mybatis的缓存机制及⽤例介绍在实际的项⽬开发中，通常对数据库的查询性能要求很⾼，⽽mybatis提供了查询缓存来缓存数据，从⽽达到提⾼查询性能的要求。
mybatis的查询缓存分为⼀级缓存和⼆级缓存，⼀级缓存是SqlSession级别的缓存，⼆级缓存时mapper级别的缓存，⼆级缓存是多个SqlSession共享的。
mybatis通过缓存机制减轻数据压⼒，提⾼数据库性能。
⼀级缓存：mybatis的⼀级缓存是SQLSession级别的缓存，在操作数据库时需要构造SqlSession对象，在对象中有⼀个HashMap⽤于存储缓存数据，不同的SqlSession之间缓存数据区域（HashMap）是互相不影响的。
⼀级缓存的作⽤域是SqlSession范围的，当在同⼀个SqlSession中执⾏两次相同的sql语句时，第⼀次执⾏完毕会将数据库中查询的数据写到缓存（内存）中，第⼆次查询时会从缓存中获取数据，不再去底层进⾏数据库查询，从⽽提⾼了查询效率。
需要注意的是：如果SqlSession执⾏了DML操作（insert、update、delete），并执⾏commit()操作，mybatis则会清空SqlSession中的⼀级缓存，这样做的⽬的是为了保证缓存数据中存储的是最新的信息，避免出现脏读现象。
当⼀个SqlSession结束后该SqlSession中的⼀级缓存也就不存在了，Mybatis默认开启⼀级缓存，不需要进⾏任何配置。
注意：Mybatis的缓存机制是基于id进⾏缓存，也就是说Mybatis在使⽤HashMap缓存数据时，是使⽤对象的id作为key，⽽对象作为value保存例⼦说明：⼯程架构图：pom.xml<project xmlns="/POM/4.0.0" xmlns:xsi="/2001/XMLSchema-instance" xsi:schemaLocation="/POM/4.0.0 /xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion><groupId>com.gm.test</groupId><artifactId>MybatisCacheTest</artifactId><version>0.0.1-SNAPSHOT</version><packaging>war</packaging><properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><!-- spring版本号 --><spring.version>4.3.10.RELEASE</spring.version><!-- mybatis版本号 --><mybatis.version>3.2.6</mybatis.version><!-- log4j⽇志⽂件管理包版本 --><slf4j.version>1.7.7</slf4j.version><log4j.version>1.2.17</log4j.version></properties><dependencies><dependency><groupId>junit</groupId><artifactId>junit</artifactId><version>4.11</version><!-- 表⽰开发的时候引⼊，发布的时候不会加载此包 --><scope>test</scope></dependency><!-- spring核⼼包 --><dependency><groupId>org.springframework</groupId><artifactId>spring-core</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-web</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-oxm</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-aop</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-tx</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-jdbc</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-webmvc</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-aop</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-context-support</artifactId><version>${spring.version}</version></dependency><dependency><groupId>org.springframework</groupId><artifactId>spring-test</artifactId><version>${spring.version}</version></dependency><!-- aop --><dependency><groupId>org.aspectj</groupId><artifactId>aspectjweaver</artifactId><version>1.8.4</version></dependency><!-- mybatis核⼼包 --><dependency><groupId>org.mybatis</groupId><artifactId>mybatis</artifactId><version>${mybatis.version}</version></dependency><!-- mybatis/spring包 --><dependency><groupId>org.mybatis</groupId><artifactId>mybatis-spring</artifactId><version>1.2.2</version></dependency><!-- 导⼊java ee jar 包 --><dependency><groupId>javax</groupId><artifactId>javaee-api</artifactId><version>7.0</version></dependency><!-- 导⼊Mysql数据库链接jar包 --><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><version>5.1.30</version></dependency><!-- 导⼊dbcp的jar包，⽤来在applicationContext.xml中配置数据库 --> <dependency><groupId>commons-dbcp</groupId><artifactId>commons-dbcp</artifactId><version>1.2.2</version></dependency><!-- JSTL标签类 --><dependency><groupId>jstl</groupId><artifactId>jstl</artifactId><version>1.2</version></dependency><!-- ⽇志⽂件管理包 --><!-- log start --><dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>${log4j.version}</version></dependency><!-- 格式化对象，⽅便输出⽇志 --><dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.1.41</version></dependency><dependency><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId><version>${slf4j.version}</version></dependency><dependency><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId><version>${slf4j.version}</version></dependency><!