Hibernate中的一对多与多对一映射

Posted 2020-07-22 十三弦

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1.需求

　　一个部门有多个员工; 【一对多】

　　多个员工，属于一个部门【多对一】

2.实体Bean设计

　　Dept：

public class Dept {
    private int depId;
    private String depName;
    private Set<Employee> emps = new HashSet<Employee>();
    set...
    get...      
}

　　Employee：

public class Employee {
    private int empId;
    private String empName;
    private double salary;
    private Dept dept;
    set...
    get...
}

3.配置映射文件

　　Dept.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.department">
    <class name="Dept" table="t_dept">
        <!--主键-->
        <id name="depId">
            <generator class="native"/>
        </id>
        <property name="depName" column="deptName" type="string"/>
        <!--一对多关联映射的配置-->
        <set name="emps" table="t_employee" >
            <!--外键字段-->
            <key column="dept_id"></key>
            <!--一对多关系-->
            <one-to-many class="Employee"/>
        </set>
    </class>

</hibernate-mapping>

　　Employee.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.juaner.hibernate.department">
    <class name="Employee" table="t_employee">
        <!--主键-->
        <id name="empId" column="empId">
            <generator class="native"/>
        </id>
        <property name="empName" column="empName" type="string"/>
        <property name="salary" column="salary"/>
        <!--多对一映射-->
        <many-to-one name="dept" column="dept_id" class="Dept"></many-to-one>
    </class>

</hibernate-mapping>

4.保存数据

使用“一”的一方来设置关联

    @Test
    public void test1(){
        Session session = sf.openSession();
        session.beginTransaction();

        //部门对象
        Dept dept = new Dept();
        dept.setDepName("开发部");
        //员工对象
        Employee employee = new Employee();
        employee.setEmpName("张三");
        employee.setSalary(1000);
        Employee employee2 = new Employee();
        employee2.setEmpName("李四");
        employee2.setSalary(2000);

        dept.getEmps().add(employee);
        dept.getEmps().add(employee2);

        session.save(dept);
        session.save(employee);
        session.save(employee2);

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

　　此时会执行5条sql语句，其中最后两条update语句用来设置关联：

        Hibernate: insert into t_dept (deptName) values (?)
        Hibernate: insert into t_employee (empName, salary, dept_id) values (?, ?, ?)
        Hibernate: insert into t_employee (empName, salary, dept_id) values (?, ?, ?)
        Hibernate: update t_employee set dept_id=? where empId=?
        Hibernate: update t_employee set dept_id=? where empId=?

使用“多”的一方来设置关联

    @Test
    public void test2(){
        Session session = sf.openSession();
        session.beginTransaction();

        //部门对象
        Dept dept = new Dept();
        dept.setDepName("人事部");
        //员工对象
        Employee employee = new Employee();
        employee.setEmpName("张三");
        employee.setSalary(1000);
        Employee employee2 = new Employee();
        employee2.setEmpName("李四");
        employee2.setSalary(2000);

        employee.setDept(dept);
        employee2.setDept(dept);
        //先保存一的一方，再保存多的一方，关系会自动维护，减少不必要的语句
        session.save(dept);
        session.save(employee);
        session.save(employee2);

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

    }

　　此时只执行三条sql语句：

Hibernate: insert into t_dept (deptName) values (?)
Hibernate: insert into t_employee (empName, salary, dept_id) values (?, ?, ?)
Hibernate: insert into t_employee (empName, salary, dept_id) values (?, ?, ?)