Spring源码解析之事务传播特性

一、使用方式

可以采用Transactional，配置propagation即可。
打开org.springframework.transaction.annotation.Transactional可见默认传播特性是REQUIRED。

/**
	 * The transaction propagation type.
	 * <p>Defaults to {@link Propagation#REQUIRED}.
	 * @see org.springframework.transaction.interceptor.TransactionAttribute#getPropagationBehavior()
	 */
	Propagation propagation() default Propagation.REQUIRED;

二、getTransaction

顾名思义，此项属性是在事务存在交互时生效。那么到底是如何生效的呢，核心源码位于org.springframework.transaction.support.AbstractPlatformTransactionManager。核心入口是getTransaction方法。

@Override
	public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
		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();
		}

		//已经存在事务 根据事务传播特性进行处理
		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.
		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.
		//当前不存在事务 MANDATORY直接抛出异常 
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}
		//REQUIRED  REQUIRES_NEW NESTED则会新建一个事务
		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);
				doBegin(transaction, definition);
				prepareSynchronization(status, definition);
				return status;
			}
			catch (RuntimeException | Error ex) {
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			xxx
		}
	}

可以看到，在当前不存在事务时，

• MANDATORY 直接抛出异常
• REQUIRED REQUIRES_NEW NESTED 自动新建一个事务。
  

那么存在事务时是如何处理的呢？

三、handleExistingTransaction

/**
	 * 处理已经存在事务的情况
	 * Create a TransactionStatus for an existing transaction.
	 */
	private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {

		//NEVER 已经存在事务 直接抛出异常
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
			throw new IllegalTransactionStateException(
					"Existing transaction found for transaction marked with propagation 'never'");
		}

		//NOT_SUPPORTED 注意prepareTransactionStatus方法参数传递事务的时候传递参数为null，所以是采用非事务方式运行。执行会挂起当前事务。
		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);
		}


		//REQUIRES_NEW 新建事务 会挂起当前事务
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}
			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;
			}
		}

		//NESTED 处理嵌套事务
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			// 检查是否支持嵌套事务
			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() + "]");
			}
			// 支持的话则采用Savepoint 否则开启新事务 并不会挂起当前事务
			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);
				//注意此处会创建savePoint
				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);
				// 注意此处newTransaction属性设置为true，说明确实采用了创建新事务方式来实现
				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);
		// 其他的传播特性则加入当前事务 不会创建新事务
		return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
	}

可以看到，当已经存在事务时，

• NEVER 直接报错，不支持事务
• NOT_SUPPORTED 默默按照非事务方式运行
• REQUIRES_NEW 新建一个事务。
• NESTED 处理嵌套事务 视情况采用savePoint或者新建事务。
• 其他的 加入当前事务

四、NESTED 嵌套事务

SavePoint

先简单说说SavePoint机制吧。这个也比较简单。
比如一个 事务比较复杂，容易出错。那么如果当前DB支持SavePoint的话，那么创建一个SavePoint就等于创建了一个快照，可以不用每次都回滚整个事务，仅回滚到指定的SavePoint即可。

五、个人理解

NESTED这个处理确实比较复杂。个人也查了很多资料。目前个人目前理解如下：
NESTED对于事务的处理主要在于级别不同。
REQUIRES_NEW创建的两个事务是平级的，一个事务的成功与否对另一个事务的成功与否不产生影响。
而NESTED创建的事务则名副其实，是受其父级事务影响的。
一句话总结就是，子事务的成功与否不影响父级事务的成功，但是父级事务的成功与否则会影响子事务的成功。
父事务回滚，子事务一定会滚。
子事务回滚，父事务不一定会滚。

六、总结

最后总结如下

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