-- log end --></dependencies><build><plugins><plugin><artifactId>maven-war-plugin</artifactId><configuration><version>3.1</version></configuration></plugin></plugins></build></project>sql语句：CREATE TABLE employee(id INT(11) PRIMARY KEY AUTO_INCREMENT,loginname VARCHAR(18),PASSWORD VARCHAR(18),NAME VARCHAR(18) DEFAULT NULL,sex CHAR(2) DEFAULT NULL,age INT(11) DEFAULT NULL,phone VARCHAR(21),sal DOUBLE,state VARCHAR(18));INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('jack','123456','杰克','男',26,'12345678936',9800,'ACTIVE'); INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('rose','123456','露丝','⼥',21,'78965412395',6800,'ACTIVE'); INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('tom','123456','汤姆','男',25,'139********',8800,'ACTIVE'); INSERT INTO employee (loginname,PASSWORD,NAME,sex,age,phone,sal,state) VALUES('alice','123456','爱丽丝','⼥',20,'74185296375',5800,'ACTIVE'); log4j.properties:log4j.rootLogger=ERROR, stdout.gm.mapper.EmployeeMapper=DEBUGlog4j.appender.console=org.apache.log4j.ConsoleAppenderyout=org.apache.log4j.PatternLayoutyout.ConversionPattern=[%-12d{HH\:mm\:ss.SS}] [%p] %l %m%nlog4j.appender.stdout=org.apache.log4j.ConsoleAppenderyout=org.apache.log4j.PatternLayoutyout.ConversionPattern=%d %p [%c] - %m%nmybatis-config.xml:<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE configuration PUBLIC "-////DTD Config 3.0//EN" "/dtd/mybatis-3-config.dtd"><configuration><!-- 全局参数 --><settings><!-- 设置但JDBC类型为空时,某些驱动程序要指定值,default:OTHER，插⼊空值时不需要指定类型 --><setting name="jdbcTypeForNull" value="NULL" /><!-- 要使延迟加载⽣效必须配置下⾯两个属性 --><setting name="lazyLoadingEnabled" value="true" /><setting name="aggressiveLazyLoading" value="false" /><setting name="logImpl" value="LOG4J" /></settings><!-- <plugins> <plugin interceptor="com.manager.util.MybatisInterceptor"></plugin></plugins> --><environments default="mysql"><environment id="mysql"><!-- 指定事务管理类型，type="JDBC"指直接简单使⽤了JDBC的提交和回滚设置 --><transactionManager type="JDBC" /><!-- dataSource指数据源配置，POOLED是JDBC连接对象的数据源连接池的实现 --><dataSource type="POOLED"><property name="driver" value="com.mysql.jdbc.Driver" /><property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis" /><property name="username" value="root" /><property name="password" value="1qaz@wsx" /></dataSource></environment></environments><mappers><mapper resource="com/gm/mapper/EmployeeMapper.xml" /></mappers></configuration>Employee.java:package com.gm.domain;import java.io.Serializable;public class Employee implements Serializable {/****/private static final long serialVersionUID = 1L;private Integer id;private String loginname;private String password;private String name;private String sex;private String age;private String phone;private Double sal;private String state;public Integer getId() {return id;}public void setId(Integer id) {this.id = id;}public String getLoginname() {return loginname;}public void setLoginname(String loginname) {this.loginname = loginname;}public String getPassword() {return password;}public void setPassword(String password) {this.password = password;}public String getName() {return name;}public void setName(String name) { = name;}public String getSex() {return sex;}public void setSex(String sex) {this.sex = sex;}public String getAge() {return age;}public void setAge(String age) {this.age = age;}public String getPhone() {return phone;}public void setPhone(String phone) {this.phone = phone;}public Double getSal() {return sal;}public void setSal(Double sal) {this.sal = sal;}public String getState() {return state;}public void setState(String state) {this.state = state;}}EmployeeMapper.xml:<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE mapper PUBLIC "-////DTD Mapper 3.0//EN" "/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.gm.mapper.EmployeeMapper"><select id="selectEmployeeById" parameterType="int"resultType="com.gm.domain.Employee">SELECT * FROM employee WHERE id = #{id}</select><!-- 查询所有Employee --><select id="selectAllEmployee" parameterType="int"resultType="com.gm.domain.Employee">SELECT * FROM employee</select><!-- 根据id删除Employee --><delete id="deleteEmployeeById" parameterType="int">DELETE FROMemployee WHERE id = #{id}</delete></mapper>EmployeeMapper.java:package com.gm.mapper;import java.util.List;import com.gm.domain.Employee;public interface EmployeeMapper {// 根据id查询EmployeeEmployee selectEmployeeById(Integer id);// 查询所有EmployeeList<Employee> selectAllEmployee();// 根据id删除Employeevoid deleteEmployeeById(Integer id);}⼯⼚⼯具类：MySqlSessionFactory.java:package com.gm.factory;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;public class MySqlSessionFactory {private static SqlSessionFactory sqlSessionFactory = null;static {try {InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) {e.printStackTrace();}}// 获取SqlSession对象的静态⽅法public static SqlSession getSqlSession() {return sqlSessionFactory.openSession();}// 获取SqlSessionFactory的静态⽅法public