MyBatis

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MyBatis-plus是完全基于MyBatis开发的一个增强工具，是在MyBatis的基础上做增强的框架，为简化开发、提高效率而生。它在MyBatis原本的框架上增加了很多实用性功能，比如乐观锁插件、字段自动填充功能、分页插件、条件构造器、sql 注入器等等。使用 MyBatis-plus 可以完全不写任何 XML 文件，直接使用继承了BaseMapper 接口的类对象完成对数据库的映射操作基于映射的原理，MyBatis-plus 必然要实现 Mapper中的方法与 SQL 语句的对应转化，以下即为 MyBatis-plus 重要流程图例。

1. Mapper 对象方法映射为 SQL 语句1）在 MyBatis-plus 中， MybatisPlusAutoConfiguration 自动配置类的 sqlSessionFactory()方法为 Spring提供创建 sqlSession的工厂类对象，对 sqlSessionFactory 进行定义的定义类变为了 MybatisSqlSessionFactoryBean。在 sqlSessionFactory()方法中，除了注入 MyBatis本身的组件，还会注入MyBatis-plus 的主键生成器、SQL 注入器等组件，最后通过 MybatisSqlSessionFactoryBean#getObject() 方法获取到 sqlSessionFactory 对象

@Bean @ConditionalOnMissingBean public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception { // TODO 使用 MybatisSqlSessionFactoryBean 而不是 SqlSessionFactoryBean MybatisSqlSessionFactoryBean factory = new MybatisSqlSessionFactoryBean(); factory.setDataSource(dataSource); factory.setVfs(SpringBootVFS.class); if (StringUtils.hasText(this.properties.getConfigLocation())) { factory.setConfigLocation(this.resourceLoader.getResource(this.properties.getConfigLocation())); } 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); } Resource[] mapperLocations = this.properties.resolveMapperLocations(); if (!ObjectUtils.isEmpty(mapperLocations)) { factory.setMapperLocations(mapperLocations); } // TODO 修改源码支持定义 TransactionFactory this.getBeanThen(TransactionFactory.class, factory::setTransactionFactory); // TODO 对源码做了一定的修改(因为源码适配了老旧的mybatis版本,但我们不需要适配) Class mapperInterface = Resources.classForName(mapperClass); configuration.addMapper(mapperInterface); } else { throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one."); } } } } }

4）、MybatisConfiguration#addMapper()方法其实是去调用 MybatisMapperRegistry#addMapper() 方法，其核心是MybatisMapperAnnotationBuilder#parse()

@Override public void addMapper(Class type) { if (type.isInterface()) { if (hasMapper(type)) { // TODO 如果之前注入直接返回 return; // TODO 这里就不抛异常了 // throw new BindingException("Type " + type + " is already known to the MapperRegistry."); } boolean loadCompleted = false; try { // TODO 这里也换成 MybatisMapperProxyFactory 而不是 MapperProxyFactory knownMappers.put(type, new MybatisMapperProxyFactory(type)); // It's important that the type is added before the parser is run // otherwise the binding may automatically be attempted by the // mapper parser. If the type is already known, it won't try. // TODO 这里也换成 MybatisMapperAnnotationBuilder 而不是 MapperAnnotationBuilder MybatisMapperAnnotationBuilder parser = new MybatisMapperAnnotationBuilder(config, type); parser.parse(); loadCompleted = true; } finally { if (!loadCompleted) { knownMappers.remove(type); } } } }

5）、MybatisMapperAnnotationBuilder#parse() 方法真正开始完成 Mapper 接口中的方法与 SQL 语句的映射，其中 parseStatement()方法是解析 @Select/@Update 等注解写入的 SQL语句，而代码 GlobalConfigUtils.getSqlInjector(configuration).inspectInject(assistant, type) 通过 MaBatis-plus的 SQL 注入器完成 Mapper 方法与 SQL 语句的转化

@Override public void parse() { String resource = type.toString(); if (!configuration.isResourceLoaded(resource)) { loadXmlResource(); configuration.addLoadedResource(resource); String mapperName = type.getName(); assistant.setCurrentNamespace(mapperName); parseCache(); parseCacheRef(); InterceptorIgnoreHelper.InterceptorIgnoreCache cache = InterceptorIgnoreHelper.initSqlParserInfoCache(type); for (Method method : type.getMethods()) { if (!canHaveStatement(method)) { continue; } if (getAnnotationWrapper(method, false, Select.class, SelectProvider.class).isPresent() && method.getAnnotation(ResultMap.class) == null) { parseResultMap(method); } try { // TODO 加入注解过滤缓存 InterceptorIgnoreHelper.initSqlParserInfoCache(cache, mapperName, method); SqlParserHelper.initSqlParserInfoCache(mapperName, method); parseStatement(method); } catch (IncompleteElementException e) { // TODO 使用 MybatisMethodResolver 而不是 MethodResolver configuration.addIncompleteMethod(new MybatisMethodResolver(this, method)); } } // TODO 注入 CURD 动态 SQL , 放在在最后, because 可能会有人会用注解重写sql try { // https://github.com/baomidou/mybatis-plus/issues/3038 if (GlobalConfigUtils.isSupperMapperChildren(configuration, type)) { parserInjector(); } } catch (IncompleteElementException e) { configuration.addIncompleteMethod(new InjectorResolver(this)); } } parsePendingMethods(); }

