JPA - 高级映射

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:

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:

            com.IT屋.eclipselink.entity.Staff      com.IT屋.eclipselink.entity.NonTeachingStaff      com.IT屋.eclipselink.entity.TeachingStaff                                                                                 

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:

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:

            com.IT屋.eclipselink.entity.Staff      com.IT屋.eclipselink.entity.NonTeachingStaff      com.IT屋.eclipselink.entity.TeachingStaff                                                                                 

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:

表格中 TeachingStaff 表的结果格式如下所示:

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:

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;@Entitypublic 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;@Entitypublic 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:

         com.IT屋.eclipselink.entity.Staff      com.IT屋.eclipselink.entity.NonTeachingStaff      com.IT屋.eclipselink.entity.TeachingStaff                                                                                 

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:

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:

上表NonTeachingStaff包含Staff和NonTeachingStaff实体的字段.