Spring源码之spring事务

目录

• Spring事务
• 事务自定义标签
  • 自定义标签
  • 解析标签
• bean 的初始化
  • InfrastructureAdvisorAutoProxyCreator
  • 获取增强方法
  • 获取所有增强中内适用于当前方法的增强
• TransactionInterceptor 事务增强的实现
  • 创建事务：
  • 回滚
  • 事务提交

Spring事务

spring声明式事务让我们从复杂的事务处理中得以脱身，我们可以不再去关注获得、关闭连接、事务提交、和回滚操作；简单来说事务可以做到在发生异常时进行回滚。

事务自定义标签

自定义标签

spring事务的开关配置是：tx:annotation-driven/ ，全局搜索："annotation-driven"，可以找到类：

    public class TxNamespaceHandler extends NamespaceHandlerSupport

    @Override
	public void init() {
		registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
		// 程序入口
		registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
		registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
	}

org\springframework\transaction\config\AnnotationDrivenBeanDefinitionParser.java 然后我们关注parse方法

public BeanDefinition parse(Element element, ParserContext parserContext) {
		registerTransactionalEventListenerFactory(parserContext);
		String mode = element.getAttribute("mode");
		if ("aspectj".equals(mode)) {// 配置mode属性可以实现通过AOP织入事务
			// mode="aspectj"
			registerTransactionAspect(element, parserContext);
		}
		else {
			// mode="proxy"
			// 根据配置文件定义工具类的 beanDefinition 并注册
			AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
		}
		return null;
	}

解析标签

找到真正的入口： AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);

public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
			// 注册 或者升级 InfrastructureAdvisorAutoProxyCreator 它实现了接口 BeanPostProcessor，
			// (最本质的逻辑是：通过 registry 注册了该后处理器，当从 beanFactory 依据 registory 初始化一个bean的时候，会调用该后处理器，对该 bean 进行事务增强）
			// 保证 bean 实例化时会调用后处理器的 postProcessBeforeInitialization 方法。
			AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);
			// beanName ??
			String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME;
			// 判断默认的beanName是否已经解析
			if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
				Object eleSource = parserContext.extractSource(element);

				// Create the TransactionAttributeSource definition.
				// 创建事务属性 beanDefinition 配置 ？？
				RootBeanDefinition sourceDef = new RootBeanDefinition(
						"org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
				sourceDef.setSource(eleSource);
				sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
				// 注册
				String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);

				// Create the TransactionInterceptor definition.
				// 拦截器 ？？ 实际调用时调用其invoke方法
				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);// 注冊

				// Create the TransactionAttributeSourceAdvisor definition.
				// 切点的 beanDefinition       PointcutAdvisor <- AbstractPointcutAdvisor <- AbstractBeanFactoryPointcutAdvisor <- BeanFactoryTransactionAttributeSourceAdvisor
				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")) {// 是否配置了order属性
					advisorDef.getPropertyValues().add("order", element.getAttribute("order"));
				}
				parserContext.getRegistry().registerBeanDefinition(txAdvisorBeanName, advisorDef);// 注册

				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);
			}
		}

上述代码的主要工作是注册了三个类：

1. 注册：InfrastructureAdvisorAutoProxyCreator

AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);

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

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

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

		// 注册或升级
		return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
	}

InfrastructureAdvisorAutoProxyCreator.java类图

查看它的类图，很快能找打我们需要重点关注的接口：BeanPostProcessor，看到它，就代表后处理器它来了

public interface BeanPostProcessor {
	default Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}
	default Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
		return bean;
	}
}

InfrastructureAdvisorAutoProxyCreator 它实现了接口 BeanPostProcessor，保证 bean 初始化时会调用后处理器中定义的方法。

(最本质的逻辑是：如果在容器组件中注册了该后处理器，当从 beanFactory 初始化一个bean的时候，就会调用该后处理器，对需要初始化 bean 进行事务增强）

1. 其中有两个类被注册到了类 BeanFactoryTransactionAttributeSourceAdvisor 中,从它的类图中我们可以看到它继承自：Advisor 故事即将开始；

