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： Compan

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

<code class=" hljs java"><span class="hljs-keyword">import</span> java.util.HashSet;
<span class="hljs-keyword">import</span> java.util.Set;

<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Company</span> {</span>
    <span class="hljs-keyword">private</span> Integer id;
    <span class="hljs-keyword">private</span> String name;
    <span class="hljs-keyword">private</span> Set<Employee> employees = <span class="hljs-keyword">new</span> HashSet<Employee>();

    <span class="hljs-keyword">public</span> <span class="hljs-title">Company</span>() {
        <span class="hljs-keyword">super</span>();
    }
    <span class="hljs-keyword">public</span> <span class="hljs-title">Company</span>(String name) {
        <span class="hljs-keyword">super</span>();
        <span class="hljs-keyword">this</span>.name = name;
    }
    <span class="hljs-keyword">public</span> Integer <span class="hljs-title">getId</span>() {
        <span class="hljs-keyword">return</span> id;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setId</span>(Integer id) {
        <span class="hljs-keyword">this</span>.id = id;
    }
    <span class="hljs-keyword">public</span> String <span class="hljs-title">getName</span>() {
        <span class="hljs-keyword">return</span> name;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setName</span>(String name) {
        <span class="hljs-keyword">this</span>.name = name;
    }
    <span class="hljs-keyword">public</span> Set<Employee> <span class="hljs-title">getEmployees</span>() {
        <span class="hljs-keyword">return</span> employees;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setEmployees</span>(Set<Employee> employees) {
        <span class="hljs-keyword">this</span>.employees = employees;
    }
}</code>

Employee.java

<code class=" hljs cs">    <span class="hljs-keyword">public</span> <span class="hljs-keyword">class</span> Employee {
    <span class="hljs-keyword">private</span> Integer id;
    <span class="hljs-keyword">private</span> String name;
    <span class="hljs-keyword">private</span> Integer age;
    <span class="hljs-keyword">private</span> Company company;
    <span class="hljs-keyword">public</span> Company <span class="hljs-title">getCompany</span>() {
        <span class="hljs-keyword">return</span> company;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setCompany</span>(Company company) {
        <span class="hljs-keyword">this</span>.company = company;
    }
    <span class="hljs-keyword">public</span> Integer <span class="hljs-title">getId</span>() {
        <span class="hljs-keyword">return</span> id;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setId</span>(Integer id) {
        <span class="hljs-keyword">this</span>.id = id;
    }
    <span class="hljs-keyword">public</span> String <span class="hljs-title">getName</span>() {
        <span class="hljs-keyword">return</span> name;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setName</span>(String name) {
        <span class="hljs-keyword">this</span>.name = name;
    }
    <span class="hljs-keyword">public</span> Integer <span class="hljs-title">getAge</span>() {
        <span class="hljs-keyword">return</span> age;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setAge</span>(Integer age) {
        <span class="hljs-keyword">this</span>.age = age;
    }

}
</code>

DayEmployee.java

<code class=" hljs java"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DayEmployee</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Employee</span>{</span>
    <span class="hljs-keyword">private</span> Double dayMoney;

    <span class="hljs-keyword">public</span> Double <span class="hljs-title">getDayMoney</span>() {
        <span class="hljs-keyword">return</span> dayMoney;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setDayMoney</span>(Double dayMoney) {
        <span class="hljs-keyword">this</span>.dayMoney = dayMoney;
    }

}
</code>

MonthEmploy.java

<code class=" hljs java">    <span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MonthEmploy</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Employee</span>{</span>
    <span class="hljs-keyword">private</span> Double monthMoney;

    <span class="hljs-keyword">public</span> Double <span class="hljs-title">getMonthMoney</span>() {
        <span class="hljs-keyword">return</span> monthMoney;
    }
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setMonthMoney</span>(Double monthMoney) {
        <span class="hljs-keyword">this</span>.monthMoney = monthMoney;
    }

