1.引

上一節(jié)分析了嵌套事物的創(chuàng)建，本節(jié)分析每種傳播特性的回滾處理過(guò)程。由于這一部分的組合情況會(huì)很多琅摩，我們只分析其中的一兩種情況，更多的大家還是要多看源碼锭硼、多測(cè)試房资！ 注意：這里最外層的事物一定要開(kāi)啟，如果將最外層的事物特性設(shè)置為PROPAGATION_NOT_SUPPORTED檀头，則不會(huì)引發(fā)嵌套事物的問(wèn)題志膀。

2.processRollback回顧

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

        try {

            // 1.事物完成之前的觸發(fā)器調(diào)用
            triggerBeforeCompletion(status);

            // 2.如果有保存點(diǎn),則調(diào)用rollbackToHeldSavepoint回滾到保存點(diǎn)
            if (status.hasSavepoint()) {
                if (status.isDebug()) {
                    logger.debug("Rolling back transaction to savepoint");
                }
                status.rollbackToHeldSavepoint();
            }
            // 3.如果當(dāng)前事物是一個(gè)新的事物,則調(diào)用doRollback執(zhí)行給定事物的回滾
            else if (status.isNewTransaction()) {
                if (status.isDebug()) {
                    logger.debug("Initiating transaction rollback");
                }
                doRollback(status);
            }
            else {
                // Participating in larger transaction
                // 4.如果當(dāng)前事物并非獨(dú)立事物,則將當(dāng)前事物的rollbackOnly屬性標(biāo)記為true,等到事物鏈完成之后,一起執(zhí)行回滾

                // 如果當(dāng)前存在事物,但是
                // 事物的rollbackOnly屬性已經(jīng)被標(biāo)記為true
                // 或者globalRollbackOnParticipationFailure(返回是否僅在參與事務(wù)失敗后才將現(xiàn)有事務(wù)全局標(biāo)記為回滾)為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("==當(dāng)前事物并非獨(dú)立事物,且RollbackOnly為true\n");
                        // 則將ConnectionHolder中的rollbackOnly標(biāo)記為true
                        doSetRollbackOnly(status);
                    }
                    else {
                        if (status.isDebug()) {
                            logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
                        }
                    }
                }
                // 5.如果當(dāng)前不存在事物,則不會(huì)回滾
                // 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.事物完成之后的觸發(fā)器調(diào)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.事物完成后清理資源
        cleanupAfterCompletion(status);
    }
}

3.NOT_SUPPORTED

BankService、PersonService鳖擒、AccountService的事物傳播特性依次是：

@Transactional(propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
public void save() throws RuntimeException {
    System.out.println("==調(diào)用BankService的save方法\n");
    System.out.println("==準(zhǔn)備調(diào)用PersonService的save方法\n");
    personService.save();
    System.out.println("==準(zhǔn)備調(diào)用PersonService的save方法\n");
    accountService.save();
    throw new RuntimeException("==AccountService的save方法手動(dòng)拋出異常");
}

@Override
@Transactional(propagation = Propagation.NOT_SUPPORTED, rollbackFor = Exception.class)
public void save() throws RuntimeException {
    System.out.println("==調(diào)用PersonService的save方法\n");
    jdbcTemplate.update(insert_sql);
    throw new RuntimeException("==PersonService手動(dòng)拋出異常");
}

@Transactional(propagation = Propagation.NOT_SUPPORTED, rollbackFor = Exception.class)
public void save() throws RuntimeException {
    System.out.println("==調(diào)用AccountService的save方法\n");
    jdbcTemplate.update(insert_sql);
    throw new RuntimeException("==AccountService的save方法手動(dòng)拋出異常");
}

該特性下PersonService溉浙、AccountService并不會(huì)開(kāi)啟事物，在processRollback方法中會(huì)走到第6點(diǎn)蒋荚，所以即便PersonService戳稽、AccountService拋出異常，也不會(huì)回滾期升。但是最外層的BankService是開(kāi)啟事物的惊奇，所以如果BankService里有針對(duì)數(shù)據(jù)庫(kù)的寫(xiě)操作并拋出異常，依然會(huì)回滾播赁。

但是這里是嵌套事物的回滾颂郎，當(dāng)內(nèi)層事物回滾之后不要忘記，外層事物還處在被掛起的狀態(tài)容为，那么外層被掛起的事物如何恢復(fù)呢乓序？就在processRollback方法中的第7點(diǎn)。