6)、AbstractSqlInjector#inspectInject() 会完成 BaseMapper 接口中提供的通用方法对应的 SQL 语句准备，这部分主要通过 AbstractMethod#inject()方法完成

@Override public void inspectInject(MapperBuilderAssistant builderAssistant, Class mapperClass) { Class modelClass = extractModelClass(mapperClass); if (modelClass != null) { String className = mapperClass.toString(); Set mapperRegistryCache = GlobalConfigUtils.getMapperRegistryCache(builderAssistant.getConfiguration()); if (!mapperRegistryCache.contains(className)) { List methodList = this.getMethodList(mapperClass); if (CollectionUtils.isNotEmpty(methodList)) { TableInfo tableInfo = TableInfoHelper.initTableInfo(builderAssistant, modelClass); // 循环注入自定义方法 methodList.forEach(m -> m.inject(builderAssistant, mapperClass, modelClass, tableInfo)); } else { logger.debug(mapperClass.toString() + ", No effective injection method was found."); } mapperRegistryCache.add(className); } } }

7)、AbstractMethod#inject()方法并没有什么特别的操作，只是调用其子类实现 injectMappedStatement()方法。以 SelectOne#injectMappedStatement() 为例，其 SQL 语句的核心在于 SqlMethod 类，这个枚举类中缓存了可以动态拼接的 SQL 语句脚本，只需要填上参数 format 就可以得到 SQL 语句的执行脚本。以上过程结束，只需要将所有信息通过 addInsertMappedStatement()方法封装成 MappedStatement对象并将其加入到容器中，这样 Mapper接口方法调用时，就可以通过动态代理的方式找到其对应执行的 SQL 脚本，至此 SQL 语句准备及配置解析就完成了。最后拼接的 SQL 语句脚本形式如下示例，实际执行数据库操作时会解析这个脚本完成变量替换，从而得到可执行的 SQL 语句。

${ew.sqlFirst} SELECT ${ew.sqlSelect} id,name,type FROM node id=#{ew.entity.id} AND name=#{ew.entity.name} AND type=#{ew.entity.type} AND ${ew.sqlSegment} ${ew.sqlSegment} ${ew.sqlComment} @Override public MappedStatement injectMappedStatement(Class mapperClass, Class modelClass, TableInfo tableInfo) { SqlMethod sqlMethod = SqlMethod.SELECT_ONE; SqlSource sqlSource = languageDriver.createSqlSource(configuration, String.format(sqlMethod.getSql(), sqlFirst(), sqlSelectColumns(tableInfo, true), tableInfo.getTableName(), sqlWhereEntityWrapper(true, tableInfo), sqlComment()), modelClass); return this.addSelectMappedStatementForTable(mapperClass, getMethod(sqlMethod), sqlSource, tableInfo); }

8)、SqlSessionFactory对象的创建需要回到 MybatisSqlSessionFactoryBean#buildSqlSessionFactory()方法中，很容易追踪到 MybatisSqlSessionFactoryBuilder#build()方法，最后其实是通过 SqlSessionFactoryBuilder#build()方法创建了一个 DefaultSqlSessionFactory 对象返回

public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config); }

2. Mapper 操作数据库的流程1）、@MapperScan 注解通过 @Import(MapperScannerRegistrar.class) 引入扫描注册的类MapperScannerRegistrar，该类实现了ImportBeanDefinitionRegistrar接口并重写registerBeanDefinitions()方法，在该方法中注册了 MapperScannerConfigurer 类

public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { if (!AutoConfigurationPackages.has(this.beanFactory)) { logger.debug("Could not determine auto-configuration package, automatic mapper scanning disabled."); return; } logger.debug("Searching for mappers annotated with @Mapper"); List packages = AutoConfigurationPackages.get(this.beanFactory); if (logger.isDebugEnabled()) { packages.forEach(pkg -> logger.debug("Using auto-configuration base package '{}'", pkg)); } BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class); builder.addPropertyValue("processPropertyPlaceHolders", true); builder.addPropertyValue("annotationClass", Mapper.class); builder.addPropertyValue("basePackage", StringUtils.collectionToCommaDelimitedString(packages)); BeanWrapper beanWrapper = new BeanWrapperImpl(MapperScannerConfigurer.class); Stream.of(beanWrapper.getPropertyDescriptors()) // Need to mybatis-spring 2.0.2+ .filter(x -> x.getName().equals("lazyInitialization")).findAny() .ifPresent(x -> builder.addPropertyValue("lazyInitialization", "${mybatis.lazy-initialization:false}")); registry.registerBeanDefinition(MapperScannerConfigurer.class.getName(), builder.getBeanDefinition()); }

