hibernate 映射继承关系

实现方式一般有三种：

1. 继承关系树每个具体类对应一张表（不介绍）

2. 继承关系树的根类对应一张表

3. 继承关系树的每个类对应一张表

先介绍关系：

DayEmployee和MonthEmploy是Employee的子类，并且Company和Employee是一对多关系：

具体代码如下：

Company.java

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

public class Company {
    private Integer id;
    private String name;
    private Set<Employee> employees = new HashSet<Employee>();

    public Company() {
        super();
    }
    public Company(String name) {
        super();
        this.name = name;
    }
    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 Set<Employee> getEmployees() {
        return employees;
    }
    public void setEmployees(Set<Employee> employees) {
        this.employees = employees;
    }
}

Employee.java

    public class Employee {
    private Integer id;
    private String name;
    private Integer age;
    private Company company;
    public Company getCompany() {
        return company;
    }
    public void setCompany(Company company) {
        this.company = company;
    }
    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 Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }

}

DayEmployee.java

public class DayEmployee extends Employee{
    private Double dayMoney;

    public Double getDayMoney() {
        return dayMoney;
    }
    public void setDayMoney(Double dayMoney) {
        this.dayMoney = dayMoney;
    }

}

MonthEmploy.java

    public class MonthEmploy extends Employee{
    private Double monthMoney;

    public Double getMonthMoney() {
        return monthMoney;
    }
    public void setMonthMoney(Double monthMoney) {
        this.monthMoney = monthMoney;
    }

}

Company.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="com.dongecs.test1.pojo">
    <class name="Company" table="COMPANY">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        <set name="employees" table="EMPLOYEE" inverse="true" lazy="true">
            <key>
                <column name="COMPANY" />
            </key>
            <one-to-many class="Employee" />
        </set>
    </class>
</hibernate-mapping>

继承关系树的根类对应一张表对应的Employee.hbm.xml的写法

Employee.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="com.dongecs.test1.pojo">
    <!-- 利用discriminator-value默认 empType 的值为emp -->
    <class name="Employee" table="EMPLOYEE" discriminator-value="emp">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
       <!--用于识别此类的类别 -->
        <discriminator column="empType" type="string"/>

        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        <property name="age" type="java.lang.Integer">
            <column name="AGE" />
        </property>
        <many-to-one name="company" class="Company" fetch="join">
            <column name="COMPANY" />
        </many-to-one>
       <!--  若为DayEmployee，那么将empType的值设为DE -->
        <subclass name="DayEmployee" discriminator-value="DE">
            <property name="dayMoney" type="double"></property>
        </subclass>
         <!--  若为MonthEmploy，那么将empType的值设为ME -->
        <subclass name="MonthEmploy" discriminator-value="ME">
            <property name="monthMoney" type="double"></property>
        </subclass>
    </class>
</hibernate-mapping>

利用上述代码作如下测试：

public class EmployeeTest {

    SessionFactory sessionFactory = null;
    Session session = null;
    Transaction transaction = null;

    @Before
    public void init() {
        Configuration configuration = new Configuration().configure();
        ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties())
                .buildServiceRegistry();
        sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        session = sessionFactory.openSession();
        transaction = session.beginTransaction();
    }

    @After
    public void after() {
        transaction.commit();
        session.close();
        sessionFactory.close();
    }

    @Test
    public void testEmployee() {
        Company company = new Company("com1");

        DayEmployee d1 = new DayEmployee();
        d1.setAge(22);
        d1.setDayMoney(50.0);
        d1.setName("d1");

        MonthEmploy d2 = new MonthEmploy();
        d2.setAge(30);
        d2.setMonthMoney(79.0);
        d2.setName("d2");

        Employee employee = new Employee();
        employee.setAge(90);
        employee.setName("employee");

        d1.setCompany(company);
        d2.setCompany(company);
        employee.setCompany(company);