BeanFactoryTransactionAttributeSourceAdvisor.java 类图

以它为根基通过AOP增强的方式最终实现事务功能，它以beanName：org.springframework.transaction.config.internalTransactionAdvisor 注册到了容器组件中。

• 被注册的两个类之一是：AnnotationTransactionAttributeSource 注册的beanName 为：transactionAttributeSource ，(它是增强和方法是否匹配的实际处理类,后续会提到它)

AnnotationTransactionAttributeSource.java类图

• 另一个类被注册到BeanFactoryTransactionAttributeSourceAdvisor 中的类是：TransactionInterceptor，注册的beanName为：adviceBeanName；
  
  TransactionInterceptor.java类图：
  
  

从它的类图我们可以找到，它实现了接口：MethodInterceptor，看到这个接口我们就该想到方法 invoke()；

bean 的初始化

InfrastructureAdvisorAutoProxyCreator

注册的三个类中，第一个登上舞台的是类 InfrastructureAdvisorAutoProxyCreator

跟随类 InfrastructureAdvisorAutoProxyCreator的继承体系解构，在它上游的类：AbstractAutoProxyCreator 中发现了如下的定义：

很容发现了我们的老朋友：wrapIfNecessary() 自打学习 AOP 我们就认识了它

// 初始化后  后处理器<增强>
	public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) throws BeansException {
		if (bean != null) {
			Object cacheKey = getCacheKey(bean.getClass(), beanName);
			if (this.earlyProxyReferences.remove(cacheKey) != bean) {// 避免循环依赖，提前暴露 ？？
				// 如果需要增强  则需要封装指定的 bean (代理该bean)
				return wrapIfNecessary(bean, beanName, cacheKey);
			}
		}
		return bean;
}

进入该方法的逻辑：

// 要么返回bean 要么返回被代理后的bean-proxy
	protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
		if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
			return bean;// 已经处理过 返回
		}
		// advisedBeans : 需要被增强的
		if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
			return bean;// 不需要增强返回
		}
		// 不需要被代理的类：基础类，或者设置了不需代理的类
		if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
			this.advisedBeans.put(cacheKey, Boolean.FALSE);
			return bean;
		}

		// Create proxy if we have advice.
		// 获取所有需要被增强的方法 ?? 获取增强切点，(且适用)
		Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
		if (specificInterceptors != DO_NOT_PROXY) {// 存在增强方法，则创建代理
			this.advisedBeans.put(cacheKey, Boolean.TRUE);
			// 对获取到的需要增强的方法进行代理
			Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));// 单例
			this.proxyTypes.put(cacheKey, proxy.getClass());
			return proxy;
		}

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

获取增强方法

我们在类AbstractAdvisorAutoProxyCreator：中找到了获取增强方法的定义

protected Object[] getAdvicesAndAdvisorsForBean(
			Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {

		// 根据 Class 以及 beanName 获取增强，(且适用)
		List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
		if (advisors.isEmpty()) {
			return DO_NOT_PROXY;
		}
		return advisors.toArray();
	}

在大体实现上根AOP 基本类似：

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
		// AnnotationAwareAspectJAutoProxyCreator 类覆盖了该方法
		List<Advisor> candidateAdvisors = findCandidateAdvisors();// 获取所有的增强 （所有  拦截||切点？？？）
		List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);// 寻找适用于当前 Bean 的增强
		extendAdvisors(eligibleAdvisors);
		if (!eligibleAdvisors.isEmpty()) {
			eligibleAdvisors = sortAdvisors(eligibleAdvisors);
		}
		return eligibleAdvisors;
	}

在获取所有增强的 findCandidateAdvisors() 方法内部找到了如下代码：

BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Advisor.class, true, false);

我们可看到，它提取了所有的实现了 Advisor 接口的类，到这里我们会议一下，解析事务开关标签的时候，是不是注册了三个类，其中有两个类就注册到了：BeanFactoryTransactionAttributeSourceAdvisor

而它的继承体系中刚好就有：Advisor，自此我们注册的另外两个类将会通过 BeanFactoryTransactionAttributeSourceAdvisor 一一登录舞台。

