Hibernate的單向一對多
阿新 • • 發佈：2022-05-05
單向一對多會多執行update語句，效率不如多對一。
一、學生表和年級表
Student表
| id | name | age | grade_id |
|---|---|---|---|
| 1 | 張三 | 20 | 1 |
| 2 | 李四 | 22 | 1 |
| 3 | 王五 | 22 | 2 |
Grade表
| id | name |
|---|---|
| 1 | 基礎 |
| 2 | 中級 |
二、Student類和Grade類
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先建立一個Student類
public class Student implements Serializable { private Integer id; private String name; private Integer age; // private Grade grade; @Override public String toString() { return "Student{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + //", grade=" + grade + '}'; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Integer getAge() { return age; } public void setAge(Integer age) { this.age = age; } // public Grade getGrade() { // return grade; // } // // public void setGrade(Grade grade) { // this.grade = grade; // } }
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建立一個Grade類，用的是set集合。
public class Grade implements Serializable { private Integer id; private String name; private Set<Student> students = new HashSet<>(); @Override public String toString() { return "Grade{" + "id=" + id + ", name='" + name + '\'' + ", students=" + students + '}'; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Set<Student> getStudents() { return students; } public void setStudents(Set<Student> students) { this.students = students; } }
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建立Student.hbm.xml
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.uestc"> <class name="Student"> <id name="id"> <generator class="native"/> </id> <property name="name"/> <property name="age"/> <!-- 多對一 name對應的欄位名稱 class對應的型別 not-null不可以為空 column外來鍵列名 foreign-key外來鍵名稱 --> <!-- <many-to-one name="grade" class="Grade" not-null="true"--> <!-- column="grade_id" foreign-key="fk_grade"/>--> </class> </hibernate-mapping>
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建立Grade.hbm.xml
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.uestc"> <class name="Grade"> <id name="id"> <generator class="native"/> </id> <property name="name"/> <!-- 單向一對多關係 set:Grade類中的集合屬性 name：集合屬性名稱 key：外來鍵 column：外來鍵列名 foreign-key:生成外來鍵約束的名字（可以不寫，預設隨機） not-null="true" 不可以為空（可以不寫，預設false） one-to-many：Grade類中屬性students所表示型別 --> <set name="students"> <key foreign-key="fk_grade" column="grade_id" not-null="true"></key> <one-to-many class="Student"/> </set> </class> </hibernate-mapping>
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在src和test下面的resources下面建立hibernate.cfg.xml
<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <!-- 初始化JDBC連線 --> <property name="connection.driver_class">com.mysql.jdbc.Driver</property> <property name="connection.url">jdbc:mysql:///hibernate</property> <property name="connection.username">root</property> <property name="connection.password">root</property> <!-- 方言 --> <property name="dialect">org.hibernate.dialect.MySQL8Dialect</property> <!-- 資料庫生成方式 --> <property name="hbm2ddl.auto">update</property> <!-- validate檢測,create-drop刪了重新建立,create重新建表,update表如果存在插資料，如果不存在建表插資料 --> <!-- 列印sql --> <property name="show_sql">true</property> <!-- 格式化sql --> <property name="format_sql">true</property> <!-- 關聯物件配置檔案 --> <mapping resource="com/uestc/Grade.hbm.xml"/> <mapping resource="com/uestc/Student.hbm.xml"/> </session-factory> </hibernate-configuration>
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測試類，測試單向一對多和是否獲取到Grade下面的基礎和名字是否對應。
/** * 單向一對多 */ @Test public void testSingleOneToMany() { Session session = null; Transaction tx = null; try { session = HibernateUtil.getSession(); tx = session.beginTransaction(); //建立例項物件 Student student = new Student(); student.setName("張三"); student.setAge(18); Student student2 = new Student(); student2.setName("李四"); student2.setAge(18); Student student3 = new Student(); student3.setName("王五"); student3.setAge(18); Grade grade1 = new Grade(); grade1.setName("基礎"); grade1.getStudents().add(student); grade1.getStudents().add(student2); Grade grade2 = new Grade(); grade2.setName("中級"); grade2.getStudents().add(student3); //儲存的順序是根據外來鍵約束而定的，如果外來鍵不可以為空，必須先儲存外來鍵的一端 //單向一對多會多執行update語句，效率不如多對一 session.save(grade1); session.save(grade2); session.save(student); session.save(student2); session.save(student3); tx.commit(); }catch (Exception e){ e.printStackTrace(); tx.rollback(); }finally { HibernateUtil.closeSession(); } } /** * 獲取 * 查詢不用開啟事務，會降低效能 */ @Test public void testSingleGetOneToMany() { Session session = null; Transaction tx = null; try { session = HibernateUtil.getSession(); tx = session.beginTransaction(); Grade grade = session.get(Grade.class,1); System.out.println("grade:" +grade.getName()); for(Student stu : grade.getStudents()){ System.out.println("stuName:" +stu.getName()); } tx.commit(); }catch (Exception e){ e.printStackTrace(); tx.rollback(); }finally { HibernateUtil.closeSession(); }
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資料庫最後得到的結果。