聯合子類策略

這種情況下子類的字段被映射到各自的表中，這些字段包括父類中的字段，并執行一個join操作來實例化子類。

舉例

如果實體類Teacher繼承實體類Person，實體類Student也繼承自實體Person，
會映射成3個表，子表不包含父表的屬性，子表只有子類的屬性，子表主鍵外鍵關聯父表的id

這種策略超類會被映射成一個單獨的表，每個子類也會映射成一個單獨的表。子類對應的表中只包括自身屬性對應的字段，默認情況下使用主鍵作為超類對應的表的外鍵。
這種策略對于實體間的多態關系提供了很好的支持。但缺點是實例化子類實例時需要一個或多個表的關聯操作。在深層次的繼承結構中，這會導致性能很低。

配置

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

ddl語句

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:父類
 */
@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:子類
 */
@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:子類
 */
@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:繼承測試
 */
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("倉老師");
    teacher.setAddress("北京");

    Student student = new Student();
    student.setName("機械師");
    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();
  }
}

其他關聯項目

JPA 系列教程15-繼承-一個表-SINGLE_TABLE
http://blog.csdn.net/je_ge/article/details/53678422

源碼地址

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

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