获取所有增强中内适用于当前方法的增强

跟随方法 findAdvisorsThatCanApply()以及参数candidateAdvisors，找到如下方法：

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
		if (candidateAdvisors.isEmpty()) {
			return candidateAdvisors;
		}
		List<Advisor> eligibleAdvisors = new ArrayList<>();
		// 处理引介增强
		for (Advisor candidate : candidateAdvisors) {
			if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
				eligibleAdvisors.add(candidate);// 适合 ？？ 拦截条件成立？？
			}
		}
		boolean hasIntroductions = !eligibleAdvisors.isEmpty();
		for (Advisor candidate : candidateAdvisors) {
			// 跳过引介增强（已处理）
			if (candidate instanceof IntroductionAdvisor) {
				// already processed
				continue;
			}
			// 普通 bean 处理
			if (canApply(candidate, clazz, hasIntroductions)) {
				eligibleAdvisors.add(candidate);
			}
		}
		return eligibleAdvisors;
	}

根据 BeanFactoryTransactionAttributeSourceAdvisor 的类图我们可以很明确知道，它的继承结构中并没有：IntroductionAdvisor，那么进入下面的逻辑：

canApply() 方法判断增强是否适用当前方法：

public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
		if (advisor instanceof IntroductionAdvisor) {
			return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
		}
		else if (advisor instanceof PointcutAdvisor) {
			PointcutAdvisor pca = (PointcutAdvisor) advisor;
			return canApply(pca.getPointcut(), targetClass, hasIntroductions);
		}
		else {
			// It doesn't have a pointcut so we assume it applies.
			return true;
		}
	}

很明显 BeanFactoryTransactionAttributeSourceAdvisor 继承自：PointcutAdvisor

else if (advisor instanceof PointcutAdvisor) {
			PointcutAdvisor pca = (PointcutAdvisor) advisor;
			return canApply(pca.getPointcut(), targetClass, hasIntroductions);
		}

跟随上述几行代码，我在类 BeanFactoryTransactionAttributeSourceAdvisor 类中找到了getPointcut() 方法：

private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
		@Override
		@Nullable
		protected TransactionAttributeSource getTransactionAttributeSource() {
			return transactionAttributeSource;
		}
};

定睛一看：你说巧不巧，它返回的：TransactionAttributeSourcePointcut，实现了一个回调 getTransactionAttributeSource()方法，

它返回的就是 transactionAttributeSource。回到开始，解析事务标签注册了三个类，两个被注册到：BeanFactoryTransactionAttributeSourceAdvisor

transactionAttributeSource 就是那二者之一，它对应的bean就是：AnnotationTransactionAttributeSource。

继续深入方法：canApply(pca.getPointcut(), targetClass, hasIntroductions);

public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {

		..........

		MethodMatcher methodMatcher = pc.getMethodMatcher();

		IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
        if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
            introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
        }
		...............

		for (Class<?> clazz : classes) {
			Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
			for (Method method : methods) {
				if (introductionAwareMethodMatcher != null ?
						introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
						methodMatcher.matches(method, targetClass)) {
					return true;
				}
			}
		}

		return false;
	}

经过上文，我们得知：pc的实际类型是： TransactionAttributeSourcePointcut

TransactionAttributeSourcePointcut.java类图

跟随它的类图，我们在类：StaticMethodMatcherPointcut 中找到了 getMethodMatcher() 方法的实现

@Override
	public final MethodMatcher getMethodMatcher() {
		return this;
	}

根据它的类型我么可以确定表达式：

introductionAwareMethodMatcher != null
    ? introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)
        : methodMatcher.matches(method, targetClass)

最终执行的是后者：methodMatcher.matches(method, targetClass)。

类 TransactionAttributeSourcePointcut中找到了 matches的实现：

@Override
	public boolean matches(Method method, @Nullable Class<?> targetClass) {
		if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
			return false;
		}
		// 注册的类定义 transactionAttributeSource AnnotationTransactionAttributeSource
		TransactionAttributeSource tas = getTransactionAttributeSource();
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}

