MyBaits自动配置原理

前言

首先我们建立一个SpringBoot工程，导入mybatis-spring-boot-starter依赖。

<dependency>
    <groupId>org.mybatis.spring.boot</groupId>
    <artifactId>mybatis-spring-boot-starter</artifactId>
    <version>2.1.4</version>
</dependency>

导入后发现这个依赖其实就是帮助我们导入了mybatis需要的依赖，其中和自动配置相关最重要的一个就是mybatis-spring-boot-autoconfigure

<dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-jdbc</artifactId>
    </dependency>
    <dependency>
      <groupId>org.mybatis.spring.boot</groupId>
      <artifactId>mybatis-spring-boot-autoconfigure</artifactId>
    </dependency>
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
    </dependency>
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis-spring</artifactId>
    </dependency>
  </dependencies>

MyBatis自动配置中是如何工作的

如上面分析自动配置的关键类我们就从mybatis-spring-boot-autoconfigure开始着手分析。

spring.factories

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

从spring.factories文件看到这里通过SPI机制加载了两个类

• MybatisAutoConfiguration
• MybatisLanguateDriverAutoConfiguration.

MybatisAutoConfiguration

//表示这是一个Spring配置类
@Configuration 
//这个类需要在classpath中存在SqlSessionFactory和SqlSessionFactoryBean时才生效
@ConditionalOnClass({SqlSessionFactory.class, SqlSessionFactoryBean.class}) --
//这个类需要有一个DataSource的Canidate注册到Spring容器中 
@ConditionalOnSingleCandidate(DataSource.class)
//使MybatisProperties注解类生效
@EnableConfigurationProperties({MybatisProperties.class})
//需要在DataSourceAutoConfiguration和MybatisLanguageDriverAutoConfiguration自动配置之后执行
@AutoConfigureAfter({DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class})
public class MybatisAutoConfiguration implements InitializingBean {
   
}

MybatisAutoConfiguration#sqlSessionFactory

@Bean
@ConditionalOnMissingBean
public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
	//创建一个SqlSessionFactoryBean, 在mybatis-spring项目下
    SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
	factory.setDataSource(dataSource);
	factory.setVfs(SpringBootVFS.class);
	if (StringUtils.hasText(this.properties.getConfigLocation())) {
		     factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation()));
	}
    
    //应用Configuration对象
	this.applyConfiguration(factory);
	if (this.properties.getConfigurationProperties() != null) {
		factory.setConfigurationProperties(this.properties.getConfigurationProperties());
	}

	if (!ObjectUtils.isEmpty(this.interceptors)) {
		factory.setPlugins(this.interceptors);
	}

	if (this.databaseIdProvider != null) {
		factory.setDatabaseIdProvider(this.databaseIdProvider);
	}

	if (StringUtils.hasLength(this.properties.getTypeAliasesPackage())) {
		factory.setTypeAliasesPackage(this.properties.getTypeAliasesPackage());
	}

	if (this.properties.getTypeAliasesSuperType() != null) {
		factory.setTypeAliasesSuperType(this.properties.getTypeAliasesSuperType());
	}

	if (StringUtils.hasLength(this.properties.getTypeHandlersPackage())) {
		factory.setTypeHandlersPackage(this.properties.getTypeHandlersPackage());
	}

	if (!ObjectUtils.isEmpty(this.typeHandlers)) {
		factory.setTypeHandlers(this.typeHandlers);
	}

	if (!ObjectUtils.isEmpty(this.properties.resolveMapperLocations())) {
		factory.setMapperLocations(this.properties.resolveMapperLocations());
	}

	Set<String> factoryPropertyNames = (Set)Stream.of((new BeanWrapperImpl(SqlSessionFactoryBean.class)).getPropertyDescriptors()).map(FeatureDescriptor::getName).collect(Collectors.toSet());
	Class<? extends LanguageDriver> defaultLanguageDriver = this.properties.getDefaultScriptingLanguageDriver();
	if (factoryPropertyNames.contains("scriptingLanguageDrivers") && !ObjectUtils.isEmpty(this.languageDrivers)) {
		factory.setScriptingLanguageDrivers(this.languageDrivers);
		if (defaultLanguageDriver == null && this.languageDrivers.length == 1) {
			defaultLanguageDriver = this.languageDrivers[0].getClass();
		}
	}

