hibernate one2many (单向关联)
2021-05-17 13:29
标签:case 就是 void man oid testcase ram enc 有关
Classes类:
package com.cnblogs.hibernate_first; import java.util.Set; public class Classes { private int id; private String name; //hibernate对Set进行了扩展,可以延时加载,在需要的时候才需要查询SET。比如有1000个学生,当需要的时候才进行加载 private Set students; public Set getStudents() { return students; } public void setStudents(Set 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:
public class Student { private int id; private String 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; } }
classes.hbm.xml
span>mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd" >package="com.cnblogs.hibernate_first"> class name="Classes" table="t_classes"> class="native"> class> class="com.cnblogs.hibernate_first.Student"/>
student.hbm.xml:
span>mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd" >package="com.cnblogs.hibernate_first"> class name="Student" table="t_student"> class="native"> class>
测试类:
package com.cnblogs.hibernate_first; import java.util.HashSet; import java.util.Iterator; import java.util.Set; import org.hibernate.Session; import junit.framework.TestCase; public class One2ManyTest extends TestCase { public void testSave1() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); // 建立关联 Student student1 = new Student(); student1.setName("张三"); Student student2 = new Student(); student2.setName("李四"); Classes classes = new Classes(); classes.setName("动力节点"); Setstudents = new HashSet (); students.add(student1); students.add(student2); classes.setStudents(students); //因为student不是持久化状态,他是Transient状态 session.save(classes); session.getTransaction().commit(); } catch (Exception e) { e.printStackTrace(); session.getTransaction().rollback(); } finally { HibernateUtils.closeSession(session); } } public void testSave2() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); // 建立关联 Student student1 = new Student(); student1.setName("张三"); session.save(student1); Student student2 = new Student(); student2.setName("李四"); session.save(student2); Classes classes = new Classes(); classes.setName("动力节点"); Set students = new HashSet (); students.add(student1); students.add(student2); classes.setStudents(students); //因为student不是持久化状态,他是Transient状态 session.save(classes); session.getTransaction().commit(); } catch (Exception e) { e.printStackTrace(); session.getTransaction().rollback(); } finally { HibernateUtils.closeSession(session); } } public void testLoad1() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); Classes classes = (Classes)session.load(Classes.class, 1); System.out.println("Classes.name = " + classes.getName()); Set students = classes.getStudents(); for(Iterator iter = students.iterator();iter.hasNext();){ Student student = (Student)iter.next(); System.out.println("Student in Classes " + student.getName()); } session.getTransaction().commit(); } catch (Exception e) { e.printStackTrace(); session.getTransaction().rollback(); } finally { HibernateUtils.closeSession(session); } } }
hibernate 一对多的关联映射(单向 Classes ——》Student)
hibernate对Set进行了扩展,可以延时加载,在需要的时候才需要查询SET。比如有1000个学生,当需要的时候才进行加载
一对多和多对一的映射原理是一样的,都是在多的一端加入一个外键指向一的一端
他们的区别在于维护的关系不用
多对一维护的关系:多指向一的关系,如果维护了多指向一的关系,那么加载多的时候会把一加载上来。
一对多维护的关系:一指向多的关系,如果维护了一指向多的关系,那么加载一的时候会把一加载上来。
在一的一端维护关系存在缺陷
*因为多的一端Student不知道Classes的存在(也就是Student没有维护与Classes的关系),所以在保存Student的时候关系字段Classid为null,
如果将该关系字段设置为非空,则将无法保存数据。
*另外因为Student不维护关系,而Classes维护关系,Classes就会发出多余的update语句,保证Classes与Student有关系,,这样加载Classes的
时候,才能把该Classes对应的学生集合加载上来。
hibernate one2many (单向关联)
标签:case 就是 void man oid testcase ram enc 有关
原文地址:http://www.cnblogs.com/Juli/p/7743938.html