JPA - 高级映射
JPA是一个以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 SINGLE_TABLE、JOINED_TABLE 和 TABLE_PER_CONCRETE_CLASS 三种继承策略。
让我们考虑一个 Staff、TeachingStaff、NonTeachingStaff 类及其关系的示例,如下所示:
![继承策略](/jpa/images/inheritance_strategy.png)
在上图中,Staff 是一个实体,TeachingStaff 和 NonTeachingStaff 是 Staff 的子实体。 这里我们将讨论上面例子中的所有三种继承策略。
SINGLE_TABLE 单表策略
单表策略采用所有类字段(超类和子类)并将它们映射到称为 SINGLE_TABLE 策略的单个表中。 在这里,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。 提醒一下继承的概念(是子类继承超类属性的机制),因此 sid、sname 是同时属于TeachingStaff 和NonTeachingStaff 的字段。 创建 JPA 项目。 本项目各模块如下:
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,@DecriminatorColumn 指定字段名称(类型),其值显示其余(Teaching 和 NonTeachingStaff)字段。
在 com.tutorialspoint.eclipselink.entity 包下创建名为 TeachingStaff.java 的 Staff 类的子类(类)。 TeaStaff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.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 类
service 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 的类来存储 Staff、TeachingStaff 和 NonTeachingStaff 类字段。 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 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,您将得到一个表,其中包含所有三个类的字段,并且与名为 "Type"(字段)的鉴别器列不同。
JOINED_TABLE 联表策略
联表策略是共享包含唯一值的引用列来联表并轻松进行事务。 让我们考虑与上面相同的示例。
创建 JPA 项目。 全部项目模块如下所示:
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 包下创建名为 TeachingStaff.java 的 Staff 类的子类(类)。 TeaStaff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.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 类
Service 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,staff表以表格形式显示的结果如下:
Sid | Dtype | Sname |
---|---|---|
1 | TeachingStaff | Gopal |
2 | TeachingStaff | Manisha |
3 | NonTeachingStaff | Satish |
4 | NonTeachingStaff | Krishna |
表格格式的TeachingStaff表结果如下:
Sid | Qualification | Subjectexpertise |
---|---|---|
1 | MSC MED | Maths |
2 | BSC BED | English |
上表中的sid为外键(参考staff表的字段)NonTeachingStaff表以表格形式显示的结果如下:
Sid | Areaexpertise |
---|---|
3 | Accounts |
4 | Office Admin |
最后,分别使用它们的字段创建三个表,并且 SID 字段由所有三个表共享。 在职员表中,SID 是主键,在其余(TeachingStaff 和 NonTeachingStaff)表中,SID 是外键。
TABLE_PER_CONCRETE_CLASS 每类表策略
每类表策略是为每个子实体创建一个表。 将创建职员表,但它将包含空记录。 Staff表的字段值必须被TeachingStaff和NonTeachingStaff表共享。
让我们考虑与上面相同的示例。 本项目各模块如下所示:
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 包下创建名为 TeachingStaff.java 的 Staff 类的子类(类)。 TeaStaff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.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 类
Service 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 的类来存储 Staff、TeachingStaff 和 NonTeachingStaff 类字段。 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。 对于输出,请检查 MySQL 工作台,如下所示:
此处创建了三个表,Staff 表包含空记录。
TeachingStaff的结果以表格形式显示如下:
Sid | Qualification | Sname | Subjectexpertise |
---|---|---|---|
1 | MSC MED | Gopal | Maths |
2 | BSC BED | Manisha | English |
上表TeachingStaff包含Staff和TeachingStaff实体的字段。
NonTeachingStaff的结果以表格形式显示如下:
Sid | Areaexpertise | Sname |
---|---|---|
3 | Accounts | Satish |
4 | Office Admin | Krishna |
上表 NonTeachingStaff 包含 Staff 和 NonTeachingStaff 实体的字段。