Spring事务管理(2)-PlatformmTransactionManager解析和事务传播方式原理

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Spring在事务管理时，对事务的处理做了极致的抽象，即PlatformTransactionManager。对事务的操作，简单地来说，只有三步操作：获取事务，提交事务，回滚事务。

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public interface PlatformTransactionManager {
	// 获取事务
	TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException;
	// 提交事务
	void commit(TransactionStatus status) throws TransactionException;
	// 回滚事务
	void rollback(TransactionStatus status) throws TransactionException;

}

当然Spring不会仅仅只提供一个接口，同时会有一个抽象模版类，实现了事务管理的具体骨架。AbstractPlatformTransactionManager类可以说是Spring事务管理的控制台，决定事务如何创建，提交和回滚。

在Spring事务管理(二)-TransactionProxyFactoryBean原理中，分析TransactionInterceptor增强时，在invoke方法中最重要的三个操作：

  1. 创建事务 createTransactionIfNecessary
  2. 异常后事务处理 completeTransactionAfterThrowing
  3. 方法执行成功后事务提交 commitTransactionAfterReturning

在具体操作中，最后都是通过事务管理器PlatformTransactionManager的接口实现来执行的，其实也就是上面列出的三个接口方法。我们分别介绍这三个方法的实现，并以DataSourceTransactionManager为实现类观察JDBC方式事务的具体实现。

0. 生成代理

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1. 获取事务

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getTransaction方法根据事务定义来获取事务状态，事务状态中记录了事务定义，事务对象及事务相关的资源信息。对于事务的获取，除了调用事务管理器的实现来获取事务对象本身外，另外的很重要的一点是处理了事务的传播方式。

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public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
  // 1.获取事务对象
  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();
  }

  // 2.如果已存在事务，根据不同的事务传播方式处理获取事务
  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());
  }

  // 3. 如果当前没有事务，不同的事务传播方式不同处理方式
  // 3.1 事务传播方式为mandatory(强制必须有事务)，则抛出异常
  // 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'");
  }
  // 3.2 事务传播方式为required或required_new或nested(嵌套)，创建一个新的事务状态
  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);
      // 创建新的事务状态对象
      DefaultTransactionStatus status = newTransactionStatus(
          definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
      // 事务初始化
      doBegin(transaction, definition);
      // 准备其他同步操作
      prepareSynchronization(status, definition);
      return status;
    }
    catch (RuntimeException | Error ex) {
      resume(null, suspendedResources);
      throw ex;
    }
  }
  // 3.3 其他事务传播方式，返回一个事务对象为null的事务状态对象
  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);
  }
}

获取事务的方法主要做两件事情：

  1. 获取事务对象
  2. 根据事务传播方式返回事务状态对象

获取事务对象，在DataSourceTransactionManager的实现中，返回一个DataSourceTransactionObject对象

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protected Object doGetTransaction() {
  DataSourceTransactionObject txObject = new DataSourceTransactionObject();
  txObject.setSavepointAllowed(isNestedTransactionAllowed());
  ConnectionHolder conHolder =
      (ConnectionHolder) 
  // 从事务同步管理器中根据DataSource获取数据库连接资源		
  TransactionSynchronizationManager.getResource(obtainDataSource());
  txObject.setConnectionHolder(conHolder, false);
  return txObject;
}

每次执行doGetTransaction方法，即会创建一个DataSourceTransactionObject对象txObject，并从事务同步管理器中根据DataSource获取数据库连接持有对象ConnectionHolder，然后存入txObject中。事务同步管理类持有一个ThreadLocal级别的resources对象，存储DataSource和ConnectionHolder的映射关系。因此返回的txObject中持有的ConnectionHolder可能有值，也可能为空。而不同的事务传播方式下，事务管理的处理根据txObejct中是否存在事务有不同的处理方式。

关于关注事务传播方式的实现，很多人对事务传播方式都是一知半解，只是因为没有了解源码的实现。现在就来看看具体的实现。事务传播方式的实现分为两种情况，事务不存在和事务已经存在。isExistingTransaction方法判断事务是否存在，默认在AbstractPlatformTransactionManager抽象类中返回false，而在DataSourceTransactionManager实现中，则根据是否有数据库连接来决定。

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protected boolean isExistingTransaction(Object transaction) {
  DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
  return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}

当前无事务

如果当前没有事务，则不同事务传播方式的处理如下：

  1. 事务传播方式为mandatory(强制必须有事务)，当前没有事务，即抛出异常。
  2. 事务传播方式为required或required_new或nested(嵌套)，当前没有事务，即会创建一个新的事务状态。
  3. 其他事务传播方式时，直接返回事务对象为null的事务状态对象，即不在事务中执行。

如何创建一个新的事务状态

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// 1. 新建事务状态,返回DefaultTransactionStatus对象
DefaultTransactionStatus status = newTransactionStatus(
    definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
// 2. 事务初始化
doBegin(transaction, definition);
// 3. 准备同步操作
prepareSynchronization(status, definition);
return status;

