Hibernate教程-使用注解实现每个具体类的表

在每个具体类的表模式中，每个类都会创建一个独立的表。因此，表中没有可空值。该方法的缺点是子类表中会创建重复的列。

在这里，我们需要在父类中使用 @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 注解，并在子类中使用 @AttributeOverrides 注解。

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) 指定我们使用的是每个具体类一个表的策略。该注解只应在父类中指定。

@AttributeOverrides 定义父类的属性将在该类中重写。在表结构中，父类表的列将添加到子类表中。

类层次结构如下所示：

每个表的表结构如下：

Employee 类的表结构

Regular_Employee 类的表结构

Contract_Employee 类的表结构

每个具体类的表的示例

在这个示例中，我们将创建三个类，并在 employee.hbm.xml 文件中提供这些类的映射。

1) 创建持久化类

你需要创建表示继承关系的持久化类。让我们为上述层次结构创建三个类：

文件: Employee.java

package cn.javatiku.mypackage;

import javax.persistence.*;

@Entity
@Table(name = "employee102")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private int id;

    @Column(name = "name")
    private String name;

    // setters and getters
}

文件: Regular_Employee.java

package cn.javatiku.mypackage;

import javax.persistence.*;

@Entity
@Table(name = "regularemployee102")
@AttributeOverrides({
    @AttributeOverride(name = "id", column = @Column(name = "id")),
    @AttributeOverride(name = "name", column = @Column(name = "name"))
})
public class Regular_Employee extends Employee {

    @Column(name = "salary")
    private float salary;

    @Column(name = "bonus")
    private int bonus;

    // setters and getters
}

文件: Contract_Employee.java

package cn.javatiku.mypackage;

import javax.persistence.*;

@Entity
@Table(name = "contractemployee102")
@AttributeOverrides({
    @AttributeOverride(name = "id", column = @Column(name = "id")),
    @AttributeOverride(name = "name", column = @Column(name = "name"))
})
public class Contract_Employee extends Employee {

    @Column(name = "pay_per_hour")
    private float pay_per_hour;

    @Column(name = "contract_duration")
    private String contract_duration;

    public float getPay_per_hour() {
        return pay_per_hour;
    }

    public void setPay_per_hour(float payPerHour) {
        pay_per_hour = payPerHour;
    }

    public String getContract_duration() {
        return contract_duration;
    }

    public void setContract_duration(String contractDuration) {
        contract_duration = contractDuration;
    }
}

2) 在 pom.xml 文件中添加项目信息和配置

打开 pom.xml 文件并点击 source。现在，在 <dependencies>....</dependencies> 标签之间添加以下依赖项。

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-core</artifactId>
    <version>5.3.1.Final</version>
</dependency>

<dependency>
    <groupId>com.oracle</groupId>
    <artifactId>ojdbc14</artifactId>
    <version>10.2.0.4.0</version>
</dependency>

3) 在配置文件中添加 hbm 文件的映射

文件: hibernate.cfg.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC 
     "-//Hibernate/Hibernate Configuration DTD 5.3//EN" 
     "http://hibernate.sourceforge.net/hibernate-configuration-5.3.dtd">

<!-- 由 MyEclipse Hibernate 工具生成 -->
<hibernate-configuration>
    <session-factory>
        <property name="hbm2ddl.auto">update</property>
        <property name="dialect">org.hibernate.dialect.Oracle9Dialect</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>
        <property name="connection.username">system</property>
        <property name="connection.password">jtp</property>
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        
        <mapping class="cn.javatiku.mypackage.Employee"/>
        <mapping class="cn.javatiku.mypackage.Contract_Employee"/>
        <mapping class="cn.javatiku.mypackage.Regular_Employee"/>
    </session-factory>
</hibernate-configuration>

hbm2ddl.auto 属性定义用于在数据库中自动创建表。

4) 创建存储持久化对象的类

在这个类中，我们将简单地将员工对象存储在数据库中。

文件: StoreData.java

package cn.javatiku.mypackage;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class StoreData {

    public static void main(String[] args) {

        StandardServiceRegistry ssr = new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
        Metadata meta = new MetadataSources(ssr).getMetadataBuilder().build();

        SessionFactory factory = meta.getSessionFactoryBuilder().build();
        Session session = factory.openSession();

        Transaction t = session.beginTransaction();

        Employee e1 = new Employee();
        e1.setName("Gaurav Chawla");

        Regular_Employee e2 = new Regular_Employee();
        e2.setName("Vivek Kumar");
        e2.setSalary(50000);
        e2.setBonus(5);

        Contract_Employee e3 = new Contract_Employee();
        e3.setName("Arjun Kumar");
        e3.setPay_per_hour(1000);
        e3.setContract_duration("15 hours");

        session.persist(e1);
        session.persist(e2);
        session.persist(e3);

        t.commit();
        session.close();
        System.out.println("success");
    }
}

运行 StoreData 类将三个员工对象持久化到数据库中，各自存储在对应的表中。