static SqlSessionFactory getSessionFactory() {return sqlSessionFactory;}}执⾏⽅法类⼀：OneLevelCacheTest.javapackage com.gm.test;import org.apache.ibatis.session.SqlSession;import com.gm.domain.Employee;import com.gm.factory.MySqlSessionFactory;import com.gm.mapper.EmployeeMapper;public class OneLevelCacheTest {public void testCache1() {// 使⽤⼯⼚类获得SqlSession对象SqlSession sqlSession = MySqlSessionFactory.getSqlSession();// 获得EmployeeMapping对象EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);Employee employee1 = employeeMapper.selectEmployeeById(1);System.out.println(employee1);// 再次查询id为1的Employee对象，因为是同⼀个SqlSession,所以会从之前的⼀级缓存中查找数据Employee employee2 = employeeMapper.selectEmployeeById(1);System.out.println(employee2);sqlSession.close();}public static void main(String[] args) {OneLevelCacheTest t = new OneLevelCacheTest();t.testCache1();}public void testCache2() {// 使⽤⼯⼚类获得SqlSession对象SqlSession sqlSession = MySqlSessionFactory.getSqlSession();// 获得EmployeeMapping对象EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);Employee employee1 = employeeMapper.selectEmployeeById(1);System.out.println(employee1);// 执⾏delete操作employeeMapper.deleteEmployeeById(4);// commit提交mit();// 再次查询id为1的Employee对象，因为DML操作会清空sqlSession缓存，所以会再次执⾏select语句Employee employee2 = employeeMapper.selectEmployeeById(1);System.out.println(employee2);sqlSession.close();}public void testCache3() {// 使⽤⼯⼚类获得SqlSession对象SqlSession sqlSession = MySqlSessionFactory.getSqlSession();// 获得EmployeeMapping对象EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);Employee employee1 = employeeMapper.selectEmployeeById(1);System.out.println(employee1);// 关闭⼀级缓存sqlSession.close();// 再次访问，需要再次获取⼀级缓存，然后才能查找数据，否则会抛出异常sqlSession = MySqlSessionFactory.getSqlSession();// 再次获得EmployeeMapper对象employeeMapper = sqlSession.getMapper(EmployeeMapper.class);Employee employee2 = employeeMapper.selectEmployeeById(1);System.out.println(employee2);sqlSession.close();}public void testCache4() {// 使⽤⼯⼚类获得SqlSession对象SqlSession sqlSession = MySqlSessionFactory.getSqlSession();// 获得EmployeeMapping对象EmployeeMapper employeeMapper = sqlSession.getMapper(EmployeeMapper.class);Employee employee1 = employeeMapper.selectEmployeeById(1);System.out.println(employee1);// 关闭⼀级缓存sqlSession.close();// 再次访问，需要再次获取⼀级缓存，然后才能查找数据，否则会抛出异常sqlSession = MySqlSessionFactory.getSqlSession();// 再次获得EmployeeMapper对象employeeMapper = sqlSession.getMapper(EmployeeMapper.class);// 再次查询id为1的Employee对象，因为DML操作会清空sqlSession缓存，所以会再次执⾏select语句Employee employee2 = employeeMapper.selectEmployeeById(1);System.out.println(employee2);sqlSession.close();}}运⾏之后结果：2018-08-22 09:35:47,858 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ? 2018-08-22 09:35:47,886 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)2018-08-22 09:35:47,899 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1com.gm.domain.Employee@56235b8ecom.gm.domain.Employee@56235b8e通过观察结果可以看出，在第⼀次查询id为1的Employee对象时执⾏了⼀条select语句，但是第⼆次获取id为1的Employee对象时并没有执⾏select语句，因为此时⼀级缓存也就是SqlSession缓存中已经缓存了id为1的Employee对象，Mybatis直接从缓存中将对象取出来，并没有再次去查询数据库，所以第⼆次也就没有执⾏select语句执⾏⽅法类⼆：public static void main(String[] args) {OneLevelCacheTest t = new OneLevelCacheTest();t.testCache2();}运⾏之后的结果：2018-08-22 09:41:17,024 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?2018-08-22 09:41:17,045 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)2018-08-22 09:41:17,058 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1com.gm.domain.Employee@56235b8e2018-08-22 09:41:17,058 DEBUG [com.gm.mapper.EmployeeMapper.deleteEmployeeById] - ==> Preparing: DELETE FROM employee WHERE id = ?2018-08-22 09:41:17,059 DEBUG [com.gm.mapper.EmployeeMapper.deleteEmployeeById] - ==> Parameters: 4(Integer)2018-08-22 09:41:17,079 DEBUG [com.gm.mapper.EmployeeMapper.deleteEmployeeById] - <== Updates: 12018-08-22 09:41:17,125 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - ==> Preparing: SELECT * FROM employee WHERE id = ?2018-08-22 09:41:17,125 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - ==> Parameters: 1(Integer)2018-08-22 09:41:17,128 DEBUG [com.gm.mapper.EmployeeMapper.selectEmployeeById] - <== Total: 1com.gm.domain.Employee@370736d9结果分析：在第⼀次查询id为1的employee对象时执⾏了⼀条select语句，接下来执⾏了⼀个delete并commit操作，Mybatis为了保证缓存中存储的是最新消息，会清空SqlSession缓存。
