一级缓存的生命周期和session的生命周期一致，当前sessioin一旦关闭，一级缓存就消失，session间不能共享一级缓存的数据，因此一级缓存也叫session级的缓存或事务级缓存。一级缓存只存实体对象的 ，它不会缓存一般的对象属性（查询缓存可以），即当获得对象后，就将该对象的缓存起来，如果在同一session中如果再去获取这个对象时，它会先判断缓存中有没有该对象的id，如果有就直接从缓存中取出，反之则去数据库中取，取的同时将该对象的缓存起来，有以下方法可以支持一级缓存：
- get()
- save()
- load()
- iterate（查询实体对象）
Query和Criteria的list()和uniqueResult()只会写缓存，但不会从缓存中读取（除非结合查询缓存）。一级缓存无法取消，但可以管理，可以使用session.clear()、session.evict()清除或驱逐。
以下对hibernate的一级缓存进行代码测试：
hibernate.cfg.xml配置：
1<!DOCTYPE hibernate-configuration PUBLIC2 "-//Hibernate/Hibernate Configuration DTD 3.0//EN"3 "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">4 5<hibernate-configuration>6 <session-factory>7 <propertyname="hibernate.connection.url">8 jdbc:mysql://localhost/hibernate_testcache9 </property>10 <propertyname="hibernate.connection.driver_class">11 com.mysql.jdbc.Driver12 </property>13 <propertyname="hibernate.connection.username">root</property>14 <propertyname="hibernate.connection.password">root</property>15 <propertyname="hibernate.dialect">16 org.hibernate.dialect.MySQLDialect17 </property>18 <propertyname="hibernate.show_sql">true</property>19 20 <mappingresource="com/sunyzc/hibernate/testcache/User.hbm.xml"/>21 </session-factory>22</hibernate-configuration>实体Bean：User.java
1packagecom.sunyzc.hibernate.testcache;2 3publicclass User {4 privateint id;5 privateString name;6 7 8 9}User.hbm.xml
1<?xmlversion="1.0"?>2<!DOCTYPE hibernate-mapping PUBLIC3 "-//Hibernate/Hibernate Mapping DTD 3.0//EN"4 "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">5<hibernate-mapping>6 <classname="com.sunyzc.hibernate.testcache.User"table="t_user">7 <idname="id">8 <generatorclass="native"/>9 </id>10 <propertyname="name"column="name"type="java.lang.String"/>11 </class>12</hibernate-mapping>创建表的工具：ExportDB.java
1packagecom.sunyzc.hibernate.testcache;2 3importorg.hibernate.cfg.Configuration;4importorg.hibernate.tool.hbm2ddl.SchemaExport;5 6publicclass ExportDB {7 publicstatic void main(String[] args) {8 9 Configuration cfg = newConfiguration().configure();10 SchemaExport export = newSchemaExport(cfg);11 export.create(true,true);12 }13}HibernateUtils.java
1packagecom.sunyzc.hibernate.testcache;2 3importorg.hibernate.Session;4importorg.hibernate.SessionFactory;5importorg.hibernate.cfg.Configuration;6 7publicclass HibernateUtils {8 privatestatic SessionFactory factory;9 static{10 try{11 factory = newConfiguration().configure().buildSessionFactory();12 }catch(Exception e) {13 e.printStackTrace();14 }15 }16 17 publicstatic Session getSession() {18 returnfactory.openSession();19 }20 21 publicstatic void closeSession(Session session) {22 if(session != null&& session.isOpen()) {23 session.close();24 }25 }26}
测试1：在同一个session中发出两次load查询
1publicvoid testCache1() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 User user = (User) session.load(User.class,1);7 System.out.println("user.name="+ user.getName());8 9 user = (User) session.load(User.class,1);10 System.out.println("user.name="+ user.getName());11 session.getTransaction().commit();12 }catch(Exception e) {13 e.printStackTrace();14 session.getTransaction().rollback();15 }finally{16 HibernateUtils.closeSession(session);17 }18}输出结果：
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from t_user user0_ where user0_.id=?
user.name=user0
user.name=user0
测试2：在同一个session中发出两次get查询
1publicvoid testCache2() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 User user = (User) session.get(User.class,1);7 System.out.println("user.name="+ user.getName());8 9 user = (User) session.get(User.class,1);10 System.out.println("user.name="+ user.getName());11 session.getTransaction().commit();12 }catch(Exception e) {13 e.printStackTrace();14 session.getTransaction().rollback();15 }finally{16 HibernateUtils.closeSession(session);17 }18}输出结果：
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from t_user user0_ where user0_.id=?
user.name=user0
user.name=user0
测试3：在同一个session中发出两次iterate查询实体对象
1publicvoid testCache3() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 String hql = "from User u where u.id=1";7 User user = (User) session.createQuery(hql).iterate().next();8 System.out.println("user.name="+ user.getName());9 10 user = (User) session.createQuery(hql).iterate().next();11 System.out.println("user.name="+ user.getName());12 session.getTransaction().commit();13 }catch(Exception e) {14 e.printStackTrace();15 session.getTransaction().rollback();16 }finally{17 HibernateUtils.closeSession(session);18 }19}输出结果：
Hibernate: select user0_.id as col_0_0_ from t_user user0_ where user0_.id=1
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from t_user user0_ where user0_.id=?
