JPA继承关系详解(1)

   这篇文章,考虑很多因素,有的地方比较详细,但配置这些并未涉及…

 

首先,我们需要了解为什么需要使用JPA继承关系!我们使用2简单的对象来阐述!学生,老师!

学生:姓名,年龄,性别,学习

老师:姓名,年龄,性别,教学

从上面2个类我们可以了解,学生与老师的3个属性是相同的,不同之处只有一处就是,学习与教学的区别!

传统方式在不使用继承关系的情况下,创建学生与老师的pojo需要这样…

package cn.vicky.test01; import java.io.Serializable; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.EnumType; import javax.persistence.Enumerated; import javax.persistence.GeneratedValue; import javax.persistence.GenerationType; import javax.persistence.Id; @Entity public class Student implements Serializable { private static final long serialVersionUID = 1L; // parameter @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id; @Column(length = 20, unique = true, nullable = false) private String name; @Enumerated(EnumType.STRING) @Column(length=5,nullable=false) private Sex sex = Sex.MAN; @Column(length = 20, nullable = false) private String study = "Java"; // getter & setter   

Teacher.java

package cn.vicky.test01; import java.io.Serializable; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.EnumType; import javax.persistence.Enumerated; import javax.persistence.GeneratedValue; import javax.persistence.GenerationType; import javax.persistence.Id; @Entity public class Teacher implements Serializable{ private static final long serialVersionUID = 1L; // parameter @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer id; @Column(length = 20, unique = true, nullable = false) private String name; @Enumerated(EnumType.STRING) @Column(length = 5, nullable = false) private Sex sex = Sex.MAN; @Column(length = 20, nullable = false) private String teach = "大学日语"; // getter & setter

编写Junit4单元测试类:JPATest.java

package cn.vicky.test01; import javax.persistence.EntityManagerFactory; import javax.persistence.Persistence; import org.apache.log4j.Logger; import org.junit.AfterClass; import org.junit.Assert; import org.junit.BeforeClass; import org.junit.Test; public class JPATest { private static final Logger logger = Logger.getLogger(JPATest.class.getName()); private static EntityManagerFactory factory; @BeforeClass public static void setUpBeforeClass() throws Exception { factory = Persistence.createEntityManagerFactory("vicky"); } @Test public void testInit(){ try { Assert.assertNotNull(factory); } catch (Exception e) { logger.error(e); } } @AfterClass public static void tearDownAfterClass() throws Exception { factory.close(); } }

执行:查看生成的数据库:

mysql> desc student;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | NO   | UNI | NULL    |                |

| sex   | varchar(5)  | NO   |     | NULL    |                |

| study | varchar(20) | NO   |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

4 rows in set (0.00 sec)

 

mysql> desc teacher;

+-------+-------------+------+-----+---------+----------------+

| Field | Type        | Null | Key | Default | Extra          |

+-------+-------------+------+-----+---------+----------------+

| id    | int(11)     | NO   | PRI | NULL    | auto_increment |

| name  | varchar(20) | NO   | UNI | NULL    |                |

| sex   | varchar(5)  | NO   |     | NULL    |                |

| teach | varchar(20) | NO   |     | NULL    |                |

+-------+-------------+------+-----+---------+----------------+

4 rows in set (0.00 sec)