Hibernate基础知识（5）

小咪咪 2022-07-19 02:27 160阅读 0赞

一、多对多关联关系映射

以 学生Student选课Course为例。

1、实体类编写和多对多配置映射配置

学生类：

package lsq.hibernate.manytomany;
    
    import java.util.HashSet;
    import java.util.Set;
    
    //学生类
    public class Student {
    	private Integer id;
    	private String sname;
    	private Set<Course> courses = new HashSet<Course>();
    
    	public Integer getId() {
    		return id;
    	}
    
    	public void setId(Integer id) {
    		this.id = id;
    	}
    
    	public String getSname() {
    		return sname;
    	}
    
    	public void setSname(String sname) {
    		this.sname = sname;
    	}
    
    	public Set<Course> getCourses() {
    		return courses;
    	}
    
    	public void setCourses(Set<Course> courses) {
    		this.courses = courses;
    	}
    
    }

课程类：

package lsq.hibernate.manytomany;
    
    import java.util.HashSet;
    import java.util.Set;
    
    //课程类
    public class Course {
    	private Integer id;
    	private String cname;
    	private Set<Student> students = new HashSet<Student>();
    
    	public Integer getId() {
    		return id;
    	}
    
    	public void setId(Integer id) {
    		this.id = id;
    	}
    
    	public String getCname() {
    		return cname;
    	}
    
    	public void setCname(String cname) {
    		this.cname = cname;
    	}
    
    	public Set<Student> getStudents() {
    		return students;
    	}
    
    	public void setStudents(Set<Student> students) {
    		this.students = students;
    	}
    
    }

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>
    	<class name="lsq.hibernate.manytomany.Student" table="students" catalog="hibernate3day2">
    			<id name="id">
    				<generator class="identity"></generator>
    			</id>
    			<property name="sname"></property>
    			
    			
    			 <set name="courses" table="student_course">
    			 	
    			 	<key column="student_id"></key>
    			 	
    			 	<many-to-many column="course_id" class="lsq.hibernate.manytomany.Course"></many-to-many>
    			 </set>
    	</class>
    </hibernate-mapping>

Course.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>
    	<class name="lsq.hibernate.manytomany.Course" table="courses" catalog="hibernate3day2">
    			<id name="id">
    				<generator class="identity"></generator>
    			</id>
    			<property name="cname"></property>
    			
    			<set name="students" table="student_course" inverse="true">
    				
    				<key column="course_id"></key>
    				<many-to-many column="student_id" class="lsq.hibernate.manytomany.Student"></many-to-many>
    			</set>
    	</class>
    </hibernate-mapping>

案例一、测试保存

多对多关联映射中，不存在一对多那样父子表关系。

在实际开发中，多对多可以不配置 cascade。

\* 只要两个对象，建立一次关系，就会向中间表插入一条数据。

student1.getCourses().add(course1);  // 产生一条 insert  
      course1.getStudents().add(student1); // 产生一条 insert   
      解决： 只需要建立一方面 关联，或者在任何一方 添加inverse=true  (推荐)。

package lsq.hibernate.manytomany;
    
    import lsq.hibernate.utils.HibernateUtils;
    
    import org.hibernate.Session;
    import org.hibernate.Transaction;
    import org.junit.Test;
    
    //多对多测试
    public class HibernateTest {
    	
    	//测试保存
    	@Test
    	public void demo1(){
    		Session session = HibernateUtils.openSession();
    		Transaction transaction = session.beginTransaction();
    		
    		//保存学生
    		Student student1 = new Student();
    		student1.setSname("关羽");
    		
    		Student student2 = new Student();
    		student2.setSname("张飞");
    		
    		session.save(student1);
    		session.save(student2);
    		
    		//保存课程
    		Course course1 = new Course();
    		course1.setCname("语文");
    		
    		Course course2 = new Course();
    		course2.setCname("数学");
    		
    		session.save(course1);
    		session.save(course2);
    		
    		//关羽选修语文
    		student1.getCourses().add(course1);
    		//course1.getStudents().add(student1);
    		
    		transaction.commit();
    		session.close();
    	}
    }

案例二、测试解除关系，删除中间表数据

@Test
    	public void demo2(){
    		Session session = HibernateUtils.openSession();
    		Transaction transaction = session.beginTransaction();
    		
    		Student student = (Student) session.get(Student.class, 1);
    		Course course = (Course) session.get(Course.class, 1);
    		
    		//解除关系
    		student.getCourses().remove(course);
    		
    		transaction.commit();
    		session.close();
    	}

案例三、改变学生选课关系

student.getCourses().remove(course); 删除选课数据  
      student.getCourses().add(course); 重新选课

案例四、学生存在选课记录，学生数据能否删除

可以，先删除关羽所有选课记录，再删除关羽数据   
      \* 如果删除课程，先删除选课记录，再删除课程

@Test
    	public void demo3(){
    		Session session = HibernateUtils.openSession();
    		Transaction transaction = session.beginTransaction();
    		
    		Student student = (Student) session.get(Student.class, 1);
    		
    		//删除学生
    		session.delete(student);
    		
    		transaction.commit();
    		session.close();
    	}

案例五、删除课程时，能否将选课学生删除 （在实际中很少用）

可以，使用 cascade 级联

\* 多对多 最好不要使用 级联操作

// 删除课程，将选该课程的学生删除
    	// 课程---> 学生 配置Course.hbm.xml
    	// <set name="students" table="student_course" cascade="delete">
    	@Test
    	public void demo4() {
    		Session session = HibernateUtils.openSession();
    		Transaction transaction = session.beginTransaction();
    
    		Course course = (Course) session.get(Course.class, 1);
    		session.delete(course);
    
    		transaction.commit();
    		session.close();
    	}

Hibernate基础知识（5）

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