@MapperScan

@MapperScan import了一个很重要的MapperScannerRegistrar，一个mapper注册处理器，主要完成 mapper接口的扫描和bean定义的注入。

MapperScan两个比较重要的配置信息：

basePackages : 指定扫描的mapper包路径。

factoryBean : 默认为MapperFactoryBean，扫描到的mapper接口最后都会被包装成一个MapperFactoryBean。

MapperScannerRegistrar

MapperScannerRegistrar 是一个实现了 ImportBeanDefinitionRegistrar接口的一个类，主要完成了registerBeanDefinitions 注册 bean的定义的功能

void registerBeanDefinitions(AnnotationAttributes annoAttrs, BeanDefinitionRegistry registry) {//用于扫描mapper接口并注册bean定义ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);// this check is needed in Spring 3.1Optional.ofNullable(resourceLoader).ifPresent(scanner::setResourceLoader);// 处理mapperScan配置信息，将这些配置都设置给scanner Class<? extends Annotation> annotationClass = annoAttrs.getClass("annotationClass");if (!Annotation.class.equals(annotationClass)) {scanner.setAnnotationClass(annotationClass);}Class<?> markerInterface = annoAttrs.getClass("markerInterface");if (!Class.class.equals(markerInterface)) {scanner.setMarkerInterface(markerInterface);}Class<? extends BeanNameGenerator> generatorClass = annoAttrs.getClass("nameGenerator");if (!BeanNameGenerator.class.equals(generatorClass)) {scanner.setBeanNameGenerator(BeanUtils.instantiateClass(generatorClass));}Class<? extends MapperFactoryBean> mapperFactoryBeanClass = annoAttrs.getClass("factoryBean");if (!MapperFactoryBean.class.equals(mapperFactoryBeanClass)) {scanner.setMapperFactoryBeanClass(mapperFactoryBeanClass);}scanner.setSqlSessionTemplateBeanName(annoAttrs.getString("sqlSessionTemplateRef"));scanner.setSqlSessionFactoryBeanName(annoAttrs.getString("sqlSessionFactoryRef"));List<String> basePackages = new ArrayList<>();basePackages.addAll(Arrays.stream(annoAttrs.getStringArray("value")).filter(StringUtils::hasText).collect(Collectors.toList()));basePackages.addAll(Arrays.stream(annoAttrs.getStringArray("basePackages")).filter(StringUtils::hasText).collect(Collectors.toList()));basePackages.addAll(Arrays.stream(annoAttrs.getClassArray("basePackageClasses")).map(ClassUtils::getPackageName).collect(Collectors.toList()));//注册过滤器scanner.registerFilters();//这里才是真正处理扫描注册过程scanner.doScan(StringUtils.toStringArray(basePackages));}

继续跟进doScan()，ClassPathMapperScanner 是继承于ClassPathBeanDefinitionScanner，所以先执行的super.doScan，这将mapper接口都包装成BeanDefinitionHolder，然后再执行processBeanDefinitions(beanDefinitions)，这一步主要是增强BeanDefinition ，注入sqlSessionFactory、sqlSessionTemplate、MapperFactoryBean 等配置。

@Overridepublic 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;}

这一步主要是增强BeanDefinition ，注入sqlSessionFactory、sqlSessionTemplate、MapperFactoryBean 等配置。

  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 MapperFactoryBeandefinition.getConstructorArgumentValues().addGenericArgumentValue(beanClassName); // issue #59// BeanDefinition 最后要实例化的对象class ，就是MapperFactoryBeandefinition.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);}}

MapperFactoryBean

下面介绍一下MapperFactoryBean。
可以看到这个该类是一个FactoryBean

Spring容器中获取Bean的时候，是调用的getObject()

  @Overridepublic T getObject() throws Exception {return getSqlSession().getMapper(this.mapperInterface);}

最终会调用到MapperRegistry的getMapper()，最终生成的是一个代理对象

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null) {throw new BindingException("Type " + type + " is not known to the MapperRegistry.");}try {return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}

可以看到new了一个MapperProxy，然后生成动态代理对象。

  public T newInstance(SqlSession sqlSession) {final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);return newInstance(mapperProxy);}protected T newInstance(MapperProxy<T> mapperProxy) {return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);}

MapperProxy是一个InvocationHandler，所以代理对象最后执行的是invoke方法

public class MapperProxy<T> implements InvocationHandler, Serializable

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

根据mapperInterface，sqlSession.getConfiguration()，创建了一个MapperMethod ，
会根据mapper的接口名和方法名找到xml解析后生成的对应MappedStatement

private MapperMethod cachedMapperMethod(Method method) {return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));}

MapperMethod最后执行的excute方法。

public Object execute(SqlSession sqlSession, Object[] args) {Object result;switch (command.getType()) {case INSERT: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));break;}case UPDATE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));break;}case DELETE: {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));break;}case SELECT:if (method.returnsVoid() && method.hasResultHandler()) {executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);} else if (method.returnsCursor()) {result = executeForCursor(sqlSession, args);} else {Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);if (method.returnsOptional()&& (result == null || !method.getReturnType().equals(result.getClass()))) {result = Optional.ofNullable(result);}}break;case FLUSH:result = sqlSession.flushStatements();break;default:throw new BindingException("Unknown execution method for: " + command.getName());}if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {throw new BindingException("Mapper method '" + command.getName()+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");}return result;}

总结

通过对 @MapperScan 的分析，最后可以简单总结为：MapperScannerRegistrar完成了mapper接口的扫描并生成BeanDefinition，BeanDefinition里面真正的beanClass是MapperFactoryBean，Spring容器在getBean的时候真正拿到的是MapperFactoryBean.getObject（），得到是一个MapperProxy代理对象。

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SpringBoot整合Mybatis源码解析之 @MapperScan

@MapperScan

MapperScannerRegistrar

MapperFactoryBean

总结

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