（九）mybatis学习之缓存
一级缓存（sqlsession缓存）
mybatis默认支持一级缓存。
在操作数据库时需要构造sqlSession对象，在sqlSession对象中有一个数据结构（hashMap）用于存储缓存数据。
不同的sqlSession之间的缓存数据区域（hashMap)是互相不影响的
示例1：
@Test
public void findUserByIdTest() throws IOException{
String resource = "sqlMapConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
//创建会话工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//根据会话工厂创建会话
SqlSession sqlSession= sqlSessionFactory.openSession();
//通过sqlSession操作数据库
//第一个参数：映射文件中的配置的id值，等于=namespace.id
//第二个参数：指定映射文件中所匹配的parameterType类型参数
User user = sqlSession.selectOne("xxxx.findUserById", "40287d8147b119bb0147b11c1e710000");
System.out.println(user);
user = sqlSession.selectOne("xxxx.findUserById", "40287d8147b119bb0147b11c1e710000");
System.out.println(user);
//释放资源
sqlSession.close();
}控制台日志：
2016-04-04 16:38:29,714 [main] DEBUG [xxxx.findUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 16:38:29,763 [main] DEBUG [xxxx.findUserById] - ==> Parameters: 40287d8147b119bb0147b11c1e710000(String) 2016-04-04 16:38:29,830 [main] DEBUG [xxxx.findUserById] - <== Total: 1 User [id=40287d8147b119bb0147b11c1e710000, userName=zero, nickName=卡洛, password=e10adc3949ba59abbe56e057f20f883e, sex=1, birthday=Sat Aug 02 00:00:00 GMT+08:00 2014, province=日本, city=大阪府, county=大阪市, createTime=Thu Aug 07 23:35:15 GMT+08:00 2014] User [id=40287d8147b119bb0147b11c1e710000, userName=zero, nickName=卡洛, password=e10adc3949ba59abbe56e057f20f883e, sex=1, birthday=Sat Aug 02 00:00:00 GMT+08:00 2014, province=日本, city=大阪府, county=大阪市, createTime=Thu Aug 07 23:35:15 GMT+08:00 2014]
同一个sqlSession里就会使用到一级缓存。
二级缓存（mapper缓存）
备注：使用缓存的pojo类需要实现序列化
二级缓存是mapper级别的缓存，多个sqlSsession去操作同一个mapper的sql语句，多个sqlSession可以共用二级缓存；
二级缓存区域是按namespace划分的，即每一个namespace的mapper都有一个二级缓存区域。
如果sqlsession去执行了commit操作，那么会清空一、二缓存中的数据。
。
二级缓存是跨sqlSession的。
如：
sqlsession1查询用户id为1的用户信息，查询到用户信息会将数据存储到二级缓存中。
sqlsession2查询用户id为1的用户信息，会先去缓存中查找是否存在数据。
二级缓存是需要配置开启。
（1）需要在settings里开启二级缓存总开关
<!-- 开启二级缓存总开关，默认是true --> <setting name="cacheEnabled" value="true"/>
（2）需要在mapper.xml里开启二级缓存（开启某个mapper的二级缓存）
在userMapper.xml里开启二级缓存
<!-- 开启本mapper的namespace下的二级缓存 --> <cache/>
用户信息pojo类（实现序列化）：
public class User implements Serializable{
private List<String> idsList;
private String id;
private String userName; //用户名
private String nickName; //昵称
private String password; //登录密码
private Integer sex; //性别
private Date birthday; //出生日期
private String province; //省
private String city; //市
private String county; //县
private Date createTime; //创建时间
//...get set
测试类：
@Test
public void testFindUserById() throws Exception {
//获取代理对象
UserMapper userMapper = (UserMapper) context.getBean("userMapper");
User user1 = userMapper.findUserById("1");
System.out.println(user1);
// User user = new User();
// user.setId("1");
// user.setUserName("222xxxxxx");
// userMapper.updateUserByTrim(user);
User user2 = userMapper.findUserById("1");
System.out.println(user2);
}
控制台日志：
2016-04-04 17:02:05,971 [main] DEBUG [mapper.UserMapper.findUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 17:02:06,017 [main] DEBUG [mapper.UserMapper.findUserById] - ==> Parameters: 1(String) 2016-04-04 17:02:06,046 [main] DEBUG [mapper.UserMapper.findUserById] - <== Total: 1 2016-04-04 17:02:06,057 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1860b23] 2016-04-04 17:02:06,058 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource User [id=1, userName=222xxxxxx, nickName=1, password=1, sex=1, birthday=null, province=null, city=null, county=null, createTime=null] 2016-04-04 17:02:06,064 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Creating a new SqlSession 2016-04-04 17:02:06,064 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1a338a0] was not registered for synchronization because synchronization is not active 2016-04-04 17:02:06,065 [main] DEBUG [mapper.UserMapper] - Cache Hit Ratio [mapper.UserMapper]: 0.5 2016-04-04 17:02:06,065 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1a338a0] User [id=1, userName=222xxxxxx, nickName=1, password=1, sex=1, birthday=null, province=null, city=null, county=null, createTime=null]由日志我们看到，第二次查询的时候是没有执行sql语句的。其中日志里有个Cache Hit Ratio [mapper.UserMapper]: 0.5，这是表示命中率。0.5的意思是，一共查询了两次（调用了两次findUserById），有一次命中（第二次从缓存中找到了）。
