hibernate单向一对多联系关系

hibernate单向一对多关联

类图如下：

hibernate单向一对多联系关系

在关系模型中，同多对一一样，还是在Student表中有一个外键指向Class表。

在配置的时候需要注意了，Student配置文件不需要进行额外的配置了，需要进行额外配置的是MyClass的配置文件，需要通过set标签配置，这里set标签也是有两个作用的，一为数据库服务，第二位程序服务。

MyClass.java

package com.org.model;

import java.util.HashSet;
import java.util.Set;



public class MyClass {
	private int id;
	private String name;
	private Set<Student> students =  new HashSet<Student>();
	
	public Set<Student> getStudents() {
		return students;
	}
	public void setStudents(Set<Student> students) {
		this.students = students;
	}
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	

}

Student.java

package com.org.model;



public class Student {
	private int id;
	private String name;
	
	public  Student(){}
	public Student(String name){
		this.name = name;
	}
	
	
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
	

}

MyClass.hbm.xml

<?xml version="1.0"?>
<!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.org.model">
	<class name="MyClass" table="class">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name" not-null="true"/>
		
		<!-- set标签中的name为MyClass类中多方对应的属性名        key标签中的column为多方数据库表中的外键字段-->
		<set name="students" cascade="save-update">
			<key column="class_id" not-null="true"></key>
			
			<one-to-many class="Student"/>      <!-- class表明set中放的是什么类型的集合 -->
		</set>
	</class>


</hibernate-mapping>

Student.hbm.xml

<?xml version="1.0"?>
<!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.org.model">
	<class name="Student" table="student">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name" column="name" not-null="true"/>
		
		</class>


</hibernate-mapping>

测试类HibernateTest.java

package com.org.model.test;

import java.util.HashSet;

import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.Test;

import com.org.model.MyClass;
import com.org.model.Student;
import com.org.util.HibernateUtil;

public class HibernateTest {

	@Test
	public void testSave1() {
		Session session = null;
		Transaction tx = null;
		try {

			// 拿到session
			session = HibernateUtil.getSession();
			System.out.println(session);
			// 开启事务
			tx = session.beginTransaction();

			// 给实体赋值
			MyClass class1 = new MyClass();
			class1.setName("09001");
			
			Student st1 = new Student("aaa");
			Student st2 = new Student("bbb");
			Student st3 = new Student("ccc");
			HashSet<Student> students = new HashSet<Student>();
			students.add(st1);
			students.add(st2);
			students.add(st3);
			
			class1.setStudents(students);
			
			session.save(class1);
			
	

			// 提交事务
			tx.commit();

		} catch (Exception e) {
			// 打印堆栈信息
			e.printStackTrace();
			// 事务回滚
			if (tx != null) {
				tx.rollback();
			}
		}
	}

}

由于配置了级联关系，Hibernate在插入班级数据（一方）时，会把它所关联的学生数据（多方）也保存到数据库中。

缺点：对象之间的关联关系由一方来维护，在插入一方数据时会更新多方的与这个“一”的对象有关联的对象。导致频繁访问数据库。

测试时，控制台输出的sql语句：

Hibernate: 
    insert 
    into
        class
        (name) 
    values
        (?)
Hibernate: 
    insert 
    into
        student
        (name, class_id) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        student
        (name, class_id) 
    values
        (?, ?)
Hibernate: 
    insert 
    into
        student
        (name, class_id) 
    values
        (?, ?)
Hibernate: 
    update
        student 
    set
        class_id=? 
    where
        id=?
Hibernate: 
    update
        student 
    set
        class_id=? 
    where
        id=?
Hibernate: 
    update
        student 
    set
        class_id=? 
    where
        id=?

hibernate单向一对多联系关系

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