JPA 系列教程16-继承-联合子类-JOINED

2023-07-30 10:22:46

联合子类策略

这种情况下子类的字段被映射到各自的表中，这些字段包括父类中的字段，并执行一个join操作来实例化子类。

举例

如果实体类Teacher继承实体类Person，实体类Student也继承自实体Person，

会映射成3个表，子表不包含父表的属性，子表只有子类的属性，子表主键外键关联父表的id

这种策略超类会被映射成一个单独的表，每个子类也会映射成一个单独的表。子类对应的表中只包括自身属性对应的字段，默认情况下使用主键作为超类对应的表的外键。

这种策略对于实体间的多态关系提供了很好的支持。但缺点是实例化子类实例时需要一个或多个表的关联操作。在深层次的继承结构中，这会导致性能很低。

配置

注解为：@Inheritance(strategy = InheritanceType.JOINED)

ddl语句

CREATE TABLE `t_person` (

  `type` int(11) NOT NULL,

  `id` bigint(20) NOT NULL AUTO_INCREMENT,

  `name` varchar(255) DEFAULT NULL,

  PRIMARY KEY (`id`)

) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

CREATE TABLE `t_teacher` (

  `address` varchar(255) DEFAULT NULL,

  `id` bigint(20) NOT NULL,

  PRIMARY KEY (`id`),

  CONSTRAINT `FK_8ik5mbfy94c9cdklxfo2wqgg9` FOREIGN KEY (`id`) REFERENCES `t_person` (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `t_student` (

  `school` varchar(255) DEFAULT NULL,

  `id` bigint(20) NOT NULL,

  PRIMARY KEY (`id`),

  CONSTRAINT `FK_bdp2c9ntgnbd0ydfj1fwqa7l2` FOREIGN KEY (`id`) REFERENCES `t_person` (`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Person

package com.jege.jpa.extend;

import javax.persistence.DiscriminatorColumn;

import javax.persistence.DiscriminatorType;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.Inheritance;

import javax.persistence.InheritanceType;

import javax.persistence.Table;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:父类

 */

@Entity

@Table(name = "t_person")

@Inheritance(strategy = InheritanceType.JOINED)

@DiscriminatorColumn(name = "type", discriminatorType = DiscriminatorType.INTEGER)

public class Person {

  @Id

  @GeneratedValue

  private Long id;

  private String name;

  public Long getId() {

    return id;

  }

  public void setId(Long id) {

    this.id = id;

  }

  public String getName() {

    return name;

  }

  public void setName(String name) {

    this.name = name;

  }

}

Teacher

package com.jege.jpa.extend;

import javax.persistence.Entity;

import javax.persistence.Table;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:子类

 */

@Entity

@Table(name = "t_teacher")

public class Teacher extends Person {

  private String address;

  public String getAddress() {

    return address;

  }

  public void setAddress(String address) {

    this.address = address;

  }

}

Student

package com.jege.jpa.extend;

import javax.persistence.Entity;

import javax.persistence.Table;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:子类

 */

@Entity

@Table(name = "t_student")

public class Student extends Person {

  private String school;

  public String getSchool() {

    return school;

  }

  public void setSchool(String school) {

    this.school = school;

  }

}

MainTest

package com.jege.jpa.extend;

import javax.persistence.EntityManager;

import javax.persistence.EntityManagerFactory;

import javax.persistence.Persistence;

import org.junit.After;

import org.junit.AfterClass;

import org.junit.Before;

import org.junit.BeforeClass;

import org.junit.Test;

/**

 * @author JE哥

 * @email 1272434821@qq.com

 * @description:继承测试

 */

public class MainTest {

  private static EntityManagerFactory entityManagerFactory = null;

  private EntityManager entityManager = null;

  @BeforeClass

  public static void setUpBeforeClass() throws Exception {

    entityManagerFactory = Persistence.createEntityManagerFactory("com.jege.jpa");

  }

  @Before

  public void setUp() throws Exception {

    entityManager = entityManagerFactory.createEntityManager();

  }

  @Test

  public void persist() {

    Person person = new Person();

    person.setName("jege");

    Teacher teacher = new Teacher();

    teacher.setName("仓老师");

    teacher.setAddress("北京");

    Student student = new Student();

    student.setName("机械师");

    student.setSchool("上海");

    entityManager.getTransaction().begin();

    entityManager.persist(student);

    entityManager.persist(teacher);

    entityManager.persist(person);

    entityManager.getTransaction().commit();

  }

  @Test

  public void find() {

    persist();

    entityManager.clear();

    Student student = entityManager.find(Student.class, 1L);

    System.out.println(student.getSchool());

  }

  @After

  public void tearDown() throws Exception {

    if (entityManager != null && entityManager.isOpen())

      entityManager.close();

  }

  @AfterClass

  public static void tearDownAfterClass() throws Exception {

    if (entityManagerFactory != null && entityManagerFactory.isOpen())

      entityManagerFactory.close();

  }

}

其他关联项目

JPA 系列教程15-继承-一个表-SINGLE_TABLE

http://blog.csdn.net/je_ge/article/details/53678422

源码地址

https://github.com/je-ge/jpa

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