第一步，新建事务状态，就是构建一个DefaultTransactionStatus对象

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protected DefaultTransactionStatus newTransactionStatus(
    TransactionDefinition definition, @Nullable Object transaction, boolean newTransaction,
    boolean newSynchronization, boolean debug, @Nullable Object suspendedResources) {

  boolean actualNewSynchronization = newSynchronization &&
      !TransactionSynchronizationManager.isSynchronizationActive();
  return new DefaultTransactionStatus(
      transaction, newTransaction, actualNewSynchronization,
      definition.isReadOnly(), debug, suspendedResources);
}

第二步，事务初始化，AbstractPlatformTransactionManager没有实现，来看DataSourceTransactionManager的实现：获取一个新的数据库连接并开启事务，完成事务的基本属性设置。

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

  try {
  	// 如果当前没有事务，由DataSource获取一个新的数据库连接，并赋予txObject
    if (!txObject.hasConnectionHolder() ||
        txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
      Connection newCon = obtainDataSource().getConnection();
      if (logger.isDebugEnabled()) {
        logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
      }
      txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
    }

	// 设置数据库连接与事务同步
    txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
    con = txObject.getConnectionHolder().getConnection();

	// 设置事务隔离级别
    Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
    txObject.setPreviousIsolationLevel(previousIsolationLevel);

    // 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).
    // 非常重要的一点，JDBC通过设置自动提交为false，开启一个新的事务
    if (con.getAutoCommit()) {
      txObject.setMustRestoreAutoCommit(true);
      if (logger.isDebugEnabled()) {
        logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
      }
      con.setAutoCommit(false);
    }

	// 如果设置事务只读属性，执行Statement设置只读
    prepareTransactionalConnection(con, definition);
    // 激活事务状态
    txObject.getConnectionHolder().setTransactionActive(true);

	// 设置超时时间
    int timeout = determineTimeout(definition);
    if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
      txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
    }

    // Bind the connection holder to the thread.
    // 如果为新连接，绑定DataSource和数据库连接持有者的映射关系
    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);
  }
}

第三步：准备同步操作，如果事务状态开启同步，则在事务同步管理器中设置事务基础属性

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protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
  if (status.isNewSynchronization()) {
    TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
    TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
        definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
            definition.getIsolationLevel() : null);
    TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
    TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
    TransactionSynchronizationManager.initSynchronization();
  }
}

当前有事务

如果当前已经有事务存在，由handleExistingTransaction方法完成事务操作。

  1. 传播方式为never（不允许事务），抛出异常
  2. 传播方式为not_supported（不支持），则挂起当前事务，以无事务方式运行
  3. 传播方式为required_new，挂起原有事务，并开启新的事务
  4. 传播方式为nested（嵌套）,创建嵌套事务。这里一般方式都是通过savepoint保存点的完成嵌套，但Spring对JTA事务单独做了另一种处理。
  5. 传播方式为supports或required，返回当前事务
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private TransactionStatus handleExistingTransaction(
    TransactionDefinition definition, Object transaction, boolean debugEnabled)
    throws TransactionException {

  // 传播方式为never（不允许事务），抛出异常
  if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
    throw new IllegalTransactionStateException(
        "Existing transaction found for transaction marked with propagation 'never'");
  }

  // 传播方式为not_supported（不支持），则挂起当前事务，以无事务方式运行
  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);
  }

  // 传播方式为required_new，挂起原有事务，并开启新的事务
  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;
    }
  }

  // 传播方式为nested（嵌套）,创建嵌套事务
  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() + "]");
    }
    // 使用保存点支持嵌套事务
    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);
      status.createAndHoldSavepoint();
      return status;
    }
    // 只适用于JTA事务：通过嵌套的begin和commit/rollback创建嵌套事务
    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.
  // 传播方式为supports或required，返回当前事务
  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);
}

这里关注两个点

第一是事务的挂起，Spring并不是真的对数据库连接做了什么挂起操作，而是在逻辑上由事务同步管理器做了事务信息和状态的重置，并将原事务信息和状态返回，并记录在新的事务状态对象中，从而形成一种链式结构。

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protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {
  if (TransactionSynchronizationManager.isSynchronizationActive()) {
    List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
    try {
      Object suspendedResources = null;
      if (transaction != null) {
        suspendedResources = doSuspend(transaction);
      }
      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);
      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;
  }
}

第二是嵌套事务设置保存点，通常由JDBC3.0支持的savepoint API完成，然后将保存点记录在事务状态中。

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DefaultTransactionStatus status =
    prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
status.createAndHoldSavepoint();
return status;

