Spring 事务源码解析

准备

• Spring版本：5.0.8

解析过程

标签解析分析

事务注解驱动标签 <tx:annotation-driven/> 解析，首先定位到 TxNamespaceHandler

// TxNamespaceHandler
public void init() {
   registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
   registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
   registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
}

// AnnotationDrivenBeanDefinitionParser
public BeanDefinition parse(Element element, ParserContext parserContext) {
   registerTransactionalEventListenerFactory(parserContext);
   String mode = element.getAttribute("mode");
   if ("aspectj".equals(mode)) {
      // mode="aspectj"
      registerTransactionAspect(element, parserContext);
   }
   else {
      // mode="proxy"
      AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
   }
   return null;
}

// AnnotationDrivenBeanDefinitionParser
private static class AopAutoProxyConfigurer {

   public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
      // 注册了InfrastructureAdvisorAutoProxyCreator类 该类实现了BeanPostProcessor接口
      AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);

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

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

         // 创建TransactionInterceptor BeanDefinition
         RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
         interceptorDef.setSource(eleSource);
         interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
         /**
          * 注册事务管理器
          *
          * 默认会寻找<tx:annotation-driven/>配置的transaction-manager属性作为事务管理器
          * 否则会寻找beanName为transactionManager的Bean作为事务管理器
          */
         registerTransactionManager(element, interceptorDef);
         // 为TransactionInterceptor添加TransactionAttributeSource引用
         interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
         String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);

         // 创建TransactionAttributeSourceAdvisor BeanDefinition
         RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);
         advisorDef.setSource(eleSource);
         advisorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
         // 为TransactionAttributeSourceAdvisor添加TransactionAttributeSource、TransactionInterceptor引用
         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);
      }
   }
}

注册 InfrastructureAdvisorAutoProxyCreator 类，查看该类继承图，发现该类实现了BeanPostProcessor 接口，在bean实例化时会调用其 postProcessAfterInitialization 方法，而其实现在父类 AbstractAutoProxyCreator 中

// AbstractAutoProxyCreator
public Object postProcessAfterInitialization(@Nullable 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;
}

// AbstractAutoProxyCreator
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
   if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
      return bean;
   }

   if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
      return bean;
   }
   // 如果是aop基础类或配置不需要代理 直接返回
   if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
      this.advisedBeans.put(cacheKey, Boolean.FALSE);
      return bean;
   }

   // 获取bean匹配的增强器
   Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
   if (specificInterceptors != DO_NOT_PROXY) {
      this.advisedBeans.put(cacheKey, Boolean.TRUE);
      // 创建代理
      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;
}

getAdvicesAndAdvisorsForBean 方法具体就是为了获取配置为增强器的bean，而在AopAutoProxyConfigurer.configureAutoProxyCreator方法注册的BeanFactoryTransactionAttributeSourceAdvisor 类实现了 Advisor，会被查找出来，另外被注入的两个也会被查找出来

// AbstractAdvisorAutoProxyCreator
protected Object[] getAdvicesAndAdvisorsForBean(
      Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {
   // 查找合适的增强器
   List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
   if (advisors.isEmpty()) {
      return DO_NOT_PROXY;
   }
   return advisors.toArray();
}

// AbstractAdvisorAutoProxyCreator
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
   // 寻找合适的增强器
   List<Advisor> candidateAdvisors = findCandidateAdvisors();
   // 确定增强器能否应用在bean上
   List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
   extendAdvisors(eligibleAdvisors);
   if (!eligibleAdvisors.isEmpty()) {
      eligibleAdvisors = sortAdvisors(eligibleAdvisors);
   }
   return eligibleAdvisors;
}

// AbstractAdvisorAutoProxyCreator
protected List<Advisor> findAdvisorsThatCanApply(
      List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {

   ProxyCreationContext.setCurrentProxiedBeanName(beanName);
   try {
      // 获取当前bean可用的增强器
      return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
   }
   finally {
      ProxyCreationContext.setCurrentProxiedBeanName(null);
   }
}

