Spring源码解析之事务传播特性

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。

@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？

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

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

来源：https://blog.csdn.net/chenpengjia006/article/details/117256167