JPA - 高级映射

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JPA 是一个随 Java 规范发布的库。因此，它支持实体持久化的所有面向对象概念。到目前为止，我们已经完成了对象关系映射的基础知识。本章将带您了解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，因此我们可以在实体之间使用继承关系或策略。JPA 支持三种类型的继承策略，例如 SINGLE_TABLE、JOINED_TABLE 和 TABLE_PER_CONCRETE_CLASS。

让我们考虑一个 Staff、TeachingStaff、NonTeachingStaff 类及其关系的示例，如下所示

Inheritance Strategy

在上图中，Staff 是一个实体，TeachingStaff 和 NonTeachingStaff 是 Staff 的子实体。在这里，我们将讨论上述示例在所有三种继承策略中的应用。

单表策略

单表策略获取所有类的字段（超类和子类），并将它们映射到一个称为 SINGLE_TABLE 策略的单个表中。这里，鉴别器值在区分一个表中三个实体的值方面起着关键作用。

让我们考虑上面的例子，TeachingStaff 和 NonTeachingStaff 是 Staff 类的子类。回想一下继承的概念（是从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所示

创建实体

在 ‘src’ 包下创建一个名为 ‘com.tutorialspoint.eclipselink.entity’ 的包。在给定包下创建一个名为 Staff.java 的新 Java 类。Staff 实体类如下所示

package com.tutorialspoint.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;
   }
}

在上面的代码中，@DescriminatorColumn 指定字段名称 (type)，其值显示其余的（Teaching 和 NonTeachingStaff）字段。

在 com.tutorialspoint.eclipselink.entity 包下，创建一个名为 TeachingStaff.java 的 Staff 类的子类（类）。TeachingStaff 实体类如下所示

package com.tutorialspoint.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;
   }
}

在 com.tutorialspoint.eclipselink.entity 包下，创建一个名为 NonTeachingStaff.java 的 Staff 类的子类（类）。NonTeachingStaff 实体类如下所示

package com.tutorialspoint.eclipselink.entity;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue( value = "NS" )

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 文件包含数据库配置信息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.tutorialspoint.eclipselink.entity.Staff</class>
      <class>com.tutorialspoint.eclipselink.entity.NonTeachingStaff</class>
      <class>com.tutorialspoint.eclipselink.entity.TeachingStaff</class>
      
      <properties>
         <property name="javax.persistence.jdbc.url" value="jdbc:mysql://: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 ‘src’ 包下创建一个名为 ‘com.tutorialspoint.eclipselink.service’ 的包。

在给定包下创建一个名为 SaveClient.java 的类，以存储 Staff、TeachingStaff 和 NonTeachingStaff 类的字段。SaveClient 类如下所示

package com.tutorialspoint.eclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.tutorialspoint.eclipselink.entity.NonTeachingStaff;
import com.tutorialspoint.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();
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。检查 MySQL Workbench 以获取输出。输出以表格格式显示如下

Sid 类型 Sname 专业领域 资格 学科专业
1 TS Gopal MSC MED 数学
2 TS Manisha BSC BED 英语
3 NS Satish 会计
4 NS Krishna 办公室行政

最后，您将获得一个包含所有三个类字段的单表，并由名为 ‘Type’（字段）的鉴别器列区分。

连接表策略

连接表策略是共享包含唯一值的引用列，以连接表并简化事务。让我们考虑与上面相同的示例。

创建一个 JPA 项目。所有项目模块如下所示

创建实体

在 ‘src’ 包下创建一个名为 ‘com.tutorialspoint.eclipselink.entity’ 的包。在给定包下创建一个名为 Staff.java 的新 Java 类。Staff 实体类如下所示

package com.tutorialspoint.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;
   }
}

在 com.tutorialspoint.eclipselink.entity 包下，创建一个名为 TeachingStaff.java 的 Staff 类的子类（类）。TeachingStaff 实体类如下所示

package com.tutorialspoint.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;
   }
}

