Staff类的子类(类) com.it1352.eclipselink.entity 包. NonTeachingStaff Entity类如下所示:

package com.it1352.eclipselink.entity; 
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@PrimaryKeyJoinColumn(referencedColumnName="sid")

public class NonTeachingStaff extends Staff {
   private String areaexpertise;

   public NonTeachingStaff( int sid, String sname, String areaexpertise ) {
      super( sid, sname );
      this.areaexpertise = areaexpertise;
   }

   public NonTeachingStaff( ) {
      super( );
   }

   public String getAreaexpertise( ) {
      return areaexpertise;
   }

   public void setAreaexpertise( String areaexpertise ) {
      this.areaexpertise = areaexpertise;
   }
}

Persistence.xml

Persistence.xml文件包含配置信息数据库和实体类的注册信息. xml文件如下所示:

<?xml version = "1.0" encoding = "UTF-8"?>

<persistence version = "2.0" xmlns = "http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   
   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.IT屋.eclipselink.entity.Staff</class>
      <class>com.IT屋.eclipselink.entity.NonTeachingStaff</class>
      <class>com.IT屋.eclipselink.entity.TeachingStaff</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://localhost:3306/jpadb"/>
         <property name = "javax.persistence.jdbc.user" value = "root"/>
         <property name = "javax.persistence.jdbc.password" value = "root"/>
         <property name = "javax.persistence.jdbc.driver" value = "com.mysql.jdbc.Driver"/>
         <property name = "eclipselink.logging.level" value = "FINE"/>
         <property name = "eclipselink.ddl-generation" value = "create-tables"/>
      </properties>
      
   </persistence-unit>
</persistence>

服务类

服务类是业务组件的实现部分.在名为'com.it1352.eclipselink.service'的'src'包下创建一个包.

创建一个名为SaveClient的类给定包下的.java存储Staff，TeachingStaff和NonTeachingStaff类字段.然后SaveClient类如下:

package com.it1352.eclipselink.service; 
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.IT屋.eclipselink.entity.NonTeachingStaff;
import com.IT屋.eclipselink.entity.TeachingStaff;

public class SaveClient {
   public static void main( String[ ] args ) {
      EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
      EntityManager entitymanager = emfactory.createEntityManager( );
      entitymanager.getTransaction( ).begin( );

      //Teaching staff entity 
      TeachingStaff ts1 = new TeachingStaff(1,"Gopal","MSc MEd","Maths");
      TeachingStaff ts2 = new TeachingStaff(2, "Manisha", "BSc BEd", "English");
      
      //Non-Teaching Staff entity
      NonTeachingStaff nts1 = new NonTeachingStaff(3, "Satish", "Accounts");
      NonTeachingStaff nts2 = new NonTeachingStaff(4, "Krishna", "Office Admin");

      //storing all entities
      entitymanager.persist(ts1);
      entitymanager.persist(ts2);
      entitymanager.persist(nts1);
      entitymanager.persist(nts2);

      entitymanager.getTransaction().commit();
      entitymanager.close();
      emfactory.close();
   }
}

编译和执行上述程序后，您将在Eclipse IDE的控制台面板中收到通知.对于输出检查MySQL工作台如下:

这里创建了三个表，表格格式的 staff 表的结果如下所示:

SidDtypeSname
1TeachingStaffGopal
2TeachingStaffManisha
3NonTeachingStaffSatish
4NonTeachingStaffKrishna

表格中 TeachingStaff 表的结果格式如下所示:

Sid资格Subjectexpertise
1MSC MEDMaths
2BSC BEDEnglish

在上表中，sid是外键(参考字段表格员工表)表格格式的 NonTeachingStaff 表的结果如下所示:

SidAreaexpertise
3Accounts
4Office Admin

最后，分别使用字段创建三个表，并且所有三个表共享SID字段.在员工表中，SID是主键，在剩余(TeachingStaff和NonTeachingStaff)表中，SID是外键.

每类策略表

每班表策略是为每个子实体创建一个表.将创建staff表，但它将包含空记录. Staff表的字段值必须由TeachingStaff和NonTeachingStaff表共享.