这里我们见到了回调方法：getTransactionAttributeSource()， 最终：tas 的实际类型为：AnnotationTransactionAttributeSource，

表达式 methodMatcher.matches(method, targetClass) 最终判定的是：

(tas == null || tas.getTransactionAttribute(method, targetClass) != null)

跟随 AnnotationTransactionAttributeSource.getTransactionAttribute 继续往下，

在父类AbstractFallbackTransactionAttributeSource中找到方法：computeTransactionAttribute() ，至此我们终于看到了提取事务声明的方法：

protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
		Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);
		// 查看方法中是否存在事务声明
		TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
		if (txAttr != null) {
			return txAttr; }
		// 查看方法所在类是否存在事务声明
		txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
		if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
			return txAttr; }
		// specificMethod != method 代表存在接口，去接口中找
		if (specificMethod != method) {
			// 查找接口实现方法 ？？
			txAttr = findTransactionAttribute(method);
			if (txAttr != null) {
				return txAttr;  }
			// 查找接口实现类 ？？
			txAttr = findTransactionAttribute(method.getDeclaringClass());
			if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
				return txAttr; }
		}
		return null;
	}

最后在 AnnotationTransactionAttributeSource 类中找到 findTransactionAttribute() 方法的定义：

    protected TransactionAttribute findTransactionAttribute(Class<?> clazz) {
		return determineTransactionAttribute(clazz);
	}

	protected TransactionAttribute findTransactionAttribute(Method method) {
		return determineTransactionAttribute(method);
	}

	protected TransactionAttribute determineTransactionAttribute(AnnotatedElement element) {
    		// annotationParsers (SpringTransactionAnnotationParser)
    		for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
    			TransactionAttribute attr = annotationParser.parseTransactionAnnotation(element);
    			if (attr != null) {
    				return attr;
    			}
    		}
    		return null;
    	}

走到此处，你可能懵逼了：this.annotationParsers是个啥，我没有注册过它啊？ 查看类 AnnotationTransactionAttributeSource 的构造器：

public AnnotationTransactionAttributeSource(boolean publicMethodsOnly) {
		this.publicMethodsOnly = publicMethodsOnly;
		if (jta12Present || ejb3Present) {
			this.annotationParsers = new LinkedHashSet<>(4);
			this.annotationParsers.add(new SpringTransactionAnnotationParser());
			if (jta12Present) {
				this.annotationParsers.add(new JtaTransactionAnnotationParser());
			}
			if (ejb3Present) {
				this.annotationParsers.add(new Ejb3TransactionAnnotationParser());
			}
		}
		else {
			this.annotationParsers = Collections.singleton(new SpringTransactionAnnotationParser());
		}
	}

最终集合 this.annotationParsers 被初始化为：SpringTransactionAnnotationParser 的集合；从中，我们来到了此行的终点：

@Override
	@Nullable
	public TransactionAttribute parseTransactionAnnotation(AnnotatedElement element) {
		// 注解工具类，提取注解 ??
		AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
				element, Transactional.class, false, false);
		if (attributes != null) {
			return parseTransactionAnnotation(attributes);
		}
		else {
			return null;
		}
	}

	// 获取注解标记
	protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
		RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();

		Propagation propagation = attributes.getEnum("propagation");
		rbta.setPropagationBehavior(propagation.value());
		Isolation isolation = attributes.getEnum("isolation");
		rbta.setIsolationLevel(isolation.value());
		rbta.setTimeout(attributes.getNumber("timeout").intValue());// 超时 ？？
		rbta.setReadOnly(attributes.getBoolean("readOnly"));// 只读 ？？
		rbta.setQualifier(attributes.getString("value"));// 

