序

本文主要研究一下spring事务的REQUIRES_NEW

TransactionDefinition

org/springframework/transaction/TransactionDefinition.java

    /**
     * Create a new transaction, suspending the current transaction if one exists.
     * Analogous to the EJB transaction attribute of the same name.
     * <p><b>NOTE:</b> Actual transaction suspension will not work out-of-the-box
     * on all transaction managers. This in particular applies to
     * {@link org.springframework.transaction.jta.JtaTransactionManager},
     * which requires the {@code javax.transaction.TransactionManager} to be
     * made available it to it (which is server-specific in standard Java EE).
     * <p>A {@code PROPAGATION_REQUIRES_NEW} scope always defines its own
     * transaction synchronizations. Existing synchronizations will be suspended
     * and resumed appropriately.
     * @see org.springframework.transaction.jta.JtaTransactionManager#setTransactionManager
     */
    int PROPAGATION_REQUIRES_NEW = 3;

PROPAGATION_REQUIRES_NEW在有事务的场景下会suspend当前事务，然后创建新事务

AbstractPlatformTransactionManager

org/springframework/transaction/support/AbstractPlatformTransactionManager.java

        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;
            }
        }

handleExistingTransaction方法在判断是PROPAGATION_REQUIRES_NEW，会执行suspend方法，然后newTransactionStatus，执行doBegin及prepareSynchronization

suspend

org/springframework/transaction/support/AbstractPlatformTransactionManager.java

    /**
     * Suspend the given transaction. Suspends transaction synchronization first,
     * then delegates to the {@code doSuspend} template method.
     * @param transaction the current transaction object
     * (or {@code null} to just suspend active synchronizations, if any)
     * @return an object that holds suspended resources
     * (or {@code null} if neither transaction nor synchronization active)
     * @see #doSuspend
     * @see #resume
     */
    @Nullable
    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;
        }
    }

suspend方法主要是执行doSuspendSynchronization方法返回suspendedSynchronizations，执行doSuspend返回suspendedResources，最后根据这两个创建SuspendedResourcesHolder

doSuspendSynchronization

    /**
     * Suspend all current synchronizations and deactivate transaction
     * synchronization for the current thread.
     * @return the List of suspended TransactionSynchronization objects
     */
    private List<TransactionSynchronization> doSuspendSynchronization() {
        List<TransactionSynchronization> suspendedSynchronizations =
                TransactionSynchronizationManager.getSynchronizations();
        for (TransactionSynchronization synchronization : suspendedSynchronizations) {
            synchronization.suspend();
        }
        TransactionSynchronizationManager.clearSynchronization();
        return suspendedSynchronizations;
    }

doSuspendSynchronization这个遍历suspendedSynchronizations，挨个执行suspend，然后clearSynchronization

ResourceHolderSynchronization

org/springframework/transaction/support/ResourceHolderSynchronization.java

    public void suspend() {
        if (this.holderActive) {
            TransactionSynchronizationManager.unbindResource(this.resourceKey);
        }
    }

ResourceHolderSynchronization的suspend执行的是TransactionSynchronizationManager.unbindResource

unbindResource

org/springframework/transaction/support/TransactionSynchronizationManager.java

    /**
     * Unbind a resource for the given key from the current thread.
     * @param key the key to unbind (usually the resource factory)
     * @return the previously bound value (usually the active resource object)
     * @throws IllegalStateException if there is no value bound to the thread
     * @see ResourceTransactionManager#getResourceFactory()
     */
    public static Object unbindResource(Object key) throws IllegalStateException {
        Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
        Object value = doUnbindResource(actualKey);
        if (value == null) {
            throw new IllegalStateException(
                    "No value for key [" + actualKey + "] bound to thread [" + Thread.currentThread().getName() + "]");
        }
        return value;
    }

