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。

---

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）

---

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) {
            @Override
            public 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 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 {
            // 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);
    }
}

---

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();
    }
}