- 前言
- 1. Mapper 对象方法映射为 SQL 语句
- 2. Mapper 操作数据库的流程
前言
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
基于 Spring Boot + MyBatis Plus + Vue & Element 实现的后台管理系统 + 用户小程序，支持 RBAC 动态权限、多租户、数据权限、工作流、三方登录、支付、短信、商城等功能。
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
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()); } ...... // TODO 自定义枚举包 if (StringUtils.hasLength(this.properties.getTypeEnumsPackage())) { factory.setTypeEnumsPackage(this.properties.getTypeEnumsPackage()); } // TODO 此处必为非 NULL GlobalConfig globalConfig = this.properties.getGlobalConfig(); // TODO 注入填充器 if (this.applicationContext.getBeanNamesForType(MetaObjectHandler.class, false, false).length > 0) { MetaObjectHandler metaObjectHandler = this.applicationContext.getBean(MetaObjectHandler.class); globalConfig.setMetaObjectHandler(metaObjectHandler); } // TODO 注入主键生成器 if (this.applicationContext.getBeanNamesForType(IKeyGenerator.class, false, false).length > 0) { IKeyGenerator keyGenerator = this.applicationContext.getBean(IKeyGenerator.class); globalConfig.getDbConfig().setKeyGenerator(keyGenerator); } // TODO 注入sql注入器 if (this.applicationContext.getBeanNamesForType(ISqlInjector.class, false, false).length > 0) { ISqlInjector iSqlInjector = this.applicationContext.getBean(ISqlInjector.class); globalConfig.setSqlInjector(iSqlInjector); } // TODO 设置 GlobalConfig 到 MybatisSqlSessionFactoryBean factory.setGlobalConfig(globalConfig); return factory.getObject(); }
2.MybatisSqlSessionFactoryBean#getObject() 执行懒加载策略，最后通过 buildSqlSessionFactory() 方法创建 SqlSessionFactory 工厂类对象。这个方法的流程很长，不过大致可以分为两个步骤：
- 创建
MybatisXMLConfigBuilder对象，调用其parse()方法去解析 XML 配置文件及 Mapper - 解析获得的信息存储在
targetConfiguration对象中，根据其信息创建SqlSessionFactory对象
protected SqlSessionFactory buildSqlSessionFactory() throws Exception { final MybatisConfiguration targetConfiguration; // TODO 使用 MybatisXmlConfigBuilder 而不是 XMLConfigBuilder MybatisXMLConfigBuilder xmlConfigBuilder = null; ...... } else if (this.configLocation != null) { // TODO 使用 MybatisXMLConfigBuilder // 1.1 创建 MybatisConfiguration 对象 xmlConfigBuilder = new MybatisXMLConfigBuilder(this.configLocation.getInputStream(), null, this.configurationProperties); // 2.1 将解析获得的信息的引用传递给 targetConfiguration 对象 targetConfiguration = xmlConfigBuilder.getConfiguration(); } else { LOGGER.debug(() -> "Property 'configuration' or 'configLocation' not specified, using default MyBatis Configuration"); // TODO 使用 MybatisConfiguration targetConfiguration = new MybatisConfiguration(); Optional.ofNullable(this.configurationProperties).ifPresent(targetConfiguration::setVariables); } // TODO 无配置启动所必须的 this.globalConfig = Optional.ofNullable(this.globalConfig).orElseGet(GlobalConfigUtils::defaults); this.globalConfig.setDbConfig(Optional.ofNullable(this.globalConfig.getDbConfig()).orElseGet(GlobalConfig.DbConfig::new)); // TODO 初始化 id-work 以及 打印骚东西 targetConfiguration.setGlobalConfig(this.globalConfig); ...... // 1.2 开始解析 XML 配置文件 及 Mapper 接口 if (xmlConfigBuilder != null) { try { xmlConfigBuilder.parse(); LOGGER.debug(() -> "Parsed configuration file: '" + this.configLocation + "'"); } catch (Exception ex) { throw new NestedIOException("Failed to parse config resource: " + this.configLocation, ex); } finally { ErrorContext.instance().reset(); } } targetConfiguration.setEnvironment(new Environment(MybatisSqlSessionFactoryBean.class.getSimpleName(), this.transactionFactory == null ? new SpringManagedTransactionFactory() : this.transactionFactory, this.dataSource)); if (this.mapperLocations != null) { if (this.mapperLocations.length == 0) { LOGGER.warn(() -> "Property 'mapperLocations' was specified but matching resources are not found."); } else { for (Resource mapperLocation : this.mapperLocations) { if (mapperLocation == null) { continue; } try { XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(), targetConfiguration, mapperLocation.toString(), targetConfiguration.getSqlFragments()); xmlMapperBuilder.parse(); } catch (Exception e) { throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e); } finally { ErrorContext.instance().reset(); } LOGGER.debug(() -> "Parsed mapper file: '" + mapperLocation + "'"); } } } else { LOGGER.debug(() -> "Property 'mapperLocations' was not specified."); } // 2.2 根据 targetConfiguration 对象中保存的信息创建 SqlSessionFactory 对象 final SqlSessionFactory sqlSessionFactory = new MybatisSqlSessionFactoryBuilder().build(targetConfiguration); ...... return sqlSessionFactory; }
