文章目录

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

1. Mapper 对象方法映射为 SQL 语句

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();
 }

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：

1.创建 MybatisXMLConfigBuilder 对象，调用其 parse() 方法去解析 XML 配置文件及 Mapper
2.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;
}

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);
       }
   }
}

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);
             }
         }
     }
 }

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();
 }

AbstractSqlInjector#inspectInject() 会完成 BaseMapper 接口中提供的 17 个默认方法与 SQL 语句的转化，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);
         }
     }
 }

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 Insert#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，只需要填上参数 format 就可以得到可执行的 SQL 语句。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 语句，至此 SQL 语句准备及配置解析就完成了

@Override
 public MappedStatement injectMappedStatement(Class<?> mapperClass, Class<?> modelClass, TableInfo tableInfo) {
     KeyGenerator keyGenerator = new NoKeyGenerator();
     SqlMethod sqlMethod = SqlMethod.INSERT_ONE;
     String columnScript = SqlScriptUtils.convertTrim(tableInfo.getAllInsertSqlColumnMaybeIf(),
         LEFT_BRACKET, RIGHT_BRACKET, null, COMMA);
     String valuesScript = SqlScriptUtils.convertTrim(tableInfo.getAllInsertSqlPropertyMaybeIf(null),
         LEFT_BRACKET, RIGHT_BRACKET, null, COMMA);
     String keyProperty = null;
     String keyColumn = null;
     // 表包含主键处理逻辑,如果不包含主键当普通字段处理
     if (StringUtils.isNotEmpty(tableInfo.getKeyProperty())) {
         if (tableInfo.getIdType() == IdType.AUTO) {
             /** 自增主键 */
             keyGenerator = new Jdbc3KeyGenerator();
             keyProperty = tableInfo.getKeyProperty();
             keyColumn = tableInfo.getKeyColumn();
         } else {
             if (null != tableInfo.getKeySequence()) {
                 keyGenerator = TableInfoHelper.genKeyGenerator(tableInfo, builderAssistant, sqlMethod.getMethod(), languageDriver);
                 keyProperty = tableInfo.getKeyProperty();
                 keyColumn = tableInfo.getKeyColumn();
             }
         }
     }
     String sql = String.format(sqlMethod.getSql(), tableInfo.getTableName(), columnScript, valuesScript);
     SqlSource sqlSource = languageDriver.createSqlSource(configuration, sql, modelClass);
     return this.addInsertMappedStatement(mapperClass, modelClass, sqlMethod.getMethod(), sqlSource, keyGenerator, keyProperty, keyColumn);
 }

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

@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());
}

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));
}

ClassPathMapperScanner#processBeanDefinitions() 将扫描到的 Mapper接口生成对应的 MapperFactoryBean形式注册到容器中，通过 MapperFactoryBean 可以获取 Mapper 实例

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);
 }
}

@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);
}
```

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);
     }
 }

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);
}

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);
 }

MyBatis-plus 源码解析

文章目录

前言

1. Mapper 对象方法映射为 SQL 语句

2. Mapper 操作数据库的流程

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