Spring5 源码阅读笔记（4.1.1）createTransactionIfNecessary(tm, txAttr, joinpointIdentification) 创建事务如有必要

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) {
			//开启事务，这里重点看
			status = tm.getTransaction(txAttr);
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
						"] because no transaction manager has been configured");
			}
		}
	}
	//创建事务信息对象，记录新老事务信息对象
	return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

跟 getTransaction：
类 PlatformTransactionManager

@Override
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
	//DataSourceTransactionObject拿到对象 见4.1.1.1
	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();
	}

	//第一次进来connectionHolder为空的，所以不存在事务
	//有连接（connectionHolder），并且活跃时，为true
	if (isExistingTransaction(transaction)) {
		// Existing transaction found -> check propagation behavior to find out how to behave.
		//见4.1.1.2
		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.
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
		throw new IllegalTransactionStateException(
				"No existing transaction found for transaction marked with propagation 'mandatory'");
	}
	//第一次进来大部分会走这里
	/** PROPAGATION_REQUIRED：如果已经存在事务，就加入存在事务的连接。
	 *  ROPAGATION_REQUIRES_NEW：如果已经存在事务，挂起当前事务，创建新的连接。
	 *  PROPAGATION_NESTED：如果存在事务，在嵌套事务内执行。
	 */
	else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
			definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
			definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
		//先挂起
		SuspendedResourcesHolder suspendedResources = suspend(null);
		if (debugEnabled) {
			logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
		}
		try {
			boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
			//创建事务状态对象，其实就是封装了事务对象的一些信息，记录事务状态的
			DefaultTransactionStatus status = newTransactionStatus(
											//true代表是最新事务
					definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);

			//开启事务 DataSourceTransactionObject 见4.1.1.3
			doBegin(transaction, definition);

			//开启事务后，改变事务状态
			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);
	}
}