注册后置处理器开启对事务的支持

@EnableTransactionManagement

@EnableTransactionManagement注解的主要作用是开启对事务的支持，源码如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

	boolean proxyTargetClass() default false;

	AdviceMode mode() default AdviceMode.PROXY;

	int order() default Ordered.LOWEST_PRECEDENCE;

}

这里最核心的是TransactionManagementConfigurationSelector类，这个类主要的作用是通过ImportSelector注册了AutoProxyRegistrar和ProxyTransactionManagementConfiguration2个组件，源码如下：

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

	@Override
	protected String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
			case PROXY:
				// 注册 InfrastructureAdvisorAutoProxyCreator 后置处理器和事务管理器组件
				return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()};
			case ASPECTJ:
				return new String[] {TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};
			default:
				return null;
		}
	}
}

AutoProxyRegistrar

AutoProxyRegistrar 的主要作用是将InfrastructureAdvisorAutoProxyCreator后置处理器注册到容器，注册这个后置处理器和上一篇Spring AOP注册AnnotationAwareAspectJAutoProxyCreator后置处理器一样，这里就不在重复说明了。InfrastructureAdvisorAutoProxyCreator 他是实现了BeanPostProcessor 接口的后置处理器，所以所有 Bean 的初始化都会调用其 postProcessAfterInitialization方法，这个方法的实现是在其父类AbstractAutoProxyCreator类中。

ProxyTransactionManagementConfiguration

我们通过ProxyTransactionManagementConfiguration来注册事务管理器组件，这个类本身也是一个配置类。在这个配置类中我们将会注册一下三个组件：

BeanFactoryTransactionAttributeSourceAdvisor：事务增强器，包含了切面组件 TransactionInterceptor和标签解析器TransactionAttributeSource
TransactionAttributeSource：@Transaction注解标签解析器
TransactionInterceptor：保存了事务属性信息，事务管理器；它本身也是一个方法拦截器，在invoke方法中进行了事务的处理。

创建代理Bean

上面我们说了所以所有 Bean 的初始化都会调用其 AbstractAutoProxyCreator#postProcessAfterInitialization方法来完成Bean的增强，我们跟进去可以看到这段代码：

@Override
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
	if (bean != null) {
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (!this.earlyProxyReferences.contains(cacheKey)) {
			return wrapIfNecessary(bean, beanName, cacheKey);
		}
	}
	return bean;
}

可以看到生代理对象是在wrapIfNecessary(bean, beanName, cacheKey);方法中完成的，源码如下：

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
	
	// 如果存在建言那么久创建代理类
	// 获取拦截链
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
	if (specificInterceptors != DO_NOT_PROXY) {
		this.advisedBeans.put(cacheKey, Boolean.TRUE);
		// 使用拦截链创建代理对象，对原有的Bean进行增强
		Object proxy = createProxy(
				bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
		this.proxyTypes.put(cacheKey, proxy.getClass());
		return proxy;
	}

	this.advisedBeans.put(cacheKey, Boolean.FALSE);
	return bean;
}

找到拦截链的的核心方法是 BeanFactoryAdvisorRetrievalHelper#findAdvisorBeans方法

findAdvisorBeans:67, BeanFactoryAdvisorRetrievalHelper (org.springframework.aop.framework.autoproxy)
findCandidateAdvisors:102, AbstractAdvisorAutoProxyCreator (org.springframework.aop.framework.autoproxy)
findEligibleAdvisors:88, AbstractAdvisorAutoProxyCreator (org.springframework.aop.framework.autoproxy)
getAdvicesAndAdvisorsForBean:70, AbstractAdvisorAutoProxyCreator (org.springframework.aop.framework.autoproxy)
wrapIfNecessary:346, AbstractAutoProxyCreator (org.springframework.aop.framework.autoproxy)
postProcessAfterInitialization:298, AbstractAutoProxyCreator (org.springframework.aop.framework.autoproxy)
applyBeanPostProcessorsAfterInitialization:423, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
initializeBean:1638, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
doCreateBean:555, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
createBean:483, AbstractAutowireCapableBeanFactory (org.springframework.beans.factory.support)
getObject:312, AbstractBeanFactory$1 (org.springframework.beans.factory.support)
getSingleton:230, DefaultSingletonBeanRegistry (org.springframework.beans.factory.support)
doGetBean:308, AbstractBeanFactory (org.springframework.beans.factory.support)
getBean:197, AbstractBeanFactory (org.springframework.beans.factory.support)
preInstantiateSingletons:761, DefaultListableBeanFactory (org.springframework.beans.factory.support)
finishBeanFactoryInitialization:867, AbstractApplicationContext (org.springframework.context.support)
refresh:543, AbstractApplicationContext (org.springframework.context.support)
<init>:84, AnnotationConfigApplicationContext (org.springframework.context.annotation)

