Hibernate（20）Hibernate如何处理继承关系？

Victor3562026-01-03 8:06

Hibernate提供了几种策略来处理继承关系，主要有三种：

  1. 单表继承（Single Table Inheritance）：在单个表中存储继承层次结构的所有类。
  2. 表连接继承（Joined Table Inheritance）：为每个类创建独立的表，并使用外键建立关联。
  3. 表每类继承（Table Per Class Inheritance）：为每个具体类创建独立的表，不存储父类信息。

下面详细介绍这三种策略，并通过具体的示例代码演示如何在Hibernate中实现它们。

单表继承（Single Table Inheritance）

在单表继承策略中，继承层次结构的所有类的属性和数据都存储在同一个表中，通过一个区分列（discriminator column）来区分具体的类型。

示例

配置文件 hibernate.cfg.xml
xml 复制代码 
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/your_database</property>
        <property name="hibernate.connection.username">your_username</property>
        <property name="hibernate.connection.password">your_password</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.format_sql">true</property>
        <property name="hibernate.hbm2ddl.auto">update</property>
        <mapping class="com.example.domain.Person"/>
        <mapping class="com.example.domain.Employee"/>
        <mapping class="com.example.domain.Student"/>
    </session-factory>
</hibernate-configuration>
实体类
父类 Person
java 复制代码 
package com.example.domain;

import javax.persistence.*;

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "person_type", discriminatorType = DiscriminatorType.STRING)
public abstract class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;

    // constructors, getters, and setters
    public Person() {}

    public Person(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
子类 Employee
java 复制代码 
package com.example.domain;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("EMPLOYEE")
public class Employee extends Person {
    private double salary;

    // constructors, getters, and setters
    public Employee() {}

    public Employee(String name, double salary) {
        super(name);
        this.salary = salary;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}
子类 Student
java 复制代码 
package com.example.domain;

import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;

@Entity
@DiscriminatorValue("STUDENT")
public class Student extends Person {
    private String major;

    // constructors, getters, and setters
    public Student() {}

    public Student(String name, String major) {
        super(name);
        this.major = major;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }
}

插入和查询示例代码

java 复制代码 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

public class HibernateSingleTableInheritanceExample {
    public static void main(String[] args) {
        // 获取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 插入数据
        insertSampleData(sessionFactory);

        // 查询数据
        queryData(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void insertSampleData(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();
        try {
            Employee employee = new Employee("John Doe", 50000.0);
            Student student = new Student("Jane Doe", "Computer Science");

            session.save(employee);
            session.save(student);

            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }

    private static void queryData(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        try {
            List<Person> persons = session.createQuery("FROM Person", Person.class).list();
            for (Person person : persons) {
                System.out.println("Person: " + person.getName());
                if (person instanceof Employee) {
                    System.out.println("Employee Salary: " + ((Employee) person).getSalary());
                } else if (person instanceof Student) {
                    System.out.println("Student Major: " + ((Student) person).getMajor());
                }
            }
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

表连接继承（Joined Table Inheritance）

在表连接继承策略中，为每个类创建独立的表，子类表包含自己的属性并通过外键引用父类表中的记录。

示例

实体类
父类 Person
java 复制代码 
package com.example.domain;

import javax.persistence.*;

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;

    // constructors, getters, and setters
    public Person() {}

    public Person(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
子类 Employee
java 复制代码 
package com.example.domain;

import javax.persistence.Entity;

@Entity
public class Employee extends Person {
    private double salary;

    // constructors, getters, and setters
    public Employee() {}

    public Employee(String name, double salary) {
        super(name);
        this.salary = salary;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}
子类 Student
java 复制代码 
package com.example.domain;

import javax.persistence.Entity;

@Entity
public class Student extends Person {
    private String major;

    // constructors, getters, and setters
    public Student() {}

    public Student(String name, String major) {
        super(name);
        this.major = major;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }
}

查询和插入数据的代码与单表继承的示例相同。

表每类继承（Table Per Class Inheritance）

在表每类继承策略中，每个具体类都有自己的独立表，不存储父类信息，所有字段（包括父类的字段）都在子类表中。

示例

实体类
父类 Person
java 复制代码 
package com.example.domain;

import javax.persistence.*;

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    private Long id;
    
    private String name;

    // constructors, getters, and setters
    public Person() {}

    public Person(String name) {
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}
子类 Employee
java 复制代码 
package com.example.domain;

import javax.persistence.Entity;

@Entity
public class Employee extends Person {
    private double salary;

    // constructors, getters, and setters
    public Employee() {}

    public Employee(String name, double salary) {
        super(name);
        this.salary = salary;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}
子类 Student
java 复制代码 
package com.example.domain;

import javax.persistence.Entity;

@Entity
public class Student extends Person {
    private String major;

    // constructors, getters, and setters
    public Student() {}

    public Student(String name, String major) {
        super(name);
        this.major = major;
    }

    public String getMajor() {
        return major;
    }
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