让我们考虑与上面相同的示例.该项目的所有模块如下所示:

创建实体

创建名为'com.it1352.eclipselink.entity的包'在'src'包下.在给定的包下创建一个名为 Staff.java 的新java类. Staff实体类如下所示:

package com.it1352.eclipselink.entity; 
import java.io.Serializable;

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.Table;

@Entity
@Table
@Inheritance( strategy = InheritanceType.TABLE_PER_CLASS )

public class Staff implements Serializable {

   @Id
   @GeneratedValue( strategy = GenerationType.AUTO )

   private int sid;
   private String sname;

   public Staff( int sid, String sname ) {
      super( );
      this.sid = sid;
      this.sname = sname;
   }

   public Staff( ) {
      super( );
   }

   public int getSid( ) {
      return sid;
   }

   public void setSid( int sid ) {
      this.sid = sid;
   }

   public String getSname( ) {
      return sname;
   }

   public void setSname( String sname ) {
      this.sname = sname;
   }
}

创建名为 TeachingStaff.java 的Staff类的子类(类) com.it1352.eclipselink.entity 包. TeachingStaff Entity类如下所示:

package com.it1352.eclipselink.entity; 
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
public class TeachingStaff extends Staff {
   private String qualification;
   private String subjectexpertise;

   public TeachingStaff( int sid, String sname, String qualification, String subjectexpertise ) {
      super( sid, sname );
      this.qualification = qualification;
      this.subjectexpertise = subjectexpertise;
   }

   public TeachingStaff( ) {
      super( );
   }

   public String getQualification( ){
      return qualification;
   }
   
   public void setQualification( String qualification ) {
      this.qualification = qualification;
   }

   public String getSubjectexpertise( ) {
      return subjectexpertise;
   }

   public void setSubjectexpertise( String subjectexpertise ){
      this.subjectexpertise = subjectexpertise;
   }
}

创建名为 NonTeachingStaff.java 的Staff类的子类(类) com.it1352.eclipselink.entity 包. NonTeachingStaff Entity类如下所示:

package com.it1352.eclipselink.entity; 
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
public class NonTeachingStaff extends Staff {
   private String areaexpertise;

   public NonTeachingStaff( int sid, String sname, String areaexpertise ) {
      super( sid, sname );
      this.areaexpertise = areaexpertise;
   }

   public NonTeachingStaff( ) {
      super( );
   }

   public String getAreaexpertise( ) {
      return areaexpertise;
   }

   public void setAreaexpertise( String areaexpertise ) {
      this.areaexpertise = areaexpertise;
   }
}

Persistence.xml

Persistence.xml文件包含配置信息数据库和实体类的注册信息. xml文件如下所示:

<?xml version="1.0" encoding = "UTF-8"?>
<persistence version = "2.0" xmlns = "http://java.sun.com/xml/ns/persistence"
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">

   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.IT屋.eclipselink.entity.Staff</class>
      <class>com.IT屋.eclipselink.entity.NonTeachingStaff</class>
      <class>com.IT屋.eclipselink.entity.TeachingStaff</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://localhost:3306/jpadb"/>
         <property name = "javax.persistence.jdbc.user" value = "root"/>
         <property name = "javax.persistence.jdbc.password" value = "root"/>
         <property name = "javax.persistence.jdbc.driver" value = "com.mysql.jdbc.Driver"/>
         <property name = "eclipselink.logging.level" value = "FINE"/>
         <property name = "eclipselink.ddl-generation" value="create-tables"/>
      </properties>
      
   </persistence-unit>
</persistence>

服务类

服务类是业务组件的实现部分.在名为'com.it1352.eclipselink.service'的'src'包下创建一个包.

创建一个名为

 
 package com.it1352.eclipselink.service; 
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.IT屋.eclipselink.entity.NonTeachingStaff;
import com.IT屋.eclipselink.entity.TeachingStaff;

public class SaveClient {
   public static void main( String[ ] args ) {
      EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
      EntityManager entitymanager = emfactory.createEntityManager( );
      entitymanager.getTransaction( ).begin( );

      //Teaching staff entity 
      TeachingStaff ts1 = new TeachingStaff(1,"Gopal","MSc MEd","Maths");
      TeachingStaff ts2 = new TeachingStaff(2, "Manisha", "BSc BEd", "English");
      
      //Non-Teaching Staff entity
      NonTeachingStaff nts1 = new NonTeachingStaff(3, "Satish", "Accounts");
      NonTeachingStaff nts2 = new NonTeachingStaff(4, "Krishna", "Office Admin");

      //storing all entities
      entitymanager.persist(ts1);
      entitymanager.persist(ts2);
      entitymanager.persist(nts1);
      entitymanager.persist(nts2);

      entitymanager.getTransaction().commit();
      entitymanager.close();
      emfactory.close();
   }
}

编译和执行上述程序后，您将在Eclipse IDE的控制台面板中收到通知.对于输出，请按如下方式检查MySQL工作台:

这里创建了三个表， Staff 表包含空记录.

表格格式的 TeachingStaff 的结果显示如下:

Sid资格SnameSubjectexpertise
1MSC MEDGopalMaths
2BSC BEDManishaEnglish

上表TeachingStaff包含Staff和TeachingStaff实体的字段.

表格格式的 NonTeachingStaff 的结果如下所示:

SidAreaexpertiseSname
3账户Satish
4Office AdminKrishna

上表NonTeachingStaff包含Staff和NonTeachingStaff实体的字段.

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