// AopUtils
public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
   if (candidateAdvisors.isEmpty()) {
      return candidateAdvisors;
   }
   List<Advisor> eligibleAdvisors = new LinkedList<>();
   // 首先处理引介增强是否可以应用在当前bean上
   for (Advisor candidate : candidateAdvisors) {
      if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
         eligibleAdvisors.add(candidate);
      }
   }
   boolean hasIntroductions = !eligibleAdvisors.isEmpty();
   for (Advisor candidate : candidateAdvisors) {
      if (candidate instanceof IntroductionAdvisor) {
         // 引介增强已处理
         continue;
      }
      // 判断当前增强是否可以应用在当前bean上
      if (canApply(candidate, clazz, hasIntroductions)) {
         eligibleAdvisors.add(candidate);
      }
   }
   return eligibleAdvisors;
}

// AopUtils
public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
   // 判断引介增强是否匹配
   if (advisor instanceof IntroductionAdvisor) {
      return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
   }
   else if (advisor instanceof PointcutAdvisor) {
      PointcutAdvisor pca = (PointcutAdvisor) advisor;
      // 判断是否可以应用在当前bean上
      return canApply(pca.getPointcut(), targetClass, hasIntroductions);
   }
   else {
      // It doesn't have a pointcut so we assume it applies.
      return true;
   }
}

// BeanFactoryTransactionAttributeSourceAdvisor
public Pointcut getPointcut() {
   return this.pointcut;
}

// AopUtils
public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
   Assert.notNull(pc, "Pointcut must not be null");
   if (!pc.getClassFilter().matches(targetClass)) {
      return false;
   }

   /**
    * 获取方法匹配器
    * 
    * TransactionAttributeSourcePointcut的getMethodMatcher()方法
    * 实现在其父类StaticMethodMatcherPointcut
       */
   MethodMatcher methodMatcher = pc.getMethodMatcher();
   if (methodMatcher == MethodMatcher.TRUE) {
      // No need to iterate the methods if we're matching any method anyway...
      return true;
   }

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

   Set<Class<?>> classes = new LinkedHashSet<>();
   if (!Proxy.isProxyClass(targetClass)) {
      classes.add(ClassUtils.getUserClass(targetClass));
   }
   classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));

   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;
}

调用的 matches 方法在 TransactionAttributeSourcePointcut 中实现

// TransactionAttributeSourcePointcut
public boolean matches(Method method, @Nullable Class<?> targetClass) {
   if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
      return false;
   }
   TransactionAttributeSource tas = getTransactionAttributeSource();
   // 判断是否有事务属性
   return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
}

// AbstractFallbackTransactionAttributeSource
public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
   if (method.getDeclaringClass() == Object.class) {
      return null;
   }

   // First, see if we have a cached value.
   Object cacheKey = getCacheKey(method, targetClass);
   Object cached = this.attributeCache.get(cacheKey);
   if (cached != null) {
      // Value will either be canonical value indicating there is no transaction attribute,
      // or an actual transaction attribute.
      if (cached == NULL_TRANSACTION_ATTRIBUTE) {
         return null;
      } else {
         return (TransactionAttribute) cached;
      }
   } else {
      // We need to work it out.
      // 获取事务属性
      TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
      // Put it in the cache.
      if (txAttr == null) {
         this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
      } else {
         String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
         if (txAttr instanceof DefaultTransactionAttribute) {
            ((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
         }
         if (logger.isDebugEnabled()) {
            logger.debug("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
         }
         this.attributeCache.put(cacheKey, txAttr);
      }
      return txAttr;
   }
}

// AbstractFallbackTransactionAttributeSource
protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
   // Don't allow no-public methods as required.
   if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
      return null;
   }

   // 根据指定的接口方法和目标类获取目标方法
   Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);

   // 查询目标类的具体方法是否有事务属性
   TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
   if (txAttr != null) {
      return txAttr;
   }