		// 回滚 ？？
		List<RollbackRuleAttribute> rollbackRules = new ArrayList<>();
		for (Class<?> rbRule : attributes.getClassArray("rollbackFor")) {
			rollbackRules.add(new RollbackRuleAttribute(rbRule));
		}
		for (String rbRule : attributes.getStringArray("rollbackForClassName")) {
			rollbackRules.add(new RollbackRuleAttribute(rbRule));
		}
		for (Class<?> rbRule : attributes.getClassArray("noRollbackFor")) {
			rollbackRules.add(new NoRollbackRuleAttribute(rbRule));
		}
		for (String rbRule : attributes.getStringArray("noRollbackForClassName")) {
			rollbackRules.add(new NoRollbackRuleAttribute(rbRule));
		}
		rbta.setRollbackRules(rollbackRules);

		return rbta;
	}

自此事务标签的解析完成,剩下的是对增强的具体实现

TransactionInterceptor 事务增强的实现

应该还记得有两个类被注册到BeanFactoryTransactionAttributeSourceAdvisor 中，我们已经讲到了，AnnotationTransactionAttributeSource，

它的作用是处理增强是否匹配当前方法。 (换句话说，方法是是否能提取到事务标签即可证明是否适用增强)

剩下一个还未提到的类是：TransactionInterceptor，

不多说，关门放类图：

TransactionInterceptor.java 类图

从继承的体系结构，可以很容易得知他是一个拦截器，那么直接看invoke的定义：

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);// 继续
	}

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

    		// If the transaction attribute is null, the method is non-transactional.
    		TransactionAttributeSource tas = getTransactionAttributeSource();
    		// 获取事务属性  <增强信息>
    		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
    		// transactionManager
    		final PlatformTransactionManager tm = determineTransactionManager(txAttr);
    		// 切点信息 ？？  - 构造方法唯一标识:  全限定类名.方法名()
    		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

    		// 声明式事务处理
    		if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
    			// Standard transaction demarcation with getTransaction and commit/rollback calls.
    			// 创建事务
    			TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);

    			Object retVal;
    			try {
    				// This is an around advice: Invoke the next interceptor in the chain.
    				// This will normally result in a target object being invoked.
    				// 执行被增强的方法
    				retVal = invocation.proceedWithInvocation();
    			}
    			catch (Throwable ex) {
    				// 出现异常，处理回滚  默认仅仅回滚 RunTimeException
    				completeTransactionAfterThrowing(txInfo, ex);
    				throw ex;
    			}
    			finally {
    				// clear
    				cleanupTransactionInfo(txInfo);
    			}
    			// 提交事务  <不论是否回滚都需要提交事务到恰当的状态>
    			// 对于内嵌事务不会直接提交，会将事务结果设置保存点，当最外层事务也正常执行后，由最外层事务统一提交
    			commitTransactionAfterReturning(txInfo);
    			return retVal;
    		}
    	}

创建事务：

// 创建事务
	protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
			@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {

		// If no name specified, apply method identification as transaction name.
		if (txAttr != null && txAttr.getName() == null) {
			// 包装器？？
			txAttr = new DelegatingTransactionAttribute(txAttr) {
				@Override
				public String getName() {
					return joinpointIdentification;
				}
			};
		}

		TransactionStatus status = null;
		if (txAttr != null) {
			if (tm != null) {
				// 事务状态 ？？
				status = tm.getTransaction(txAttr);// DataSourceTransactionManager.getTransaction();
			}
			else {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
							"] because no transaction manager has been configured");
				}
			}
		}
		// 准备并封装事务信息
		return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
	}

处理事务准备工作，事务获取以及信息构建：

public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
		Object transaction = doGetTransaction();// 获取事务 ，基于JDBC创建事务实例，如果当前线程已经记录了连接，可以复用 

		// Cache debug flag to avoid repeated checks.
		boolean debugEnabled = logger.isDebugEnabled();

		if (definition == null) {
			// Use defaults if no transaction definition given.
			definition = new DefaultTransactionDefinition();
		}

		if (isExistingTransaction(transaction)) {// 当前线程已经存在事务，且连接不为空，连接中的 transactionActive 不为空
			// Existing transaction found -> check propagation behavior to find out how to behave.
			return handleExistingTransaction(definition, transaction, debugEnabled);// 当前线程存在事务，转向嵌套事务的处理
		}

		// Check definition settings for new transaction.
		// 事务超时设置验证
		if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
			throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
		}

