目录一、使用方式二、getTransaction三、handleExistingTransaction四、NESTED 嵌套事务五、个人理解六、总结
一、使用方式
可以采用Transactional，配置propagation即可。打开org.springframework.transaction.annotation.Transactional可见默认传播特性是REQUIRED。
/** * The transaction propagation type. * <p>Defaults to {@link Propagation#REQUIRED}. * @see org.springframework.transaction.interceptor.TransactionAttribute#getPropagationBehavior() */Propagation propagation() default Propagation.REQUIRED;
二、getTransaction
顾名思义，此项属性是在事务存在交互时生效。那么到底是如何生效的呢，核心源码位于org.springframework.transaction.support.AbstractPlatformTransactionManager。核心入口是getTransaction方法。
@Overridepublic final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {Object transaction = doGetTransaction();// Cache debug flag to avoid repeated checks.boolean debugEnabled = logger.isDebugEnabled();//if (definition == null) {// Use defaults if no transaction definition given.definition = new DefaultTransactionDefinition();}//已经存在事务 根据事务传播特性进行处理if (isExistingTransaction(transaction)) {// Existing transaction found -> check propagation behavior to find out how to behave.return handleExistingTransaction(definition, transaction, debugEnabled);}// Check definition settings for new transaction.if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());}// No existing transaction found -> check propagation behavior to find out how to proceed.//当前不存在事务 MANDATORY直接抛出异常 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {throw new IllegalTransactionStateException("No existing transaction found for transaction marked with propagation 'mandatory'");}//REQUIRED REQUIRES_NEW NESTED则会新建一个事务else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {SuspendedResourcesHolder suspendedResources = suspend(null);if (debugEnabled) {logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);}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 ex) {resume(null, suspendedResources);throw ex;}}else {xxx}}
可以看到，在当前不存在事务时，
MANDATORY 直接抛出异常 REQUIRED REQUIRES_NEW NESTED 自动新建一个事务。
那么存在事务时是如何处理的呢？
三、handleExistingTransaction
/** * 处理已经存在事务的情况 * Create a TransactionStatus for an existing transaction. */private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)throws TransactionException {//NEVER 已经存在事务 直接抛出异常if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {throw new IllegalTransactionStateException("Existing transaction found for transaction marked with propagation 'never'");}//NOT_SUPPORTED 注意prepareTransactionStatus方法参数传递事务的时候传递参数为null，所以是采用非事务方式运行。执行会挂起当前事务。if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {if (debugEnabled) {logger.debug("Suspending current transaction");}Object suspendedResources = suspend(transaction);boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);return prepareTransactionStatus(definition, null, false, newSynchronization, debugEnabled, suspendedResources);}//REQUIRES_NEW 新建事务 会挂起当前事务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;}}//NESTED 处理嵌套事务if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {// 检查是否支持嵌套事务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() + "]");}// 支持的话则采用Savepoint 否则开启新事务 并不会挂起当前事务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);//注意此处会创建savePointstatus.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);// 注意此处newTransaction属性设置为true，说明确实采用了创建新事务方式来实现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");}}}boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);// 其他的传播特性则加入当前事务 不会创建新事务return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);}
可以看到，当已经存在事务时，
NEVER 直接报错，不支持事务 NOT_SUPPORTED 默默按照非事务方式运行 REQUIRES_NEW 新建一个事务。 NESTED 处理嵌套事务 视情况采用savePoint或者新建事务。 其他的 加入当前事务
四、NESTED 嵌套事务
SavePoint
先简单说说SavePoint机制吧。这个也比较简单。比如一个 事务比较复杂，容易出错。那么如果当前DB支持SavePoint的话，那么创建一个SavePoint就等于创建了一个快照，可以不用每次都回滚整个事务，仅回滚到指定的SavePoint即可。
五、个人理解
NESTED这个处理确实比较复杂。个人也查了很多资料。目前个人目前理解如下：NESTED对于事务的处理主要在于级别不同。REQUIRES_NEW创建的两个事务是平级的，一个事务的成功与否对另一个事务的成功与否不产生影响。而NESTED创建的事务则名副其实，是受其父级事务影响的。一句话总结就是，子事务的成功与否不影响父级事务的成功，但是父级事务的成功与否则会影响子事务的成功。父事务回滚，子事务一定会滚。子事务回滚，父事务不一定会滚。
六、总结
最后总结如下
名称 说明 PROPAGATION_REQUIRED 方法被调用时自动开启事务，在事务范围内使用则使用同一个事务，否则开启新事务。 默认选项。 PROPAGATION_SUPPORTS 支持当前事务，如果当前没有事务，就以非事务方式执行。 PROPAGATION_MANDATORY 支持当前事务，如果当前没有事务，就抛出异常。 PROPAGATION_REQUIRES_NEW 新建事务，如果当前存在事务，把当前事务挂起。 PROPAGATION_NOT_SUPPORTED 以非事务方式执行操作，如果当前存在事务，就把当前事务挂起。 PROPAGATION_NEVER 以非事务方式执行，如果当前存在事务，则抛出异常。 PROPAGATION_NESTED 如果一个活动的事务存在，则运行在一个嵌套的事务中. 如果没有活动事务, 则按TransactionDefinition.PROPAGATION_REQUIRED 属性执行。需要JDBC3.0以上支持。到此这篇关于Spring源码解析之事务传播特性的文章就介绍到这了,更多相关Spring事务传播特性内容请搜索以前的文章或继续浏览下面的相关文章希望大家以后多多支持！
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