final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);

protected TransactionAttribute computeTransactionAttribute(Method method, Class<?> targetClass) {
 // Don't allow no-public methods as required.
 //1. allowPublicMethodsOnly()返回true，只能是公共方法
 if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
  return null;
 }

 // Ignore CGLIB subclasses - introspect the actual user class.
 Class<?> userClass = ClassUtils.getUserClass(targetClass);
 // The method may be on an interface, but we need attributes from the target class.
 // If the target class is null, the method will be unchanged.
 //method代表接口中的方法、specificMethod代表實(shí)現(xiàn)類的方法
 Method specificMethod = ClassUtils.getMostSpecificMethod(method, userClass);
 // If we are dealing with method with generic parameters, find the original method.
 //處理泛型
 specificMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);

 // First try is the method in the target class.
 //查看方法中是否存在事務(wù)
 TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
 if (txAttr != null) {
  return txAttr;
 }

 // Second try is the transaction attribute on the target class.
 //查看方法所在類是否存在事務(wù)聲明
 txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
 if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
  return txAttr;
 }

 //如果存在接口，則在接口中查找
 if (specificMethod != method) {
  // Fallback is to look at the original method.
  //查找接口方法
  txAttr = findTransactionAttribute(method);
  if (txAttr != null) {
   return txAttr;
  }
  // Last fallback is the class of the original method.
  //到接口類中尋找
  txAttr = findTransactionAttribute(method.getDeclaringClass());
  if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
   return txAttr;
  }
 }

 return null;
}

protected PlatformTransactionManager determineTransactionManager(TransactionAttribute txAttr) {
 // Do not attempt to lookup tx manager if no tx attributes are set
 if (txAttr == null || this.beanFactory == null) {
  return getTransactionManager();
 }
 String qualifier = txAttr.getQualifier();
 if (StringUtils.hasText(qualifier)) {
  return determineQualifiedTransactionManager(qualifier);
 }
 else if (StringUtils.hasText(this.transactionManagerBeanName)) {
  return determineQualifiedTransactionManager(this.transactionManagerBeanName);
 }
 else {
  //常用的會(huì)走到這里
  PlatformTransactionManager defaultTransactionManager = getTransactionManager();
  if (defaultTransactionManager == null) {
   defaultTransactionManager = this.transactionManagerCache.get(DEFAULT_TRANSACTION_MANAGER_KEY);
   if (defaultTransactionManager == null) {
    //從beanFactory獲取PlatformTransactionManager類型的bean
    defaultTransactionManager = this.beanFactory.getBean(PlatformTransactionManager.class);
    this.transactionManagerCache.putIfAbsent(
      DEFAULT_TRANSACTION_MANAGER_KEY, defaultTransactionManager);
   }
  }
  return defaultTransactionManager;
 }
}

@Bean
public PlatformTransactionManager txManager() {
 return new DataSourceTransactionManager(dataSource());
}

@Override
 public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
 //1.獲取事務(wù)
 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();
 }

 //判斷當(dāng)前線程是否存在事務(wù)，判斷依據(jù)為當(dāng)前線程記錄連接不為空且連接中的(connectionHolder)中的transactionActive屬性不為空
 if (isExistingTransaction(transaction)) {
  // Existing transaction found -> check propagation behavior to find out how to behave.
  return handleExistingTransaction(definition, transaction, debugEnabled);
 }

 // Check definition settings for new transaction.
 //事務(wù)超時(shí)設(shè)置驗(yàn)證
 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.
 //如果當(dāng)前線程不存在事務(wù)，但是@Transactional卻聲明事務(wù)為PROPAGATION_MANDATORY拋出異常
 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
  throw new IllegalTransactionStateException(
    "No existing transaction found for transaction marked with propagation 'mandatory'");
 }
 //如果當(dāng)前線程不存在事務(wù)，PROPAGATION_REQUIRED、PROPAGATION_REQUIRES_NEW、PROPAGATION_NESTED都得創(chuàng)建事務(wù)
 else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
   definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
   definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
  //空掛起
  SuspendedResourcesHolder suspendedResources = suspend(null);
  if (debugEnabled) {
   logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
  }
  try {
   //默認(rèn)返回true
   boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
   //構(gòu)建事務(wù)狀態(tài)
   DefaultTransactionStatus status = newTransactionStatus(
     definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
   //構(gòu)造transaction、包括設(shè)置connectionHolder、隔離級(jí)別、timeout
   //如果是新事務(wù)，綁定到當(dāng)前線程
   doBegin(transaction, definition);
   //新事務(wù)同步設(shè)置，針對(duì)當(dāng)前線程
   prepareSynchronization(status, definition);
   return status;
  }
  catch (RuntimeException ex) {
   resume(null, suspendedResources);
   throw ex;
  }
  catch (Error err) {
   resume(null, suspendedResources);
   throw err;
  }
 }
 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);
  }
  //聲明事務(wù)是PROPAGATION_SUPPORTS
  boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
  return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
 }
}