mybatis详解-(18)一级缓存

我会带着你远行 2022-06-03 05:24 194阅读 0赞

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。MyBatis 3 中的缓存实现的很多改进都已经实现了,使得它更加强大而且易于配置。

默认情况下是没有开启缓存（二级缓存-全局缓存）的,除了局部的 session 缓存,可以增强变现而且处理循环依赖也是必须的。  
局部session也就是说的一级缓存（本地缓存），本篇内容将对一级缓存做一下展示和介绍

# 1.一级缓存效果 #

下面先看一下一级缓存的效果：

**创建实体类**  
Employee

package org.mybatis.cache;
    
    public class Employee { 
        private Integer id;
        private String lastName;
        private String email;
        private String gender;
    
        public Integer getId() {
            return id;
        }
        public void setId(Integer id) {
            this.id = id;
        }
        public String getLastName() {
            return lastName;
        }
        public void setLastName(String lastName) {
            this.lastName = lastName;
        }
        public String getEmail() {
            return email;
        }
        public void setEmail(String email) {
            this.email = email;
        }
        public String getGender() {
            return gender;
        }
        public void setGender(String gender) {
            this.gender = gender;
        }
        public String toString() {
            return "Employee [id=" + id + ", lastName=" + lastName + ", email="
                    + email + ", gender=" + gender + "]";
        }
    
    }

**创建接口**  
EmployeeMapper

package org.mybatis.cache;
    
    public interface EmployeeMapper { 
    
        public Employee getEmpById(Integer id);
    
    }

**创建映射配置文件**  
cachemapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
    
    <mapper namespace="org.mybatis.cache.EmployeeMapper">
    
        <select id="getEmpById" resultType="org.mybatis.cache.Employee">
            select * from mybatis_employee me where me.id = #{id}
        </select>
    
    </mapper>

**创建主配置文件**  
mybatis-config-cache.xml

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">
    <configuration>
    
        <properties resource="db.properties"></properties>
    
        <settings>
            <setting name="mapUnderscoreToCamelCase" value="true"/>
            <setting name="lazyLoadingEnabled" value="true" />
            <setting name="aggressiveLazyLoading" value="false"/>
        </settings>
    
        <typeAliases>
            
            <package name="org.mybatis.cache"/>
        </typeAliases>
    
    
        
        <environments default="dev_mysql">
            <environment id="dev_mysql">
                <transactionManager type="JDBC" />
                <dataSource type="POOLED">
                    <property name="driver" value="${mysql.driver}" />
                    <property name="url" value="${mysql.url}" />
                    <property name="username" value="${mysql.username}" />
                    <property name="password" value="${mysql.password}" />
                </dataSource>
            </environment>
    
            <environment id="test_mysql">
                <transactionManager type="JDBC"></transactionManager>
                <dataSource type="POOLED">
                    <property name="driver" value="${mysql.drivertest}" />
                    <property name="url" value="${mysql.urltest}" />
                    <property name="username" value="${mysql.usernametest}" />
                    <property name="password" value="${mysql.passwordtest}" />
                </dataSource>
            </environment>
    
        </environments>
    
        <mappers>
            <mapper resource="mapper/cachemapper.xml"/>
        </mappers>
    
    
    </configuration>

**创建测试类**

package org.mybatis.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;
    
    public class MybatisTest { 
    
        @Test
        public void testMybatis() {
            String resource = "mybatis-config-cache.xml";//全局配置文件
    
            InputStream inputStream = null;
            SqlSessionFactory sqlSessionFactory = null;
            SqlSession sqlSession = null;
            try {
                inputStream = Resources.getResourceAsStream(resource);
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
                sqlSession = sqlSessionFactory.openSession();
    
                EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
    
    
                Employee emp1 = mapper.getEmpById(2);
                System.out.println(emp1);
                System.out.println("`######");
                Employee emp2 = mapper.getEmpById(2);
                System.out.println(emp2);
    
    
                sqlSession.close();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
            }
        }
    }

查询id=2的Employee在测试类中操作了两次，测试结果：

> DEBUG - ooo Using Connection \[com.mysql.jdbc.JDBC4Connection@47db50c5\]  
> DEBUG - ==> Preparing: select \* from mybatis\_employee me where me.id = ?  
> DEBUG - ==> Parameters: 2(Integer)  
> Employee \[id=2, lastName=lisi, email=tang\_greatman@qq.com, gender=2\]  
> \`\#\#\#\#\#\#  
> Employee \[id=2, lastName=lisi, email=tang\_greatman@qq.com, gender=2\]  
> true  
> DEBUG - Resetting autocommit to true on JDBC Connection \[com.mysql.jdbc.JDBC4Connection@47db50c5\]  
> DEBUG - Closing JDBC Connection \[com.mysql.jdbc.JDBC4Connection@47db50c5\]  
> DEBUG - Returned connection 1205555397 to pool.

