Hibernate - 批次處理

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考慮這樣一種情況：您需要使用 Hibernate 將大量記錄上傳到資料庫。以下是使用 Hibernate 實現此目的的程式碼片段：

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();

預設情況下，Hibernate 會將所有持久化物件快取在會話級快取中，最終您的應用程式會在大約第 50,000 行出現 OutOfMemoryException 錯誤。如果您使用 Hibernate 的批次處理功能，可以解決此問題。

要使用批次處理功能，首先將hibernate.jdbc.batch_size屬性設定為批處理大小，例如 20 或 50（具體取決於物件大小）。這將告訴 Hibernate 容器每 X 行作為一個批次進行插入。為了在程式碼中實現這一點，我們需要進行一些修改，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();

上面的程式碼對於 INSERT 操作可以正常工作，但如果您想要執行 UPDATE 操作，則可以使用以下程式碼：

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();

批次處理示例

讓我們修改配置檔案以新增hibernate.jdbc.batch_size屬性：

<?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>

考慮以下 POJO Employee 類：

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;
   }
}

讓我們建立以下 EMPLOYEE 表來儲存 Employee 物件：

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)
);

以下將是將 Employee 物件與 EMPLOYEE 表對映的對映檔案：

<?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>

最後，我們將建立包含 main() 方法的應用程式類來執行應用程式，在其中我們將使用 Session 物件提供的flush()和clear()方法，以便 Hibernate 將這些記錄寫入資料庫，而不是將它們快取在記憶體中。

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 ;
   }
}

編譯和執行

以下是編譯和執行上述應用程式的步驟。在進行編譯和執行之前，請確保已正確設定了 PATH 和 CLASSPATH。

  • 建立如上所述的 hibernate.cfg.xml 配置檔案。

  • 建立如上所示的 Employee.hbm.xml 對映檔案。

  • 建立如上所示的 Employee.java 原始檔並編譯它。

  • 建立如上所示的 ManageEmployee.java 原始檔並編譯它。

  • 執行 ManageEmployee 二進位制檔案以執行程式，該程式將在 EMPLOYEE 表中建立 100000 條記錄。

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