背景
小白：师傅，现在的java开发，统一的都是spring boot，使用原生mybatis或者spring+mybatis的都很少，能否讲讲sprng boot怎么集成mybaits的？集成的原理是什么？
扫地僧：得益于spring boot各种开发好的starters，spring boot集成mybatis相对容易。其中的原理则是利用Spring boot的自动注册原理，来集成SqlSessionFactory，最终在spring boot容器中生成各种mapper的bean。代码撸下，看看就知道了。
Spring Boot集成mybatis实例
准备工作
mysql数据库,本实例的版本为:8.0.16
mysql客户端SQLyog(免费，不需要注册码)或者navicat for mysql
创建数据库davidwang456和表
CREATEdatabase davidwang456;use davidwang456;DROPTABLEIFEXISTSstudent;CREATETABLE`student`(`id`int(11)NOTNULLAUTO_INCREMENT,`first_name`varchar(100)DEFAULTNULL,`last_name`varchar(100)DEFAULTNULL,`age`int(11)DEFAULTNULL,PRIMARYKEY(`id`))ENGINE=InnoDBDEFAULTCHARSET=utf8mb4;mock数据
INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(1,'wang1','david1',21);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(2,'wang2','david2',22);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(3,'wang3','david3',23);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(4,'wang4','david4',24);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(5,'wang5','david5',25);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(6,'wang6','david6',26);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(7,'wang7','david7',27);INSERTINTO`student`(`id`,`first_name`,`last_name`,`age`)VALUES(8,'wang8','david8',28);创建maven项目
添加依赖
pom.xml
<?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>2.5.1</version> <relativePath/> <!-- lookup parent from repository --> </parent> <groupId>com.davidwang456.integrate</groupId> <artifactId>SpringBootIntegrateMybatis</artifactId> <version>3.8.0-SNAPSHOT</version> <name>SpringBootIntegrateMybatis</name> <description>Spring Boot integrate mybatis</description> <properties> <java.version>1.8</java.version> </properties> <dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <dependency> <groupId>org.mybatis.spring.boot</groupId> <artifactId>mybatis-spring-boot-starter</artifactId> <version>2.1.3</version> </dependency> <dependency> <groupId>com.zaxxer</groupId> <artifactId>HikariCP</artifactId> </dependency> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <version>1.18.16</version> <scope>provided</scope> </dependency> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-test</artifactId> <scope>test</scope> </dependency> </dependencies> <build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build> </project>实体
数据库实体
packagecom.davidwang456.mybatis.integrate;importjava.io.Serializable;importlombok.Data;@DatapublicclassStudentDTOimplementsSerializable{privatestaticfinallongserialVersionUID=1L;//字段privateIntegerid;privateStringfirstName;privateStringlastName;privateIntegerage;@OverridepublicStringtoString(){return"student [id="+id+", firstName="+firstName+", lastName="+lastName+", age="+age+']';}}查询实体
packagecom.davidwang456.mybatis.integrate;importlombok.Data;@DatapublicclassStudentQueryDTO{//字段privateIntegerid;privateStringfirstName;privateStringlastName;privateIntegerage;//排序列privateStringsort;//排序 DESC|ASCprivateStringorderBy;}配置
数据库配置
application.properties
mybatis.mapper-locations=classpath:mapper/*.xml mybatis.configuration.map-underscore-to-camel-case=true mybatis.configuration.log-impl=org.apache.ibatis.logging.stdout.StdOutImpl spring.datasource.url=jdbc:mysql://localhost:3306/davidwang456?characterEncoding=UTF-8&useSSL=false&useLegacyDatetimeCode=false&serverTimezone=UTC spring.datasource.username=root spring.datasource.password=wangwei456 spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver # Hikari will use the above plus the following to setup connection pooling spring.datasource.type=com.zaxxer.hikari.HikariDataSource spring.datasource.hikari.minimum-idle=5 spring.datasource.hikari.maximum-pool-size=15 spring.datasource.hikari.auto-commit=true spring.datasource.hikari.idle-timeout=30000 spring.datasource.hikari.pool-name=HikariCP spring.datasource.hikari.max-lifetime=1800000 spring.datasource.hikari.connection-timeout=30000其中，配置mybatis.configuration.map-underscore-to-camel-case=true定义了支持驼峰形式，配置mybatis.configuration.log-impl=org.apache.ibatis.logging.stdout.StdOutImpl定义了日志打印的类，类似与xml配置：