源码如下：

public List<Advisor> findAdvisorBeans() {
	// Determine list of advisor bean names, if not cached already.
	String[] advisorNames = null;
	synchronized (this) {
		advisorNames = this.cachedAdvisorBeanNames;
		if (advisorNames == null) {
			// 获取所有增强器的名称
			advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
					this.beanFactory, Advisor.class, true, false);
			this.cachedAdvisorBeanNames = advisorNames;
		}
	}
	if (advisorNames.length == 0) {
		return new LinkedList<Advisor>();
	}

	List<Advisor> advisors = new LinkedList<Advisor>();
	for (String name : advisorNames) {
		if (isEligibleBean(name)) {
			if (this.beanFactory.isCurrentlyInCreation(name)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipping currently created advisor '" + name + "'");
				}
			}
			else {
				try {
					// 根据名称增强器
					advisors.add(this.beanFactory.getBean(name, Advisor.class));
				}
				...
			}
		}
	}
	// 返回拦截链
	return advisors;
}

创建代理Bean的核心流程：

单例Bean初始化完成后执行后置处理器 AbstractAutoProxyCreator#postProcessAfterInitialization方法

在容器中找Advisor类型的所有增强器名称，这就会将与事务相关的增强器BeanFactoryTransactionAttributeSourceAdvisor找出来

根据增强器名称获取对应的实例，并生成拦截链

判断代理类型

根据不同的代理类型和拦截链创建代理对象

执行业务方法进行拦截

前面AOP说过不管理是JdkDynamicAopProxy还是CglibAopProxy代理，他们的执行最终都会去调用MethodInterceptor.invoke()方法，而我们事务对应的方法拦截器是TransactionInterceptor类。也就是说我们对事务的增强起始是在TransactionInterceptor的invoke方法中。源码如下：

@Override
public Object invoke(final MethodInvocation invocation) throws Throwable {
	...
	return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
		@Override
		public Object proceedWithInvocation() throws Throwable {
			return invocation.proceed();
		}
	});
}

protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
			throws Throwable {

	// 获取事务属性
	final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
	// 获取事务管理器
	final PlatformTransactionManager tm = determineTransactionManager(txAttr);
	// 构造方法唯一标示
	final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

	// 声明式事务
	if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
		// 创建事务
		TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
		Object retVal = null;
		try {
			// 执行被增强的方法
			retVal = invocation.proceedWithInvocation();
		}
		catch (Throwable ex) {
			// 异常回滚
			completeTransactionAfterThrowing(txInfo, ex);
			throw ex;
		}
		finally {
			// 清除信息
			cleanupTransactionInfo(txInfo);
		}
		// 提交事务
		commitTransactionAfterReturning(txInfo);
		return retVal;
	}
	// 编程式事务
	...
}

从上面我们的源码可以看出，一个事务处理的标准流程：

createTransactionIfNecessary创建一个事务
invocation.proceedWithInvocation();执行业务方法
completeTransactionAfterThrowing(txInfo, ex);如果遇到异常，事务回滚
commitTransactionAfterReturning(txInfo);如果没有异常就提交事务

在创建，回滚和提交事务方法中还有的很多对嵌套事务的逻辑，比如事务的传递性，事务回滚的条件判断等，这里就不说了，有兴趣自己去跟下源码。

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