user.name=user0
Hibernate: select user0_.id as col_0_0_ from t_user user0_ where user0_.id=1
user.name=user0
测试4：在通一个session中发出两次uniqueResult查询实体对象不会缓存，如果第一次用uniqueResult第二次用iterate查询是会缓存的
1publicvoid testCache4() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 String hql = "from User u where u.id=1";7 User user = (User) session.createQuery(hql).uniqueResult();8 System.out.println("user.name="+ user.getName());9 user = (User) session.createQuery(hql).uniqueResult();10 System.out.println("user.name="+ user.getName());11 session.getTransaction().commit();12 }catch(Exception e) {13 e.printStackTrace();14 session.getTransaction().rollback();15 }finally{16 HibernateUtils.closeSession(session);17 }18}输出结果：
Hibernate: select user0_.id as id0_, user0_.name as name0_ from t_user user0_ where user0_.id=1
user.name=user0
Hibernate: select user0_.id as id0_, user0_.name as name0_ from t_user user0_ where user0_.id=1
user.name=user0
测试5：在通一个session中发出两次list查询实体对象不会缓存
1publicvoid testCache4() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 List list = session.createQuery("from User u").list();7 System.out.println("user.name="+ list.get(0).getName());8 list = session.createQuery("from User u").list();9 System.out.println("user.name="+ list.get(0).getName());10 session.getTransaction().commit();11 }catch(Exception e) {12 e.printStackTrace();13 session.getTransaction().rollback();14 }finally{15 HibernateUtils.closeSession(session);16 }17}输出结果：
Hibernate: select user0_.id as id0_, user0_.name as name0_ from t_user user0_
user.name=user0
Hibernate: select user0_.id as id0_, user0_.name as name0_ from t_user user0_
user.name=user0
测试6：在同一个session中发出两次iterate查询普通属性不会缓存
1publicvoid testCache4() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 String hql = "select u.name from User u where u.id=1";7 String name = (String) session.createQuery(hql).iterate().next();8 System.out.println("user.name="+ name);9 10 11 name = (String) session.createQuery(hql).iterate().next();12 System.out.println("user.name="+ name);13 session.getTransaction().commit();14 }catch(Exception e) {15 e.printStackTrace();16 session.getTransaction().rollback();17 }finally{18 HibernateUtils.closeSession(session);19 }20}输出结果：
Hibernate: select user0_.name as col_0_0_ from t_user user0_ where user0_.id=1
user.name=user0
Hibernate: select user0_.name as col_0_0_ from t_user user0_ where user0_.id=1
user.name=user0
测试7：开启两个session中发出load查询
1publicvoid testCache7() {2 Session session1 = null;3 Session session2 = null;4 try{5 session1 = HibernateUtils.getSession();6 session2 = HibernateUtils.getSession();7 session1.beginTransaction();8 session2.beginTransaction();9 10 User user1 = (User) session1.load(User.class,1);11 System.out.println("user1.name="+ user1.getName());12 13 14 User user2 = (User) session2.load(User.class,1);15 System.out.println("user2.name="+ user2.getName());16 17 session1.getTransaction().commit();18 session2.getTransaction().commit();19 }catch(Exception e) {20 e.printStackTrace();21 session1.getTransaction().rollback();22 session2.getTransaction().rollback();23 }finally{24 HibernateUtils.closeSession(session1);25 HibernateUtils.closeSession(session2);26 }27}输出结果：
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from t_user user0_ where user0_.id=?
user1.name=user0
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_ from t_user user0_ where user0_.id=?
user2.name=user0
测试8：在同一个session中先save，再发出load查询save过的数据
1publicvoid testCache8() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 User u = newUser();7 u.setName("张三");8 Serializable id = session.save(u);9 10 User user = (User) session.load(User.class, id);11 System.out.println("user.name="+ user.getName());12 session.getTransaction().commit();13 }catch(Exception e) {14 e.printStackTrace();15 session.getTransaction().rollback();16 }finally{17 HibernateUtils.closeSession(session);18 }19}输出结果：
Hibernate: insert into t_user (name) values (?)
user.name=张三
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()。 save方法会将save的对象放入一级缓存中，如果要save大批对象，则应该要及时清空一级缓存，可以采用Session的clear()方法。若不清理缓存，可发现javaw进程占用内存持续增长，到一定程度会内存溢出的。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。
测试9：向数据库中批量加入1000条数据
1publicvoid testCache9() {2 Session session = null;3 try{4 session = HibernateUtils.getSession();5 session.beginTransaction();6 for(inti = 0; i < 1000; i++) {7 User user = newUser();8 user.setName("user"+ i);9 session.save(user);10 11 12 if(i % 20== 0) {13 session.flush();14 session.clear();15 }16 }17 session.getTransaction().commit();18 }catch(Exception e) {19 e.printStackTrace();20 session.getTransaction().rollback();21 }finally{22 HibernateUtils.closeSession(session);23 }24} 0 0
