spring-tx的事务拦截逻辑在TransactionInterceptor类，本文将详细分析其实现方式。

事务拦截器TransactionInterceptor

spring-tx的事务拦截逻辑在TransactionInterceptor类，它实现了MethodInterceptor接口。

MethodInterceptor接口

MethodInterceptor接口的实现类封装aop切面拦截逻辑：

public interface MethodInterceptor extends Interceptor {

	/**
	 * Implement this method to perform extra treatments before and after the invocation.
	 */
	Object invoke(MethodInvocation invocation) throws Throwable;
}

TransactionInterceptor类

TransactionInterceptor类封装了事务拦截逻辑：

public class TransactionInterceptor 
    extends TransactionAspectSupport implements MethodInterceptor, Serializable {

	// ...

	@Override
	public Object invoke(MethodInvocation invocation) throws Throwable {
		// Work out the target class: may be {@code null}.
		// The TransactionAttributeSource should be passed the target class
		// as well as the method, which may be from an interface.
		Class<?> targetClass = 
            (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

		// Adapt to TransactionAspectSupport's invokeWithinTransaction...
		return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
	}

	// ...
}

事务逻辑在父类TransactionAspectSupport的invokeWithinTransaction方法中。

invokeWithinTransaction方法

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

	// 如果方法没有被Transactional注解标注，则返回null
	// 返回的是AnnotationTransactionAttributeSource对象
	// 用于获取Transactional注解相关属性，
	// 比如rollbackOn, propagationBehavior, isolationLevel等
	TransactionAttributeSource tas = getTransactionAttributeSource();
	TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);

	// 获取事务管理器
	// 后面会转型成PlatformTransactionManager对象，可以开启事务、提交、回滚
    TransactionManager tm = determineTransactionManager(txAttr);

	// Reactive事务，略

	// 转型成PlatformTransactionManager对象，可以开启事务、提交、回滚
	PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
	// 获取事务方法的唯一标识
	// 格式为"类名.方法名"
	final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

	if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
		// 创建事务
		TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

		Object retVal;
		try {
			// This is an around advice: Invoke the next interceptor in the chain.
			// This will normally result in a target object being invoked.
			retVal = invocation.proceedWithInvocation();
		} catch (Throwable ex) {
			// Handle a throwable, completing the transaction.
			// We may commit or roll back, depending on the configuration.
			completeTransactionAfterThrowing(txInfo, ex);
			throw ex;
		} finally {
			// Reset the TransactionInfo ThreadLocal.
			// Call this in all cases: exception or normal return!
			cleanupTransactionInfo(txInfo);
		}

		if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
			// Set rollback-only in case of Vavr failure matching our rollback rules...
			TransactionStatus status = txInfo.getTransactionStatus();
			if (status != null && txAttr != null) {
				retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
			}
		}

		// 提交事务
		commitTransactionAfterReturning(txInfo);

		return retVal;
	} else {
		// CallbackPreferringPlatformTransactionManager事务管理器逻辑，略
	}
}

创建事务

Create a transaction if necessary based on the given TransactionAttribute. Allows callers to perform custom TransactionAttribute lookups through the TransactionAttributeSource.

protected TransactionInfo createTransactionIfNecessary(PlatformTransactionManager tm,
		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) {
			// 创建新事务或者返回已存在事务, 这取决于传播级别。
			// 隔离级别或超时等参数只在新事务时生效，已存在事务会忽略这些参数。
			status = tm.getTransaction(txAttr);
		}
	}

	// 使用指定的事务属性和TransactionStatus创建TransactionInfo
	return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

getTransaction(txAttr)

这个方法创建新事务或者返回已存在事务，这取决于传播级别。隔离级别或超时等参数只在新事务时生效，已存在事务会忽略这些参数。

public final TransactionStatus getTransaction(TransactionDefinition definition)
		throws TransactionException {

	// Use defaults if no transaction definition given.
	TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

	// DataSourceTransactionManager实现类返回DataSourceTransactionObject对象
	// DataSourceTransactionObject对象封装着数据库连接、previousIsolationLevel、readOnly、savepointAllowed等
	Object transaction = doGetTransaction();
	boolean debugEnabled = logger.isDebugEnabled();

	if (isExistingTransaction(transaction)) {
		// Existing transaction found -> check propagation behavior to find out how to behave.
		return handleExistingTransaction(def, transaction, debugEnabled);
	}