   // 查询目标类上是否有事务属性
   txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
   if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
      return txAttr;
   }

   if (specificMethod != method) {
      // 查看原始方法上是否有事务属性
      txAttr = findTransactionAttribute(method);
      if (txAttr != null) {
         return txAttr;
      }
      // 查看原始方法所属的类上是否有事务属性
      txAttr = findTransactionAttribute(method.getDeclaringClass());
      if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
         return txAttr;
      }
   }

   return null;
}

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

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

protected TransactionAttribute determineTransactionAttribute(AnnotatedElement ae) {
   for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
      // 解析事务注解
      TransactionAttribute attr = annotationParser.parseTransactionAnnotation(ae);
      if (attr != null) {
         return attr;
      }
   }
   return null;
}

// SpringTransactionAnnotationParser
public TransactionAttribute parseTransactionAnnotation(AnnotatedElement ae) {
   AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
         ae, Transactional.class, false, false);
   if (attributes != null) {
      return parseTransactionAnnotation(attributes);
   }
   else {
      return null;
   }
}

解析 @Transactional 注解属性

// SpringTransactionAnnotationParser
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());
   rbta.setReadOnly(attributes.getBoolean("readOnly"));
   rbta.setQualifier(attributes.getString("value"));
   ArrayList<RollbackRuleAttribute> rollBackRules = new ArrayList<>();
   Class<?>[] rbf = attributes.getClassArray("rollbackFor");
   for (Class<?> rbRule : rbf) {
      RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
      rollBackRules.add(rule);
   }
   String[] rbfc = attributes.getStringArray("rollbackForClassName");
   for (String rbRule : rbfc) {
      RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
      rollBackRules.add(rule);
   }
   Class<?>[] nrbf = attributes.getClassArray("noRollbackFor");
   for (Class<?> rbRule : nrbf) {
      NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
      rollBackRules.add(rule);
   }
   String[] nrbfc = attributes.getStringArray("noRollbackForClassName");
   for (String rbRule : nrbfc) {
      NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
      rollBackRules.add(rule);
   }
   rbta.getRollbackRules().addAll(rollBackRules);
   return rbta;
}

执行过程解析

TransactionInterceptor 实现了 MethodInterceptor，在执行通知方法时会调用 TransactionInterceptor 的 invoke 方法

// TransactionInterceptor
public Object invoke(final MethodInvocation invocation) throws Throwable {
   Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

   // 在事务中调用方法
   return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}

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

   TransactionAttributeSource tas = getTransactionAttributeSource();
   // 获取事务属性
   final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
   // 获取平台事务管理器
   final PlatformTransactionManager tm = determineTransactionManager(txAttr);
   final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

   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 = null;
      try {
         // This is an around advice: Invoke the next interceptor in the chain.
         // This will normally result in a target object being invoked.
         // 处理调用
         retVal = invocation.proceedWithInvocation();
      }
      catch (Throwable ex) {
         // target invocation exception
         // 事务异常处理
         completeTransactionAfterThrowing(txInfo, ex);
         throw ex;
      }
      finally {
         // 清理事务信息
         cleanupTransactionInfo(txInfo);
      }
      // 提交事务
      commitTransactionAfterReturning(txInfo);
      return retVal;
   }

   else {
      final ThrowableHolder throwableHolder = new ThrowableHolder();

      // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
      try {
         Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
            TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
            try {
               return invocation.proceedWithInvocation();
            }
            catch (Throwable ex) {
               if (txAttr.rollbackOn(ex)) {
                  // A RuntimeException: will lead to a rollback.
                  if (ex instanceof RuntimeException) {
                     throw (RuntimeException) ex;
                  }
                  else {
                     throw new ThrowableHolderException(ex);
                  }
               }
               else {
                  // A normal return value: will lead to a commit.
                  throwableHolder.throwable = ex;
                  return null;
               }
            }
            finally {
               cleanupTransactionInfo(txInfo);
            }
         });