2)、MapperScannerConfigurer是 Mapper接口的扫描配置类，实现了 BeanDefinitionRegistryPostProcessor 接口，其 postProcessBeanDefinitionRegistry()方法会在容器启动过程中被回调，通过 ClassPathMapperScanner#scan()方法完成 Mapper 的扫描注册

@Override public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) { if (this.processPropertyPlaceHolders) { 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(lazyInitialization)) { scanner.setLazyInitialization(Boolean.valueOf(lazyInitialization)); } if (StringUtils.hasText(defaultScope)) { scanner.setDefaultScope(defaultScope); } scanner.registerFilters(); scanner.scan( StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS)); }

3）、ClassPathMapperScanner#processBeanDefinitions() 将扫描到的 Mapper接口生成的对应 BeanDefinition 的 beanClass 属性替换为 MapperFactoryBean，这样每次获取 Mapper 实例实际是通过 MapperFactoryBean 的实例去获取此处体现了 FactoryBean 的定位，即用于获取同一类 bean 的工厂 bean

private void processBeanDefinitions(Set beanDefinitions) { AbstractBeanDefinition definition; BeanDefinitionRegistry registry = getRegistry(); for (BeanDefinitionHolder holder : beanDefinitions) { 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(() -> new IllegalStateException( "The target bean definition of scoped proxy bean not found. Root bean definition[" + holder + "]")); scopedProxy = true; } String beanClassName = definition.getBeanClassName(); LOGGER.debug(() -> "Creating MapperFactoryBean with name '" + holder.getBeanName() + "' and '" + beanClassName + "' mapperInterface"); // the mapper interface is the original class of the bean // but, the actual class of the bean is MapperFactoryBean definition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // issue #59 definition.setBeanClass(this.mapperFactoryBeanClass); definition.getPropertyValues().add("addToConfig", this.addToConfig); // Attribute for MockitoPostProcessor // https://github.com/mybatis/spring-boot-starter/issues/475 definition.setAttribute(FACTORY_BEAN_OBJECT_TYPE, beanClassName); 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) { LOGGER.debug(() -> "Enabling autowire by type for MapperFactoryBean with name '" + holder.getBeanName() + "'."); definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE); } definition.setLazyInit(lazyInitialization); if (scopedProxy) { continue; } if (ConfigurableBeanFactory.SCOPE_SINGLETON.equals(definition.getScope()) && defaultScope != null) { definition.setScope(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()); } } }

4）、@Autowired 自动注入 Mapper 触发容器获取 bean 的方法，调用到 MapperFactoryBean#getObject()方法，最终调用到 sqlSessionTemplate#getMapper()方法

@Override public T getMapper(Class type) { return getConfiguration().getMapper(type, this); }

5）、MyBatis-plus 使用的配置类是MybatisConfiguration，最终调用到 MybatisMapperRegistry#getMapper()方法，这里就进入了动态代理获取 MapperProxy 实例的流程

@Override public T getMapper(Class type, SqlSession sqlSession) { // TODO 这里换成 MybatisMapperProxyFactory 而不是 MapperProxyFactory final MybatisMapperProxyFactory mapperProxyFactory = (MybatisMapperProxyFactory) knownMappers.get(type); if (mapperProxyFactory == null) { throw new BindingException("Type " + type + " is not known to the MybatisPlusMapperRegistry."); } try { return mapperProxyFactory.newInstance(sqlSession); } catch (Exception e) { throw new BindingException("Error getting mapper instance. Cause: " + e, e); } }

6）、MybatisMapperProxyFactory#newInstance()方法给自动注入返回一个 MybatisMapperProxy 代理对象

protected T newInstance(MybatisMapperProxy mapperProxy) { return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy); }

7）、调用 Mapper 接口的方法触发代理对象的 MybatisMapperProxy#invoke()，此时根据 Mapper 对象被调用的方法生成 MybatisMapperMethod 对象，通过MybatisMapperMethod#execute()去真正地执行 SQL 语句，从而完成数据库操作。此后的流程本文就不再分析，具体可参考文章 MyBatis Mapper 简要总结

@Override public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { try { if (Object.class.equals(method.getDeclaringClass())) { return method.invoke(this, args); } else if (method.isDefault()) { return invokeDefaultMethod(proxy, method, args); } } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } final MybatisMapperMethod mapperMethod = cachedMapperMethod(method); return mapperMethod.execute(sqlSession, args); }

总结：

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