}
</code>

Company.hbm.xml

<code class=" hljs xml"><span class="hljs-pi"><?xml version="1.0"?></span>
<span class="hljs-doctype"><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"></span>
<span class="hljs-tag"><<span class="hljs-title">hibernate-mapping</span> <span class="hljs-attribute">package</span>=<span class="hljs-value">"com.dongecs.test1.pojo"</span>></span>
    <span class="hljs-tag"><<span class="hljs-title">class</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"Company"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"COMPANY"</span>></span>
        <span class="hljs-tag"><<span class="hljs-title">id</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"id"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span>
            <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"ID"</span> /></span>
            <span class="hljs-tag"><<span class="hljs-title">generator</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"native"</span> /></span>
        <span class="hljs-tag"></<span class="hljs-title">id</span>></span>
        <span class="hljs-tag"><<span class="hljs-title">property</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"name"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.String"</span>></span>
            <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"NAME"</span> /></span>
        <span class="hljs-tag"></<span class="hljs-title">property</span>></span>
        <span class="hljs-tag"><<span class="hljs-title">set</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"employees"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"EMPLOYEE"</span> <span class="hljs-attribute">inverse</span>=<span class="hljs-value">"true"</span> <span class="hljs-attribute">lazy</span>=<span class="hljs-value">"true"</span>></span>
            <span class="hljs-tag"><<span class="hljs-title">key</span>></span>
                <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"COMPANY"</span> /></span>
            <span class="hljs-tag"></<span class="hljs-title">key</span>></span>
            <span class="hljs-tag"><<span class="hljs-title">one-to-many</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"Employee"</span> /></span>
        <span class="hljs-tag"></<span class="hljs-title">set</span>></span>
    <span class="hljs-tag"></<span class="hljs-title">class</span>></span>
<span class="hljs-tag"></<span class="hljs-title">hibernate-mapping</span>></span></code>

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

<code class=" hljs xml"><span class="hljs-pi"><?xml version="1.0"?></span>
<span class="hljs-doctype"><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"></span>
<span class="hljs-tag"><<span class="hljs-title">hibernate-mapping</span> <span class="hljs-attribute">package</span>=<span class="hljs-value">"com.dongecs.test1.pojo"</span>></span>
    <span class="hljs-comment"><!-- 利用discriminator-value默认 empType 的值为emp --></span>
    <span class="hljs-tag"><<span class="hljs-title">class</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"Employee"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"EMPLOYEE"</span> <span class="hljs-attribute">discriminator-value</span>=<span class="hljs-value">"emp"</span>></span>
        <span class="hljs-tag"><<span class="hljs-title">id</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"id"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span>
            <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"ID"</span> /></span>
            <span class="hljs-tag"><<span class="hljs-title">generator</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"native"</span> /></span>
        <span class="hljs-tag"></<span class="hljs-title">id</span>></span>
       <span class="hljs-comment"><!--用于识别此类的类别 --></span>
        <span class="hljs-tag"><<span class="hljs-title">discriminator</span> <span class="hljs-attribute">column</span>=<span class="hljs-value">"empType"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"string"</span>/></span>

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

利用上述代码作如下测试：

<code class=" hljs java"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">EmployeeTest</span> {</span>

    SessionFactory sessionFactory = <span class="hljs-keyword">null</span>;
    Session session = <span class="hljs-keyword">null</span>;
    Transaction transaction = <span class="hljs-keyword">null</span>;

    <span class="hljs-annotation">@Before</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">init</span>() {
        Configuration configuration = <span class="hljs-keyword">new</span> Configuration().configure();
        ServiceRegistry serviceRegistry = <span class="hljs-keyword">new</span> ServiceRegistryBuilder().applySettings(configuration.getProperties())
                .buildServiceRegistry();
        sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        session = sessionFactory.openSession();
        transaction = session.beginTransaction();
    }

    <span class="hljs-annotation">@After</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">after</span>() {
        transaction.commit();
        session.close();
        sessionFactory.close();
    }

    <span class="hljs-annotation">@Test</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">testEmployee</span>() {
        Company company = <span class="hljs-keyword">new</span> Company(<span class="hljs-string">"com1"</span>);

        DayEmployee d1 = <span class="hljs-keyword">new</span> DayEmployee();
        d1.setAge(<span class="hljs-number">22</span>);
        d1.setDayMoney(<span class="hljs-number">50.0</span>);
        d1.setName(<span class="hljs-string">"d1"</span>);

        MonthEmploy d2 = <span class="hljs-keyword">new</span> MonthEmploy();
        d2.setAge(<span class="hljs-number">30</span>);
        d2.setMonthMoney(<span class="hljs-number">79.0</span>);
        d2.setName(<span class="hljs-string">"d2"</span>);