3.MybatisXMLConfigBuilder#parse() 会去解析配置文件，最后会调用到其内部方法 mapperElement()。这个方法完成解析 Mapper工作，并将其添加到配置类 MybatisConfiguration 中
private void mapperElement(XNode parent) throws Exception { /* * 定义集合 用来分类放置mybatis的Mapper与XML 按顺序依次遍历 */ if (parent != null) { //指定在classpath中的mapper文件 Set<String> resources = new HashSet<>(); //指向一个mapper接口 Set<Class<?>> mapperClasses = new HashSet<>(); setResource(parent, resources, mapperClasses); // 依次遍历 首先 resource 然后 mapper for (String resource : resources) { ErrorContext.instance().resource(resource); InputStream inputStream = Resources.getResourceAsStream(resource); //TODO XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments()); mapperParser.parse(); } for (Class<?> mapper : mapperClasses) { // 主要关注此处 configuration.addMapper(mapper); } } }
4.MybatisConfiguration#addMapper()方法其实是去调用 MybatisMapperRegistry#addMapper() 方法，其核心是MybatisMapperAnnotationBuilder#parse()
public <T> void addMapper(Class<T> 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); final String typeName = type.getName(); assistant.setCurrentNamespace(typeName); parseCache(); parseCacheRef(); SqlParserHelper.initSqlParserInfoCache(type); Method[] methods = type.getMethods(); for (Method method : methods) { try { // issue #237 if (!method.isBridge()) { // 解析 @Select 注解写入的 SQL parseStatement(method); SqlParserHelper.initSqlParserInfoCache(typeName, 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 if (GlobalConfigUtils.isSupperMapperChildren(configuration, type)) { GlobalConfigUtils.getSqlInjector(configuration).inspectInject(assistant, type); } } 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<String> mapperRegistryCache = GlobalConfigUtils.getMapperRegistryCache(builderAssistant.getConfiguration()); if (!mapperRegistryCache.contains(className)) { List<AbstractMethod> 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
<script> <choose> <when test="ew != null and ew.sqlFirst != null"> ${ew.sqlFirst} </when> <otherwise></otherwise> </choose> SELECT <choose> <when test="ew != null and ew.sqlSelect != null"> ${ew.sqlSelect} </when> <otherwise>id,name,type</otherwise> </choose> FROM node <if test="ew != null"> <where> <if test="ew.entity != null"> <if test="ew.entity.id != null">id=#{ew.entity.id}</if> <if test="ew.entity['name'] != null">AND name=#{ew.entity.name}</if> <if test="ew.entity['type'] != null">AND type=#{ew.entity.type}</if> </if> <if test="ew.sqlSegment != null and ew.sqlSegment != '' and ew.nonEmptyOfWhere"> <if test="ew.nonEmptyOfEntity and ew.nonEmptyOfNormal">AND</if> ${ew.sqlSegment} </if> </where> <if test="ew.sqlSegment != null and ew.sqlSegment != '' and ew.emptyOfWhere"> ${ew.sqlSegment} </if> </if> <choose> <when test="ew != null and ew.sqlComment != null"> ${ew.sqlComment} </when> <otherwise></otherwise> </choose> </script> @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); }
基于微服务的思想，构建在 B2C 电商场景下的项目实战。核心技术栈，是 Spring Boot + Dubbo 。未来，会重构成 Spring Cloud Alibaba 。
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
void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry, String beanName) { BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(MapperScannerConfigurer.class); ...... registry.registerBeanDefinition(beanName, 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
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)); } 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<BeanDefinitionHolder> beanDefinitions) { GenericBeanDefinition definition; for (BeanDefinitionHolder holder : beanDefinitions) { definition = (GenericBeanDefinition) holder.getBeanDefinition(); 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); 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); } }
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> T getMapper(Class<T> 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> T getMapper(Class<T> type, SqlSession sqlSession) { // TODO 这里换成 MybatisMapperProxyFactory 而不是 MapperProxyFactory final MybatisMapperProxyFactory<T> mapperProxyFactory = (MybatisMapperProxyFactory<T>) 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<T> 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。
@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); }