         // Check result state: It might indicate a Throwable to rethrow.
         if (throwableHolder.throwable != null) {
            throw throwableHolder.throwable;
         }
         return result;
      }
      catch (ThrowableHolderException ex) {
         throw ex.getCause();
      }
      catch (TransactionSystemException ex2) {
         if (throwableHolder.throwable != null) {
            logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
            ex2.initApplicationException(throwableHolder.throwable);
         }
         throw ex2;
      }
      catch (Throwable ex2) {
         if (throwableHolder.throwable != null) {
            logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
         }
         throw ex2;
      }
   }
}

// TransactionAspectSupport
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) {
      txAttr = new DelegatingTransactionAttribute(txAttr) {
         @Override
         public String getName() {
            return joinpointIdentification;
         }
      };
   }

   TransactionStatus status = null;
   if (txAttr != null) {
      if (tm != null) {
         // 获取事务状态
         status = tm.getTransaction(txAttr);
      }
      else {
         if (logger.isDebugEnabled()) {
            logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
                  "] because no transaction manager has been configured");
         }
      }
   }
   return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

// AbstractPlatformTransactionManager
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
   // 获取事务
   Object transaction = doGetTransaction();

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

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

   // 判断是否存在事务
   if (isExistingTransaction(transaction)) {
      // Existing transaction found -> check propagation behavior to find out how to behave.
      //处理已经存在的事务
      return handleExistingTransaction(definition, transaction, debugEnabled);
   }

   // Check definition settings for new transaction.
   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.
   // 当前没有事务 且 事务传播设置为mandatory类型 抛出异常
   if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
      throw new IllegalTransactionStateException(
            "No existing transaction found for transaction marked with propagation 'mandatory'");
   }
   else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
         definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
         definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
      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);
         // 开启事务
         doBegin(transaction, definition);
         // 同步事务状态
         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);
   }
}

// DataSourceTransactionManager
protected Object doGetTransaction() {
   DataSourceTransactionObject txObject = new DataSourceTransactionObject();
   txObject.setSavepointAllowed(isNestedTransactionAllowed());
   ConnectionHolder conHolder =
         (ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
   txObject.setConnectionHolder(conHolder, false);
   return txObject;
}

// AbstractPlatformTransactionManager
private TransactionStatus handleExistingTransaction(
      TransactionDefinition definition, Object transaction, boolean debugEnabled)
      throws TransactionException {

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

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

   // 事务传播类型为REQUIRES_NEW 
   if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
      if (debugEnabled) {
         logger.debug("Suspending current transaction, creating new transaction with name [" +
               definition.getName() + "]");
      }
      // 挂起旧事务 创建新事务
      SuspendedResourcesHolder suspendedResources = suspend(transaction);
      try {
         boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
         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) {
      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.
         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.
         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);
}

// AbstractPlatformTransactionManager
protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {
   if (TransactionSynchronizationManager.isSynchronizationActive()) {
      List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
      try {
         Object suspendedResources = null;
         if (transaction != null) {
            // 执行挂起操作
            suspendedResources = doSuspend(transaction);
         }
         String name = TransactionSynchronizationManager.getCurrentTransactionName();
         TransactionSynchronizationManager.setCurrentTransactionName(null);
         boolean readOnly = TransactionSynchronizationManager.isCurrentTransactionReadOnly();
         TransactionSynchronizationManager.setCurrentTransactionReadOnly(false);
         Integer isolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
         TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(null);
         boolean wasActive = TransactionSynchronizationManager.isActualTransactionActive();
         TransactionSynchronizationManager.setActualTransactionActive(false);
         return new SuspendedResourcesHolder(
               suspendedResources, suspendedSynchronizations, name, readOnly, isolationLevel, wasActive);
      }
      catch (RuntimeException | Error ex) {
         // doSuspend failed - original transaction is still active...
         doResumeSynchronization(suspendedSynchronizations);
         throw ex;
      }
   }
   else if (transaction != null) {
      // Transaction active but no synchronization active.
      Object suspendedResources = doSuspend(transaction);
      return new SuspendedResourcesHolder(suspendedResources);
   }
   else {
      // Neither transaction nor synchronization active.
      return null;
   }
}

