Hibernate实体关系映射(OneToMany、ManyToOne双边)——完整实例

双边关系是最常见的配置。在多方有一方的引用，一方也有多方的引用。双边关系能够很方便地查询数据。看一个班级与学生的双边关系。

班级（Clazz类）与学生（Student类）：一对多关系。班级中有学生属性，学生中有班级属性，二者是双边关系。（Class是关键字，不能用）

分析：Clazz为一方，Student为多方。Clazz中有Student集合属性，Student中也有Clazz类型属性。

环境：MyEclipse10，新建Web Project名为hibernate

　　　　

1、Clazz.java

Clazz类中有一个id主键、一个name属性、一个List<Student>集合代表该班级内的学生。班级与学生关系是一对多，这里使用@OneToMany配置，并用mappedBy属性配置与该List<Student>属性对应的是Student类中的clazz属性。具体的关联配置都在Student的clazz属性上。

双边关系中，控制权一般交给多方，因此这里的@OneToMany没有设置数据库的外键列，而只配置了一个mappedBy属性，值为clazz，告诉hibernate，配置信息要到Student类的clazz属性中找。

package com.hibernate.bean;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Map;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;

@Entity
@Table(name = "tb_class")
public class Clazz {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    private String name;

    @OneToMany(mappedBy = "clazz")
    @Cascade(value = CascadeType.DELETE_ORPHAN)
    private List<Student> students = new ArrayList<Student>();

    // @OneToMany(mappedBy = "clazz")
    // @MapKey(name="name")
    // private Map<String, Student> students = new HashSet<String, Student>();

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

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

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

}

2、Student.java

　　Student类中有一个id主键以及name姓名、sex性别两个普通属性，还有一个Clazz类型的属性clazz，代表所在的班级。该属性与Clazz中的List<Student>集合属性是对应的。从逻辑上来讲，某student的clazz属性为某clazz，该clazz的List<Student>中必定还有改student。配置了mappedBy后，hibernate能自动维护这种关系。

　　Student与Clazz是多对一的关系，多对一使用@ManyToOne配置，另外用@JoinColumn配置关联的列名。这里用tb_student表的class_id列与tb_class的id主键列连接。这里同时配置了级联保存类型为PERSIST，创建student时，会自动将clazz也写进数据库。

package com.hibernate.bean;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name = "tb_student")
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @ManyToOne(fetch = FetchType.EAGER, cascade = { CascadeType.PERSIST })
    @JoinColumn(name = "class_id")　　//外键为class_id
    private Clazz clazz;

    private String name;

    private String sex;

    public Clazz getClazz() {
        return clazz;
    }

    public void setClazz(Clazz clazz) {
        this.clazz = clazz;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

}

3、hibernate.cfg.xml(src目录下)

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>
      <property name="show_sql">true</property>
      <property name="format_sql">true</property>
      <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
      <property name="connection.url">jdbc:oracle:thin:@192.168.1.2::orcl</property>
      <property name="connection.username">daym2</property>
      <property name="connection.password">daym2</property>
      <property name="connection.isolation"></property>
       <property name="hbm2ddl.auto">create</property>
       <property name="javax.persistence.validation.mode">none</property>

      <!-- SQL方言，这边设定的是Oracle -->
      <property name="dialect">org.hibernate.dialect.Oracle10gDialect</property>

      <property name="current_session_context">thread</property>
      <mapping class="com.hibernate.bean.Cat" />
      <mapping class="com.hibernate.bean.Person" />
      <mapping class="com.hibernate.bean.Email" />
      <mapping class="com.hibernate.bean.Clazz" />
      <mapping class="com.hibernate.bean.Student" />
    </session-factory>
</hibernate-configuration>

4、TestClazzStudent.java

package com.hibernate.bean;
import java.util.List;

import org.hibernate.Session;

import com.hibernate.bean.Clazz;
import com.hibernate.bean.Student;
import com.hibernate.bean.HibernateSessionFactory;

public class TestClazzStudent {

    @SuppressWarnings("all")
    public static void main(String[] args) throws Exception {

        Clazz clazz = new Clazz();
        clazz.setName("三年二班");

        Student student1 = new Student();
        student1.setName("周周");
        student1.setSex("男");

        Student student2 = new Student();
        student2.setName("李四");
        student2.setSex("女");

        Session session = HibernateSessionFactory.getSession();
        session.beginTransaction();

        // 保存进数据库
        session.persist(clazz);
        session.persist(student1);
        session.persist(student2);

        // 设置班级
        student1.setClazz(clazz);
        student2.setClazz(clazz);
        // clazz.getStudents().add(student1);
        // clazz.getStudents().add(student2);

        session.getTransaction().commit();
        session.close();

        session = HibernateSessionFactory.getSession();
        session.beginTransaction();

        // 查询名为“三年二班”的班级 然后输出学生
        Clazz c = (Clazz) session.createQuery(
                " select c from Clazz c where c.name = :name ").setParameter(
                "name", "三年二班").uniqueResult();

        System.out.println("三年二班 的所有学生：");
        for (Student s : c.getStudents()) {
            System.out.println("\t姓名：" + s.getName() + ", 性别：" + s.getSex());
        }

        // 直接查询班级为“三年二班”的学生
        List<Student> students = session.createQuery(
                " select s from Student s where s.clazz.name = :name ")
                .setParameter("name", "三年二班").list();

        System.out.println("三年二班 的所有学生：");
        for (Student s : students) {
            System.out.println("\t姓名：" + s.getName() + ", 性别：" + s.getSex());
        }

        session.getTransaction().commit();
        session.close();

    }

}

5、log4j.properties (src目录下)

log4j.rootLogger=INFO,stdout
log4j.category.org.hibernate.tool.hbn2ddl=DEBUG
log4j.category.org.hibernate.SQL=DEBUG
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%-d{yyyy-MM-dd HH\:mm\:ss,SSS} [%c]-[%p] %m%n