这篇文章主要介绍Spring中事务传播特性的示例分析，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！
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;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。
@Override
public 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？
/**
* 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savePoint
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);
// 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
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。
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。
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。 |
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