Hibernate 简明教程

Hibernate - Batch Processing

设想一下这种情况：你需要使用 Hibernate 上传大量记录到你的数据库。以下是使用 Hibernate 实现此目标的代码段：

Consider a situation when you need to upload a large number of records into your database using Hibernate. Following is the code snippet to achieve this using Hibernate −

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，最终你的应用程序会在大约第 50,000 行时因 OutOfMemoryException 而崩溃。如果你将 batch processing 与 Hibernate 一起使用，则可以解决此问题。

By default, Hibernate will cache all the persisted objects in the session-level cache and ultimately your application would fall over with an OutOfMemoryException somewhere around the 50,000th row. You can resolve this problem, if you are using batch processing with Hibernate.

要使用批处理功能，首先将 hibernate.jdbc.batch_size 设置为批量大小，以 20 或 50 的数字作为对象大小。这将告知 Hibernate 容器将每 X 行插入为一批。要在你的代码中实现此功能，我们需要进行如下小修改：

To use the batch processing feature, first set hibernate.jdbc.batch_size as batch size to a number either at 20 or 50 depending on object size. This will tell the hibernate container that every X rows to be inserted as batch. To implement this in your code, we would need to do little modification as follows −

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，但如果你愿意执行 UPDATE 操作，则可以使用以下代码来实现：

Above code will work fine for the INSERT operation, but if you are willing to make UPDATE operation, then you can achieve using the following code −

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();

Batch Processing Example

让我们修改配置文件以添加 hibernate.jdbc.batch_size 属性：

Let us modify the configuration file to add hibernate.jdbc.batch_size property −

<?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：

Consider the following POJO Employee class −

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：

Let us create the following EMPLOYEE table to store the Employee objects −

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)
);

以下是将 Employee 对象与 EMPLOYEE 表映射的映射文件：

Following will be the mapping file to map the Employee objects with EMPLOYEE table −

<?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，我们将创建带有 main() 方法的应用程序类来运行应用程序，在应用程序中，我们将使用 Session 对象附带的 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内存中。

Finally, we will create our application class with the main() method to run the application where we will use flush() and clear() methods available with Session object so that Hibernate keeps writing these records into the database instead of caching them in the memory.

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 ;
   }
}

Compilation and Execution

以下是编译和运行上述应用程序的步骤。请确保在继续编译和执行之前，已适当地设置 PATH 和 CLASSPATH。

Here are the steps to compile and run the above mentioned application. Make sure, you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.

Create hibernate.cfg.xml configuration file as explained above.
Create Employee.hbm.xml mapping file as shown above.
Create Employee.java source file as shown above and compile it.
Create ManageEmployee.java source file as shown above and compile it.
Execute ManageEmployee binary to run the program, which will create 100000 records in EMPLOYEE table.