第三节 二级缓存
这里我们使用的是Mybatis自带的二级缓存。
1、代码测试二级缓存
①开启二级缓存功能
在想要使用二级缓存的Mapper配置文件中加入cache标签
<!-- 加入cache标签启用二级缓存功能 --><br /> <cache/>
②让实体类支持序列化
public class Employee implements Serializable {
③junit测试
这个功能的测试操作需要将SqlSessionFactory对象设置为成员变量
@Test
public void testSecondLevelCacheExists() {
SqlSession session = factory.openSession();
EmployeeMapper mapper = session.getMapper(EmployeeMapper.class);
Employee employee = mapper.selectEmployeeById(2);
System.out.println("employee = " + employee);
// 在执行第二次查询前,关闭当前SqlSession<br /> session.close();
// 开启一个新的SqlSession<br /> session = factory.openSession();
mapper = session.getMapper(EmployeeMapper.class);
employee = mapper.selectEmployeeById(2);
System.out.println("employee = " + employee);
session.close();
}
打印效果:
DEBUG 12-01 09:44:27,057 Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.0 (LoggingCache.java:62)
DEBUG 12-01 09:44:27,459 ==> Preparing: select emp_id,emp_name,emp_salary,emp_gender,emp_age from t_emp where emp_id=? (BaseJdbcLogger.java:145)
DEBUG 12-01 09:44:27,510 ==> Parameters: 2(Integer) (BaseJdbcLogger.java:145)
DEBUG 12-01 09:44:27,536 <== Total: 1 (BaseJdbcLogger.java:145)
employee = Employee{empId=2, empName=’AAAAAA’, empSalary=6666.66, empAge=20, empGender=’male’}
DEBUG 12-01 09:44:27,622 Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.5 (LoggingCache.java:62)
employee = Employee{empId=2, empName=’AAAAAA’, empSalary=6666.66, empAge=20, empGender=’male’}
④缓存命中率
日志中打印的Cache Hit Ratio叫做缓存命中率
Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.0(0/1)
Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.5(1/2)
Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.6666666666666666(2/3)
Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.75(3/4)
Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.8(4/5)
缓存命中率=命中缓存的次数/查询的总次数
2、查询结果存入二级缓存的时机
结论:SqlSession关闭的时候,一级缓存中的内容会被存入二级缓存
// 1.开启两个SqlSession
SqlSession session01 = factory.openSession();
SqlSession session02 = factory.openSession();
// 2.获取两个EmployeeMapper
EmployeeMapper employeeMapper01 = session01.getMapper(EmployeeMapper.class);
EmployeeMapper employeeMapper02 = session02.getMapper(EmployeeMapper.class);
// 3.使用两个EmployeeMapper做两次查询,返回两个Employee对象
Employee employee01 = employeeMapper01.selectEmployeeById(2);
Employee employee02 = employeeMapper02.selectEmployeeById(2);
// 4.比较两个Employee对象
System.out.println(“employee02.equals(employee01) = “ + employee02.equals(employee01));
上面代码打印的结果是:
DEBUG 12-01 10:10:32,209 Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.0 (LoggingCache.java:62)
DEBUG 12-01 10:10:32,570 ==> Preparing: select emp_id,emp_name,emp_salary,emp_gender,emp_age from t_emp where emp_id=? (BaseJdbcLogger.java:145)
DEBUG 12-01 10:10:32,624 ==> Parameters: 2(Integer) (BaseJdbcLogger.java:145)
DEBUG 12-01 10:10:32,643 <== Total: 1 (BaseJdbcLogger.java:145)
DEBUG 12-01 10:10:32,644 Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.0 (LoggingCache.java:62)
DEBUG 12-01 10:10:32,661 ==> Preparing: select emp_id,emp_name,emp_salary,emp_gender,emp_age from t_emp where emp_id=? (BaseJdbcLogger.java:145)
DEBUG 12-01 10:10:32,662 ==> Parameters: 2(Integer) (BaseJdbcLogger.java:145)
DEBUG 12-01 10:10:32,665 <== Total: 1 (BaseJdbcLogger.java:145)
employee02.equals(employee01) = false
修改代码:
// 1.开启两个SqlSession
SqlSession session01 = factory.openSession();
SqlSession session02 = factory.openSession();
// 2.获取两个EmployeeMapper
EmployeeMapper employeeMapper01 = session01.getMapper(EmployeeMapper.class);
EmployeeMapper employeeMapper02 = session02.getMapper(EmployeeMapper.class);
// 3.使用两个EmployeeMapper做两次查询,返回两个Employee对象
Employee employee01 = employeeMapper01.selectEmployeeById(2);
// ※第一次查询完成后,把所在的SqlSession关闭,使一级缓存中的数据存入二级缓存
session01.close();
Employee employee02 = employeeMapper02.selectEmployeeById(2);
// 4.比较两个Employee对象
System.out.println(“employee02.equals(employee01) = “ + employee02.equals(employee01));
// 5.另外一个SqlSession用完正常关闭
session02.close();
打印结果:
DEBUG 12-01 10:14:06,804 Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.0 (LoggingCache.java:62)
DEBUG 12-01 10:14:07,135 ==> Preparing: select emp_id,emp_name,emp_salary,emp_gender,emp_age from t_emp where emp_id=? (BaseJdbcLogger.java:145)
DEBUG 12-01 10:14:07,202 ==> Parameters: 2(Integer) (BaseJdbcLogger.java:145)
DEBUG 12-01 10:14:07,224 <== Total: 1 (BaseJdbcLogger.java:145)
DEBUG 12-01 10:14:07,308 Cache Hit Ratio [com.atguigu.mybatis.EmployeeMapper]: 0.5 (LoggingCache.java:62)
employee02.equals(employee01) = false
3、二级缓存相关配置
在Mapper配置文件中添加的cache标签可以设置一些属性:
- eviction属性:缓存回收策略
- LRU(Least Recently Used) – 最近最少使用的:移除最长时间不被使用的对象。
- FIFO(First in First out) – 先进先出:按对象进入缓存的顺序来移除它们。
- SOFT – 软引用:移除基于垃圾回收器状态和软引用规则的对象。
- WEAK – 弱引用:更积极地移除基于垃圾收集器状态和弱引用规则的对象。
- 默认的是 LRU。
- flushInterval属性:刷新间隔,单位毫秒
- 默认情况是不设置,也就是没有刷新间隔,缓存仅仅调用语句时刷新
- size属性:引用数目,正整数
- 代表缓存最多可以存储多少个对象,太大容易导致内存溢出
- readOnly属性:只读,true/false
- true:只读缓存;会给所有调用者返回缓存对象的相同实例。因此这些对象不能被修改。这提供了很重要的性能优势。
- false:读写缓存;会返回缓存对象的拷贝(通过序列化)。这会慢一些,但是安全,因此默认是 false。
- 是否使用阻塞性缓存BlockingCache
- true:在查询缓存时锁住对应的Key,如果缓存命中了则会释放对应的锁,否则会在查询数据库以后再释放锁,保证只有一个线程到数据库中查找指定key对应的数据
- false:不使用阻塞性缓存,性能更好