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 会将所有持久化的对象缓存在会话级缓存中，最终您的应用程序将在第 50,000 行附近出现 OutOfMemoryException。 如果您将批处理与 Hibernate 一起使用，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。 要在您的代码中实现这一点，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，但是如果您愿意进行 UPDATE 操作，那么您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) );
以下是用于将 Employee 对象映射到 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();
/* 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 ;
}
}
编译与执行
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，请确保您已正确设置 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。
如上图创建 ManageEmployee.java源文件并编译。
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。
