MyBatis-缓存机制

MyBatis-缓存机制         
    MyBatis 包含一个非常强大的查询缓存特性,它可以非常方便地配置和定制。缓存可以极大的提升查询效率。
    MyBatis系统中默认定义了两级缓存。
    一级缓存和二级缓存：
        一级缓存：(本地缓存):SqlSession级别的缓存，一级缓存是一致开启的，没法关闭。方法之间不共用！
            与数据库同一次会话期间查询到的数据放在本地缓存中。
            以后如果需要获取相同的数据，直接从缓存中拿，没必要再去查询数据库；
        二级缓存(全局缓存)：
        
        ①默认情况下，只有一级缓存（SqlSession级别的缓存，也称为本地缓存）开启。
        ②二级缓存需要手动开启和配置，他是基于namespace级别的缓存。
        ③为了提高扩展性。MyBatis定义了缓存接口Cache。我们可以通过实现Cache接口来自定义二级缓存。

1.测试一级缓存【默认是开启的本地缓存的】

public class EmployeeMapperTest {

    
    public SqlSessionFactory getSqlSessionFactory() throws IOException{
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        return sqlSessionFactory;
    }
    
    @Test
    public void testFirstLevelCache() throws Exception{
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        
        SqlSession sqlSession = sqlSessionFactory.openSession();

        EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
        Employee emp = mapper.getEmpById(1);
        System.out.println(emp);

        EmployeeMapper mapper2 = sqlSession.getMapper(EmployeeMapper.class);
        Employee emp2 = mapper2.getEmpById(1);
       26 　　　　System.out.println(emp==emp2);
        sqlSession.close();

    }
}

因此运行结果为true：

DEBUG 12-26 10:18:07,715 ==>  Preparing: select id,last_name,gender,email from tbl_emp where id = ?   (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:18:07,781 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:18:07,809 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee [id=1, lastName=zhangsan, gender=true, email=zhangsan@163.com]
true

emp==emp2比较两个对象，相当于比较他们在堆中的地址值


一级缓存失效的情况【4种】(没有使用到当前一级缓存的情况，效果就是,还需要再向数据库发出查询)

package com.neuedu.cache;

import java.io.IOException;
import java.io.InputStream;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import com.neuedu.entity.Department;
import com.neuedu.entity.Employee;
import com.neuedu.mapper.EmployeeMapper;

public class testLevelCache {

    public SqlSessionFactory getSqlSessionFactory() throws IOException{
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        return sqlSessionFactory;
    }
    
    
    
    //一级缓存    缓存是SqlSession对象独有的，每一个SqlSession对象都有自己的缓存空间
        //1.sqlSession不同。
        @Test
        public void testFirstLevelCache() throws Exception{
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            
            SqlSession sqlSession = sqlSessionFactory.openSession();

            EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
            Employee emp = mapper.getEmployeeById(1);
            System.out.println(emp);
            
            
            sqlSession=sqlSessionFactory.openSession();
            EmployeeMapper mapper2 = sqlSession.getMapper(EmployeeMapper.class);
            Employee emp2 = mapper2.getEmployeeById(1);
            System.out.println(emp==emp2);

            sqlSession.close();

        }
        
        //2.SqlSession相同，但是查询条件不一样[当前缓存中还没有这个数据]
        @Test
        public void testFirstLevelCache2() throws IOException{
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            
            SqlSession sqlSession = sqlSessionFactory.openSession();
            EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
            Employee emp = mapper.getEmployeeById(2);
            System.out.println(emp);

            Employee emp2 = mapper.getEmployeeById(3);
            System.out.println(emp2);    
            System.out.println(emp == emp2);
            sqlSession.close();
        }
        
        
        
        
        //3.SqlSession相同，但是两次查询之间执行了增删改操作【这次增删改可能对当前数据有影响】
        @Test
        public void testFirstLevelCache3() throws IOException{
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            
            SqlSession sqlSession = sqlSessionFactory.openSession(true);
            EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
        
            Employee emp = mapper.getEmployeeById(2);
            System.out.println(emp);
            mapper.insertEmployee(new Employee(null, "孙悟空", true, "sunwukong@163.com", new Department(1,"开发部")));
            Employee emp2 = mapper.getEmployeeById(2);
            System.out.println(emp2);    
            System.out.println(emp == emp2);
            sqlSession.close();
            
        }
        
        
        //4.SqlSession相同,手动清除了一级缓存[缓存清空]。     
        @Test
        public void testGetEmployee() throws IOException{
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            
            SqlSession sqlSession = sqlSessionFactory.openSession(true);
            EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
            Employee emp = mapper.getEmployeeById(2);
            System.out.println(emp);
            //手动清空缓存
            sqlSession.clearCache();
            Employee emp2 = mapper.getEmployeeById(2);
            System.out.println(emp2);    
            System.out.println(emp == emp2);
            sqlSession.close();
        }
}

运行结果：

DEBUG 12-26 10:16:30,839 Cache Hit Ratio [com.neuedu.mapper.EmployeeMapper]: 0.0  (LoggingCache.java:62) 
DEBUG 12-26 10:16:31,271 ==>  Preparing: select * from employee where id=?   (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:16:31,334 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:16:31,360 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee [id=1, username=张三丰, gender=true, email=zhngsanfeng@163.com, dept=null]
DEBUG 12-26 10:16:31,361 Cache Hit Ratio [com.neuedu.mapper.EmployeeMapper]: 0.0  (LoggingCache.java:62) 
DEBUG 12-26 10:16:31,457 ==>  Preparing: select * from employee where id=?   (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:16:31,458 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:16:31,461 <==      Total: 1  (BaseJdbcLogger.java:145) 
false

都是执行两次sql语句

2.二级缓存：【全局缓存】：基于namespace级别的缓存:一个namespace对应一个二级缓存。
          【一级缓存的范围还是太小了，每次SqlSession一关闭，一级缓存中的数据就消失】
          所以从这个角度讲:能跨sqlSession的缓存为二级缓存！

mybatis-config.xml：

<!-- 开启二级缓存 -->
<setting name="cacheEnabled" value="true"/>

EmployeeMapper.xml：

<!-- 
    二级缓存是namespace级别的，一个namespace【mapper接口】对应一个二级缓存
 -->
<cache></cache>

实体类还需要实现Serializable接口：

public class Employee implements Serializable{

}

//二级缓存    
        @Test
        public void testLevelCache() throws Exception{
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            
            SqlSession sqlSession = sqlSessionFactory.openSession();

            EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
            Employee emp = mapper.getEmployeeById(1);
            System.out.println(emp);
            sqlSession.close();
            
            SqlSession sqlSession2 = sqlSessionFactory.openSession();
            EmployeeMapper mapper2 = sqlSession2.getMapper(EmployeeMapper.class);
            Employee emp2 = mapper2.getEmployeeById(1);
            System.out.println(emp==emp2);

            sqlSession.close();

        }

运行结果：

DEBUG 12-26 10:26:31,261 Cache Hit Ratio [com.neuedu.mapper.EmployeeMapper]: 0.0  (LoggingCache.java:62) 
DEBUG 12-26 10:26:31,731 ==>  Preparing: select * from employee where id=?   (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:26:31,800 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 12-26 10:26:31,830 <==      Total: 1  (BaseJdbcLogger.java:145) 
Employee [id=1, username=张三丰, gender=true, email=zhngsanfeng@163.com, dept=null]
DEBUG 12-26 10:26:31,844 Cache Hit Ratio [com.neuedu.mapper.EmployeeMapper]: 0.5  (LoggingCache.java:62) 
false

执行一次sql语句