		// No existing transaction found -> check propagation behavior to find out how to proceed.
		// 当前线程不存在事务，且 propagationBehavior 被声明为：PROPAGATION_MANDATORY 则抛出异常
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {// 强制性的，但是当前线程没有事务，所以抛出异常
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}
		else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||// 需要？？
				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||// 需要一个新的？？
				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {// 嵌套
			// 上述类型，他们都需要新建事务 

			//  空挂起 ？？
			SuspendedResourcesHolder suspendedResources = suspend(null);
			if (debugEnabled) {
				logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
			}
			try {
				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
				DefaultTransactionStatus status = newTransactionStatus(
						definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
				// 构造事务，设置connectionHolder、隔离级别、超时、如果是新连接则需要绑定到当前线程   (委托给低层的连接完成)
				doBegin(transaction, definition);// 数据库连接设置，新(新生成，或者记录被清空无记录) 连接记录到当前线程
				// 新同步事务的设置？？  针对当前线程进行设置
				prepareSynchronization(status, definition);
				return status;
			}
			catch (RuntimeException | Error ex) {
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			// Create "empty" transaction: no actual transaction, but potentially synchronization.
			if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
				logger.warn("Custom isolation level specified but no actual transaction initiated; " +
						"isolation level will effectively be ignored: " + definition);
			}
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
			return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
		}
	}

数据库连接设置：

protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			// 当前线程没有绑定连接或者设置了同步事务
			if (!txObject.hasConnectionHolder() ||
					txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
				Connection newCon = obtainDataSource().getConnection();
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);// 新连接
			}

			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);// 修改connectionHolder 的事务标记
			con = txObject.getConnectionHolder().getConnection();

			// 设置隔离级别
			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			txObject.setPreviousIsolationLevel(previousIsolationLevel);

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			if (con.getAutoCommit()) {// 更改自动提交设置，由spring控制提交
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
				con.setAutoCommit(false);
			}

			prepareTransactionalConnection(con, definition);
			txObject.getConnectionHolder().setTransactionActive(true);// 设置当前线程存在事务 

			int timeout = determineTimeout(definition);
			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {// 设置连接绑定到事务的过期时间 ？？
				txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
			}

			// Bind the connection holder to the thread.
			if (txObject.isNewConnectionHolder()) {// 连接是否为新的连接 ？？ 如果是绑定到当前线程
				TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
			}
		}

		catch (Throwable ex) {
			if (txObject.isNewConnectionHolder()) {
				DataSourceUtils.releaseConnection(con, obtainDataSource());
				txObject.setConnectionHolder(null, false);
			}
			throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
		}
	}

处理已经存在的事务：

private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {// 处理已经存在的事务

		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {// never 抛异常
			throw new IllegalTransactionStateException(
					"Existing transaction found for transaction marked with propagation 'never'");
		}

		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {// supported
			if (debugEnabled) {
				logger.debug("Suspending current transaction");
			}
			Object suspendedResources = suspend(transaction);
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);// always
			return prepareTransactionStatus(
					definition, null, false, newSynchronization, debugEnabled, suspendedResources);
		}

		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {// requires_new
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}
			SuspendedResourcesHolder suspendedResources = suspend(transaction);// 挂起旧事务
			try {
				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);// 创建新事务
				DefaultTransactionStatus status = newTransactionStatus(
						definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
				doBegin(transaction, definition);
				prepareSynchronization(status, definition);
				return status;
			}
			catch (RuntimeException | Error beginEx) {
				resumeAfterBeginException(transaction, suspendedResources, beginEx);
				throw beginEx;
			}
		}