    //设置默认的语言驱动类
	if (factoryPropertyNames.contains("defaultScriptingLanguageDriver")) {
		factory.setDefaultScriptingLanguageDriver(defaultLanguageDriver);
	}
  
    //这里默认会返回一个DefaultSqlSessionFactory对象
	return factory.getObject();
}

MybatisAutoConfiguration#sqlSessionTemplate

@Bean
@ConditionalOnMissingBean
public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
	ExecutorType executorType = this.properties.getExecutorType();
	return executorType != null ? new SqlSessionTemplate(sqlSessionFactory, executorType) : new SqlSessionTemplate(sqlSessionFactory);
}

到这里也就知道了MyBatis自动配置其实就是替我们完成了SqlSessionFactory和SqlSessionTempate的创建, 省去了自己导入相关依赖和配置相关Bean的麻烦.

MybatisLanguageDriverAutoConfiguration

这个类的配置是对各个语言的支持，比如Thymeleaf， Velocity，LegacyVelociy， FreeMarker等视图组件的支持。

@Configuration
@ConditionalOnClass({LanguageDriver.class})
public class MybatisLanguageDriverAutoConfiguration {
    private static final String CONFIGURATION_PROPERTY_PREFIX = "mybatis.scripting-language-driver";

    public MybatisLanguageDriverAutoConfiguration() {
    }

    @Configuration
    @ConditionalOnClass({ThymeleafLanguageDriver.class})
    public static class ThymeleafConfiguration {
        public ThymeleafConfiguration() {
        }
    }

    @Configuration
    @ConditionalOnClass({VelocityLanguageDriver.class, VelocityLanguageDriverConfig.class})
    public static class VelocityConfiguration {
        public VelocityConfiguration() {
        }
    }

    @Configuration
    @ConditionalOnClass({Driver.class})
         @ConditionalOnMissingClass({"org.mybatis.scripting.velocity.VelocityLanguageDriverConfig"})
    public static class LegacyVelocityConfiguration {
        public LegacyVelocityConfiguration() {
        }
    }

    @Configuration
    @ConditionalOnClass({FreeMarkerLanguageDriver.class, FreeMarkerLanguageDriverConfig.class})
    public static class FreeMarkerConfiguration {
        public FreeMarkerConfiguration() {
        }
    }

    @Configuration
    @ConditionalOnClass({FreeMarkerLanguageDriver.class})
    @ConditionalOnMissingClass({"org.mybatis.scripting.freemarker.FreeMarkerLanguageDriverConfig"})
    public static class LegacyFreeMarkerConfiguration {
        public LegacyFreeMarkerConfiguration() {
        }
    }
}

MybatisLanguageDriverAutoConfiguration类在org.mybatis.spring.boot.autoconfigure包下，我删掉了内部静态类下的代码，为了保持这个类看起来更直观

自定义Mapper是如何被扫描的

业务开发中，我们是声明接口（Mapper），那么我们自定义的Mapper是如何被扫描的呢, 我们继续顺着MybatisAutoConfiguration代码分析,其内部包含了一个AutoConfiguredMapperScannerRegistrar的内部静态类.

AutoConfiguredMapperScannerRegistrar

registerBeanDefinitions

public static class AutoConfiguredMapperScannerRegistrar 
    implements BeanFactoryAware, ImportBeanDefinitionRegistrar {
	private BeanFactory beanFactory;

	public AutoConfiguredMapperScannerRegistrar() {
	}

	public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
		if (!AutoConfigurationPackages.has(this.beanFactory)) {
			
		} else {
	        //1.获取到SpringBoot的基础包路径
			List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
			
            //2.生成一个BeanDefinition的构造器，用于构建MapperScannerConfigurer的                         //BeanDefinition
			BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
			builder.addPropertyValue("processPropertyPlaceHolders", true);
            //3.设置@Mapper注解的接口才会被当成Mapper接口
			builder.addPropertyValue("annotationClass", Mapper.class);
            
			builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));
			BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class);
			//4.获取MapperScannerConfigurer的属性名称
            Set<String> propertyNames = (Set)Stream.of(beanWrapper.getPropertyDescriptors()).map(FeatureDescriptor::getName).collect(Collectors.toSet());
			if (propertyNames.contains("lazyInitialization")) {
				builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}");
			}

			if (propertyNames.contains("defaultScope")) {
				builder.addPropertyValue("defaultScope", "${mybatis.mapper-default-scope:}");
			}
            //5.这里添加一个MapperScannerConfigurer的BeanDefinition对象，也就是注入一个
            //MapperScannerConfigurer对象
			registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
		}
	}

	public void setBeanFactory(BeanFactory beanFactory) {
		this.beanFactory = beanFactory;
	}
}

