首先我们建立一个SpringBoot工程，然后导入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); }
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()); } } } }
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。
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