        Employee employee = <span class="hljs-keyword">new</span> Employee();
        employee.setAge(<span class="hljs-number">90</span>);
        employee.setName(<span class="hljs-string">"employee"</span>);

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

        <span class="hljs-comment">//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</span>
<span class="hljs-comment">//      company.getEmployees().add(employee);</span>
<span class="hljs-comment">//      company.getEmployees().add(d1);</span>
<span class="hljs-comment">//      company.getEmployees().add(d2);</span>

        <span class="hljs-comment">//把一的一段放到最前保存，可以减小3条更新操作</span>
        session.save(company);

        session.save(employee);
        session.save(d2);
        session.save(d1);
    }
}</code>

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

<code class=" hljs xml"><span class="hljs-pi"><?xml version="1.0"?></span>
<span class="hljs-doctype"><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"></span>
<span class="hljs-tag"><<span class="hljs-title">hibernate-mapping</span> <span class="hljs-attribute">package</span>=<span class="hljs-value">"com.dongecs.test1.pojo"</span>></span>
    <span class="hljs-tag"><<span class="hljs-title">class</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"Employee"</span> <span class="hljs-attribute">table</span>=<span class="hljs-value">"EMPLOYEE"</span>></span>
        <span class="hljs-tag"><<span class="hljs-title">id</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"id"</span> <span class="hljs-attribute">type</span>=<span class="hljs-value">"java.lang.Integer"</span>></span>
            <span class="hljs-tag"><<span class="hljs-title">column</span> <span class="hljs-attribute">name</span>=<span class="hljs-value">"ID"</span> /></span>
            <span class="hljs-tag"><<span class="hljs-title">generator</span> <span class="hljs-attribute">class</span>=<span class="hljs-value">"native"</span> /></span>
        <span class="hljs-tag"></<span class="hljs-title">id</span>></span>

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

测试代码：

<code class=" hljs java"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">EmployeeTest</span> {</span>

    SessionFactory sessionFactory = <span class="hljs-keyword">null</span>;
    Session session = <span class="hljs-keyword">null</span>;
    Transaction transaction = <span class="hljs-keyword">null</span>;

    <span class="hljs-annotation">@Before</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">init</span>() {
        Configuration configuration = <span class="hljs-keyword">new</span> Configuration().configure();
        ServiceRegistry serviceRegistry = <span class="hljs-keyword">new</span> ServiceRegistryBuilder().applySettings(configuration.getProperties())
                .buildServiceRegistry();
        sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        session = sessionFactory.openSession();
        transaction = session.beginTransaction();
    }

    <span class="hljs-annotation">@After</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">after</span>() {
        transaction.commit();
        session.close();
        sessionFactory.close();
    }

    <span class="hljs-annotation">@Test</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">testEmployee</span>() {
        Company company = <span class="hljs-keyword">new</span> Company(<span class="hljs-string">"com1"</span>);

        DayEmployee d1 = <span class="hljs-keyword">new</span> DayEmployee();
        d1.setAge(<span class="hljs-number">22</span>);
        d1.setDayMoney(<span class="hljs-number">50.0</span>);
        d1.setName(<span class="hljs-string">"d1"</span>);

        MonthEmploy d2 = <span class="hljs-keyword">new</span> MonthEmploy();
        d2.setAge(<span class="hljs-number">30</span>);
        d2.setMonthMoney(<span class="hljs-number">79.0</span>);
        d2.setName(<span class="hljs-string">"d2"</span>);

        Employee employee = <span class="hljs-keyword">new</span> Employee();
        employee.setAge(<span class="hljs-number">90</span>);
        employee.setName(<span class="hljs-string">"employee"</span>);

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

        <span class="hljs-comment">//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</span>
<span class="hljs-comment">//      company.getEmployees().add(employee);</span>
<span class="hljs-comment">//      company.getEmployees().add(d1);</span>
<span class="hljs-comment">//      company.getEmployees().add(d2);</span>

        <span class="hljs-comment">//把一的一段放到最前保存，可以减小3条更新操作</span>
        session.save(company);

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

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

    <span class="hljs-annotation">@Test</span>
    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">findFromDayEmployeeTable</span>(){
        String hql = <span class="hljs-string">"from DayEmployee"</span>;
        <span class="hljs-comment">//通过内连接找</span>
        List<Employee> list = session.createQuery(hql).list();
        System.out.println(list.size());
    }
}
</code>

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