hibernate的映射之三(多对多单向关联)

Posted 2020-10-19 努力奋斗吧

Many-to-Many 多对多的映射可以使用一组Java集合不包含任何重复的元素来实现。我们已经看到了Hibernate如何设置映射集合。

集(SET)被映射到与映射表中<set>元素，并以java.util.HashSet初始化。您可以使用Set集合在类中时，集合不需要重复的元素。

在现实生活中多对多的例子挺多的。

例如：学生与老师，订单与商品等

下面我就以员工和项目来做例子，一个员工可以做多个项目，一个项目可以被多个员工做。

1.准备JavaBean

项目·实体·类

public class Project {
    private Integer proid;
    private String proname;
  
    public Integer getProid() {
        return proid;
    }

    public void setProid(Integer proid) {
        this.proid = proid;
    }

    public String getProname() {
        return proname;
    }

    public void setProname(String proname) {
        this.proname = proname;
    }
}

员工实体类（员工实体类中需要植入一个项目集合。）

public class Employee {
    private Integer empid;
    private String empname;
    //植入项目集合   一个员工可以参与多个项目
    private Set<Project> projects=new HashSet<Project>();

    public Integer getEmpid() {
        return empid;
    }

    public void setEmpid(Integer empid) {
        this.empid = empid;
    }

    public String getEmpname() {
        return empname;
    }

    public void setEmpname(String empname) {
        this.empname = empname;
    }

    public Set<Project> getProjects() {
        return projects;
    }

    public void setProjects(Set<Project> projects) {
        this.projects = projects;
    }
}

2.持久化配置文件
Employee.hbm.xml配置文件

<?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">
<!-- 映射文件开始 -->
<hibernate-mapping package="cn.day04mapping.manytomany.entity">
    <!--表名称-->
    <class name="Employee" table="EMPLOYEE" schema="root">
        <!--列名-->
       <id name="empid" column="EMPID">
           <!--主键生成的策略  native：自动生成主键字段-->
           <generator class="native"></generator>
       </id>
     <property name="empname" column="EMPNAME"></property>
        <!--单相关联-->
        <!--植入set 标签  table 中间表的表名  -->
        <set name="projects"  table="PROEMP">
            <!--column 多的一方表的外键列-->
            <key column="REMPID"></key>

            <!--column  中间表的另外一个外键列-->
            <many-to-many column="RPROID" class="Project"></many-to-many>
        </set>
    </class>
</hibernate-mapping>

Project.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">
<!-- 映射文件开始 -->
<hibernate-mapping package="cn.day04mapping.manytomany.entity">
    <!--表名称-->
    <class name="Project" table="PROJECT" schema="root">
        <!--列名-->
       <id name="proid" column="PROID">
           <!--主键生成的策略  native：自动生成主键字段-->
           <generator class="native"></generator>
       </id>
     <property name="proname" column="PRONAME"></property>
    </class>
</hibernate-mapping>


3.hibernate.cfg.xml主配置文件

<?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>
        <!-- 指定数据库所用到的驱动 -->
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <!-- 指定数据库链接的url，hibernate链接的数据库名 -->
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:orcl</property>
        <!-- 指定连接数据库的用户名 -->
        <property name="connection.username">root</property>
        <!-- 指定连接数据库的用户口令 -->
        <property name="connection.password">root</property>
        <!-- Enable Hibernate‘s automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!--格式化sql -->
        <property name="format_sql ">true</property>
        <!-- 打印sql 控制台-->
        <property name="show_sql">true</property>
        <!-- 指定数据库方言 -->
        <property name="dialect">org.hibernate.dialect.Oracle10gDialect</property>
        <!-- 根据需要自动创建数据库表 -->
        <property name="hbm2ddl.auto">update</property>

        <!--多对多-->
        <mapping resource="cn/day04mapping/manytomany/entity/Project.hbm.xml"></mapping>
        <mapping resource="cn/day04mapping/manytomany/entity/Employee.hbm.xml"></mapping>


        <!--延迟加载-->
        <mapping resource="cn/day05lazy/entity/Student.hbm.xml"></mapping>

    </session-factory>
</hibernate-configuration>


4.书写测试类和方法

public class HibernateUtil {
      static Configuration cfg;
      static SessionFactory factory;
      static ThreadLocal<Session> td;
    //静态代码块
    static {
        cfg=new Configuration().configure();
        factory=cfg.buildSessionFactory();
        td=new ThreadLocal<Session>();
    }
    public static Session getSession(){
         //看看有无线程变量
        Session session = td.get();
        if (session==null){
            //线程中没有session对象 创建一个
             session = factory.openSession();
             td.set(session);
        }
        //没有和当前线程绑定
        return session;
    }
    //关闭session
    public static void closeSession(){
        Session session = td.get();
        td.set(null);
        session.close();
    }
}

//多对多单向关联的查询
@Test
public void test01(){
    Session session = HibernateUtil.getSession();    //得到session
    Employee employee = session.get(Employee.class, 1);
    System.out.println(employee.getEmpname());
    System.out.println("=============");
    for (Project project:employee.getProjects()){
        System.out.println(project.getProname());
    }
}

//多对多单向关联的添加
@Test
public void test03(){
    Session session = HibernateUtil.getSession();
    //开启事务
    Transaction transaction = session.beginTransaction();
    Employee employee=new Employee();
    employee.setEmpname("微冷的雨");

    Project project=new Project();
    project.setProname("开发部");
    Project project1=new Project();
    project1.setProname("研发部");

    employee.getProjects().add(project);
    employee.getProjects().add(project1);

    //保存数据
    session.save(employee);
    session.save(project);
    session.save(project1);
    //提交事务
    transaction.commit();
    session.close();
}