    /**
     * Actually remove the value of the resource that is bound for the given key.
     */
    @Nullable
    private static Object doUnbindResource(Object actualKey) {
        Map<Object, Object> map = resources.get();
        if (map == null) {
            return null;
        }
        Object value = map.remove(actualKey);
        // Remove entire ThreadLocal if empty...
        if (map.isEmpty()) {
            resources.remove();
        }
        // Transparently suppress a ResourceHolder that was marked as void...
        if (value instanceof ResourceHolder && ((ResourceHolder) value).isVoid()) {
            value = null;
        }
        if (value != null && logger.isTraceEnabled()) {
            logger.trace("Removed value [" + value + "] for key [" + actualKey + "] from thread [" +
                    Thread.currentThread().getName() + "]");
        }
        return value;
    }

unbindResource主要是执行doUnbindResource，从resources中移除

cleanupAfterCompletion

org/springframework/transaction/support/AbstractPlatformTransactionManager.java

    /**
     * Clean up after completion, clearing synchronization if necessary,
     * and invoking doCleanupAfterCompletion.
     * @param status object representing the transaction
     * @see #doCleanupAfterCompletion
     */
    private void cleanupAfterCompletion(DefaultTransactionStatus status) {
        status.setCompleted();
        if (status.isNewSynchronization()) {
            TransactionSynchronizationManager.clear();
        }
        if (status.isNewTransaction()) {
            doCleanupAfterCompletion(status.getTransaction());
        }
        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());
        }
    }

在内嵌事务执行完之后，会判断是否有suspendedResources，如果有则执行resume，恢复之前suspend的事务

resume

org/springframework/transaction/support/AbstractPlatformTransactionManager.java

    /**
     * Resume the given transaction. Delegates to the {@code doResume}
     * template method first, then resuming transaction synchronization.
     * @param transaction the current transaction object
     * @param resourcesHolder the object that holds suspended resources,
     * as returned by {@code suspend} (or {@code null} to just
     * resume synchronizations, if any)
     * @see #doResume
     * @see #suspend
     */
    protected final void resume(@Nullable Object transaction, @Nullable SuspendedResourcesHolder resourcesHolder)
            throws TransactionException {

        if (resourcesHolder != null) {
            Object suspendedResources = resourcesHolder.suspendedResources;
            if (suspendedResources != null) {
                doResume(transaction, suspendedResources);
            }
            List<TransactionSynchronization> suspendedSynchronizations = resourcesHolder.suspendedSynchronizations;
            if (suspendedSynchronizations != null) {
                TransactionSynchronizationManager.setActualTransactionActive(resourcesHolder.wasActive);
                TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(resourcesHolder.isolationLevel);
                TransactionSynchronizationManager.setCurrentTransactionReadOnly(resourcesHolder.readOnly);
                TransactionSynchronizationManager.setCurrentTransactionName(resourcesHolder.name);
                doResumeSynchronization(suspendedSynchronizations);
            }
        }
    }

resume方法执行doResume，然后恢复之前的TransactionSynchronizationManager的一些设置

doResume

org/springframework/orm/jpa/JpaTransactionManager.java

    @Override
    protected void doResume(@Nullable Object transaction, Object suspendedResources) {
        SuspendedResourcesHolder resourcesHolder = (SuspendedResourcesHolder) suspendedResources;
        TransactionSynchronizationManager.bindResource(
                obtainEntityManagerFactory(), resourcesHolder.getEntityManagerHolder());
        if (getDataSource() != null && resourcesHolder.getConnectionHolder() != null) {
            TransactionSynchronizationManager.bindResource(getDataSource(), resourcesHolder.getConnectionHolder());
        }
    }

doResume这里就是给bind回来

小结

spring事务的REQUIRES_NEW传播级别的实现就是对当前事务进行suspend，底层是unbind，然后创建新事务，执行完毕判断是否有suspend的事务，有则执行resume，底层是bind。具体对于mysql来讲，它不感知这些嵌套事务，它先接收到的是内嵌的新事务的sql，然后提交，最后接收到了外层resume回来的事务。