结果中只执行了一次sql查询，剩下的一次查询实际上就是从mybatis缓存中获取到的数据。

# 2.一级缓存失效情况 #

**1）sqlSession不同**

@Test
    public void testMybatis() {
        String resource = "mybatis-config-cache.xml";//全局配置文件
    
        InputStream inputStream = null;
        SqlSessionFactory sqlSessionFactory = null;
    
        try {
            inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    
            SqlSession sqlSession1 = sqlSessionFactory.openSession();
            EmployeeMapper mapper1 = sqlSession1.getMapper(EmployeeMapper.class);
    
    
            Employee emp1 = mapper1.getEmpById(2);
            System.out.println(emp1);
            System.out.println("`######");
    
            SqlSession sqlSession2 = sqlSessionFactory.openSession();
            EmployeeMapper mapper2 = sqlSession2.getMapper(EmployeeMapper.class);
    
            Employee emp2 = mapper2.getEmpById(2);
            System.out.println(emp2);
    
    
    
            sqlSession1.close();
            sqlSession2.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
        }
    }

**2）sqlSession相同，查询条件不同（一级缓存中还没有缓存该数据）**

@Test
    public void testMybatis() {
        String resource = "mybatis-config-cache.xml";//全局配置文件
    
        InputStream inputStream = null;
        SqlSessionFactory sqlSessionFactory = null;
    
        try {
            inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    
            SqlSession sqlSession1 = sqlSessionFactory.openSession();
            EmployeeMapper mapper1 = sqlSession1.getMapper(EmployeeMapper.class);
    
    
            Employee emp1 = mapper1.getEmpById(2);
            System.out.println(emp1);
    
            System.out.println("`######");
    
            Employee emp2 = mapper1.getEmpById(3);
            System.out.println(emp2);
    
            System.out.println(emp1 == emp2);
    
            sqlSession1.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
        }
    }

**3）sqlSession相同，两次查询之间做了增删改操作**

EmployeeMapper接口添加新增方法

public void addEmp(Employee emp);

cachemapper.xml配置文件中添加新增方法配置

<insert id="addEmp" parameterType="org.mybatis.cache.Employee" useGeneratedKeys="true" keyProperty="id">
            insert into mybatis_employee 
            (last_name,email,gender) 
            values 
            (#{lastName},#{email},#{gender});
        </insert>

@Test
        public void testMybatis() {
            String resource = "mybatis-config-cache.xml";//全局配置文件
    
            InputStream inputStream = null;
            SqlSessionFactory sqlSessionFactory = null;
    
            try {
                inputStream = Resources.getResourceAsStream(resource);
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    
                SqlSession sqlSession1 = sqlSessionFactory.openSession();
                EmployeeMapper mapper1 = sqlSession1.getMapper(EmployeeMapper.class);
    
    
                Employee emp1 = mapper1.getEmpById(2);
                System.out.println(emp1);
    
                System.out.println("`######");
    
                //执行增删改操作
                mapper1.addEmp(new Employee(null, "xiaohua", "295273263@qq.com", "1"));
                sqlSession1.commit();
    
                Employee emp2 = mapper1.getEmpById(2);
                System.out.println(emp2);
    
                System.out.println(emp1 == emp2);
    
                sqlSession1.close();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
    
            }
        }

4）sqlSession相同，手动清除了一级缓存

@Test
    public void testMybatis() {
        String resource = "mybatis-config-cache.xml";//全局配置文件
    
        InputStream inputStream = null;
        SqlSessionFactory sqlSessionFactory = null;
    
        try {
            inputStream = Resources.getResourceAsStream(resource);
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    
            SqlSession sqlSession1 = sqlSessionFactory.openSession();
            EmployeeMapper mapper1 = sqlSession1.getMapper(EmployeeMapper.class);
    
    
            Employee emp1 = mapper1.getEmpById(2);
            System.out.println(emp1);
    
            System.out.println("`######");
    
            //手动清楚sqlSession缓存
            sqlSession1.clearCache();
    
            Employee emp2 = mapper1.getEmpById(2);
            System.out.println(emp2);
    
            System.out.println(emp1 == emp2);
    
            sqlSession1.close();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
    
        }
    }

以上介绍了一级缓存失效或者不起作用的几种情况。  
当然还有一点可能会被忽视：在执行操作未完成情况下数据库该条内容发生改变，mybatis是无法判别的，mybatis还是取sqlSession缓存操作

@Test
        public void testMybatis() {
            String resource = "mybatis-config-cache.xml";//全局配置文件
    
            InputStream inputStream = null;
            SqlSessionFactory sqlSessionFactory = null;
    
            try {
                inputStream = Resources.getResourceAsStream(resource);
                sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    
                SqlSession sqlSession1 = sqlSessionFactory.openSession();
                EmployeeMapper mapper1 = sqlSession1.getMapper(EmployeeMapper.class);
    
    
                Employee emp1 = mapper1.getEmpById(2);
                System.out.println(emp1);
    
                System.out.println("`######");
    
                try {
                    Thread.sleep(1000*20);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
    
                Employee emp2 = mapper1.getEmpById(2);
                System.out.println(emp2);
    
                System.out.println(emp1 == emp2);
    
                sqlSession1.close();
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
    
            }
        }

# 3.一级缓存原理 #

一级缓存原理实现：实际在sqlsession中有一个数据区域，是map结构，这个区域就是一级缓存区域。  
一级缓存中的key是由sql语句、条件、statement等信息组成一个唯一值。一级缓存中的value，就是查询出的结果对象。  
![这里写图片描述][SouthEast]  
图片引自[mybatis–查询缓存][mybatis]

[SouthEast]: /images/20220603/d6dd3ea1374249c99467e21f18ddb2b7.png
[mybatis]: http://blog.csdn.net/csdn_gia/article/details/54948791