	// No existing transaction found -> check propagation behavior to find out how to proceed.
	if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
		// 传播级别设置为PROPAGATION_MANDATORY时，如果当前没有事务，则抛出异常
		throw new IllegalTransactionStateException(
				"No existing transaction found for transaction marked with propagation 'mandatory'");
	} else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
			def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
			def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
		SuspendedResourcesHolder suspendedResources = suspend(null);
		// Creating new transaction
		try {
			return startTransaction(def, transaction, debugEnabled, suspendedResources);
		} catch (RuntimeException | Error ex) {
			resume(null, suspendedResources);
			throw ex;
		}
	} else {
		// Create "empty" transaction: no actual transaction, but potentially synchronization.
		if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
			logger.warn("Custom isolation level specified but no actual transaction initiated; " +
					"isolation level will effectively be ignored: " + def);
		}
		boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
		return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
	}
}

doGetTransaction()

protected Object doGetTransaction() {
	// DataSourceTransactionObject封装着数据库连接、previousIsolationLevel、readOnly、savepointAllowed等
	DataSourceTransactionObject txObject = new DataSourceTransactionObject();
	// 是否允许设置保存点，NESTED传播级别时使用，DataSourceTransactionManager类型该属性为true
	txObject.setSavepointAllowed(isNestedTransactionAllowed());
	// 从ThreadLocal获取当前线程上绑定的ConnectionHolder
	// ConnectionHolder对象保存着数据库连接
	// 业务方法第一次执行时为null
	ConnectionHolder conHolder =
			(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
	txObject.setConnectionHolder(conHolder, false);
	return txObject;
}

当前有事务

handleExistingTransaction方法

if (isExistingTransaction(transaction)) {
	// Existing transaction found -> check propagation behavior to find out how to behave.
	return handleExistingTransaction(def, transaction, debugEnabled);
}

isExistingTransaction方法判断当前是否存在事务：

protected boolean isExistingTransaction(Object transaction) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
	// 判断存在数据库连接且开启了事务
	return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}

handleExistingTransaction方法：

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

	// 传播级别为NEVER
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
		// 传播级别设置为NEVER时，如果当前有事务，则抛出异常
		throw new IllegalTransactionStateException(
				"Existing transaction found for transaction marked with propagation 'never'");
	}

	// 传播级别为NOT_SUPPORTED
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
		// 挂起当前事务
		Object suspendedResources = suspend(transaction);
		boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
		return prepareTransactionStatus(
				definition, null, false, newSynchronization, debugEnabled, suspendedResources);
	}

	// 传播级别为REQUIRES_NEW
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
		// 挂起当前事务，然后创建新事务
		SuspendedResourcesHolder suspendedResources = suspend(transaction);
		try {
			return startTransaction(definition, transaction, debugEnabled, suspendedResources);
		} catch (RuntimeException | Error beginEx) {
			resumeAfterBeginException(transaction, suspendedResources, beginEx);
			throw beginEx;
		}
	}

	// 传播级别为NESTED
	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'");
		}
		// Creating nested transaction
		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.
			return startTransaction(definition, transaction, debugEnabled, null);
		}
	}

	// 传播级别为SUPPORTS/REQUIRED
	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);
}

传播级别为NEVER

传播级别设置为NEVER时，如果当前有事务，抛出异常：

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

传播级别为NOT_SUPPORTED

挂起当前事务，业务方法以无事务方式执行：

if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
	// 挂起当前事务
	Object suspendedResources = suspend(transaction);
	boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
	return prepareTransactionStatus(
			definition, null, false, newSynchronization, debugEnabled, suspendedResources);
}

prepareTransactionStatus方法：

1. 创建DefaultTransactionStatus对象，把SuspendedResources封装进去，以便后续恢复旧事务
2. 使用TransactionSynchronizationManager将事务属性绑定到当前线程
3. 初始化当前线程TransactionSynchronization集

由于传播级别为NOT_SUPPORTED所以此处不会开启事务。

传播级别为REQUIRES_NEW

挂起当前事务，创建新事务：

if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
	// 挂起当前事务
	SuspendedResourcesHolder suspendedResources = suspend(transaction);
	try {
		// 开启新事务
		return startTransaction(definition, transaction, debugEnabled, suspendedResources);
	} catch (RuntimeException | Error beginEx) {
		resumeAfterBeginException(transaction, suspendedResources, beginEx);
		throw beginEx;
	}
}

开启新事务：

private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,
		boolean debugEnabled, SuspendedResourcesHolder suspendedResources) {

	// 值为true
	boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
	DefaultTransactionStatus status = newTransactionStatus(
			definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
	// 开启新事务
	doBegin(transaction, definition);
	// 初始化当前线程TransactionSynchronization集
	prepareSynchronization(status, definition);
	return status;
}

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

	try {
		if (!txObject.hasConnectionHolder() ||
				txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
			// 打开一个新连接
			Connection newCon = obtainDataSource().getConnection();
			txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
		}

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

		Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
		txObject.setPreviousIsolationLevel(previousIsolationLevel);
		txObject.setReadOnly(definition.isReadOnly());