当在测试类里加入一个更新操作时，此时会清空缓存。
测试代码：
@Test
public void testFindUserById() throws Exception {
//获取代理对象
UserMapper userMapper = (UserMapper) context.getBean("userMapper");
User user1 = userMapper.findUserById("1");
System.out.println(user1);
User user = new User();
user.setId("1");
user.setUserName("222xxxxxx");
userMapper.updateUserByTrim(user);
User user2 = userMapper.findUserById("1");
System.out.println(user2);
}
控制台日志：
2016-04-04 17:04:48,229 [main] DEBUG [com.mchange.v2.resourcepool.BasicResourcePool] - awaitAvailable(): [unknown] 2016-04-04 17:04:48,570 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@f6b73d [wrapping: com.mysql.jdbc.JDBC4Connection@17845e3]] will not be managed by Spring 2016-04-04 17:04:48,579 [main] DEBUG [mapper.UserMapper.findUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 17:04:48,627 [main] DEBUG [mapper.UserMapper.findUserById] - ==> Parameters: 1(String) 2016-04-04 17:04:48,661 [main] DEBUG [mapper.UserMapper.findUserById] - <== Total: 1 2016-04-04 17:04:48,673 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1456008] 2016-04-04 17:04:48,673 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource User [id=1, userName=222xxxxxx, nickName=1, password=1, sex=1, birthday=null, province=null, city=null, county=null, createTime=null] 2016-04-04 17:04:48,675 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Creating a new SqlSession 2016-04-04 17:04:48,675 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1e19787] was not registered for synchronization because synchronization is not active 2016-04-04 17:04:48,720 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 17:04:48,720 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@1952a24 [wrapping: com.mysql.jdbc.JDBC4Connection@17845e3]] will not be managed by Spring 2016-04-04 17:04:48,720 [main] DEBUG [mapper.UserMapper.updateUserByTrim] - ==> Preparing: update tab_user set userName=? where id=? 2016-04-04 17:04:48,721 [main] DEBUG [mapper.UserMapper.updateUserByTrim] - ==> Parameters: 222xxxxxx(String), 1(String) 2016-04-04 17:04:48,722 [main] DEBUG [mapper.UserMapper.updateUserByTrim] - <== Updates: 1 2016-04-04 17:04:48,723 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@1e19787] 2016-04-04 17:04:48,723 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource 2016-04-04 17:04:48,723 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Creating a new SqlSession 2016-04-04 17:04:48,723 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@160eaa6] was not registered for synchronization because synchronization is not active 2016-04-04 17:04:48,723 [main] DEBUG [mapper.UserMapper] - Cache Hit Ratio [mapper.UserMapper]: 0.0 2016-04-04 17:04:48,723 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Fetching JDBC Connection from DataSource 2016-04-04 17:04:48,723 [main] DEBUG [org.mybatis.spring.transaction.SpringManagedTransaction] - JDBC Connection [com.mchange.v2.c3p0.impl.NewProxyConnection@ec9d4b [wrapping: com.mysql.jdbc.JDBC4Connection@17845e3]] will not be managed by Spring 2016-04-04 17:04:48,723 [main] DEBUG [mapper.UserMapper.findUserById] - ==> Preparing: select * from tab_user where id=? 2016-04-04 17:04:48,723 [main] DEBUG [mapper.UserMapper.findUserById] - ==> Parameters: 1(String) 2016-04-04 17:04:48,726 [main] DEBUG [mapper.UserMapper.findUserById] - <== Total: 1 2016-04-04 17:04:48,726 [main] DEBUG [org.mybatis.spring.SqlSessionUtils] - Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@160eaa6] 2016-04-04 17:04:48,727 [main] DEBUG [org.springframework.jdbc.datasource.DataSourceUtils] - Returning JDBC Connection to DataSource User [id=1, userName=222xxxxxx, nickName=1, password=1, sex=1, birthday=null, province=null, city=null, county=null, createTime=null]
禁用二级缓存
设置useCache=false来禁用二级缓存。useCache默认值是true，即默认是先从缓存查询<!-- 使用useCache="false"禁用缓存，默认值是true -->
<select id="findUserById" parameterType="java.lang.String" resultType="User" useCache="false">
select * from tab_user where id=#{id}
</select>此时使用该查询时，每次都会从数据库中查询获取数据了。
刷新缓存
刷新缓存就是清空缓存
使用flushCache="true"来配置刷新缓存（清空缓存），默认值就是true
一般执行完commit操作完都需要刷新缓存，是因为<insert>、<update>、<delete>标签中有个flushCache的属性，该属性值默认是true。表示执行完刷新缓存。