4.恢復(fù)掛起事物

private void cleanupAfterCompletion(DefaultTransactionStatus status) {
    // 1.將當(dāng)前事物狀態(tài)標(biāo)記為已完成
    status.setCompleted();
    // 2.清除synchronization
    if (status.isNewSynchronization()) {
        TransactionSynchronizationManager.clear();
    }
    // 3.事務(wù)完成后清理資源坎背。
    if (status.isNewTransaction()) {
        doCleanupAfterCompletion(status.getTransaction());
    }
    // 4.從嵌套事物中恢復(fù)被掛起的資源
    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());
    }
}

關(guān)于其他的資源清理涉及的代碼很多替劈，留在后面介紹，這里我們只看如何恢復(fù)被掛起的事物得滤。

protected final void resume(@Nullable Object transaction, @Nullable SuspendedResourcesHolder resourcesHolder) throws TransactionException {
    if (resourcesHolder != null) {
        Object suspendedResources = resourcesHolder.suspendedResources;
        if (suspendedResources != null) {
            // 恢復(fù)掛起資源
            doResume(transaction, suspendedResources);
        }
        List<TransactionSynchronization> suspendedSynchronizations = resourcesHolder.suspendedSynchronizations;
        // 恢復(fù)掛起的事物同步回調(diào)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() + "]");
    }
}

該方法比較簡(jiǎn)單陨献，獲取被掛起事物后重新綁定到resources對(duì)象即可。resources的定義如下：

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

5.PROPAGATION_REQUIRES_NEW

該情況下懂更，因?yàn)镻ROPAGATION_REQUIRES_NEW每次都會(huì)新建事物眨业，并掛起已有事物急膀，所以在該傳播特性下，如果PersonService或AccountService發(fā)生異常龄捡，就會(huì)執(zhí)行回滾操作卓嫂。當(dāng)PersonService或AccountService完成回滾之后，恢復(fù)并回滾之前掛起的事物即可墅茉。至于外層被掛起的事物會(huì)不會(huì)回滾命黔，則要根據(jù)外層事物的傳播特性而定。假如外層事物的傳播特性為NOT_SUPPORTED就斤，那么即便外層事物操作了數(shù)據(jù)庫(kù)并拋出異常也不會(huì)被回滾悍募。而且不會(huì)有嵌套事物的問(wèn)題。

6.PROPAGATION_NESTED

該特性下就有可能會(huì)走到processRollback方法的弟2步洋机，即回滾到保存點(diǎn)坠宴。前提是外層開(kāi)啟事物，且內(nèi)層事物必須有異常拋出绷旗。針對(duì)本例即BankService必須開(kāi)啟事物喜鼓，PersonService或AccountService拋出異常。如果PersonService或AccountService沒(méi)有拋出異常衔肢，雖然兩者存在保存點(diǎn)庄岖，但是并不會(huì)回滾，這樣一來(lái)即便BankService拋出異常并回滾也不會(huì)回滾到保存點(diǎn)角骤，因?yàn)楸４纥c(diǎn)是建立在PersonService和AccountService對(duì)應(yīng)的事物上的隅忿。

下面來(lái)看回滾到保存點(diǎn)的實(shí)現(xiàn)：

/**
 * 回滾到事物的保存點(diǎn)并釋放保存點(diǎn)資源
 * 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");
    }
    // 回滾到保存點(diǎn)
    getSavepointManager().rollbackToSavepoint(savepoint);
    // 釋放保存點(diǎn)資源
    getSavepointManager().releaseSavepoint(savepoint);
    setSavepoint(null);
}

public void rollbackToSavepoint(Object savepoint) throws TransactionException {
    // 回滾到保存點(diǎn)并重置RollbackOnly屬性
    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 {
        // 釋放保存點(diǎn)資源
        conHolder.getConnection().releaseSavepoint((Savepoint) savepoint);
    }
    catch (Throwable ex) {
        logger.debug("Could not explicitly release JDBC savepoint", ex);
    }
}

7.PROPAGATION_SUPPORTS和PROPAGATION_REQUIRED

對(duì)于這兩種特性，既不會(huì)開(kāi)啟一個(gè)新的事物邦尊，也不會(huì)在原有事物中嵌套運(yùn)行背桐，而是把本身的事物交給已有事物。該情況下會(huì)執(zhí)行到processRollback方法的第4點(diǎn)蝉揍，將rollbackOnly標(biāo)記為true链峭，等到最外層事物事物回滾的時(shí)候一起回滾。即使最外層的事物沒(méi)有拋出異常又沾，內(nèi)層事物的異常也會(huì)被外層事物截獲并將整個(gè)事物進(jìn)行回滾弊仪。

8.總結(jié)

關(guān)于嵌套事物的處理，就先分析到這里捍掺，大家還是要多結(jié)合事物創(chuàng)建過(guò)程撼短、事物傳播特性等多多分析。