AutoConfiguredMapperScannerRegistrar类是MybatisAutoConfiguration的内部静态类，位于包org.mybatis.spring.boot.autoconfigure下。

可以看到这个类实现了ImportBeanDefinitionRegistrar接口，ImportBeanDefinitionRegistrar接口是Spring用来动态注册Bean的，也就是会向Spring容器中注入一个BeanDefinition， 这个BeanDefinition就是MapperScannerConfigurer。
ImportBeanDefinitionRegistrar实现类只能通过其他类@Import的方式来加载，通常是配置类或者启动类，所以MybatisAutoConfiguration类下还有一个内部类MapperScannerRegistrarNotFoundConfiguration如下。
​

@Configuration
@Import({MybatisAutoConfiguration.AutoConfiguredMapperScannerRegistrar.class})
@ConditionalOnMissingBean({MapperFactoryBean.class, MapperScannerConfigurer.class})
public static class MapperScannerRegistrarNotFoundConfiguration implements InitializingBean {
        
}

​

这个方法里最后会调用接口BeanDefinitionRegistry.registerBeanDefinition， beanName是"org.mybatis.spring.mapper.MapperScannerConfigurer", registerBeanDefinition方法实际会调用DefaultListableBeanFactory.registerBeanDefinition。DefaultListableBeanFactory是BeanDefinitionRegistry接口的实现类。

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory 
    implements ConfigurableListableBeanFactory, 
               BeanDefinitionRegistry, 
               Serializable {

}

AutoConfiguredMapperScannerRegistrar类和MapperScanner注解的作用是一样的，如果你没有通过以下三种配置方式扫描Mapper接口的包路径

• 配置MapperScannerConfigurer扫描器类型的Spring Bean
• @Mapper注解
• <mybatis: scan/>标签

那么这里就会通过AutoConfiguredMapperScannerRegistrar类添加一个MapperScannerConfigurer扫描器对象，去扫描SpringBoot包设置的基础包路径，也就是启动类的同级目录。 如果设置了@Mapper注解，则会当成Mapper接口解析，那么这里自动配置则不生效。
​

MapperScannerConfigurer

MapperScannerConfigurer

MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口，BeanDefinitionRegistryPostProcessor 接口又继承了BeanFactoryPostProcessor接口， 也就是说在MapperScannerConfigurer类里需要实现这两个接口的方法。

public interface BeanDefinitionRegistryPostProcessor extends BeanFactoryPostProcessor {
    void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry var1) throws BeansException;
}

@FunctionalInterface
public interface BeanFactoryPostProcessor {
    void postProcessBeanFactory(ConfigurableListableBeanFactory var1) throws BeansException;
}

在MapperScannerConfigurer类里可以看到这里只实现了postProcessBeanDefinitionRegistry。

BeanDefinitionRegistryPostProcessor

Spring里有两个用来动态注册Bean到容器中（BeanDefinitionRegistryPostProcessor和ImportBeanDefinitionRegistrar）。ImportBeanDefinitionRegistrar上文中有提到。
BeanDefinitionRegistryPostProcessor接口实现了BeanFactoryPostProcessor接口，是Spring框架的BeanDefinitionRegistry的后置处理器，用来注册额外的BeanDefinition，postProcessBeanDefinitionRegistry方法会在所有的beanDefinition被加载了，但是所有的Bean还没创建前调用。BeanDefinitionRegistryPostProcessor经常被用来注册BeanFactoryPostProcessor的BeanDefinition。
​

​

postProcessBeanDefinitionRegistry

public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
    
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        if (this.processPropertyPlaceHolders) {
            this.processPropertyPlaceHolders();
        }

        ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
        scanner.setAddToConfig(this.addToConfig);
        scanner.setAnnotationClass(this.annotationClass);
        scanner.setMarkerInterface(this.markerInterface);
        scanner.setSqlSessionFactory(this.sqlSessionFactory);
        scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
        scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
        scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
        scanner.setResourceLoader(this.applicationContext);
        scanner.setBeanNameGenerator(this.nameGenerator);
        scanner.setMapperFactoryBeanClass(this.mapperFactoryBeanClass);
        if (StringUtils.hasText(this.lazyInitialization)) {
            scanner.setLazyInitialization(Boolean.valueOf(this.lazyInitialization));
        }

        if (StringUtils.hasText(this.defaultScope)) {
            scanner.setDefaultScope(this.defaultScope);
        }

        scanner.registerFilters();
        scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ",; \t\n"));
    }
}

MapperScannerConfigurer在包org.mybatis.spring.mapper下.