		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {// nested 嵌套式事务处理
			if (!isNestedTransactionAllowed()) {
				throw new NestedTransactionNotSupportedException(
						"Transaction manager does not allow nested transactions by default - " +
						"specify 'nestedTransactionAllowed' property with value 'true'");
			}
			if (debugEnabled) {
				logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
			}
			if (useSavepointForNestedTransaction()) {
				// Create savepoint within existing Spring-managed transaction,
				// through the SavepointManager API implemented by TransactionStatus.
				// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
				// 没有保存点，在嵌套式事务建立初始保存点  存档？？
				DefaultTransactionStatus status =
						prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
				status.createAndHoldSavepoint();
				return status;
			}
			else {
				// Nested transaction through nested begin and commit/rollback calls.
				// Usually only for JTA: Spring synchronization might get activated here
				// in case of a pre-existing JTA transaction.
				// 不能使用保存点的时候，新建事务
				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
				DefaultTransactionStatus status = newTransactionStatus(
						definition, transaction, true, newSynchronization, debugEnabled, null);
				doBegin(transaction, definition);
				prepareSynchronization(status, definition);
				return status;
			}
		}

		// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
		if (debugEnabled) {
			logger.debug("Participating in existing transaction");
		}
		if (isValidateExistingTransaction()) {
			if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
				Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
				if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
					Constants isoConstants = DefaultTransactionDefinition.constants;
					throw new IllegalTransactionStateException("Participating transaction with definition [" +
							definition + "] specifies isolation level which is incompatible with existing transaction: " +
							(currentIsolationLevel != null ?
									isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
									"(unknown)"));
				}
			}
			if (!definition.isReadOnly()) {
				if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
					throw new IllegalTransactionStateException("Participating transaction with definition [" +
							definition + "] is not marked as read-only but existing transaction is");
				}
			}
		}
		boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
		return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
	}

回滚

protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {
		if (txInfo != null && txInfo.getTransactionStatus() != null) {//  判断存在事务
			if (logger.isTraceEnabled()) {
				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
						"] after exception: " + ex);
			}
			// 默认实现： (ex instanceof RuntimeException || ex instanceof Error)
			if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
				try {
					txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
				}
				catch (TransactionSystemException ex2) {
					logger.error("Application exception overridden by rollback exception", ex);
					ex2.initApplicationException(ex);
					throw ex2;
				}
				catch (RuntimeException | Error ex2) {
					logger.error("Application exception overridden by rollback exception", ex);
					throw ex2;
				}
			}
			else {
				// We don't roll back on this exception.
				// Will still roll back if TransactionStatus.isRollbackOnly() is true.
				// 当不满足 rollbackOn 的回滚条件时，抛异常也会提交事务
				try {
					txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
				}
				catch (TransactionSystemException ex2) {
					logger.error("Application exception overridden by commit exception", ex);
					ex2.initApplicationException(ex);
					throw ex2;
				}
				catch (RuntimeException | Error ex2) {
					logger.error("Application exception overridden by commit exception", ex);
					throw ex2;
				}
			}
		}
	}

回滚处理：

private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
		try {
			boolean unexpectedRollback = unexpected;

			try {
				// 自定义触发器的调用
				// 激活所有 TransactionSynchronization 中的 beforeCompletion方法
				triggerBeforeCompletion(status);

				if (status.hasSavepoint()) {// 回滚到保存点 ??  常用于嵌套事务，内层事务异常回滚不会影响到外层事务
					if (status.isDebug()) {
						logger.debug("Rolling back transaction to savepoint");
					}
					// 保存点依赖于底层的数据库连接
					status.rollbackToHeldSavepoint();// 如果有保存点，当前事务为单独的线程(单独的事务)，会退到保存点
				}
				else if (status.isNewTransaction()) {// 当前事务为独立的新事务，直接回滚
					if (status.isDebug()) {
						logger.debug("Initiating transaction rollback");
					}
					doRollback(status);
				}
				else {// 没有保存点，且不是独立的新事务，那么标记状态，等到事务链执行完后统一回滚  (都不提交)
					// Participating in larger transaction
					if (status.hasTransaction()) {
						if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
							if (status.isDebug()) {
								logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
							}
							doSetRollbackOnly(status);
						}
						else {
							if (status.isDebug()) {
								logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
							}
						}
					}
					else {
						logger.debug("Should roll back transaction but cannot - no transaction available");
					}
					// Unexpected rollback only matters here if we're asked to fail early
					if (!isFailEarlyOnGlobalRollbackOnly()) {
						unexpectedRollback = false;
					}
				}
			}
			catch (RuntimeException | Error ex) {
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
				throw ex;
			}

