PROPAGATION_REQUIRED：如果當前沒有事務，就新建一個事務，如果已經存在一個事務，就加入到這個事務中。這是最常見的選擇。
PROPAGATION_SUPPORTS：支援當前事務，如果當前沒有事務，就以非事務方式執行。
PROPAGATION_MANDATORY：使用當前的事務，如果當前沒有事務，就丟擲異常。
PROPAGATION_REQUIRES_NEW：新建事務，如果當前存在事務，把當前事務掛起。
PROPAGATION_NOT_SUPPORTED：以非事務方式執行操作，如果當前存在事務，就把當前事務掛起。
PROPAGATION_NEVER：以非事務方式執行，如果當前存在事務，則丟擲異常。
PROPAGATION_NESTED：如果當前存在事務，則在巢狀事務內執行。如果當前沒有事務，則執行PROPAGATION_REQUIRED類似的操作。

	public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
		//獲取事物物件（內建conection)
		Object transaction = doGetTransaction();

		boolean debugEnabled = logger.isDebugEnabled();

		if (definition == null) {
			definition = new DefaultTransactionDefinition();
		}

		//這裡判斷是否已經存在一個事物，如果存在走其他邏輯
		if (isExistingTransaction(transaction)) {
			return handleExistingTransaction(definition, transaction, debugEnabled);
		}

		if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
			throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
		}

		//使用當前的事務，如果當前沒有事務，就丟擲異常
		//程式碼走到這裡，說明isExistingTransaction返回false，當前上下文沒有事物。
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}

		//PROPAGATION_REQUIRED，PROPAGATION_REQUIRES_NEW，PROPAGATION_NESTED
		//在沒有事物的情況下，都會新建一個新的事物。
		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);
				//這裡建立一個新的事物
				DefaultTransactionStatus status = newTransactionStatus(
						definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
				doBegin(transaction, definition);
				prepareSynchronization(status, definition);
				return status;
			}
			catch (RuntimeException | Error ex) {
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			//PROPAGATION_SUPPORTS，支援當前事務，如果當前沒有事務，就以非事務方式執行。
			//下面是如果當前沒有事物，就建立一個空事物。其實就是已非事物方式執行。
			// 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);
		}
	}
 

上面程式碼主要是在當前上下文沒有事物的情況下的處理邏輯，

如果PROPAGATION_MANDATORY，則丟擲異常

如果PROPAGATION_REQUIRED，PROPAGATION_REQUIRES_NEW，PROPAGATION_NESTED，則建立一個新事物。

最後就是PROPAGATION_SUPPORTS，有就支援，沒有就已非事物執行。

	/**
	 * Create a TransactionStatus for an existing transaction.
	 */
	private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {

		//PROPAGATION_NEVER，如果是這個，那麼直接拋異常
		//以非事務方式執行，如果當前存在事務，則丟擲異常。
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
			throw new IllegalTransactionStateException(
					"Existing transaction found for transaction marked with propagation 'never'");
		}

		//PROPAGATION_NOT_SUPPORTED：以非事務方式執行操作，如果當前存在事務，就把當前事務掛起。
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction");
			}
			//這裡掛起之前的事物，掛起的邏輯就是把之前事物上下文的資源封裝成一個物件，放到新事物當中（空事物），
			//並且清空當前的上下文環境
			Object suspendedResources = suspend(transaction);
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);

			//返回空事物
			return prepareTransactionStatus(
					definition, null, false, newSynchronization, debugEnabled, suspendedResources);
		}

		//PROPAGATION_REQUIRES_NEW：新建事務，如果當前存在事務，把當前事務掛起。
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}

			//掛起之前的事物。
			SuspendedResourcesHolder suspendedResources = suspend(transaction);
			try {
				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);

				//建立一個新事物，把之前掛機的資源繫結到新事物當中去。
				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;
			}
		}

		//如果當前存在事務，則在巢狀事務內執行。如果當前沒有事務，則執行PROPAGATION_REQUIRED類似的操作
		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);
				//所謂巢狀事物，其實就是JDBC的還原點，在當前位置建立還原點，
				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);
	}
 

上面程式碼大題邏輯就是如果已經存在一個事物，那麼如何處理的邏輯。