// DataSourceTransactionManager
protected Object doSuspend(Object transaction) {
   DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
   // 重置connectionHolder为null
   txObject.setConnectionHolder(null);
   return TransactionSynchronizationManager.unbindResource(obtainDataSource());
}

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

   try {
      // suspend操作中已经重置connectionHolder
      if (!txObject.hasConnectionHolder() ||
            txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
         // 获取新的数据库连接
         Connection newCon = obtainDataSource().getConnection();
         if (logger.isDebugEnabled()) {
            logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
         }
         txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
      }

      txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
      con = txObject.getConnectionHolder().getConnection();

      Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
      // 设置事务隔离级别
      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()) {
         txObject.setMustRestoreAutoCommit(true);
         if (logger.isDebugEnabled()) {
            logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
         }
         // 设置事务自动提交为false 交由spring管理
         con.setAutoCommit(false);
      }

      prepareTransactionalConnection(con, definition);
      txObject.getConnectionHolder().setTransactionActive(true);

      int timeout = determineTimeout(definition);
      if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
         txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
      }

      // Bind the connection holder to the thread.
      if (txObject.isNewConnectionHolder()) {
         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);
   }
}

// TransactionAspectSupport
protected TransactionInfo prepareTransactionInfo(@Nullable PlatformTransactionManager tm,
      @Nullable TransactionAttribute txAttr, String joinpointIdentification,
      @Nullable TransactionStatus status) {

   TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
   if (txAttr != null) {
      // We need a transaction for this method...
      if (logger.isTraceEnabled()) {
         logger.trace("Getting transaction for [" + txInfo.getJoinpointIdentification() + "]");
      }
      // The transaction manager will flag an error if an incompatible tx already exists.
      txInfo.newTransactionStatus(status);
   }
   else {
      // The TransactionInfo.hasTransaction() method will return false. We created it only
      // to preserve the integrity of the ThreadLocal stack maintained in this class.
      if (logger.isTraceEnabled())
         logger.trace("Don't need to create transaction for [" + joinpointIdentification +
               "]: This method isn't transactional.");
   }

   // We always bind the TransactionInfo to the thread, even if we didn't create
   // a new transaction here. This guarantees that the TransactionInfo stack
   // will be managed correctly even if no transaction was created by this aspect.
   txInfo.bindToThread();
   return txInfo;
}

事务提交

查看 TransactionAspectSupport 的 commitTransactionAfterReturning 方法

// TransactionAspectSupport
protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) {
   if (txInfo != null && txInfo.getTransactionStatus() != null) {
      if (logger.isTraceEnabled()) {
         logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
      }
      txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
   }
}

// AbstractPlatformTransactionManager
public final void commit(TransactionStatus status) throws TransactionException {
   if (status.isCompleted()) {
      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()) {
      if (defStatus.isDebug()) {
         logger.debug("Transactional code has requested rollback");
      }
      // 执行回滚操作
      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");
      }
      // 执行回滚操作
      processRollback(defStatus, true);
      return;
   }
   // 执行提交操作
   processCommit(defStatus);
}

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

      try {
         boolean unexpectedRollback = false;
         prepareForCommit(status);
         triggerBeforeCommit(status);
         triggerBeforeCompletion(status);
         beforeCompletionInvoked = true;

         if (status.hasSavepoint()) {
            if (status.isDebug()) {
               logger.debug("Releasing transaction savepoint");
            }
            unexpectedRollback = status.isGlobalRollbackOnly();
            status.releaseHeldSavepoint();
         }
         else if (status.isNewTransaction()) {
            if (status.isDebug()) {
               logger.debug("Initiating transaction commit");
            }
            unexpectedRollback = status.isGlobalRollbackOnly();
            // 执行提交操作
            doCommit(status);
         }
         else if (isFailEarlyOnGlobalRollbackOnly()) {
            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) {
            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);
         throw ex;
      }

      // Trigger afterCommit callbacks, with an exception thrown there
      // propagated to callers but the transaction still considered as committed.
      try {
         triggerAfterCommit(status);
      }
      finally {
         triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
      }

