JPA高级映射

JPA是一个随Java规范发布的库。因此，它支持所有面向对象的实体持久化概念。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类及其关系如下：

传承战略

在上面的图表中，员工是一个实体，TeachingStaff和NonTeachingStaff是员工的子实体。这里我们将在所有三种继承策略中讨论上面的例子。

单桌战略

单表战略采用所有类字段（超类和子类）并将它们映射到一个称为SINGLE_TABLE策略的表中。这里，鉴别器价值在区分一个表中三个实体的价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。提醒继承的概念（是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

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

package com.CodingDict.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 指定字段名称 （类型） ，它的值显示剩余（Teaching和NonTeachingStaff）字段。

在 com.CodingDict.eclipselink.entity 包下创建一个名为 TeachingStaff.java的 Staff类的子类（class）。TeachingStaff实体类如下所示： **

package com. CodingDict.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.CodingDict.eclipselink.entity 包下创建一个名为 NonTeachingStaff.java的 Staff类的子类（class）。NonTeachingStaff实体类如下所示： **

package com. CodingDict.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文件包含数据库的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.CodingDict.eclipselink.entity.Staff</class>
      <class>com.CodingDict.eclipselink.entity.NonTeachingStaff</class>
      <class>com.CodingDict.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.CodingDict.eclipselink.service'的 'src' 包下创建一个包。 **

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

package com.CodingDict.eclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.CodingDict.eclipselink.entity.NonTeachingStaff;
import com.CodingDict.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工作台的输出。表格格式的输出显示如下：

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

最后，你将得到单个表，其中包含所有三个类的字段，并且与名为 '类型' （字段）的鉴别器列不同。

加入桌面策略

联合表策略是共享包含唯一值的引用列以加入表并轻松实现交易。让我们考虑一下上面的例子。

创建一个JPA项目。所有的项目模块如下所示：

创建实体

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

package com.CodingDict.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.CodingDict.eclipselink.entity 包下创建一个名为 TeachingStaff.java的 Staff类的子类（class）。TeachingStaff实体类如下所示： **

package com. CodingDict.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.CodingDict.eclipselink.entity 包下创建一个名为 NonTeachingStaff.java的 Staff类的子类（class）。NonTeachingStaff实体类如下所示： **

package com. CodingDict.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文件包含数据库的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.CodingDict.eclipselink.entity.Staff</class>
      <class>com.CodingDict.eclipselink.entity.NonTeachingStaff</class>
      <class>com.CodingDict.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.CodingDict.eclipselink.service'的 'src' 包下创建一个包。 **

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

package com.CodingDict.eclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.CodingDict.eclipselink.entity.NonTeachingStaff;
import com.CodingDict.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。对于输出检查MySQL工作台，如下所示：

这里创建了三个表格，表格格式的 工作人员 表格的结果如下所示：

希德	D型	SNAME
1	教学人员	戈帕尔
2	教学人员	MANISHA
3	NonTeachingStaff	萨蒂什
4	NonTeachingStaff	克里希纳

以表格格式显示 TeachingStaff 表的结果如下所示：

希德	合格	Subjectexpertise
1	MSC MED	数学
2	BSC BED	英语

在上表中sid是外键（参考字段形式staff表）表格格式的 NonTeachingStaff 表的结果如下所示：

希德	Areaexpertise
3	帐号
4	Office Admin

最后，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。在员工表中，SID是主键，其余（TeachingStaff和NonTeachingStaff）表中的SID是外键。

表每类战略

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

让我们考虑一下上面的例子。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.CodingDict.eclipselink.entity' 的包。在给定的包下创建一个名为 Staff.java 的新Java类。职员实体类显示如下：

package com.CodingDict.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.CodingDict.eclipselink.entity 包下创建一个名为 TeachingStaff.java的 Staff类的子类（class）。TeachingStaff实体类如下所示： **

package com. CodingDict.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.CodingDict.eclipselink.entity 包下创建一个名为 NonTeachingStaff.java的 Staff类的子类（class）。NonTeachingStaff实体类如下所示： **

package com. CodingDict.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文件包含数据库的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.CodingDict.eclipselink.entity.Staff</class>
      <class>com.CodingDict.eclipselink.entity.NonTeachingStaff</class>
      <class>com.CodingDict.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.CodingDict.eclipselink.service'的 'src' 包下创建一个包。 **

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

package com.CodingDict.eclipselink.service;

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

import com.CodingDict.eclipselink.entity.NonTeachingStaff;
import com.CodingDict.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。对于输出，请按照以下方式检查MySQL工作台：

这里创建了三个表并且 Staff 表包含空记录。

以表格格式显示 TeachingStaff 的结果如下所示：

希德	合格	SNAME	Subjectexpertise
1	MSC MED	戈帕尔	数学
2	BSC BED	MANISHA	英语

上表TeachingStaff包含Staff和TeachingStaff实体的字段。

的结果 NonTeachingStaff 以表格格式如下所示：

希德	Areaexpertise	SNAME
3	帐号	萨蒂什
4	Office Admin	克里希纳

上表NonTeachingStaff包含Staff和NonTeachingStaff实体的字段。