Spring事务实现原理

Spring事务的原理是基于AOP实现的，所以流程也可以理解为与AOP一样分为3步：解析切面、织入通知和运行时增强。

1、解析切面

Srping事务的是通过@EnableTransactionManagement注解开启的，该注解往IoC容器中注入了四个对象：InfrastructureAdvisorAutoProxyCreator（类似于AnnotationAwareAspectJAutoProxyCreator，用于创建代理类的）、BeanFactoryTransactionAttributeSourceAdvisor、TransactionAttributeSource、TransactionInterceptor。
其中InfrastructureAdvisorAutoProxyCreator实现了BeanPostProcessor的接口，因此会在第一个Bean实例化之前调用applyBeanPostProcessorsBeforeInstantiation方法，里面会跑到InfrastructureAdvisorAutoProxyCreator的postProcessBeforeInstantiation方法里面：

	@Nullable
	protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) {
   
		for (BeanPostProcessor bp : getBeanPostProcessors()) {
   
			if (bp instanceof InstantiationAwareBeanPostProcessor) {
   
				InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
				Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName);
				if (result != null) {
   
					return result;
				}
			}
		}
		return null;
	}

然后会调用AbstractAutoProxyCreator的postProcessBeforeInstantiation方法对切面进行初始化，其流程和AOP初始化流程一致（这里要注意事务的切面是AnnotationAwareAspectJAutoProxyCreator调用findCandidateAdvisors跑到父类AbstractAdvisorAutoProxyCreator的findCandidateAdvisors方法里面调用BeanFactoryAdvisorRetrievalHelper的findAdvisorBeans方法加载BeanFactoryTransactionAttributeSourceAdvisor缓存起来的）被切面类顺便加载

	public List<Advisor> fi