jpa继承映射

继承映射在 Annotation 中使用 @Inheritance 注解，并且需要使用 strategy 属性指定继承策略，继承策略有 SINGLE_TABLE、TABLE_PER_CLASS 和 JOINED 三种。

一、SINGLE_TABLE

SINGLE_TABLE 是将父类和其所有的子类集合在一块，存在一张表中，并创建一个新的字段来判断对象的类型。

Person.java：

   1:  @Entity

   2:  @Inheritance(strategy=InheritanceType.SINGLE_TABLE)

   3:  @DiscriminatorColumn(name="discriminator",discriminatorType=DiscriminatorType.STRING)

   4:  @DiscriminatorValue("person")

   5:  public class Person {

   6:      @Id

   7:      @GeneratedValue

   8:      private int id;

   9:      private String name;

  10:  //Getters and setters omitted here...

  11:  }

@Inheritance 的 strategy 属性是指定继承关系的生成策略，@DiscriminatorColumn 注解作用是指定生成的新的判断对象类型的字段的名称和类型，@DiscriminatorValue 注解是确定此类（Person）的标示，即 DiscriminatorColumn 的值。

Student.java：

   1:  @Entity

   2:  @DiscriminatorValue("student")

   3:  public class Student extends Person{

   4:      private int score;

   5:  //Getters and setters omitted here...

   6:  }

Teacher.java：

   1:  @Entity

   2:  @DiscriminatorValue("teacher")

   3:  public class Teacher extends Person{

   4:      private String title;

   5:  //Getters and setters omitted here...

   6:  }

生成的数据库表如下：

存入数据后结构如下：

二、TABLE_PER_CLASS

TABLE_PER_CLASS 是为每一个类创建一个表，这些表是相互独立的。

Person.java：

   1:  @Entity

   2:  @Inheritance(strategy=InheritanceType.TABLE_PER_CLASS)

   3:  public class Person {

   4:      @Id

   5:      private int id;

   6:      private String name;

   7:  //Getters and setters omitted here...

   8:  }

Student.java：

   1:  @Entity

   2:  public class Student extends Person{

   3:      private int score;

   4:  //Getters and setters omitted here...

   5:  }

Teacher.java：

   1:  @Entity

   2:  public class Teacher extends Person{

   3:      private String title;

   4:  //Getters and setters omitted here...

   5:  }

创建的数据库表如下：

三、JOINED

JOINED 是将父类、子类分别存放在不同的表中，并且建立相应的外键，以确定相互之间的关系。

Person.java：

   1:  @Entity

   2:  @Inheritance(strategy=InheritanceType.JOINED)

   3:  public class Person {

   4:      @Id

   5:      @GeneratedValue

   6:      private int id;

   7:      private String name;

   8:  //Getters and setters omitted here...

   9:  }

子类中只需声明与父类不同的域即可：

Student.java：

   1:  @Entity

   2:  public class Student extends Person{

   3:      private int coat;

   4:  //Getters and setters omitted here...

   5:  }

Teacher.java 类似。

生成的数据库表如下：

上部分为转载的地址

https://blog.csdn.net/benjamin_whx/article/details/44521315

现在通过一个（join）实例说明

package com.example.demo.entity;

import org.hibernate.annotations.DynamicInsert;
import org.hibernate.annotations.DynamicUpdate;

import javax.persistence.*;
import java.io.Serializable;
@Entity
@Table(name = "document")
@Inheritance(strategy = InheritanceType.JOINED)
@DynamicInsert
@DynamicUpdate
public class Document implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String fullName;
    private String path;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getFullName() {
        return fullName;
    }

    public void setFullName(String fullName) {
        this.fullName = fullName;
    }

    public String getPath() {
        return path;
    }

    public void setPath(String path) {
        this.path = path;
    }

}

package com.example.demo.entity;

import org.hibernate.annotations.DynamicInsert;
import org.hibernate.annotations.DynamicUpdate;

import javax.persistence.*;

@Entity
@Table(name = "image")
@PrimaryKeyJoinColumn(name = "ig_id")
@DynamicInsert
@DynamicUpdate
public class Image extends Document{

    @OneToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "st_id")
    private Student student;

    public Student getStudent() {
        return student;
    }

    public void setStudent(Student student) {
        this.student = student;
    }
}

生成的数据表

实现保存

public void saveImage(){
        Student student = new Student();
        student.setName("iuiuiuu");
        student.setBirthday(new Date());
        studentRepository.save(student);
        Image image = new Image();
        image.setStudent(student);
        image.setFullName("image.png");
        image.setPath("d://image");
        imageRepository.save(image);

    }

学生表

文件表

image表

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