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由于事务部分代码在设计上整体比较简单，我自己觉得它在设计上没有什么特别让我眼前一亮的东西，所以下文更多的是侧重执行流程，能理解事务管理器等一众概念以及相关的变量含义，真正遇到Bug会调试，知道在什么地方打断点就行。

后文更多的是代码+注释的形式呈现（注意，而且根据Spring的不同版本，代码实现上也略有差异），请配合自己项目源码慢慢食用。

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一、@EnableTransactionManagement

利用Spring的特性和扩展接口完成Bean的注入，该注入方式也常用于一些分布式中间件的整合

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
}

1、使用@EnableTransactionManagement，解析@Import注解上的信息

2、TransactionManagementConfigurationSelector实现了AdviceModeImportSelector扩展类，达到注解Bean的效果

3、如果使用默认形式代理，则会初始化类ProxyTransactionManagementConfiguration，在该类的主要作用是初始化一个advisor。熟悉AOP的小伙伴应该都知道，Spring中的事务是基于AOP的，为什么这么说呢，因为Spring在解析AOP相关的逻辑的时候是将所有的切面都解析为一个advisor，然后将所有的advisor串起来，递归调用。而此处事务要做的也就是初始化这个么一个advisor对象，然后添加到AOP的递归调用链中（实例化过程参考下图）

4、所以问题的关键就来到了setAdvice这个属性上（AOP的概念）

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二、TransactionInterceptor

@Transactional注解对应的advice切面类为TransactionInterceptor类

• TransactionInterceptor实现了MethodInterceptor（该接口是AOP的接口，执行AOP逻辑的时候会触发相关的回调）
• 常规的afterAdvice、beforeAdvice都是切面的增强逻辑，MethodAdvice是切面逻辑。即TransactionInterceptor类中的invoke方法，就是代理逻辑。，所以我们直接看invoke方法

该部分的整体执行流程如下图所示，这里对应的就是代码的创建事务，提交事务，遇到异常回滚事务的逻辑。

1、invokeWithinTransaction

@Nullable
// 方法执行到这了这一步，说明类上有@Transactional注解
protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,final InvocationCallback invocation) throws Throwable {// If the transaction attribute is null, the method is non-transactional.TransactionAttributeSource tas = getTransactionAttributeSource();// 获取@Transactional注解中配置的值final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);// 获取配置的事务管理器对象final PlatformTransactionManager tm = determineTransactionManager(txAttr);// joinpoint的唯一标识，就是当前正在执行的方法名字final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {// Standard transaction demarcation with getTransaction and commit/rollback calls.// 如果有必要就要创建事务，这里涉及到事务的传播机制实现TransactionInfo txInfo = createTransactionIfNecessary(tm, 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.// 具体的业务逻辑retVal = invocation.proceedWithInvocation();}catch (Throwable ex) {// target invocation exception// 发生异常，事务回滚completeTransactionAfterThrowing(txInfo, ex);throw ex;}finally {cleanupTransactionInfo(txInfo);}// 正常结束，事务提交commitTransactionAfterReturning(txInfo);return retVal;}else {Object result;final ThrowableHolder throwableHolder = new ThrowableHolder();// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.try {result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);try {return invocation.proceedWithInvocation();}catch (Throwable ex) {if (txAttr.rollbackOn(ex)) {// A RuntimeException: will lead to a rollback.if (ex instanceof RuntimeException) {throw (RuntimeException) ex;}else {throw new ThrowableHolderException(ex);}}else {// A normal return value: will lead to a commit.throwableHolder.throwable = ex;return null;}}finally {cleanupTransactionInfo(txInfo);}});}catch (ThrowableHolderException ex) {throw ex.getCause();}catch (TransactionSystemException ex2) {if (throwableHolder.throwable != null) {logger.error("Application exception overridden by commit exception", throwableHolder.throwable);ex2.initApplicationException(throwableHolder.throwable);}throw ex2;}catch (Throwable ex2) {if (throwableHolder.throwable != null) {logger.error("Application exception overridden by commit exception", throwableHolder.throwable);}throw ex2;}// Check result state: It might indicate a Throwable to rethrow.if (throwableHolder.throwable != null) {throw throwableHolder.throwable;}return result;}
}