​

​

这里会调用ClassPathMapperScanner.scan()，而ClassPathMapperScanner又继承了ClassPathBeanDefinitionScanner，所以这里scan()会调用ClassPathBeanDefinitionScanner.scan()， 而ClassPathBeanDefinitionScanner.scan() 第二句代码又调用了this.doScan(basePackages)， this.doScan()又调用了ClassPathMapperScanner.doScan(), 而这个方法第一句代码又调用了super.doScan(basePackages)，父子类来回互相调用，有点晕头转向的。

org.mybatis.spring.mapper.ClassPathMapperScanner
org.springframework.context.annotation.ClassPathBeanDefinitionScanner这个类在spring-context.jar

​

ClassPathMapperScanner

ClassPathBeanDefinitionScanner#scan

public int scan(String... basePackages) {
    int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
    this.doScan(basePackages);
    if (this.includeAnnotationConfig) {
        AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
    }

    return this.registry.getBeanDefinitionCount() - beanCountAtScanStart;
}

ClassPathMapperScanner#doScan()
这个方法里在mybatis自动配置算比较重要的一个方法，也就是帮助我们自动配置MapperFactoryBean， 会把根据basePackage注册进Spring容器的BeanDefinition的beanClass设置成MapperFactoryBean。

public Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);
    if (beanDefinitions.isEmpty()) {
        LOGGER.warn(() -> {
            return "No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.";
        });
    } else {
        //这是一个关键方法，会处理已经注册进Spring容器的beanDefinition，也就是会把
        //已经注册进Spring容器的beanDefinitiond的beanClass为MapperFactoryBean
        this.processBeanDefinitions(beanDefinitions);
    }

    return beanDefinitions;
}

ClassPathBeanDefinitionScanner#doScan()

protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
    Assert.notEmpty(basePackages, "At least one base package must be specified");
    Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet();
    String[] var3 = basePackages;
    int var4 = basePackages.length;
    
    for(int var5 = 0; var5 < var4; ++var5) {
        String basePackage = var3[var5];
        //这个方法会扫描指定basePackage下被@Mapper注解标注的接口
        Set<BeanDefinition> candidates = this.findCandidateComponents(basePackage);
        Iterator var8 = candidates.iterator();

        while(var8.hasNext()) {
            BeanDefinition candidate = (BeanDefinition)var8.next();
            ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
            candidate.setScope(scopeMetadata.getScopeName());
            //这里获取beanName, 默认值是类名首字母小写
            String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
            if (candidate instanceof AbstractBeanDefinition) {
                this.postProcessBeanDefinition((AbstractBeanDefinition)candidate, beanName);
            }

            if (candidate instanceof AnnotatedBeanDefinition) {
                AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition)candidate);
            }
            //检查对应的Mapper接口是否被注册进Spring容器中。
            if (this.checkCandidate(beanName, candidate)) {
                BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                beanDefinitions.add(definitionHolder);
                this.registerBeanDefinition(definitionHolder, this.registry);
            }
        }
    }
    
    //这个集合返回以后 Spring容器会将里面的所有内容注册到容器中
    return beanDefinitions;
}

ClassPathMapperScanner#processBeanDefinitions

private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
	BeanDefinitionRegistry registry = this.getRegistry();
	Iterator var4 = beanDefinitions.iterator();

	while(var4.hasNext()) {
		BeanDefinitionHolder holder = (BeanDefinitionHolder)var4.next();
		AbstractBeanDefinition definition = (AbstractBeanDefinition)holder.getBeanDefinition();
		boolean scopedProxy = false;
		if (ScopedProxyFactoryBean.class.getName().equals(definition.getBeanClassName())) {
			definition = (AbstractBeanDefinition)Optional.ofNullable(((RootBeanDefinition)definition).getDecoratedDefinition()).map(BeanDefinitionHolder::getBeanDefinition).orElseThrow(() -> {
				return new IllegalStateException("The target bean definition of scoped proxy bean not found. Root bean definition[" + holder + "]");
			});
			scopedProxy = true;
		}

