Hibernate 批处理

考虑需要使用 Hibernate 将大量记录上传到数据库中的情况。 以下是使用 Hibernate 实现此目的的代码片段

Session session = SessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
   Employee employee = new Employee(.....);
   session.save(employee);
}
tx.commit();
session.close();

默认情况下，Hibernate 会将所有持久化对象缓存在会话级缓存中，最终我们的应用程序将在第 50,000 行左右出现 OutOfMemoryException 异常。 如果在 Hibernate 中使用批处理，则可以解决此问题。

要使用批处理功能，首先将 hibernate.jdbc.batch_size 设置为批处理大小，根据对象大小设置为 20 或 50。 这将告诉休眠容器每 X 行作为批处理插入。 要在代码中实现这一点，我们需要做一些修改，如下所示

Session session = SessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
   Employee employee = new Employee(.....);
   session.save(employee);
   if( i % 50 == 0 ) { // Same as the JDBC batch size
      //flush a batch of inserts and release memory:
      session.flush();
      session.clear();
   }
}
tx.commit();
session.close();

上面的代码对于 INSERT 操作可以正常工作，但是如果愿意进行 UPDATE 操作，那么可以使用以下代码来实现

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

ScrollableResults employeeCursor = session.createQuery("FROM EMPLOYEE").scroll();
int count = 0;

while ( employeeCursor.next() ) {
   Employee employee = (Employee) employeeCursor.get(0);
   employee.updateEmployee();
   seession.update(employee); 
   if ( ++count % 50 == 0 ) {
      session.flush();
      session.clear();
   }
}
tx.commit();
session.close();

批处理示例

让我们修改配置文件来添加 hibernate.jdbc.batch_size 属性

<?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 = "hibernate.dialect">
         org.hibernate.dialect.MySQLDialect
      </property>
   
      <property name = "hibernate.connection.driver_class">
         com.mysql.jdbc.Driver
      </property>

      <!-- Assume students is the database name -->
   
      <property name = "hibernate.connection.url">
         jdbc:mysql://localhost/hibernate_demo
      </property>
   
      <property name = "hibernate.connection.username">
         root
      </property>
   
      <property name = "hibernate.connection.password">
         123456
      </property>
   
      <property name = "hibernate.jdbc.batch_size">
         50
      </property>

      <!-- List of XML mapping files -->
      <mapping resource = "Employee.hbm.xml"/>

   </session-factory>
</hibernate-configuration>

考虑以下 POJO Employee 类

public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;  

   public Employee() {}
   
   public Employee(String fname, String lname, int salary) {
      this.firstName = fname;
      this.lastName = lname;
      this.salary = salary;
   }
   
   public int getId() {
      return id;
   }
   
   public void setId( int id ) {
      this.id = id;
   }
   
   public String getFirstName() {
      return firstName;
   }
   
   public void setFirstName( String first_name ) {
      this.firstName = first_name;
   }
   
   public String getLastName() {
      return lastName;
   }
   
   public void setLastName( String last_name ) {
      this.lastName = last_name;
   }
   
   public int getSalary() {
      return salary;
   }
   
   public void setSalary( int salary ) {
      this.salary = salary;
   }
}

让我们创建以下 employee 表来存储 Employee 对象

create table employee (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,
   PRIMARY KEY (id)
);

以下将是映射 Employee 对象与 employee 表的映射文件

<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> 

<hibernate-mapping>
   <class name = "Employee" table = "employee">
      
      <meta attribute = "class-description">
         This class contains the employee detail. 
      </meta>
      
      <id name = "id" type = "int" column = "id">
         <generator class="native"/>
      </id>
      
      <property name = "firstName" column = "first_name" type = "string"/>
      <property name = "lastName" column = "last_name" type = "string"/>
      <property name = "salary" column = "salary" type = "int"/>
      
   </class>
</hibernate-mapping>

最后，我们将使用 main() 方法创建我们的应用程序类来运行应用程序，我们将使用 Session 对象可用的 flush() 和 clear() 方法，以便 Hibernate 不断将这些记录写入数据库，而不是将它们缓存在内存中。

import java.util.*; 
 
import org.hibernate.HibernateException; 
import org.hibernate.Session; 
import org.hibernate.Transaction;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class ManageEmployee {
   private static SessionFactory factory; 
   public static void main(String[] args) {
      
      try {
         factory = new Configuration().configure().buildSessionFactory();
      } catch (Throwable ex) { 
         System.err.println("Failed to create sessionFactory object." + ex);
         throw new ExceptionInInitializerError(ex); 
      }
      ManageEmployee ME = new ManageEmployee();

      /* 批量添加员工记录 */
      ME.addEmployees( );
   }
   
   /* 批量创建员工记录的方法 */
   public void addEmployees( ){
      Session session = factory.openSession();
      Transaction tx = null;
      Integer employeeID = null;
      
      try {
         tx = session.beginTransaction();
         for ( int i=0; i<100000; i++ ) {
            String fname = "First Name " + i;
            String lname = "Last Name " + i;
            Integer salary = i;
            Employee employee = new Employee(fname, lname, salary);
            session.save(employee);
             if( i % 50 == 0 ) {
               session.flush();
               session.clear();
            }
         }
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
      return ;
   }
}

编译和执行

以下是编译和运行上述应用程序的步骤。 在继续编译和执行之前，请确保我们已正确设置 PATH 和 CLASSPATH。

• 如上所述创建 hibernate.cfg.xml 配置文件。
• 如上所示创建 Employee.hbm.xml 映射文件。
• 如上所示创建 Employee.java 源文件并编译它。
• 如上所示创建 ManageEmployee.java 源文件并编译它。
• 执行 ManageEmployee 二进制文件来运行程序，它将在 EMPLOYEE 表中创建 100000 条记录。