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

mybatis-spring-boot-starter将mybatis需要的依赖全部引入。
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。
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。
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。
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