		String beanClassName = definition.getBeanClassName();
		LOGGER.debug(() -> {
			return "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName + "' mapperInterface";
		});
		definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName);
		//1.这里把普通接口设置成MapperFactoryBean
		definition.setBeanClass(this.mapperFactoryBeanClass);
        //2.是否把Mapper接口加入到Mybatis的Config当中去， 这里设置为true
		definition.getPropertyValues().add("addToConfig", this.addToConfig);
		definition.setAttribute("factoryBeanObjectType", beanClassName);
		boolean explicitFactoryUsed = false;
		
		//2.从核心容器里获取SqlSessionFactory赋值给MapperFactoryBean
		if (StringUtils.hasText(this.sqlSessionFactoryBeanName)) {
			definition.getPropertyValues().add("sqlSessionFactory", new RuntimeBeanReference(this.sqlSessionFactoryBeanName));
			explicitFactoryUsed = true;
		} else if (this.sqlSessionFactory != null) {
			definition.getPropertyValues().add("sqlSessionFactory", this.sqlSessionFactory);
			explicitFactoryUsed = true;
		}
		
		//3.从核心容器里获取SqlSessionTemplate赋值给MapperFactoryBean
		if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
			if (explicitFactoryUsed) {
				LOGGER.warn(() -> {
					return "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.";
				});
			}

			definition.getPropertyValues().add("sqlSessionTemplate", new RuntimeBeanReference(this.sqlSessionTemplateBeanName));
			explicitFactoryUsed = true;
		} else if (this.sqlSessionTemplate != null) {
			if (explicitFactoryUsed) {
				LOGGER.warn(() -> {
					return "Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.";
				});
			}

			definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
			explicitFactoryUsed = true;
		}

		if (!explicitFactoryUsed) {
			LOGGER.debug(() -> {
				return "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.";
			});
			definition.setAutowireMode(2);
		}

		definition.setLazyInit(this.lazyInitialization);
		if (!scopedProxy) {
			if ("singleton".equals(definition.getScope()) && this.defaultScope != null) {
				definition.setScope(this.defaultScope);
			}

			if (!definition.isSingleton()) {
				BeanDefinitionHolder proxyHolder = ScopedProxyUtils.createScopedProxy(holder, registry, true);
				if (registry.containsBeanDefinition(proxyHolder.getBeanName())) {
					registry.removeBeanDefinition(proxyHolder.getBeanName());
				}

				registry.registerBeanDefinition(proxyHolder.getBeanName(), proxyHolder.getBeanDefinition());
			}
		}
	}
}

​

再看spring.factories文件

上面只是完成了MyBatis 自动配置的工作，那么这些自动配置是如何在SpringBoot启动时执行呢？ 还记得SpringBoot的EnableAutoConfiguration注解吧，它是利用SpringFactoriesLoader机制加载所有的AutoConfiguration类，所以我们需要把编写好的自动配置类放在META-INF/spring.factories文件中，这也就是我们一开始分析的mybatis-spring-boot-starter-autoconfigure的文件结构的原因。

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

​

总结

1. mybatis-spring-boot-starter将mybatis需要的依赖全部引入。
2. starter通过SPI机制引入了一个配置的Class（MyBatisAutoConfiguration）。它负责注册SqlSessionFactory和SqlSessionTemplate到Spring容器中，使用MyBatis开发时绝大部分功能要使用这两个类来完成。
3. 注入了AutoConfiguredMapperScannerRegistrar这个Bean到Spring容器，它负责将MapperScanner引入到Spring容器，然后MapperScanner会将工程中指定package下的Mapper转化为BeanDefinition并且注册到Spring容器中。
4. 在开发中使用某个具体的Mapper时，Spring能够从容器中找到这个Mapper对应的BeanDefinition，将其实例化并且注入，这样开发者就可以使用了。
5. SpringBoot+MyBatis自动配置涉及到Spring中两个自动注册Bean的关键接口（BeanDefinitionRegistryPostProcessor和ImportBeanDefinitionRegistrar），也是我们业务开发中需要重点关注的地方。

参考链接

https://www.cnblogs.com/nullifier/p/11967659.html
https://zhuanlan.zhihu.com/p/30123517
https://www.cnblogs.com/daxin/p/3545040.html