50--Spring @Transactional声明式事物（七

1.引

上一节分析了嵌套事物的创建，本节分析每种传播特性的回滚处理过程。由于这一部分的组合情况会很多，我们只分析其中的一两种情况，更多的大家还是要多看源码、多测试！ 注意：这里最外层的事物一定要开启，如果将最外层的事物特性设置为PROPAGATION_NOT_SUPPORTED，则不会引发嵌套事物的问题。

2.processRollback回顾

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

        try {

            // 1.事物完成之前的触发器调用
            triggerBeforeCompletion(status);

            // 2.如果有保存点,则调用rollbackToHeldSavepoint回滚到保存点
            if (status.hasSavepoint()) {
                if (status.isDebug()) {
                    logger.debug("Rolling back transaction to savepoint");
                }
                status.rollbackToHeldSavepoint();
            }
            // 3.如果当前事物是一个新的事物,则调用doRollback执行给定事物的回滚
            else if (status.isNewTransaction()) {
                if (status.isDebug()) {
                    logger.debug("Initiating transaction rollback");
                }
                doRollback(status);
            }
            else {
                // Participating in larger transaction
                // 4.如果当前事物并非独立事物,则将当前事物的rollbackOnly属性标记为true,等到事物链完成之后,一起执行回滚

                // 如果当前存在事物,但是
                // 事物的rollbackOnly属性已经被标记为true
                // 或者globalRollbackOnParticipationFailure(返回是否仅在参与事务失败后才将现有事务全局标记为回滚)为true
                if (status.hasTransaction()) {
                    if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
                        if (status.isDebug()) {
                            logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
                        }
                        System.out.println("==当前事物并非独立事物,且RollbackOnly为true\n");
                        // 则将ConnectionHolder中的rollbackOnly标记为true
                        doSetRollbackOnly(status);
                    }
                    else {
                        if (status.isDebug()) {
                            logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
                        }
                    }
                }
                // 5.如果当前不存在事物,则不会回滚
                // 例如配置了 @Transactional(propagation = Propagation.NOT_SUPPORTED)
                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;
        }

        // 6.事物完成之后的触发器调用
        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 {
        //7.事物完成后清理资源
        cleanupAfterCompletion(status);
    }
}

3.NOT_SUPPORTED

BankService、PersonService、AccountService的事物传播特性依次是：

@Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
public void save() throws RuntimeException {
    System.out.println("==调用BankService的save方法\n");
    System.out.println("==准备调用PersonService的save方法\n");
    personService.save();
    System.out.println("==准备调用PersonService的save方法\n");
    accountService.save();
    throw new RuntimeException("==AccountService的save方法手动抛出异常");
}

@Override
@Transactional(propagation = Propagation.NOT_SUPPORTED, rollbackFor = Exception.class)
public void save() throws RuntimeException {
    System.out.println("==调用PersonService的save方法\n");
    jdbcTemplate.update(insert_sql);
    throw new RuntimeException("==PersonService手动抛出异常");
}

@Transactional(propagation = Propagation.NOT_SUPPORTED, rollbackFor = Exception.class)
public void save() throws RuntimeException {
    System.out.println("==调用AccountService的save方法\n");
    jdbcTemplate.update(insert_sql);
    throw new RuntimeException("==AccountService的save方法手动抛出异常");
}

该特性下PersonService、AccountService并不会开启事物，在processRollback方法中会走到第6点，所以即便PersonService、AccountService抛出异常，也不会回滚。但是最外层的BankService是开启事物的，所以如果BankService里有针对数据库的写操作并抛出异常，依然会回滚。

但是这里是嵌套事物的回滚，当内层事物回滚之后不要忘记，外层事物还处在被挂起的状态，那么外层被挂起的事物如何恢复呢？就在processRollback方法中的第7点。

4.恢复挂起事物

private void cleanupAfterCompletion(DefaultTransactionStatus status) {
    // 1.将当前事物状态标记为已完成
    status.setCompleted();
    // 2.清除synchronization
    if (status.isNewSynchronization()) {
        TransactionSynchronizationManager.clear();
    }
    // 3.事务完成后清理资源。
    if (status.isNewTransaction()) {
        doCleanupAfterCompletion(status.getTransaction());
    }
    // 4.从嵌套事物中恢复被挂起的资源
    if (status.getSuspendedResources() != null) {
        if (status.isDebug()) {
            logger.debug("Resuming suspended transaction after completion of inner transaction");
        }
        Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
        resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
    }
}