在 com.tutorialspoint.eclipselink.entity 包下，创建一个名为 NonTeachingStaff.java 的 Staff 类的子类（类）。NonTeachingStaff 实体类如下所示

package com.tutorialspoint.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 文件包含数据库配置信息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.tutorialspoint.eclipselink.entity.Staff</class>
      <class>com.tutorialspoint.eclipselink.entity.NonTeachingStaff</class>
      <class>com.tutorialspoint.eclipselink.entity.TeachingStaff</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://: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 ‘src’ 包下创建一个名为 ‘com.tutorialspoint.eclipselink.service’ 的包。

在给定包下创建一个名为 SaveClient.java 的类，以存储 Staff、TeachingStaff 和 NonTeachingStaff 类的字段。然后 SaveClient 类如下所示

package com.tutorialspoint.eclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.tutorialspoint.eclipselink.entity.NonTeachingStaff;
import com.tutorialspoint.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();
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。要查看输出，请检查 MySQL Workbench，如下所示

这里创建了三个表，staff 表的结果以表格格式显示如下

Sid Dtype Sname
1 TeachingStaff Gopal
2 TeachingStaff Manisha
3 NonTeachingStaff Satish
4 NonTeachingStaff Krishna

TeachingStaff 表的结果以表格格式显示如下

Sid 资格 学科专业
1 MSC MED 数学
2 BSC BED 英语

在上表中，sid 是外键（来自 staff 表的引用字段）。NonTeachingStaff 表的结果以表格格式显示如下

Sid 专业领域
3 会计
4 办公室行政

最后，使用各自的字段创建了三个表，并且 SID 字段由所有三个表共享。在 staff 表中，SID 是主键，在其余的（TeachingStaff 和 NonTeachingStaff）表中，SID 是外键。

每个类表策略

每个类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

在 ‘src’ 包下创建一个名为 ‘com.tutorialspoint.eclipselink.entity’ 的包。在给定包下创建一个名为 Staff.java 的新 Java 类。Staff 实体类如下所示

package com.tutorialspoint.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;
   }
}

在 com.tutorialspoint.eclipselink.entity 包下，创建一个名为 TeachingStaff.java 的 Staff 类的子类（类）。TeachingStaff 实体类如下所示

package com.tutorialspoint.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;
   }
}

在 com.tutorialspoint.eclipselink.entity 包下，创建一个名为 NonTeachingStaff.java 的 Staff 类的子类（类）。NonTeachingStaff 实体类如下所示

package com.tutorialspoint.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 文件包含数据库配置信息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.tutorialspoint.eclipselink.entity.Staff</class>
      <class>com.tutorialspoint.eclipselink.entity.NonTeachingStaff</class>
      <class>com.tutorialspoint.eclipselink.entity.TeachingStaff</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://: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 ‘src’ 包下创建一个名为 ‘com.tutorialspoint.eclipselink.service’ 的包。

在给定包下创建一个名为 SaveClient.java 的类，以存储 Staff、TeachingStaff 和 NonTeachingStaff 类的字段。SaveClient 类如下所示

package com.tutorialspoint.eclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.tutorialspoint.eclipselink.entity.NonTeachingStaff;
import com.tutorialspoint.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();
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。要查看输出，请检查 MySQL Workbench，如下所示

这里创建了三个表，Staff 表包含空记录。

TeachingStaff 的结果以表格格式显示如下

Sid 资格 Sname 学科专业
1 MSC MED Gopal 数学
2 BSC BED Manisha 英语

上表 TeachingStaff 包含 Staff 和 TeachingStaff 实体的字段。

NonTeachingStaff 的结果以表格格式显示如下

Sid 专业领域 Sname
3 会计 Satish
4 办公室行政 Krishna

上表 NonTeachingStaff 包含 Staff 和 NonTeachingStaff 实体的字段。

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