		// 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);
			// 设置手动提交
			con.setAutoCommit(false);
		}

		// The default implementation executes a "SET TRANSACTION READ ONLY" statement 
		// if the "enforceReadOnly" flag is set to true and the transaction definition 
		// indicates a read-only transaction.
		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);
	}
}

传播级别为NESTED

为当前连接设置保存点，如果业务方法出现异常，会回滚到该保存点位置：

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'");
	}
	// Creating nested transaction
	// 默认就是true
	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);
		// 设置保存点
		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.
		return startTransaction(definition, transaction, debugEnabled, null);
	}
}

设置保存点：

public void createAndHoldSavepoint() throws TransactionException {
	setSavepoint(getSavepointManager().createSavepoint());
}

// JdbcTransactionObjectSupport#createSavepoint
public Object createSavepoint() throws TransactionException {
	ConnectionHolder conHolder = getConnectionHolderForSavepoint();
	try {
		if (!conHolder.supportsSavepoints()) {
			throw new NestedTransactionNotSupportedException("不支持");
		}
		if (conHolder.isRollbackOnly()) {
			throw new CannotCreateTransactionException("只读");
		}
		// 使用jdbc设置保存点
		return conHolder.createSavepoint();
	} catch (SQLException ex) {
		throw new CannotCreateTransactionException("Could not create JDBC savepoint", ex);
	}
}

传播级别为SUPPORTS/REQUIRED/MANDATORY

// 默认false
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);

事务挂起

把当前线程上绑定的资源、事务配置信息移除封装到SuspendedResourcesHolder对象，传递给新创建的TransactionStatus对象，以便在业务方法执行结束后恢复旧事务：

protected final SuspendedResourcesHolder suspend(Object transaction) throws TransactionException {
	// 判断当前线程的Set<TransactionSynchronization>已经存在
	// TransactionSynchronization: 事务回调同步器，定义了事务挂起、恢复等方法
	// 例如mybatis-spring中有SqlSessionSynchronization实现类
	if (TransactionSynchronizationManager.isSynchronizationActive()) {
		List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
		try {
			Object suspendedResources = null;
			if (transaction != null) {
				// 挂起事务
				suspendedResources = doSuspend(transaction);
			}

			// 清除当前线程事务配置参数：事务名、只读属性、隔离级别等
			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);

			// 把当前线程的事务相关信息封装起来以便后续恢复
			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;
	}
}

private List<TransactionSynchronization> doSuspendSynchronization() {
	List<TransactionSynchronization> suspendedSynchronizations =
			TransactionSynchronizationManager.getSynchronizations();
	// 挂起所有的TransactionSynchronization
	// 比如SqlSessionSynchronization实现类会清除当前线程的SessionFactory
	for (TransactionSynchronization synchronization : suspendedSynchronizations) {
		synchronization.suspend();
	}
	// 清除线程上的TransactionSynchronization集
	TransactionSynchronizationManager.clearSynchronization();
	return suspendedSynchronizations;
}

protected Object doSuspend(Object transaction) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
	txObject.setConnectionHolder(null);
	// ConnectionHolder对象
	return TransactionSynchronizationManager.unbindResource(obtainDataSource());
}

事务恢复

protected final void resume(Object transaction, SuspendedResourcesHolder resourcesHolder)
		throws TransactionException {

	if (resourcesHolder != null) {
		Object suspendedResources = resourcesHolder.suspendedResources;
		if (suspendedResources != null) {
			// 恢复之前挂起的是ConnectionHolder对象
			doResume(transaction, suspendedResources);
		}
		List<TransactionSynchronization> suspendedSynchronizations =
            resourcesHolder.suspendedSynchronizations;
		if (suspendedSynchronizations != null) {
			TransactionSynchronizationManager.setActualTransactionActive(resourcesHolder.wasActive);
			TransactionSynchronizationManager
                .setCurrentTransactionIsolationLevel(resourcesHolder.isolationLevel);
			TransactionSynchronizationManager.setCurrentTransactionReadOnly(resourcesHolder.readOnly);
			TransactionSynchronizationManager.setCurrentTransactionName(resourcesHolder.name);
			doResumeSynchronization(suspendedSynchronizations);
		}
	}
}

protected void doResume(Object transaction, Object suspendedResources) {
	// 恢复之前挂起的是ConnectionHolder对象
	TransactionSynchronizationManager.bindResource(obtainDataSource(), suspendedResources);
}

