Hibernate多对多关系映射学习笔记

Student.java

package cn.hai.domain;

import java.util.Set;

public class Student {
    private int id;
    private String name;
    private Set<Teacher> teachers;//多对多关系一般有一个中间表用于相关联，Hibernate面向对象，更注重于对象关系

    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;
    }

    public Set<Teacher> getTeachers() {
        return teachers;
    }

    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }

}

Teacher.jaca

package cn.hai.domain;

import java.util.Set;

public class Teacher {
    private int id;
    private String name;
    private Set<Student> 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;
    }

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

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

}

Student.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.hai.domain">
    <class name="Student">
        <id name="id">
            <generator class="native" />                      <!--主键，自增长-->
        </id>
        <property name="name" />
        <!-- 声明中间表teacher_student,声明Teacher表主键teacher_id,声明多对多对应的表Student,声明多对多对应表的主键student_id -->
        <set name="teachers" table="teacher_student">
            <key column="student_id" />
            <many-to-many class="Teacher" column="Teacher_id" />
        </set>
    </class>

</hibernate-mapping>

Teacher.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="cn.hai.domain">
    <class name="Teacher">
        <id name="id">
            <generator class="native" />
        </id>
        <property name="name" />
        <set name="students" table="teacher_student">
            <key column="teacher_id" />
            <many-to-many class="Student" column="student_id" />
        </set>
    </class>

</hibernate-mapping>

ManyToManyTest.java

package cn.hai;

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

import org.hibernate.Session;
import org.hibernate.Transaction;

import cn.hai.dao.HibernateUtil;
import cn.hai.domain.Student;
import cn.hai.domain.Teacher;

public class ManyToMany {

    /**
     * @param args
*/
    public static void main(String[] args) {
        Session s = null;
        Transaction tx = null;
        try {
            // 建立关系
            Set<Teacher> ts = new HashSet<Teacher>();
            Teacher t1 = new Teacher();
            t1.setName("XiangfGang1");
            ts.add(t1);

            Teacher t2 = new Teacher();
            t2.setName("ZhengHui1");
            ts.add(t2);

            Teacher t3 = new Teacher();
            t3.setName("WangJinFeng1");
            ts.add(t3);

            Set<Student> ss = new HashSet<Student>();
            Student sd1 = new Student();
            sd1.setName("ChengHai1");
            ss.add(sd1);

            Student sd2 = new Student();
            sd2.setName("JiangKui1");
            ss.add(sd2);

            Student sd3 = new Student();
            sd3.setName("YanJiaYang1");
            ss.add(sd3);

            t1.setStudents(ss);
            t2.setStudents(ss);
            t3.setStudents(ss);
            // 告诉学生有哪些老师，与上面告诉老师有哪些学生任选一种即可
// sd1.setTeachers(ts);
// sd2.setTeachers(ts);
// sd3.setTeachers(ts);

            s = HibernateUtil.getSession();
            tx = s.beginTransaction();
            s.save(t1);
            s.save(t2);
            s.save(t3);
            s.save(sd1);
            s.save(sd2);
            s.save(sd3);
            tx.commit();
        } finally {
            if (s != null) {
                s.close();
            }
        }

    }

//根据Teacher，id查学生的数量，左右连接查询导致效率低，少用
    static void query(int id){
        Session s=null;
        Transaction tx=null;
        try{    
            s=HibernateUtil.getSession();
            tx=s.beginTransaction();
            Teacher t=(Teacher)s.get(Teacher.class, id);
            System.out.println("students:"+t.getStudents().size());
            //Hibernate.initialize(t.getStudents());
            tx.commit();
        }finally{
            if(s!=null){
                s.close();
            }
        }
    }

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