2、createTransactionIfNecessary

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) {@Overridepublic String getName() {return joinpointIdentification;}};}// 每个逻辑事务都会创建一个TransactionStatus，但是TransactionStatus中有一个属性代表当前逻辑事务底层的物理事务是不是最新的TransactionStatus status = null;if (txAttr != null) {if (tm != null) {// 开启事务// status：含有挂起资源的对象status = tm.getTransaction(txAttr);}else {if (logger.isDebugEnabled()) {logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +"] because no transaction manager has been configured");}}}return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

3、commitTransactionAfterReturning

以常见的DataSourceTransactionManager为例，进入txInfo.getTransactionManager().commit(txInfo.getTransactionStatus())方法，核心代码如下

@Override
public final void commit(TransactionStatus status) throws TransactionException {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;// 可以通过TransactionAspectSupport.currentTransactionStatus().setRollbackOnly() 来设置// 事务本来是可以要提交的，但是可以强制回滚// 我们业务逻辑报错，设置事务回滚，设置backOnly就是在这个位置进行判定的if (defStatus.isLocalRollbackOnly()) {if (defStatus.isDebug()) {logger.debug("Transactional code has requested rollback");}processRollback(defStatus, false);return;}// 判断此事务在之前是否设置了需要回滚，跟globalRollbackOnParticipationFailure有关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processCommit(defStatus);
}

processCommit

private void processCommit(DefaultTransactionStatus status) throws TransactionException {try {boolean beforeCompletionInvoked = false;try {boolean unexpectedRollback = false;prepareForCommit(status);// 拿到相关的同步器，完成同步器终得额外的业务逻辑// 提交前的方法triggerBeforeCommit(status);// 完成前的方法（回滚的地方也会调用该方法，回滚也算是完成）triggerBeforeCompletion(status);beforeCompletionInvoked = true;if (status.hasSavepoint()) {if (status.isDebug()) {logger.debug("Releasing transaction savepoint");}unexpectedRollback = status.isGlobalRollbackOnly();status.releaseHeldSavepoint();}// 当前事务是自己新建的，才能提交，否则什么都不做else if (status.isNewTransaction()) {if (status.isDebug()) {logger.debug("Initiating transaction commit");}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// 事务同步器，完成后回调接口triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);throw ex;}catch (TransactionException ex) {// can only be caused by doCommitif (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 {// 触发提交后回调接口triggerAfterCommit(status);}finally {// 事务同步器，完成后回调接口triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);}}finally {// 完成后清理，含有挂起前一个事务的，恢复。将之前挂起的对象，又绑定到当前对象cleanupAfterCompletion(status);}
}

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三、getTransaction

同样还是以DataSourseTransactionManager实现为例。该部分可以理解为spring-tx提供了一个事务的基础模板，用于规范事务的执行框架，但是对于事务具体的一些对象创建，如事务管理器、事务ConnectionHolder则根据项目使用的ORM框架决定，整体骨架是一个典型的模板方法模式。

1、doGetTransaction

@Override
protected Object doGetTransaction() {// 1、新创建一个dataSourceTransaction对象DataSourceTransactionObject txObject = new DataSourceTransactionObject();txObject.setSavepointAllowed(isNestedTransactionAllowed());// 2、根据当前的datasource对象，去我们的ThrealLocalMap中获取对应的ConnextionHolder对象ConnectionHolder conHolder =(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());// 3、把获取到的ConnextionHolder对象，设置到创建出来的DataSourceTransactionObject对象上// false表示当前的conHolder对象不是新建的，是我们从ThrealLocal中直接拿的txObject.setConnectionHolder(conHolder, false);return txObject;
}