当前无事务

// No existing transaction found -> check propagation behavior to find out how to proceed.
if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
	// 传播级别设置为PROPAGATION_MANDATORY时，如果当前没有事务，则抛出异常
	throw new IllegalTransactionStateException(
			"No existing transaction found for transaction marked with propagation 'mandatory'");
} else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
		def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
		def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
	SuspendedResourcesHolder suspendedResources = suspend(null);
	// Creating new transaction
	try {
		return startTransaction(def, transaction, debugEnabled, suspendedResources);
	} catch (RuntimeException | Error ex) {
		resume(null, suspendedResources);
		throw ex;
	}
} else {
	// Create "empty" transaction: no actual transaction, but potentially synchronization.
	if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
		logger.warn("Custom isolation level specified but no actual transaction initiated; " +
				"isolation level will effectively be ignored: " + def);
	}
	boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
	return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
}

创建TransactionInfo

protected TransactionInfo prepareTransactionInfo(PlatformTransactionManager tm,
		TransactionAttribute txAttr, String joinpointIdentification,
		TransactionStatus status) {

	TransactionInfo txInfo = new TransactionInfo(tm, txAttr, joinpointIdentification);
	if (txAttr != null) {
		// 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.
	}

	// 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;
}

异常处理

protected void completeTransactionAfterThrowing(TransactionInfo txInfo, Throwable ex) {
	if (txInfo != null && txInfo.getTransactionStatus() != null) {
		if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
			try {
				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 {
				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;
			}
		}
	}
}

回滚：

public final void rollback(TransactionStatus status) throws TransactionException {
	if (status.isCompleted()) {
		throw new IllegalTransactionStateException("Transaction is already completed");
	}

	DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
	processRollback(defStatus, false);
}

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

		try {
			triggerBeforeCompletion(status);

			if (status.hasSavepoint()) {
				// 回滚到指定保存点
				status.rollbackToHeldSavepoint();
			} else if (status.isNewTransaction()) {
				// 事务回滚
				doRollback(status);
			} else {
				// Participating in larger transaction
				if (status.hasTransaction()) {
					if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
						// 设置rollback-only
						doSetRollbackOnly(status);
					}
				}
				// 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 {
		// 这里面有恢复挂起事务的逻辑
		cleanupAfterCompletion(status);
	}
}

// 回滚到指定保存点
public void rollbackToSavepoint(Object savepoint) throws TransactionException {
	ConnectionHolder conHolder = getConnectionHolderForSavepoint();
	try {
		conHolder.getConnection().rollback((Savepoint) savepoint);
		conHolder.resetRollbackOnly();
	} catch (Throwable ex) {
		throw new TransactionSystemException("Could not roll back to JDBC savepoint", ex);
	}
}
// 释放保存点
public void releaseSavepoint(Object savepoint) throws TransactionException {
	ConnectionHolder conHolder = getConnectionHolderForSavepoint();
	try {
		conHolder.getConnection().releaseSavepoint((Savepoint) savepoint);
	} catch (Throwable ex) {
		logger.debug("Could not explicitly release JDBC savepoint", ex);
	}
}

// 事务回滚
protected void doRollback(DefaultTransactionStatus status) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
	Connection con = txObject.getConnectionHolder().getConnection();
	try {
		con.rollback();
	} catch (SQLException ex) {
		throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
	}
}

private void cleanupAfterCompletion(DefaultTransactionStatus status) {
	status.setCompleted();
	if (status.isNewSynchronization()) {
		TransactionSynchronizationManager.clear();
	}
	if (status.isNewTransaction()) {
		// 恢复连接的事务属性，比如自动提交方式、隔离级别、只读属性等
		// 将连接归还给数据源，清除ConnectionHolder的conn
		doCleanupAfterCompletion(status.getTransaction());
	}
	if (status.getSuspendedResources() != null) {
		// 恢复之前挂起的事务
		Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
		resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
	}
}

事务清理

private void restoreThreadLocalStatus() {
	// Use stack to restore old transaction TransactionInfo.
	// Will be null if none was set.
	transactionInfoHolder.set(this.oldTransactionInfo);
}

事务提交

protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) {
	if (txInfo != null && txInfo.getTransactionStatus() != null) {
		txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
	}
}

提交：

public final void commit(TransactionStatus status) throws TransactionException {
	if (status.isCompleted()) {
		throw new IllegalTransactionStateException("Transaction is already completed");
	}

	DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
	if (defStatus.isLocalRollbackOnly()) {
		// Transactional code has requested rollback
		// 回滚
		processRollback(defStatus, false);
		return;
	}

	if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
		// Global transaction is marked as rollback-only but transactional code requested commit
		// 回滚
		processRollback(defStatus, true);
		return;
	}

	// 提交事务
	processCommit(defStatus);
}

// 提交事务
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()) {
				unexpectedRollback = status.isGlobalRollbackOnly();
				// 释放保存点
				status.releaseHeldSavepoint();
			} else if (status.isNewTransaction()) {
				unexpectedRollback = status.isGlobalRollbackOnly();
				// 提交事务
				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);
	}
}

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