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;@Entitypublic 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;@Entitypublic 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;@BeforeClasspublic static void setUpBeforeClass() throws Exception {factory = Persistence.createEntityManagerFactory("vicky");}@Testpublic void testInit(){try {Assert.assertNotNull(factory);} catch (Exception e) {logger.error(e);}}@AfterClasspublic 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)