Hibernate - 每个子类一张表

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简介

在 Hibernate 中，Java 类映射到数据库表。假设我们有一个类，它有几个子类，Hibernate 提供了三种方法将这些类映射到表中：

  • 每个层次结构一张表

  • 每个具体类一张表

  • 每个子类一张表

让我们详细讨论一下“每个子类一张表”策略，并通过一个示例进行说明。

每个子类一张表

假设我们有一个类“Shape”，它有两个子类，“Circle”和“Rectangle”。在这种策略中，我们为每个类创建一个新表。此外，在子类表中，会添加父类所有字段。因此，我们不需要连接父类表，因为子类表包含了父类所有的字段。

Class Hierachy

层次结构中共有三个类。Shape 是父类，Circle 和 Rectangle 是子类。为了映射这些类，我们将使用 *.hbm.xml 文件中的 joined-subclass 元素，如下所示：

<class name="com.tutorialspoint.Shape" table="shape">
...
      <joined-subclass name="com.tutorialspoint.Circle"
         table="circle">
		 <key column="sid"></key>
         <property name="radius"></property>
      </joined-subclass>
	  <joined-subclass name="com.tutorialspoint.Rectangle"
         table="rectangle">
		 <key column="rid"></key>
         <property name="length"></property>
         <property name="width"></property>
      </joined-subclass>
...

创建映射类

让我们创建需要将数据持久化到数据库中的 POJO 类。

Shape.java

package com.tutorialspoint;

public class Shape {
   public int Id;
   public String type;
   public double area;

   public Shape() {
      this.type = "Shape";
   }

   public double calculateArea() {
      this.area= 0;
      setArea(area);
      return area;
   }

   public int getId() {
      return Id;
   }

   public void setId(int i) {
      this.Id = i;
   }

   public String getType() {
      return this.type;
   }
   public double getArea() {
      return area;
   }
   public void setArea(double i) {
      area = i;
   }
}

Circle.java

package com.tutorialspoint;

import java.math.*;

public class Circle extends Shape {

   private double radius;

   public Circle(double rad ) {
      this.radius = rad;
      this.type = "Circle";
   }

   @Override
   public double calculateArea() {
      area =  Math.PI * radius * radius;
      return area;
   }
   public void setArea() {
      this.area = calculateArea();
   }
   public double getArea() {
      return area;
   }
}

Rectangle.java

package com.tutorialspoint;

public class Rectangle extends Shape {

   private double length, width;

   public Rectangle(double length, double width) {
      this.length = length;
      this.width = width;
      this.type = "Rectangle";
   }

   @Override
   public double calculateArea() {
      return length * width;
   }
   public void setArea() {
      this.area = calculateArea();
   }
   public double getArea() {
      return area;
   }
}

创建数据库表

让我们在数据库中创建表。将会有三个表对应于上述对象，您希望提供持久化。假设上述对象需要存储到以下 RDBMS 表中：

CREATE TABLE students.shape( 
   id int NOT NULL, 
   type VARCHAR(20),
   area float
);

CREATE TABLE students.circle( 
   sid int NOT NULL, 
   radius float
);

CREATE TABLE students.rectangle( 
   rid int NOT NULL,
   length float,
   width float
);

创建映射配置文件

现在创建一个映射文件，该文件指示 Hibernate 如何将定义的类映射到数据库表。

shape.hbm.xml

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC
   "-//Hibernate/Hibernate Mapping DTD 5.3//EN"
   "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
   <class name="com.tutorialspoint.Shape" table="shape">
      <id name="id">
         <generator class="increment"></generator>
      </id>
      <property name="type"></property>
      <property name="area"></property>
      <joined-subclass name="com.tutorialspoint.Circle"
         table="circle">
		 <key column="sid"></key>
         <property name="radius"></property>
      </joined-subclass>
	  <joined-subclass name="com.tutorialspoint.Rectangle"
         table="rectangle">
		 <key column="rid"></key>
         <property name="length"></property>
         <property name="width"></property>
      </joined-subclass>
   </class>
</hibernate-mapping>

创建 Hibernate 配置文件

现在为数据库和其他详细信息创建一个 Hibernate 配置文件。

hibernate.cfg.xml

<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-configuration SYSTEM 
   "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
   <session-factory>
      <property name="hbm2ddl.auto">update</property>
      <property name = "hibernate.dialect">
         org.hibernate.dialect.MySQL8Dialect
      </property>
      <property name = "hibernate.connection.driver_class">
         com.mysql.cj.jdbc.Driver
      </property>
      <!—'students' is the database name -->
      <property name = "hibernate.connection.url">
         jdbc:mysql://127.0.0.1/students
      </property>
      <property name = "hibernate.connection.username">
         root
      </property>
      <property name = "hibernate.connection.password">
         guest123
      </property>
      <!-- List of XML mapping files -->
      <mapping resource = "shape.hbm.xml"/>
   </session-factory>
</hibernate-configuration>

将属性 hbm2ddlauto 设置为 update 将在程序执行期间创建表（如果尚未创建）。

创建应用程序类

最后，我们将创建我们的应用程序类，其中包含 main() 方法以运行应用程序。我们将使用此应用程序来测试“每个具体类一张表”映射。

TestTablePerSubclass.java

package com.tutorialspoint;

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 TestTablePerSubclass{ {    
   public static void main(String[] args) {    
      // create a hibernate configuration 
      StandardServiceRegistry ssr=new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();  
      Metadata meta=new MetadataSources(ssr).getMetadataBuilder().build();  
      // get the sessionfactory and open the session
      SessionFactory factory=meta.getSessionFactoryBuilder().build();  
      Session session=factory.openSession();  

      // begin the transaction
      Transaction t=session.beginTransaction();    

      // create Shape instance and set details to persist
      Shape s1=new Shape();    
      s1.setType(s1.getType());
      s1.setArea(0);

      // create Circle instance and set details to persist
      Circle c1=new Circle(2);
      c1.setArea();  

      // create Rectangle instance and set details to persist
      Rectangle r1 = new Rectangle(3,4);
      r1.setArea();

      // persist all instances
      session.persist(s1);    
      session.persist(c1);    
      session.persist(r1);    

      // commit the transaction and close the session
      t.commit();    
      session.close();    
      System.out.println(" Successfully persisted 3 classes. Please check your database for results.");    
   } 
}

编译和执行

执行 TestTablePerHierarchy 二进制文件以运行程序。

输出

您将获得以下结果，并且记录将创建在 Shape 表中。

$java TestTablePerHierarchy
 Successfully persisted 3 classes. Please check your database for results.

如果您检查您的表，它们应该包含以下记录：

mysql> select * from shape;
+----+-----------+-------+
| id | type      | area  |
+----+-----------+-------+
|  1 | Shape     |     0 |
|  2 | Circle    | 12.56 |
|  3 | Rectangle |    12 |
+----+-----------+-------+
3 rows in set (0.00 sec)

mysql> select * from circle;
+-----+--------+
| sid | radius |
+-----+--------+
|   2 |      2 |
+-----+--------+
1 row in set (0.00 sec)

mysql> select * from rectangle;
+-----+--------+--------+
| rid | length | height |
+-----+--------+--------+
|   3 |      3 |      4 |
+-----+--------+--------+
1 row in set (0.00 sec)
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