Mybatis在SpringBoot中是如何被加载执行

首先依赖于springboot的自动装配@EnableAutoConfiguration注解，这个注解最终帮助我们读取mybatis-spring-boot-autoconfigure-x.x.x.jar中的META-INF\spring.factories配置类：

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

来看MybatisAutoConfiguration的源码，最主要的就是AutoConfiguredMapperScannerRegistrar。

@Configuration
@ConditionalOnClass({SqlSessionFactory.class, SqlSessionFactoryBean.class})
@ConditionalOnSingleCandidate(DataSource.class)
@EnableConfigurationProperties({MybatisProperties.class})
@AutoConfigureAfter({DataSourceAutoConfiguration.class, MybatisLanguageDriverAutoConfiguration.class})
public class MybatisAutoConfiguration implements InitializingBean {

    //...

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

        public void afterPropertiesSet() {
            MybatisAutoConfiguration.logger.debug("Not found configuration for registering mapper bean using @MapperScan, MapperFactoryBean and MapperScannerConfigurer.");
        }
    }

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

        public AutoConfiguredMapperScannerRegistrar() {
        }

        public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
            if (!AutoConfigurationPackages.has(this.beanFactory)) {
                MybatisAutoConfiguration.logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled.");
            } else {
                // 这里获取要扫描的包名
                MybatisAutoConfiguration.logger.debug("Searching for mappers annotated with @Mapper");
                List<String> packages = AutoConfigurationPackages.get(this.beanFactory);
                if (MybatisAutoConfiguration.logger.isDebugEnabled()) {
                    packages.forEach((pkg) -> {
                        MybatisAutoConfiguration.logger.debug("Using auto-configuration base package '{}'", pkg);
                    });
                }

                // 下面这些代码主要就是定义一个bean的定义，添加到BeanFactory中 BeanDefinitionBuilder
                BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class);
                builder.addPropertyValue("processPropertyPlaceHolders", true);
                // 这就是要扫描的注解类型，就是@Mapper
                builder.addPropertyValue("annotationClass", Mapper.class);
                // //这里是要扫描的包的路径
                builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages));
                BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class);
                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:}");
                }

                registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition());
            }
        }

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

AutoConfiguredMapperScannerRegistrar主要就是把MapperScannerConfigurer这个类注册到spring中，来看MapperScannerConfigurer的代码：

public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {

    //...
    
    @Override
    public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
        if (this.processPropertyPlaceHolders) {
            //这个是主要设置一些属性，比如上面包名，要扫描的注解类名称等等
            processPropertyPlaceHolders();
        }
        //这个类看名字都知道是干什么的, 主要就是扫描mapper注解的类
        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.registerFilters();
        scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
    }

    // ...
}

由于MapperScannerConfigurer这个类实现了BeanDefinitionRegistryPostProcessor，所以它就会被生成bean之前加载，调用它的postProcessBeanDefinitionRegistry方法，postProcessBeanDefinitionRegistry里面就会scan我们指定的package里的@Mapper类。再来看ClassPathMapperScanner:

public class ClassPathMapperScanner extends ClassPathBeanDefinitionScanner {

    //...

    @Override
    public Set<BeanDefinitionHolder> doScan(String... basePackages) {
        Set<BeanDefinitionHolder> beanDefinitions = super.doScan(basePackages);

        if (beanDefinitions.isEmpty()) {
            logger.warn("No MyBatis mapper was found in '" + Arrays.toString(basePackages) + "' package. Please check your configuration.");
        } else {
            processBeanDefinitions(beanDefinitions);
        }

        return beanDefinitions;
    }

    private void processBeanDefinitions(Set<BeanDefinitionHolder> beanDefinitions) {
        GenericBeanDefinition definition;
        for (BeanDefinitionHolder holder : beanDefinitions) {
            definition = (GenericBeanDefinition) holder.getBeanDefinition();

            if (logger.isDebugEnabled()) {
                logger.debug("Creating MapperFactoryBean with name '" + holder.getBeanName() 
                             + "' and '" + definition.getBeanClassName() + "' mapperInterface");
            }

            // the mapper interface is the original class of the bean
            // but, the actual class of the bean is MapperFactoryBean

            //这里是给bean定义的添加一个构造方法参数，就是我们扫描出来mapper注解类的类名
            definition.getConstructorArgumentValues().addGenericArgumentValue(definition.getBeanClassName()); // issue #59
            // 这个就是设置对应bean的类的类，这里设置成了org.mybatis.spring.mapper.MapperFactoryBean这个类，这注意这个类实现了FactoryBean接口
            definition.setBeanClass(this.mapperFactoryBean.getClass());

            definition.getPropertyValues().add("addToConfig", this.addToConfig);

            boolean explicitFactoryUsed = false;
            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;
            }

            if (StringUtils.hasText(this.sqlSessionTemplateBeanName)) {
                if (explicitFactoryUsed) {
                    logger.warn("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("Cannot use both: sqlSessionTemplate and sqlSessionFactory together. sqlSessionFactory is ignored.");
                }
                definition.getPropertyValues().add("sqlSessionTemplate", this.sqlSessionTemplate);
                explicitFactoryUsed = true;
            }

            if (!explicitFactoryUsed) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'.");
                }
                definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
            }
        }
    }

    //...
}

最终在获取bean的时候，会调用MapperFactoryBean这个类的getObject()：

public class MapperFactoryBean<T> extends SqlSessionDaoSupport implements FactoryBean<T> {

  private Class<T> mapperInterface;

  // ...
  
  public MapperFactoryBean(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public T getObject() throws Exception {
    return getSqlSession().getMapper(this.mapperInterface);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public Class<T> getObjectType() {
    return this.mapperInterface;
  }

  //....
}

getObject()里会返回代理对象，对象的类型即 构造函数里传进来的 mapperInterface 。

总结：

1. MyBatis实现了BeanDefinitionRegistryPostProcessor可以动态注册BeanDefinition；
2. 但是接口虽然注册成了BeanDefinition但是无法实例化Bean因为接口无法实例化；
3. 需要将BeanDefinition的BeanClass替换成JDK动态代理的实例（MyBatis的Mapper接口核心是JDK动态代理）；
4. Mybatis通过Factory Bean的工厂方法设计模式可以自由控制Bean的实例化过程，可以在getObject方法中创建JDK动态代理；