2、isExistingTransaction

@Override
protected boolean isExistingTransaction(Object transaction) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;// 判断对应的连接holder上是否存在事务return (txObject.hasConnectionHolder() && txObject.getConnectionHolder().isTransactionActive());
}

3、handleExistingTransaction

private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)throws TransactionException {// 由于这个方法是在存在事务的判定方法里面，所以如果传播行为是never则会直接抛异常if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {throw new IllegalTransactionStateException("Existing transaction found for transaction marked with propagation 'never'");}// 该注解的形式，数据库连接不会再开一个（因为没有deBegin），但是相关的事物信息会重新设置一遍，这里也会将之前的事务信息进行挂起，然后将新的事务信息完善到新的对象上// 此时，如果需要执行sql，就会由对应的事务管理器自己去创建数据库连接对象if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {if (debugEnabled) {logger.debug("Suspending current transaction");}// 把当前事务挂起，其中就会把数据库连接对象从ThrealLocal中移除Object suspendedResources = suspend(transaction);boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);return prepareTransactionStatus(definition, null, false, newSynchronization, debugEnabled, suspendedResources);}if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {if (debugEnabled) {logger.debug("Suspending current transaction, creating new transaction with name [" +definition.getName() + "]");}// 把当前事务挂起SuspendedResourcesHolder suspendedResources = suspend(transaction);try {boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);DefaultTransactionStatus status = newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);// 开启事务doBegin(transaction, definition);// 初始化相关的属性prepareSynchronization(status, definition);return status;}catch (RuntimeException | Error beginEx) {// 发生异常，设置相关参数信息resumeAfterBeginException(transaction, suspendedResources, beginEx);throw beginEx;}}// 此时，仅仅保存一个savePoint点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);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");}}}// 默认的传播机制（@Transactional注解的嵌套，相当于什么都没干），仅设置相关的参数状态boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}

4、suspend

@Nullable
protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {// synchronizations是一个ThrealLocal<Set<TransactioonSynchronization>>// 我们可以在任何地方，通过TransactionSynchronizationManager给当前线程添加TransactionSynchronization// 这个判断可以理解为，如果创建过事务就不会为null。因为第一次进来的时候就是null，当我们创建过事务后，就会调用init方法if (TransactionSynchronizationManager.isSynchronizationActive()) {// 调用TransactionSynchronization的suspend方法，并清空和返回当前线程中所有的TransactionSynchronization对象List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();try {Object suspendedResources = null;if (transaction != null) {// 挂起事务，把transaction中的connection清空，并把resources中的key-value进行移除，并返回数据库连接connection对象suspendedResources = doSuspend(transaction);}// 获取并清空当前线程中关于TransactionSynchronizationManage的设置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);// 将当前线程中的数据库连接对象，TransactionSynchronizationManage对象、TransactionSynchronization对象终得设置构造成一个对象// 表示被挂起的资源持有对象，持有了当前线程中的事务对象、TransactionSynchronization对象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;}
}

5、doBegin

@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;Connection con = null;try {// 如果当前线程中所使用的DataSource还没有创建过数据库连接，就获取一个新的数据库连接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);}txObject.getConnectionHolder().setSynchronizedWithTransaction(true);con = txObject.getConnectionHolder().getConnection();// 根据@Transactional注解中的设置，设置Connection的readOnly与隔离界别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).if (con.getAutoCommit()) {txObject.setMustRestoreAutoCommit(true);if (logger.isDebugEnabled()) {logger.debug("Switching JDBC Connection [" + con + "] to manual commit");}con.setAutoCommit(false);}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.// 这里判断，我们的这个conHolder是不是新建的（在外层如果是存在事务的情况下， 这个字段是false。当外层没有事务，才会进入该方法的上层方法，继而进入该方法，然后new一个对象，设置相关的参数未true）// 往resource Map中添加数据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);}
}

6、prepareSynchronization

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();}
}