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PROPAGATION_REQUIRED：如果当前没有事务，就新建一个事务，如果已经存在一个事务，就加入到这个事务中。这是最常见的选择。
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：使用当前的事务，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：如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则执行PROPAGATION_REQUIRED类似的操作。
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
//获取事物对象（内置conection)
Object transaction = doGetTransaction();
boolean debugEnabled = logger.isDebugEnabled();
if (definition == null) {
definition = new DefaultTransactionDefinition();
}
//这里判断是否已经存在一个事物，如果存在走其他逻辑
if (isExistingTransaction(transaction)) {
return handleExistingTransaction(definition, transaction, debugEnabled);
}
if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
}
//使用当前的事务，如果当前没有事务，就抛出异常
//代码走到这里，说明isExistingTransaction返回false，当前上下文没有事物。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
//PROPAGATION_REQUIRED，PROPAGATION_REQUIRES_NEW，PROPAGATION_NESTED
//在没有事物的情况下，都会新建一个新的事物。
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);
//这里创建一个新的事物
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 {
//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。
//下面是如果当前没有事物，就创建一个空事物。其实就是已非事物方式执行。
// 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);
}
}上面代码主要是在当前上下文没有事物的情况下的处理逻辑，
如果PROPAGATION_MANDATORY，则抛出异常
如果PROPAGATION_REQUIRED，PROPAGATION_REQUIRES_NEW，PROPAGATION_NESTED，则创建一个新事物。
最后就是PROPAGATION_SUPPORTS，有就支持，没有就已非事物运行。
/**
* Create a TransactionStatus for an existing transaction.
*/
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
//PROPAGATION_NEVER，如果是这个，那么直接抛异常
//以非事务方式执行，如果当前存在事务，则抛出异常。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
//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。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
if (debugEnabled) {
logger.debug("Suspending current transaction");
}
//这里挂起之前的事物，挂起的逻辑就是把之前事物上下文的资源封装成一个对象，放到新事物当中（空事物），
//并且清空当前的上下文环境
Object suspendedResources = suspend(transaction);
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
//返回空事物
return prepareTransactionStatus(
definition, null, false, newSynchronization, debugEnabled, suspendedResources);
}
//PROPAGATION_REQUIRES_NEW：新建事务，如果当前存在事务，把当前事务挂起。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
if (debugEnabled) {
logger.debug("Suspending current transaction, creating new transaction with name [" +
definition.getName() + "]");
}
//挂起之前的事物。
SuspendedResourcesHolder suspendedResources = suspend(transaction);
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
//创建一个新事物，把之前挂机的资源绑定到新事物当中去。
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;
}
}
//如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则执行PROPAGATION_REQUIRED类似的操作
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);
//所谓嵌套事物，其实就是JDBC的还原点，在当前位置创建还原点，
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");
}
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);
}上面代码大题逻辑就是如果已经存在一个事物，那么如何处理的逻辑。