			// 自定义触发器的调用
			// 激活所有 TransactionSynchronization 中的 afterCompletion方法
			triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);

			// Raise UnexpectedRollbackException if we had a global rollback-only marker
			if (unexpectedRollback) {
				throw new UnexpectedRollbackException(
						"Transaction rolled back because it has been marked as rollback-only");
			}
		}
		finally {
			cleanupAfterCompletion(status);// 清空记录的资源，例如记录到线程的连接 ；   将挂起的资源恢复，例如：嵌套事务。
		}
	}

事务提交

public final void commit(TransactionStatus status) throws TransactionException {
		if (status.isCompleted()) {// 事务已经完成，在此提交抛出异常
			throw new IllegalTransactionStateException(
					"Transaction is already completed - do not call commit or rollback more than once per transaction");
		}

		DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
		if (defStatus.isLocalRollbackOnly()) {// 事务链 中标记了回滚，那么直接回滚，不再尝试提交事务
			if (defStatus.isDebug()) {
				logger.debug("Transactional code has requested rollback");
			}
			processRollback(defStatus, false);
			return;
		}

		if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {//
			if (defStatus.isDebug()) {
				logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
			}
			processRollback(defStatus, true);
			return;
		}

		// 处理提交逻辑
		processCommit(defStatus);
	}

处理提交逻辑：

private void processCommit(DefaultTransactionStatus status) throws TransactionException {
		try {
			boolean beforeCompletionInvoked = false;

			try {
				boolean unexpectedRollback = false;
				prepareForCommit(status);// 预留钩子
				triggerBeforeCommit(status);// 提交前触发器激活
				triggerBeforeCompletion(status);// 完成前触发器 ？？
				beforeCompletionInvoked = true;

				if (status.hasSavepoint()) {// 存在保存点？？
					if (status.isDebug()) {// 事务正常执行完毕，释放(清除)保存点
						logger.debug("Releasing transaction savepoint");
					}
					unexpectedRollback = status.isGlobalRollbackOnly();
					status.releaseHeldSavepoint();
				}
				else if (status.isNewTransaction()) {// 独立的新事务，直接提交
					if (status.isDebug()) {
						logger.debug("Initiating transaction commit");
					}
					unexpectedRollback = status.isGlobalRollbackOnly();
					doCommit(status);// 提交事务 调用底层的数据库连接
				}// 不被spring管理的事务 ？？ 无法设置保存点的事务 ？？ 设置回滚标识
				else if (isFailEarlyOnGlobalRollbackOnly()) {// 事务链中的某个事务被设置了回滚标记，那么直接回滚
					unexpectedRollback = status.isGlobalRollbackOnly();
				}

				// Throw UnexpectedRollbackException if we have a global rollback-only
				// marker but still didn't get a corresponding exception from commit.
				if (unexpectedRollback) {// 回滚标识
					throw new UnexpectedRollbackException(
							"Transaction silently rolled back because it has been marked as rollback-only");
				}
			}
			catch (UnexpectedRollbackException ex) {
				// can only be caused by doCommit
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
				throw ex;
			}
			catch (TransactionException ex) {
				// can only be caused by doCommit
				if (isRollbackOnCommitFailure()) {
					doRollbackOnCommitException(status, ex);
				}
				else {
					triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
				}
				throw ex;
			}
			catch (RuntimeException | Error ex) {
				if (!beforeCompletionInvoked) {
					triggerBeforeCompletion(status);
				}
				doRollbackOnCommitException(status, ex);// 提交过程中出现异常，回滚
				throw ex;
			}

			// Trigger afterCommit callbacks, with an exception thrown there
			// propagated to callers but the transaction still considered as committed.
			try {
				triggerAfterCommit(status);// 提交事务后的触发器
			}
			finally {
				// 事务完成后的触发器
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
			}

		}
		finally {
			cleanupAfterCompletion(status);// 事务完成后进行清理
		}
	}