关于其他的资源清理涉及的代码很多，留在后面介绍，这里我们只看如何恢复被挂起的事物。

protected final void resume(@Nullable Object transaction, @Nullable SuspendedResourcesHolder resourcesHolder) throws TransactionException {
    if (resourcesHolder != null) {
        Object suspendedResources = resourcesHolder.suspendedResources;
        if (suspendedResources != null) {
            // 恢复挂起资源
            doResume(transaction, suspendedResources);
        }
        List<TransactionSynchronization> suspendedSynchronizations = resourcesHolder.suspendedSynchronizations;
        // 恢复挂起的事物同步回调接口
        if (suspendedSynchronizations != null) {
            TransactionSynchronizationManager.setActualTransactionActive(resourcesHolder.wasActive);
            TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(resourcesHolder.isolationLevel);
            TransactionSynchronizationManager.setCurrentTransactionReadOnly(resourcesHolder.readOnly);
            TransactionSynchronizationManager.setCurrentTransactionName(resourcesHolder.name);
            doResumeSynchronization(suspendedSynchronizations);
        }
    }
}

protected void doResume(@Nullable Object transaction, Object suspendedResources) {
    TransactionSynchronizationManager.bindResource(obtainDataSource(), suspendedResources);
}

public static void bindResource(Object key, Object value) throws IllegalStateException {
    Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
    Assert.notNull(value, "Value must not be null");
    Map<Object, Object> map = resources.get();
    // set ThreadLocal Map if none found
    if (map == null) {
        map = new HashMap<>();
        resources.set(map);
    }
    Object oldValue = map.put(actualKey, value);
    // Transparently suppress a ResourceHolder that was marked as void...
    if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
        oldValue = null;
    }
    if (oldValue != null) {
        throw new IllegalStateException("Already value [" + oldValue + "] for key [" +actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
    }
    if (logger.isTraceEnabled()) {
        logger.trace("Bound value [" + value + "] for key [" + actualKey + "] to thread [" +Thread.currentThread().getName() + "]");
    }
}

该方法比较简单，获取被挂起事物后重新绑定到resources对象即可。resources的定义如下：

private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<>("Transactional resources");

5.PROPAGATION_REQUIRES_NEW

该情况下，因为PROPAGATION_REQUIRES_NEW每次都会新建事物，并挂起已有事物，所以在该传播特性下，如果PersonService或AccountService发生异常，就会执行回滚操作。当PersonService或AccountService完成回滚之后，恢复并回滚之前挂起的事物即可。至于外层被挂起的事物会不会回滚，则要根据外层事物的传播特性而定。假如外层事物的传播特性为NOT_SUPPORTED，那么即便外层事物操作了数据库并抛出异常也不会被回滚。而且不会有嵌套事物的问题。

6.PROPAGATION_NESTED

该特性下就有可能会走到processRollback方法的弟2步，即回滚到保存点。前提是外层开启事物，且内层事物必须有异常抛出。针对本例即BankService必须开启事物，PersonService或AccountService抛出异常。如果PersonService或AccountService没有抛出异常，虽然两者存在保存点，但是并不会回滚，这样一来即便BankService抛出异常并回滚也不会回滚到保存点，因为保存点是建立在PersonService和AccountService对应的事物上的。

下面来看回滚到保存点的实现：

/**
 * 回滚到事物的保存点并释放保存点资源
 * Roll back to the savepoint that is held for the transaction
 * and release the savepoint right afterwards.
 */
public void rollbackToHeldSavepoint() throws TransactionException {
    Object savepoint = getSavepoint();
    if (savepoint == null) {
        throw new TransactionUsageException("Cannot roll back to savepoint - no savepoint associated with current transaction");
    }
    // 回滚到保存点
    getSavepointManager().rollbackToSavepoint(savepoint);
    // 释放保存点资源
    getSavepointManager().releaseSavepoint(savepoint);
    setSavepoint(null);
}

public void rollbackToSavepoint(Object savepoint) throws TransactionException {
    // 回滚到保存点并重置RollbackOnly属性
    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);
    }
}

public void releaseSavepoint(Object savepoint) throws TransactionException {
    ConnectionHolder conHolder = getConnectionHolderForSavepoint();
    try {
        // 释放保存点资源
        conHolder.getConnection().releaseSavepoint((Savepoint) savepoint);
    }
    catch (Throwable ex) {
        logger.debug("Could not explicitly release JDBC savepoint", ex);
    }
}

7.PROPAGATION_SUPPORTS和PROPAGATION_REQUIRED

对于这两种特性，既不会开启一个新的事物，也不会在原有事物中嵌套运行，而是把本身的事物交给已有事物。该情况下会执行到processRollback方法的第4点，将rollbackOnly标记为true，等到最外层事物事物回滚的时候一起回滚。即使最外层的事物没有抛出异常，内层事物的异常也会被外层事物截获并将整个事物进行回滚。

8.总结

关于嵌套事物的处理，就先分析到这里，大家还是要多结合事物创建过程、事物传播特性等多多分析。