[Java知识库]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> findAdvisorBeans() {
		// Determine list of advisor bean names, if not cached already.
		String[] advisorNames = this.cachedAdvisorBeanNames;
		if (advisorNames == null) {
			// Do not initialize FactoryBeans here: We need to leave all regular beans
			// uninitialized to let the auto-proxy creator apply to them!
			advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
					this.beanFactory, Advisor.class, true, false);
			this.cachedAdvisorBeanNames = advisorNames;
		}
		if (advisorNames.length == 0) {
			return new ArrayList<>();
		}

		List<Advisor> advisors = new ArrayList<>();
		for (String name : advisorNames) {
			if (isEligibleBean(name)) {
				if (this.beanFactory.isCurrentlyInCreation(name)) {
					if (logger.isTraceEnabled()) {
						logger.trace("Skipping currently created advisor '" + name + "'");
					}
				}
				else {
					try {
						advisors.add(this.beanFactory.getBean(name, Advisor.class));
					}
					catch (BeanCreationException ex) {
						Throwable rootCause = ex.getMostSpecificCause();
						if (rootCause instanceof BeanCurrentlyInCreationException) {
							BeanCreationException bce = (BeanCreationException) rootCause;
							String bceBeanName = bce.getBeanName();
							if (bceBeanName != null && this.beanFactory.isCurrentlyInCreation(bceBeanName)) {
								if (logger.isTraceEnabled()) {
									logger.trace("Skipping advisor '" + name +
											"' with dependency on currently created bean: " + ex.getMessage());
								}
								// Ignore: indicates a reference back to the bean we're trying to advise.
								// We want to find advisors other than the currently created bean itself.
								continue;
							}
						}
						throw ex;
					}
				}
			}
		}
		return advisors;
	}

2、织入

根据AOP的原理，在创建代理对象的时候，会将BeanFactoryTransactionAttributeSourceAdvisor织入到代理对象中，真正织入的通知则是封装在BeanFactoryTransactionAttributeSourceAdvisor里面的TransactionInterceptor

	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource());
		advisor.setAdvice(transactionInterceptor());
		if (this.enableTx != null) {
			advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		}
		return advisor;
	}

3、运行时增强

实际上是调用TransactionInterceptor的invoke方法，TransactionInterceptor的invoke方法会开启事务

	@Override
	@Nullable
	public Object invoke(MethodInvocation invocation) throws Throwable {
		// Work out the target class: may be {@code null}.
		// The TransactionAttributeSource should be passed the target class
		// as well as the method, which may be from an interface.
		Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

		// Adapt to TransactionAspectSupport's invokeWithinTransaction...
		return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
	}