spring支持的事务传播行为
spring将事务传播行为声明定义在TransactionDefinition中，有如下7种传播行为：
- PROPAGATION_REQUIRED 支持当前事务，没有则创建事务并以事务方式运行
- PROPAGATION_SUPPORTS 支持当前事务，没有则以非事务方式运行
- PROPAGATION_MANDATORY 支持当前事务，如果当前没有事务则抛出异常
- PROPAGATION_REQUIRES_NEW 创建一个新的事务，如果当前有事务将其挂起
- PROPAGATION_NOT_SUPPORTED 不支持当前事务，并总是以非事务运行
- PROPAGATION_NEVER 不支持当前事务，如果有事务抛出异常
- PROPAGATION_NESTED 如果当前事务存在，则执行一个嵌套事务(只支持jdbc)
上面的传播行为主要是围绕着对当前事务是否支持，是否需要新建、挂起、嵌套等。
接下来我们看看事务的传播行为是如何实现的。
获取一个事务
事务的传播行为主要体现在如何获取一个事务，代码在AbstractPlatformTransactionManager.getTransaction()中：
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
// 获取当前的事务
Object transaction = doGetTransaction();
...
// 没有事务定义，直接创建一个默认的
if (definition == null) {
// Use defaults if no transaction definition given.
definition = new DefaultTransactionDefinition();
}
// 存在事务的处理流程
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 // 检查事务属性定义
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.
// 当前没有事务根据传播行为进行处理
...
}
上面的代码主要逻辑是：
- 获取到当前的事务，以及设置相关的事务定义属性
- 如果存在事务进行处理
- 如果当前没有事务进行处理
我们先看看第3步没有事务的处理，还是在getTransaction()中：
// 判断当前如果没有事务直接抛出异常
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
// 开启一个新的事务
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;
}
}
// 创建一个空的事务，没有实际的事务作用
else {
// Create "empty" transaction: no actual transaction, but potentially synchronization.
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
logger.warn("Custom isolation level specified but no actual transaction initiated; " +
"isolation level will effectively be ignored: " + definition);
}
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
}
上面的代码主要有三个分支处理：
- 如果是PROPAGATION_MANDATORY，如果没有事务直接抛异常
- 如果是PROPAGATION_REQUIRED/PROPAGATION_REQUIRES_NEW/PROPAGATION_NESTED新建一个事务
- 创建空的事务，没有实际事务
当前存在事务的处理逻辑
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
// PROPAGATION_NEVER行为不能有事务，直接抛异常
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
// 不在事务中运行，挂起当前的事务
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);
}
// 创建新事物运行
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;
}
}
// 嵌套事务（会判断子类事务管理器是否支持）
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
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() + "]");
}
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);
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);
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");
}
// PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED在已有事务中运行
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);
return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}
在存在事务的情况下，上面的处理逻辑也印证了每个传播行为的描述：
- 判断是PROPAGATION_NEVER直接抛出异常，不允许存在当前事务.
- 判断是PROPAGATION_NOT_SUPPORTED则挂起当前事务，以非事务方式运行。
- 判断是PROPAGATION_REQUIRES_NEW，挂起当前事务并创建一个新的事务。
- 判断PROPAGATION_NESTED，会检查事务管理器是否支持，如果不支持抛出异常。（如jdbc的话通过Savepoint来实现）
- 其它PROPAGATION_SUPPORTS&PROPAGATION_REQUIRED两个行为，在当前事务运行。（检查事务传播级别是否为ISOLATION_DEFAULT，非只读事务）