至此，完成了获取事务

2.提交事务

首先要说的，commit方法并不是一定提交事务，也可能回滚。

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public final void commit(TransactionStatus status) throws TransactionException {
  // 事务已经提交，再次提交抛出异常
  if (status.isCompleted()) {
    throw new IllegalTransactionStateException(
        "Transaction is already completed - do not call commit or rollback more than once per transaction");
  }

  DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
  // 如果事务状态设置了回滚标识，则执行回滚
  if (defStatus.isLocalRollbackOnly()) {
    if (defStatus.isDebug()) {
      logger.debug("Transactional code has requested rollback");
    }
    processRollback(defStatus, false);
    return;
  }

  // 设置全局回滚标识为true，则执行回滚
  if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
    if (defStatus.isDebug()) {
      logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
    }
    processRollback(defStatus, true);
    return;
  }

  // 提交事务
  processCommit(defStatus);
}

processCommit执行事务的提交，但事务的提交也分为几种情况：

  1. 存在保存点，即嵌套事务，则释放保存点
  2. 如果事务是由当前事务状态开启的，即事务传播的第一层，执行事务提交
  3. 其他情况（比如事务是继承自上一层），则不做任何操作

且在processCommit方法中，不同时候设置了不同状态的触发监控，用来提示事务同步相关资源，触发需要的操作。

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private void processCommit(DefaultTransactionStatus status) throws TransactionException {
  try {
    boolean beforeCompletionInvoked = false;

    try {
      boolean unexpectedRollback = false;
      prepareForCommit(status);
      // 提交前提示
      triggerBeforeCommit(status);
      // 完成前提示
      triggerBeforeCompletion(status);
      beforeCompletionInvoked = true;

      if (status.hasSavepoint()) {
        if (status.isDebug()) {
          logger.debug("Releasing transaction savepoint");
        }
        unexpectedRollback = status.isGlobalRollbackOnly();
        status.releaseHeldSavepoint();
      }
      else if (status.isNewTransaction()) {
        if (status.isDebug()) {
          logger.debug("Initiating transaction commit");
        }
        unexpectedRollback = status.isGlobalRollbackOnly();
        // 执行事务提交
        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
      // 回滚完成提示
      triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
      throw ex;
    }
    catch (TransactionException ex) {
      // can only be caused by doCommit
      if (isRollbackOnCommitFailure()) {
        doRollbackOnCommitException(status, ex);
      }
      else {
        // 未知状态完成提示
        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 {
      // 事务提交完成提示
      triggerAfterCommit(status);
    }
    finally {
      // 操作完成完成提示
      triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
    }

  }
  finally {
    // 完成后清理
    cleanupAfterCompletion(status);
  }
}

DataSourceTransactionManager对doCommit的实现，就是执行数据库连接的提交

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protected void doCommit(DefaultTransactionStatus status) {
  DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
  Connection con = txObject.getConnectionHolder().getConnection();
  if (status.isDebug()) {
    logger.debug("Committing JDBC transaction on Connection [" + con + "]");
  }
  try {
    // 提交事务
    con.commit();
  }
  catch (SQLException ex) {
    throw new TransactionSystemException("Could not commit JDBC transaction", ex);
  }
}

3.回滚事务

回滚事务时也分为几种情况：

  1. 存在保存点（嵌套事务），则回滚到保存点
  2. 如果事务是由当前事务状态开启的，则执行回滚操作
  3. 其他情况下，如果事务状态设置了回滚标识，则设置事务对象的状态也为回滚，否则不做任何操作

企业微信截图_f7e675ea-1e73-45e5-88bf-806a7f785722

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

      try {
         triggerBeforeCompletion(status);

         if (status.hasSavepoint()) {
            if (status.isDebug()) {
               logger.debug("Rolling back transaction to savepoint");
            }
            // 回滚保存点
            status.rollbackToHeldSavepoint();
         }
         else if (status.isNewTransaction()) {
            if (status.isDebug()) {
               logger.debug("Initiating transaction rollback");
            }
            // 回滚事务
            doRollback(status);
         }
         else {
            // Participating in larger transaction
            if (status.hasTransaction()) {
               if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
                  if (status.isDebug()) {
                     logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
                  }
                  doSetRollbackOnly(status);
               }
               else {
                  if (status.isDebug()) {
                     logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
                  }
               }
            }
            else {
               logger.debug("Should roll back transaction but cannot - no transaction available");
            }
            // 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 {
      cleanupAfterCompletion(status);
   }
}

对于DataSourceTransactionManager实现，回滚保存点和回滚事务都由JDBC的API来完成。

至此，事务管理器对事务的三种操作就简单地介绍完了，但其中事务同步资源的控制十分精妙，这里就不做详细的介绍。有兴趣的自己去研究源码。于我而言，任何强大的机制都是由一行行地源码精妙地组建出来的，深入进去，一切都将真相大白。

参考

Spring事务管理(三)-PlatformmTransactionManager解析和事务传播方式原理

spring事务原理和事务同步器使用简介