        //inverse = true 时可以关联， 当相对多出3条update,因此建议使用多的一端关联
//      company.getEmployees().add(employee);
//      company.getEmployees().add(d1);
//      company.getEmployees().add(d2);

        //把一的一段放到最前保存，可以减小3条更新操作
        session.save(company);

        session.save(employee);
        session.save(d2);
        session.save(d1);
    }
}

会得出如下结果：

继承关系树的每个类对应一张表Employee.hbm.xml的写法

<?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="com.dongecs.test1.pojo">
    <class name="Employee" table="EMPLOYEE">
        <id name="id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>

        <property name="name" type="java.lang.String">
            <column name="NAME" />
        </property>
        <property name="age" type="java.lang.Integer">
            <column name="AGE" />
        </property>
        <many-to-one name="company" class="Company" fetch="join">
            <column name="COMPANY" />
        </many-to-one>
        <!--   利用joined-subclass表签外键关联MonthEmploy -->
        <joined-subclass name="MonthEmploy" table="MONEMPLOY">
            <key column="EMPLOYEE_ID"></key>
            <property name="monthMoney" type="double"></property>
        </joined-subclass>
        <joined-subclass name="DayEmployee" table="DAYEMPLOY">
            <key column="EMPLOYEE_ID"></key>
            <property name="dayMoney" type="double"></property>
        </joined-subclass>
    </class>
</hibernate-mapping>

测试代码：

public class EmployeeTest {

    SessionFactory sessionFactory = null;
    Session session = null;
    Transaction transaction = null;

    @Before
    public void init() {
        Configuration configuration = new Configuration().configure();
        ServiceRegistry serviceRegistry = new ServiceRegistryBuilder().applySettings(configuration.getProperties())
                .buildServiceRegistry();
        sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        session = sessionFactory.openSession();
        transaction = session.beginTransaction();
    }

    @After
    public void after() {
        transaction.commit();
        session.close();
        sessionFactory.close();
    }

    @Test
    public void testEmployee() {
        Company company = new Company("com1");

        DayEmployee d1 = new DayEmployee();
        d1.setAge(22);
        d1.setDayMoney(50.0);
        d1.setName("d1");

        MonthEmploy d2 = new MonthEmploy();
        d2.setAge(30);
        d2.setMonthMoney(79.0);
        d2.setName("d2");

        Employee employee = new Employee();
        employee.setAge(90);
        employee.setName("employee");

        d1.setCompany(company);
        d2.setCompany(company);
        employee.setCompany(company);

        //inverse = true 时可以关联， 当相对多出3条update,因此建议使用多的一端关联
//      company.getEmployees().add(employee);
//      company.getEmployees().add(d1);
//      company.getEmployees().add(d2);

        //把一的一段放到最前保存，可以减小3条更新操作
        session.save(company);

        session.save(employee);
        session.save(d2);
        session.save(d1);
    }

    @Test
    public void findFromEmployeeTable(){
        String hql = "from Employee";
        //通过左外链接找
        List<Employee> list = session.createQuery(hql).list();
        for (Employee emp : list){
            if (emp instanceof DayEmployee){
                System.out.println("day");
            } else if (emp instanceof MonthEmploy){
                System.out.println("mon");
            } else {
                System.out.println("emp");
            }
        }
    }

    @Test
    public void findFromDayEmployeeTable(){
        String hql = "from DayEmployee";
        //通过内连接找
        List<Employee> list = session.createQuery(hql).list();
        System.out.println(list.size());
    }
}

得出如下结果：

结论：

1.使用根类对应一张表

好处：只需创建一张表，不用外键关联

缺点：如果子类属性多，会导致另一子类有过多的空值，而且不能设置一些字段的not null约束,因此不能保证数据库的完整性

2.每个类对应一张表（外键关联）

好处：如果某个类的属性发生变化，只需修改和这个类对应的表

缺点：利用外键关联查询

ref:

孙卫琴.精通Hibernate：Java对象持久化技术详解