批处理（Batch Processing）

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2023-12-01

考虑一种需要使用Hibernate将大量记录上传到数据库的情况。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将缓存会话级缓存中的所有持久对象，最终您的应用程序将在第50,000行左右的某个地方出现OutOfMemoryException 。如果您使用Hibernate进行batch processing ，则可以解决此问题。

要使用批处理功能，首先将hibernate.jdbc.batch_size作为批处理大小设置为20或50的数字，具体取决于对象大小。这将告诉hibernate容器每个X行都要作为批处理插入。要在您的代码中实现这一点，我们需要进行如下的少量修改 -

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();

上面的代码适用于INSERT操作，但是如果你愿意进行UPDATE操作，那么你可以使用以下代码实现 -

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以添加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/test
      </property>
      <property name = "hibernate.connection.username">
         root
      </property>
      <property name = "hibernate.connection.password">
         root123
      </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)
);

以下将是使用EMPLOYEE表映射Employee对象的映射文件 -

<?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/>
      </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（）方法创建应用程序类来运行应用程序，我们将使用可用于Session对象的flush()和clear()方法，以便Hibernate继续将这些记录写入数据库，而不是将它们缓存在记忆。

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();
      /* Add employee records in batches */
      ME.addEmployees( );
   }
   /* Method to create employee records in batches */
   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 (Compilation and Execution)

以下是编译和运行上述应用程序的步骤。确保在进行编译和执行之前已适当地设置了PATH和CLASSPATH。

如上所述创建hibernate.cfg.xml配置文件。
创建Employee.hbm.xml映射文件，如上所示。
如上所示创建Employee.java源文件并进行编译。
创建ManageEmployee.java源文件，如上所示并编译它。
执行ManageEmployee二进制文件以运行该程序，该程序将在EMPLOYEE表中创建100000条记录。