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&minus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会缓存会话级缓存中的所有持久对象,最终你的应用程序会因 OutOfMemoryException而崩溃大约第50,000排的某个地方.如果您使用Hibernate 批处理,则可以解决此问题.
要使用批处理功能,请先设置 hibernate.jdbc.batch_size 批量大小为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,那么你可以使用下面的代码 :
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();
让我们修改配置文件以添加 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员工类 :
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 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()方法创建应用程序类来运行应用程序,我们将使用 flush()和Session语句可用的 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.
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 ; } }
以下是编译和运行的步骤上述申请.确保在进行编译和执行之前已经适当地设置了PATH和CLASSPATH.
创建hibernate.cfg.xml配置文件如上所述.
创建如上所示的Employee.hbm.xml映射文件.
如上所示创建Employee.java源文件并进行编译.
如上所示创建ManageEmployee.java源文件并进行编译.
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.