JPA一對一映射

一對一映射映射表示一個單值關聯,其中一個實體的實例與另一個實體的實例相關聯。在這種類型的關聯中,源實體的一個實例可以映射到目標實體的一個實例。

完整的項目目錄結構如下所示 -
JPA一對一映射

@OneToOne示例

在這個例子中,我們將創建一個學生和圖書庫之間的一對一關係,這樣一個學生只能發放一種類型的圖書。

這個例子包含以下步驟 -

第1步:com.yiibai.jpa包下創建一個實體類Employee.java,在com.yiibai.mapping包下創建一個實體類Student.java,含學生ID(s_id)和學生姓名(s_name)。

文件:Student.java 的代碼如下所示 -

package com.yiibai.mapping;

import javax.persistence.*;

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int s_id;
    private String s_name;

    public int getS_id() {
        return s_id;
    }

    public void setS_id(int s_id) {
        this.s_id = s_id;
    }

    public String getS_name() {
        return s_name;
    }

    public void setS_name(String s_name) {
        this.s_name = s_name;
    }

}

第2步:com.yiibai.mapping包下創建另一個實體類Library.java,其中包含book id(b_id),book name(b_name)以及標記有[@OneToOne](https://github.com/OneToOne "@OneToOne")註釋的學生類型對象。

文件:Library.java 的代碼如下所示 -

package com.yiibai.mapping;

import javax.persistence.*;

@Entity
public class Library {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int b_id;
    private String b_name;

    @OneToOne
    private Student stud;

    public Library(int b_id, String b_name, Student stud) {
        super();
        this.b_id = b_id;
        this.b_name = b_name;
        this.stud = stud;
    }

    public Library() {
        super();

    }

    public int getB_id() {
        return b_id;
    }

    public void setB_id(int b_id) {
        this.b_id = b_id;
    }

    public String getB_name() {
        return b_name;
    }

    public void setB_name(String b_name) {
        this.b_name = b_name;
    }

    public Student getStud() {
        return stud;
    }

    public void setStud(Student stud) {
        this.stud = stud;
    }

}

第3步: 將實體類和其他數據庫配置映射到persistence.xml文件中。

文件:persistence.xml 的代碼如下所示 -

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1"
    xmlns="http://xmlns.jcp.org/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
    <persistence-unit name="Book_issued">
        <class>com.yiibai.mapping.Student</class>
        <class>com.yiibai.mapping.Library</class>
        <properties>
            <property name="javax.persistence.jdbc.driver"
                value="com.mysql.jdbc.Driver" />
            <property name="javax.persistence.jdbc.url"
                value="jdbc:mysql://localhost:3306/testdb?serverTimezone=UTC" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password"
                value="123456" />
            <property name="eclipselink.logging.level" value="SEVERE" />
            <property name="eclipselink.ddl-generation"
                value="create-or-extend-tables" />
        </properties>
    </persistence-unit>

</persistence>

com.yiibai.onetoone包下創建一個持久化類OneToOneExample,用於將實體對象與數據保持一致。

文件:OneToOneExample.java 的代碼如下所示 -

package com.yiibai.mapping;

import javax.persistence.*;  
import com.yiibai.mapping.*;

public class OneToOneExample {

    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("Book_issued");
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();

        Student st1 = new Student();
        st1.setS_id(1);
        st1.setS_name("Maxsu");

        Student st2 = new Student();
        st2.setS_id(2);
        st2.setS_name("James");

        em.persist(st1);
        em.persist(st2);

        Library lib1 = new Library();
        lib1.setB_id(101);
        lib1.setB_name("Data Structure");
        lib1.setStud(st1);

        Library lib2 = new Library();
        lib2.setB_id(102);
        lib2.setB_name("DBMS");
        lib2.setStud(st2);

        em.persist(lib1);
        em.persist(lib2);

        em.getTransaction().commit();

        em.close();
        emf.close();
    }

}

程序輸出結果

程序執行完成後,下面的表格在MySQL工作臺下生成。

  • Student表 - 此表包含學生詳細信息。要獲取數據,請在MySQL中運行select * from student查詢。結果如下所示 -
mysql> select * from student;
+------+-------+--------+
| S_ID | S_AGE | S_NAME |
+------+-------+--------+
|    2 | NULL  | James  |
|    1 | NULL  | Maxsu  |
+------+-------+--------+
2 rows in set
  • Library表 - 這個表格代表學生和圖書庫之間的映射。 要獲取數據,請在MySQL中運行select * from library查詢。結果如下所示 -
mysql> select * from library;
+------+----------------+-----------+
| B_ID | B_NAME         | STUD_S_ID |
+------+----------------+-----------+
|  102 | DBMS           |         2 |
|  101 | Data Structure |         1 |
+------+----------------+-----------+
2 rows in set
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