19. 一級緩衝（session級緩衝）

hibernate一級緩衝，又稱為sesssion級緩衝

      

一級緩衝生命週期很短和session的生命週期一致，一級緩衝也叫session級的緩衝或事務級緩衝。主要用來緩衝實體物件的，而不會對實體物件的普通屬性進行緩衝

 

支援一級緩衝的方法：

       * get()

       * load()

       * iterate（查詢實體物件）

      

管理一級緩衝的方法：

       * session.clear(),session.evict()

      

如何避免一次性大量的實體資料入庫導致記憶體溢出

       * 先flush，再clear

      

如果資料量特別大，考慮採用jdbc實現，如果jdbc也不能滿足要求可以考慮採用資料本身的特定匯入工具     

測試案例（該測試案例基於一對多雙向關聯映射）：

package com.bjsxt.hibernate;

 

import java.io.Serializable;

import org.hibernate.Session;

import junit.framework.TestCase;

 

public class CacheLevel1Test extends TestCase {

 

       /**

        * 在同一個session中發出兩次load查詢

        */

       public void testCache1() {

              Session session = null;

              try {

                     session = HibernateUtils.getSession();

                     session.beginTransaction();

                    

                     Student student = (Student)session.load(Student.class, 1);

                     System.out.println("student.name=" + student.getName());

                    

                     //不會發出sql，因為load使用緩衝

                     student = (Student)session.load(Student.class, 1);

                     System.out.println("student.name=" + student.getName());

                    

                     session.getTransaction().commit();

              }catch(Exception e) {

                     e.printStackTrace();

                     session.getTransaction().rollback();

              }finally {

                     HibernateUtils.closeSession(session);

              }

       }           

 

       /**

        * 在同一個session中發出兩次get查詢

        */

       public void testCache2() {

              Session session = null;

              try {

                     session = HibernateUtils.getSession();

                     session.beginTransaction();

                    

                     Student student = (Student)session.get(Student.class, 1);

                     System.out.println("student.name=" + student.getName());

                    

                     //不會發出sql，因為get使用緩衝

                     student = (Student)session.get(Student.class, 1);

                     System.out.println("student.name=" + student.getName());

                    

                     session.getTransaction().commit();

              }catch(Exception e) {

                     e.printStackTrace();

                     session.getTransaction().rollback();

              }finally {

                     HibernateUtils.closeSession(session);

              }

       }    

      

       /**

        * 在同一個session中發出兩次iterate查詢實體物件

        */

       public void testCache3() {

              Session session = null;

              try {

                     session = HibernateUtils.getSession();

                     session.beginTransaction();

                    

                     Student student = (Student)session.createQuery("from Student s where s.id=1").iterate().next();

                     System.out.println("student.name=" + student.getName());

                    

                     //會發出查詢id的sql，不會發出查詢實體物件的sql，因為iterate使用緩衝

                     student = (Student)session.createQuery("from Student s where s.id=1").iterate().next();

                     System.out.println("student.name=" + student.getName());

                    

                     session.getTransaction().commit();

              }catch(Exception e) {

                     e.printStackTrace();

                     session.getTransaction().rollback();

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