JPA Advanced Mappings(映射)

JPA是一个使用java规范发布的库。因此，它支持所有面向对象的实体持久性概念。

原文链接：http://blogxinxiucan.sh1.newtouch.com/2017/07/30/JPA-Advanced-Mappings-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，如SINGLE_TABLE，JOINED_TABLE和TABLE_PER_CONCRETE_CLASS。

Staff, TeachingStaff, NonTeachingStaff关系：

在上图中，Staff是一个实体，TeachingStaff和NonTeachingStaff是员工的子实体。在这里我们将讨论上述三个继承的策略。

单表策略

单表策略采用所有类字段（超类和子类），并将其映射到称为SINGLE_TABLE策略的单个表中。鉴别器值在区分一个表中三个实体的值时起关键作用。

让我们考虑上面的例子，TeachingStaff和NonTeachingStaff是类员工的子类。提醒继承的概念（是通过子类继承超类的属性的机制），因此sid，sname是属于TeachingStaff和NonTeachingStaff的字段。创建一个JPA项目。本项目的所有模块如下：

创建实体

在“src”包下创建一个名为“com.tutorialspoint.eclipselink.entity” 的包。在给定的包下创建一个名为Staff.java的新Java类。Staff实体类显示如下：

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;

   }

}

在上述代码中，@DescriminatorColumn指定字段名称（type），其值显示剩余的（Teaching and NonTeachingStaff）字段。

在com.tutorialspoint.eclipselink.entity包下创建一个名为TeachingStaff.java的 Staff类的子类（class）。TeachingStaff实体类显示如下：

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（class）。NonTeachingStaff实体类显示如下：

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中

Persistence.xml文件包含实体类的数据库和注册信息的配置信息。xml文件显示如下：

<?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://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>

Service class

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

在给定的包下创建一个名为SaveClient.java的类来存储Staff，TeachingStaff和NonTeachingStaff类字段。SaveClient类显示如下：

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();

   }

}

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

Sid Type Sname Areaexpertise Qualification Subjectexpertise
1 TS Gopal - MSC MED Maths
2 TS Manisha - BSC BED English
3 NS Satish Accounts
4 NS Krishna Office Admin

最后，您将获得包含所有三类字段的单表，并与名为“Type”（字段）的discriminator列不同。

加盟表策略

加入表策略是共享引用的列，其中包含唯一值以加入表并进行简单的事务。让我们考虑与上述相同的例子。

创建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。在给定的包下创建一个名为Staff.java的新Java类。Staff实体类显示如下：

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（class）。TeachingStaff实体类显示如下：

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（class）。NonTeachingStaff实体类显示如下：

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中

Persistence.xml文件包含实体类的数据库和注册信息的配置信息。xml文件显示如下：

<?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://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.tutorialspoint.eclipselink.service'的'src'包下创建一个包。

在给定的包下创建一个名为SaveClient.java的类来存储Staff，TeachingStaff和NonTeachingStaff类字段。然后SaveClient类如下：

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();

   }

}

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

这里创建了三个表格，表格格式的员工表格的结果如下所示：

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

TeachingStaff表的结果如表格所示：

Sid Qualification Subjectexpertise
1 MSC MED Maths
2 BSC BED English

在上表中sid是外键（参考字段表单staff表）NonTeachingStaff表的结果如表格所示：

Sid Areaexpertise
3 Accounts
4 Office Admin

最后，这三个表分别使用它们的字段创建，SID字段由所有三个表共享。在员工表SID中是主键，在剩余的（TeachingStaff和NonTeachingStaff）表中SID是外键。

每个类策略表

每个类策略的表是为每个子实体创建一个表。工作人员表将被创建，但它将包含空记录。Staff表的字段值必须由TeachingStaff和NonTeachingStaff表共享。

让我们考虑与上述相同的例子。本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。在给定的包下创建一个名为Staff.java的新Java类。Staff实体类显示如下：

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（class）。TeachingStaff实体类显示如下：

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（class）。NonTeachingStaff实体类显示如下：

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中

Persistence.xml文件包含实体类的数据库和注册信息的配置信息。xml文件显示如下：

<?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://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.tutorialspoint.eclipselink.service'的'src'包下创建一个包。

在给定的包下创建一个名为SaveClient.java的类来存储Staff，TeachingStaff和NonTeachingStaff类字段。SaveClient类显示如下：

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();

   }

}

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

这里创建了三个表，Staff表包含空记录。

TeachingStaff的结果如表格所示：

Sid Qualification Sname Subjectexpertise
1 MSC MED Gopal Maths
2 BSC BED Manisha English

上表TeachingStaff包含员工和教学实体的领域。

NonTeachingStaff的结果如表格所示：

Sid Areaexpertise Sname
3 Accounts Satish
4 Office Admin Krishna

以上表格NonTeachingStaff包含Staff和NonTeachingStaff实体的字段。

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