Hibernate关系映射（三） 多对一和一对多

package com.lxit.entity;

import java.io.Serializable;

public class Student implements Serializable {
    
    public Student() {
    }

    public Student(String sname, String sex, String address) {
        this.sname = sname;
        this.sex = sex;
        this.address = address;
    }

    public Student(String sname, String sex, String address, Grade grade) {
        this.sname = sname;
        this.sex = sex;
        this.address = address;
        this.grade = grade;
    }

    private int sid;
    private String sname;
    private String sex;
    private String address;
    
    //单向多对一：在多的一方封装一的一方的实体对象
    private Grade grade;
    
    public int getSid() {
        return sid;
    }
    public void setSid(int sid) {
        this.sid = sid;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }

    public Grade getGrade() {
        return grade;
    }

    public void setGrade(Grade grade) {
        this.grade = grade;
    }

    @Override
    public String toString() {
        return "Student [sid=" + sid + ", sname=" + sname + ", sex=" + sex
                + ", address=" + address + "]";
    }

<?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>
    <!-- 实体映射文件：将实体对象映射为数据库中的字段 -->
     <class name="com.lxit.entity.Student" table="Student">
         <id name="sid" column="sid">
             <generator class="native"></generator>
         </id>
         <property name="sname" column="sname" type="java.lang.String"></property>
         <property name="sex" column="sex"></property>
         <property name="address" column="address"></property>
         <!-- 实现多对一的映射关系 -->
         <many-to-one name="grade" class="com.lxit.entity.Grade" column="gid" lazy="false"></many-to-one>
     </class>

package com.lxit.entity;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

public class Grade implements Serializable{
    
    public Grade() {
    }
    
    public Grade(String gname) {
        this.gname = gname;
    }
    
    //在一方添加多方的集合
    public Set<Student> students = new HashSet<Student>();


    private int gid;
    private String gname;
    
    public int getGid() {
        return gid;
    }
    public void setGid(int gid) {
        this.gid = gid;
    }
    public String getGname() {
        return gname;
    }
    public void setGname(String gname) {
        this.gname = gname;
    }

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    } 

} 

<?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>
    <!-- 实体映射文件：将实体对象映射为数据库中的字段 -->
     <class name="com.lxit.entity.Grade" table="Grade">
         <id name="gid" column="gid">
             <generator class="native"></generator>
         </id>
         <property name="gname" type="java.lang.String">
             <column name="gname" length="20"></column>
         </property>
         <!-- 
             inverse:表示将维护外键的职责交友多方来维护，默认为false
             cascade：表示级联效果，当保存班级时，班级对应的学生一起保存
         -->
         <set name="students" table="Student" cascade="all">
             <key column="gid"></key>
             <one-to-many class="com.lxit.entity.Student"/>
         </set>
     </class>

</hibernate-mapping>