这一章我们来看看事务的具体参数在项目启动后，是在哪个入口加载和解析的

入口

TxNamespaceHandler类，TxNamespaceHandler类是spring-tx包的自定义解析器，在它的init方法中，有一个AnnotationDrivenBeanDefinitionParser类，

    @Override
    public void init() {
        registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
        //＜tx:annotation-driven开头的配置， Spring会使用AnnotationDrivenBeanDefinitionParser类的 parse方法进行解析
        registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
        registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
    }

再看AnnotationDrivenBeanDefinitionParser类中的parse方法

    @Override
    @Nullable
    public BeanDefinition parse(Element element, ParserContext parserContext) {
        registerTransactionalEventListenerFactory(parserContext);
        String mode = element.getAttribute("mode");
        if ("aspectj".equals(mode)) {
            //类型为"aspectj"
            registerTransactionAspect(element, parserContext);
            if (ClassUtils.isPresent("javax.transaction.Transactional", getClass().getClassLoader())) {
                registerJtaTransactionAspect(element, parserContext);
            }
        }
        else {
            //类型为"proxy" 默认
            AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
        }
        return null;
    }

private static class AopAutoProxyConfigurer {

        public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
            AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);

            String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME;
            if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
                Object eleSource = parserContext.extractSource(element);

                // Create the TransactionAttributeSource definition.
                //创建TransactionAttributeSource的bean
                RootBeanDefinition sourceDef = new RootBeanDefinition(
                        "org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
                sourceDef.setSource(eleSource);
                sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);

                // Create the TransactionInterceptor definition.
                //创建TransactionInterceptor的bean 后面使用TransactionInterceptor作为事务增强器实现事务的逻辑
                RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
                interceptorDef.setSource(eleSource);
                interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                registerTransactionManager(element, interceptorDef);
                interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);

                // Create the TransactionAttributeSourceAdvisor definition.
                //创建TransactionAttributeSourceAdvisor的bean
                RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);
                advisorDef.setSource(eleSource);
                advisorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                advisorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                advisorDef.getPropertyValues().add("adviceBeanName", interceptorName);
                if (element.hasAttribute("order")) {
                    advisorDef.getPropertyValues().add("order", element.getAttribute("order"));
                }
                parserContext.getRegistry().registerBeanDefinition(txAdvisorBeanName, advisorDef);

                CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), eleSource);
                compositeDef.addNestedComponent(new BeanComponentDefinition(sourceDef, sourceName));
                compositeDef.addNestedComponent(new BeanComponentDefinition(interceptorDef, interceptorName));
                compositeDef.addNestedComponent(new BeanComponentDefinition(advisorDef, txAdvisorBeanName));
                parserContext.registerComponent(compositeDef);
            }
        }
    }

一步步往下走，在configureAutoProxyCreator方法中我们目前只关注registerAutoProxyCreatorIfNecessary方法

    public static void registerAutoProxyCreatorIfNecessary(
            ParserContext parserContext, Element sourceElement) {

        BeanDefinition beanDefinition = AopConfigUtils.registerAutoProxyCreatorIfNecessary(
                parserContext.getRegistry(), parserContext.extractSource(sourceElement));
        useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
        registerComponentIfNecessary(beanDefinition, parserContext);
    }

然后在AopConfigUtils.registerAutoProxyCreatorIfNecessary方法中

    @Nullable
    public static BeanDefinition registerAutoProxyCreatorIfNecessary(
            BeanDefinitionRegistry registry, @Nullable Object source) {

        return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
    }

那么在registerAutoProxyCreatorIfNecessary方法中，我们能看到的是注册了一个InfrastructureAdvisorAutoProxyCreator类型的bean而已，那么属于事务关键流程的又是哪一步呢？
其实上面这一串代码流程走下来，主要往正常的bean注册的流程靠近，InfrastructureAdvisorAutoProxyCreator类继承了AbstractAdvisorAutoProxyCreator类，而AbstractAdvisorAutoProxyCreator类又继承了AbstractAutoProxyCreator类，而AbstractAutoProxyCreator类就是bean实例化的重要类，其中的postProcessAfterInitialization方法才是接下来流程的关键

   @Override
   public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
       if (bean != null) {
           // 根据给定的 bean的class和name 构建一个 key
           Object cacheKey = getCacheKey(bean.getClass(), beanName);
           if (this.earlyProxyReferences.remove(cacheKey) != bean) {
               // 如果它适合被代理 则要封装指定 bean
               return wrapIfNecessary(bean, beanName, cacheKey);
           }
       }
       return bean;
   }