   }
   finally {
      cleanupAfterCompletion(status);
   }
}

// DataSourceTransactionManager
protected void doCommit(DefaultTransactionStatus status) {
   DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
   Connection con = txObject.getConnectionHolder().getConnection();
   if (status.isDebug()) {
      logger.debug("Committing JDBC transaction on Connection [" + con + "]");
   }
   try {
      // 执行事务提交
      con.commit();
   }
   catch (SQLException ex) {
      throw new TransactionSystemException("Could not commit JDBC transaction", ex);
   }
}

事务回滚

查看 TransactionAspectSupport 的 completeTransactionAfterThrowing 方法

// TransactionAspectSupport
protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {
   // 判断是否存在事务
   if (txInfo != null && txInfo.getTransactionStatus() != null) {
      if (logger.isTraceEnabled()) {
         logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
               "] after exception: " + ex);
      }
      // 是否满足回滚条件
      if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
         try {
            // 执行回滚操作
            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.
         try {
            // 存在异常也提交事务 因为不满足回滚条件
            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;
         }
      }
   }
}

// AbstractPlatformTransactionManager
public final void rollback(TransactionStatus status) throws TransactionException {
   if (status.isCompleted()) {
      throw new IllegalTransactionStateException(
            "Transaction is already completed - do not call commit or rollback more than once per transaction");
   }

   DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
   // 执行回滚操作
   processRollback(defStatus, false);
}

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

      try {
         triggerBeforeCompletion(status);

         if (status.hasSavepoint()) {
            if (status.isDebug()) {
               logger.debug("Rolling back transaction to savepoint");
            }
            // 回滚至指定保存点
            status.rollbackToHeldSavepoint();
         }
         else if (status.isNewTransaction()) {
            if (status.isDebug()) {
               logger.debug("Initiating transaction rollback");
            }
            // 新事务执行回滚操作
            doRollback(status);
         }
         else {
            // 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;
      }

      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 {
      // 完成之后清理并恢复挂起事务
      cleanupAfterCompletion(status);
   }
}

// DataSourceTransactionManager
protected void doRollback(DefaultTransactionStatus status) {
   DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
   Connection con = txObject.getConnectionHolder().getConnection();
   if (status.isDebug()) {
      logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
   }
   try {
      // 执行回滚操作
      con.rollback();
   }
   catch (SQLException ex) {
      throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
   }
}

// AbstractPlatformTransactionManager
private void cleanupAfterCompletion(DefaultTransactionStatus status) {
   // 设置事务状态为完成
   status.setCompleted();
   if (status.isNewSynchronization()) {
      TransactionSynchronizationManager.clear();
   }
   if (status.isNewTransaction()) {
      // 事务完成后清理操作
      doCleanupAfterCompletion(status.getTransaction());
   }
   if (status.getSuspendedResources() != null) {
      if (status.isDebug()) {
         logger.debug("Resuming suspended transaction after completion of inner transaction");
      }
      Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
      // 恢复之前挂起事务
      resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
   }
}

// DataSourceTransactionManager
protected void doCleanupAfterCompletion(Object transaction) {
   DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;

   // Remove the connection holder from the thread, if exposed.
   // 解除数据库连接与线程绑定
   if (txObject.isNewConnectionHolder()) {
      TransactionSynchronizationManager.unbindResource(obtainDataSource());
   }

   // Reset connection.
   Connection con = txObject.getConnectionHolder().getConnection();
   try {
      if (txObject.isMustRestoreAutoCommit()) {
         // 设置事务自动提交属性为true
         con.setAutoCommit(true);
      }
      // 重置数据库连接属性
      DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
   }
   catch (Throwable ex) {
      logger.debug("Could not reset JDBC Connection after transaction", ex);
   }

   if (txObject.isNewConnectionHolder()) {
      if (logger.isDebugEnabled()) {
         logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
      }
      DataSourceUtils.releaseConnection(con, this.dataSource);
   }

   txObject.getConnectionHolder().clear();
}

至此，所有过程分析完成。

参考资料

Spring源码深度解析
https://blog.csdn.net/dalinsi/article/details/53203540