hibernate一级缓存 一级缓存很短和session的生命周期一致，一级缓存也叫session级的缓存或事务级缓存 那些方法支持一级缓存： * get() * load() * iterate（查询实体对象） 如何管理一级缓存： * session.clear(),session.evict() 如何避免一次性大量的实体数据入库导致内存溢出 * 先flush，再clear 如果数据量特别大，考虑采用jdbc实现，如果jdbc也不能满足要求可以考虑采用数据本身的特定导入工具 什么时候用一级缓存? 不是经常改变的，相对静止的 eternal= " false "不是 永恒的 expires有效期 ExportDB: create table t_classes (id integer not null auto_increment, name varchar( 255), primary key (id)) create table t_student (id integer not null auto_increment, name varchar( 255), classesid integer, primary key (id)) alter table t_student add index FK4B9075708EBC77F7 (classesid), add constraint FK4B9075708EBC77F7 foreign key (classesid) references t_classes (id) mysql> create database hibernate_cache; Query OK, 1 row affected ( 0.11 sec) mysql> use hibernate_cache; Database changed InitData: Hibernate: insert into t_classes (name) values (?) Hibernate: insert into t_student (name, classesid) values (?, ?) package com.bjsxt.hibernate; import java.io.Serializable; import org.hibernate.Session; import junit.framework.TestCase; public class CacheLevel1Test extends TestCase { /* * * 在同一个session中发出两次load查询 */ public void testCache1() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); // 不会发出sql,因为load支持lazy, Student student = (Student)session.load(Student. class, 1); System. out.println( " student.name= " + student.getName()); // 不会发出sql，因为load使用缓存 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); } } /* * * 在同一个session中发出两次get查询 */ public void testCache2() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); // 马上会发出sql,因为get不支持lazy, Student student = (Student)session. get(Student. class, 1); System. out.println( " student.name= " + student.getName()); // 不会发出sql，因为get使用缓存 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); } } /* * * 在同一个session中发出两次iterate查询实体对象 */ public void testCache3() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); // 会发出N——1条查询sql(此处为二条) Student student = (Student)session.createQuery( " from Student s where s.id=1 ").iterate().next(); System. out.println( " student.name= " + student.getName()); // 会发出查询id的sql，不会发出查询实体对象的sql，因为iterate使用缓存 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(); } finally { HibernateUtils.closeSession(session); } } /* * * 在同一个session中发出两次iterate查询普通属性对象 */ public void testCache4() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); String name = (String)session.createQuery( " select s.name from Student s where s.id=1 ").iterate().next(); System. out.println( " student.name= " + name); // iterate查询普通属性，一级缓存不会缓存，所以发出sql // 一级缓存只是缓存实体对象的 name = (String)session.createQuery( " select s.name from Student s where s.id=1 ").iterate().next(); System. out.println( " student.name= " + name); session.getTransaction().commit(); } catch(Exception e) { e.printStackTrace(); session.getTransaction().rollback(); } finally { HibernateUtils.closeSession(session); } } /* * * 开启两个session中发出load查询 */ public void testCache5() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); Student 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); } // 第二个session中 try { session = HibernateUtils.getSession(); session.beginTransaction(); // 会发出查询语句，session间不能共享一级缓存的数据 // 因为它会伴随session的生命周期存在和消亡 Student 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); } } /* * * 在同一个session中先save，再发出load查询save过的数据 */ public void testCache6() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); Student stu = new Student(); stu.setName( " 王五 "); Serializable id = session.save(stu); // 不会发出查询sql，因为save是使用缓存的(在数据库中有相应的记录，与一级缓存session有关，纳入它的管理) Student student = (Student)session.load(Student. class, id); System. out.println( " student.name= " + student.getName()); session.getTransaction().commit(); } catch(Exception e) { e.printStackTrace(); session.getTransaction().rollback(); } finally { HibernateUtils.closeSession(session); } } /* * * 向数据库中批量加入1000条数据 */ public void testCache7() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); for ( int i= 0; i< 1000; i++) { Student student = new Student(); student.setName( " s_ " + i); session.save(student); // 每20条数据就强制session将数据持久化 // 同时清除缓存，避免大量数据造成内存溢出 if ( i % 20 == 0) { session.flush(); session.clear(); } } session.getTransaction().commit(); } catch(Exception e) { e.printStackTrace(); session.getTransaction().rollback(); } finally { HibernateUtils.closeSession(session); } } /* * * 在同一个session中依次load clear load */ public void testCache8() { Session session = null; try { session = HibernateUtils.getSession(); session.beginTransaction(); // 不会马上发出sql,因为load支持lazy, Student student = (Student)session.load(Student. class, 1); System. out.println( " student.name= " + student.getName()); // 我们可以用clear或者evict管理一级缓存 session.clear(); // 清除缓存中的数据 // 会发出sql，因为load使用缓存 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); } } }