Hibernate - 批量处理

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考虑这样一种情况：您需要使用 Hibernate 将大量记录上传到数据库。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，最终您的应用程序会在大约第 50,000 行出现 OutOfMemoryException 错误。如果您使用 Hibernate 的批量处理功能，可以解决此问题。

要使用批量处理功能，首先将hibernate.jdbc.batch_size属性设置为批处理大小，例如 20 或 50（具体取决于对象大小）。这将告诉 Hibernate 容器每 X 行作为一个批次进行插入。为了在代码中实现这一点，我们需要进行一些修改，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，但如果您想要执行 UPDATE 操作，则可以使用以下代码：

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();

批量处理示例

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:///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)
);

以下将是将 Employee 对象与 EMPLOYEE 表映射的映射文件：

<?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，我们将创建包含 main() 方法的应用程序类来运行应用程序，在其中我们将使用 Session 对象提供的flush()和clear()方法，以便 Hibernate 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();

      /* 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

以下是编译和运行上述应用程序的步骤。在进行编译和执行之前，请确保已正确设置了 PATH 和 CLASSPATH。

  • 创建如上所述的 hibernate.cfg.xml 配置文件。

  • 创建如上所示的 Employee.hbm.xml 映射文件。

  • 创建如上所示的 Employee.java 源文件并编译它。

  • 创建如上所示的 ManageEmployee.java 源文件并编译它。

  • 执行 ManageEmployee 二进制文件以运行程序，该程序将在 EMPLOYEE 表中创建 100000 条记录。

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