<settings><settingname="logImpl"value="STDOUT_LOGGING"/><settingname="mapUnderscoreToCamelCase"value="true"/></settings>在src/main/resources目录下，定义**mapper.xml文件。
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPEmapperPUBLIC"-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mappernamespace="com.davidwang456.mybatis.integrate.StudentMapper"><selectid="getStudentInfoByCondition"parameterType="com.davidwang456.mybatis.integrate.StudentQueryDTO"resultType="com.davidwang456.mybatis.integrate.StudentDTO"useCache="false"><bindname="first"value="'%'+firstName+'%'"/><bindname="last"value="'%'+lastName+'%'"/>select id, first_name , last_name , age from student where 1=1<iftest="id!=null">and id=#{id}</if><iftest="firstName!=null and firstName!=''">and first_name like #{first}</if><iftest="lastName!=null and lastName!=''">and last_name like #{last}</if><iftest="age!=null and age!=0">and age=#{age}</if></select></mapper>Mapper文件
packagecom.davidwang456.mybatis.integrate;importjava.util.List;importorg.apache.ibatis.annotations.Mapper;@MapperpublicinterfaceStudentMapper{publicList<StudentDTO>getStudentInfoByCondition(StudentQueryDTOstudentQueryDTO);}测试程序
packagecom.davidwang456.mybatis.integrate;importjava.util.List;importorg.springframework.beans.factory.annotation.Autowired;importorg.springframework.stereotype.Controller;importorg.springframework.web.bind.annotation.PostMapping;importorg.springframework.web.bind.annotation.RequestBody;importorg.springframework.web.bind.annotation.ResponseBody;@ControllerpublicclassTestController{@AutowiredprivateStudentMapperstudentMapper;@PostMapping("/list")@ResponseBodypublicList<StudentDTO>list(@RequestBodyStudentQueryDTOstudentQueryDTO){returnstudentMapper.getStudentInfoByCondition(studentQueryDTO);}}启动spring主程序：
packagecom.davidwang456.mybatis.integrate;importorg.springframework.boot.SpringApplication;importorg.springframework.boot.autoconfigure.SpringBootApplication;@SpringBootApplicationpublicclassSpringBootIntegrateMybatisApplication{publicstaticvoidmain(String[]args){SpringApplication.run(SpringBootIntegrateMybatisApplication.class,args);}}此时默认启动端口8080.使用postman进行测试：
curl--location--requestPOST'http://127.0.0.1:8080/list' \--header 'Content-Type:application/json' \--data-raw '{"id":null,"firstName":"wang","lastName":"david","age":null}'返回结果符合预期:
[{"id":1,"firstName":"wang1","lastName":"david1","age":21},{"id":2,"firstName":"wang2","lastName":"david2","age":22},{"id":3,"firstName":"wang3","lastName":"david3","age":23},{"id":4,"firstName":"wang4","lastName":"david4","age":24},{"id":5,"firstName":"wang5","lastName":"david5","age":25},{"id":6,"firstName":"wang6","lastName":"david6","age":26},{"id":7,"firstName":"wang7","lastName":"david7","age":27},{"id":8,"firstName":"wang8","lastName":"david8","age":28}]至此，spring boot集成mybatis就完成了。如果是老系统升级到spring boot，则可以利用配置属性mybatis.config-location来定义MyBatis.xml配置文件。其它mybatis的属性可自行参考。接下来我们看看spring boot集成mybatis的内部原理吧。
spring boot集成mybatis的内部原理
我们接下来说：springboot是如何和mybatis进行整合的
1.首先，springboot中使用mybatis需要用到mybatis-spring-boot-start，可以理解为mybatis开发的整合springboot的jar包
有一个关键点先说明：前面也提到过，不管是mybatis和spring整合，还是和springboot整合，都需要做两个操作：
1.把当前接口和对应的mapperProxyFactory存入到knownMappers中，
2.把sql包装成mappedStatement，存入到mappedStatements这个map中
1.springboot项目中，@SpringbootApplication注解上，有一个@EnableAutoConfiguration注解，而@EnableAutoConfiguration注解又利用了@Import注解，注入了一个ImportSelector的实现类 AutoConfigurationImportSelector.class
2.AutoConfigurationImportSelector在selectImports()方法中有一行重要的代码：
List configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.getBeanClassLoader());
这行代码内部，会从所有jar包中，META-INF/spring.factories文件中，加载EnableAutoConfiguration对应的实现类，那mybatis-spring-boot-autoconfigure.jar包中，配置了两个实现类，
1 org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ 2 org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,\ 3 org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration3.我们来说 MybatisAutoConfiguration.class
1 @org.springframework.context.annotation.Configuration 2 @ConditionalOnClass({ SqlSessionFactory.class, SqlSessionFactoryBean.class }) 3 @ConditionalOnSingleCandidate(DataSource.class) 4 @EnableConfigurationProperties(MybatisProperties.class) 5 @AutoConfigureAfter({ DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class }) 6 public class MybatisAutoConfiguration implements InitializingBean { 7 8 9 }这里有一个点，就是 @EnableConfigurationProperties(MybatisProperties.class)；点开MybatisProperties.class文件会发现，这里面声明的就是，在application.properties配置文件中，mybatis提供的配置信息：比如mybatis.mapper-locations=classpath:mapping/*Mapper.xml；这个点，不细说了，后面可能会写一篇自定义starter的学习笔记，到时候 再详细写
4.在MybatisAutoConfiguration中有一个静态内部类 AutoConfiguredMapperScannerRegistrar 实现了ImportBeanDefinitionRegistrar;
所以，spring在refresh的时候，会执行这个类的 registerBeanDefinitions()方法，将 MapperScannerConfigurer存到了beanDefinitionMap中
1 @Override 2 public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { 3 4 5 if (!AutoConfigurationPackages.has(this.beanFactory)) { 6 logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled."); 7 return; 8 } 9 //中间删除了部分代码 10 registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition()); 11 }5.MapperScannerConfigurer是BeanDefinitionRegistryPostProcessor的实现类，在refresh() --> invokeBeanFactoryPostProcessors(beanFactory)中，会遍历所有beanFactoryPostProcessor和BeanDefinitionRegistrtPostProcessor的实现类，依次执行postProcessorBeanDefinitionRegistrar()方法
MapperScannerConfigurer的postProcessBeanDefinitionRegistry()方法，会执行扫描方法，这里的扫描方法，和mapperScannerRegistrar的registerBeanDefinitions中的doScan是一样的，这里扫描的包是在，初始化MapperScannerConfigurer的时候，在执行完属性注入之后，调用了截图中的方法，把当前pvs中的basePackage传到MapperScannerConfigurer中，这里是如何传过去的，待研究
6.在将mapper扫描完之后，需要进行sql的解析，在和springboot整合之后，需要在配置文件中配置当前要扫描的mapper.xml文件，
mybatis.mapper-locations=classpath:mapping/*Mapper.xml
这里的mapperLocation,是在sqlSesionFactorybean中进行解析的，在第3步中的自动配置类中，通过@Bean,注入了SqlSessionFactory,
在sqlSessionFactory()方法最后，会调用factoryBean.getObject()方法，这里其实调用的就是SqlSessionFactory的getObject()方法，
1 @Override 2 public SqlSessionFactory getObject() throws Exception { 3 if (this.sqlSessionFactory == null) { 4 afterPropertiesSet(); 5 } 6 7 8 return this.sqlSessionFactory; 9 }从afterPropertiesSet()方法，一直往下追，会追到同类中的buildSqlSessionFactory()方法，在这个方法中，判断如果当前mapperlocation不为null，就进行解析
1 if (this.mapperLocations != null) { 2 if (this.mapperLocations.length == 0) { 3 LOGGER.warn(() -> "Property 'mapperLocations' was specified but matching resources are not found."); 4 } else { 5 for (Resource mapperLocation : this.mapperLocations) { 6 if (mapperLocation == null) { 7 continue; 8 } 9 try { 10 XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(), 11 targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments()); 12 xmlMapperBuilder.parse(); 13 } catch (Exception e) { 14 throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e); 15 } finally { 16 ErrorContext.instance().reset(); 17 } 18 LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'"); 19 } 20 } 21 }在xmlMapperBuilder.parse()就是原生mybatis在解析xml文件时，需要调用的方法
7.在service中注入mapper接口，在初始化service，注入依赖的mapper接口时，还是调用的mapperFactorybean.getObject()方法来获取代理对象
springboot整合mybatis 和 spring+mybatis整合时，解析xml文件有一个区别：
spring-mybatis是利用mapperFactorybean的checkDao()方法来解析xml，put数据到mappedStatement和knowmappers
springboot是利用SqlSessionFactoryBean的getObject()来解析xml,put数据到mappedStatement和knowMappers
