@Service
public class A {

? ? @Autowired
? ? private B b;

? ? @Autowired
? ? private C c;

? ? @Transactional(propagation = Propagation.REQUIRED, isolation = Isolation.DEFAULT)
? ? public void operate() {
? ? ? ? try {
? ? ? ? ? ? b.insertB();
? ? ? ? ? ? c.insertC();
? ? ? ? }catch (Exception e) {
? ? ? ? ? ? e.printStackTrace();
? ? ? ? }
? ? }
}

@Service
public class B {

? ? @Autowired
? ? private BM bm;

? ? @Transactional(propagation = Propagation.REQUIRED)
? ? public int insertB() {
? ? ? ? return bm.insert("B");
? ? }
}

@Service
public class C {

? ? @Autowired
? ? private CM cm;

? ? @Transactional(propagation = Propagation.REQUIRED)
? ? public int insertC() {
? ? ? ? return cm.insert("C");
? ? }
}

//獲取事務屬性類 AnnotationTransactionAttributeSource
TransactionAttributeSource tas = getTransactionAttributeSource();
//獲取事務屬性
final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
//獲取事務管理器
final TransactionManager tm = determineTransactionManager(txAttr);

PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
//獲取joinpoint
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
//注解事務會走這里
if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
?? ?// Standard transaction demarcation with getTransaction and commit/rollback calls.
?? ?//開啟事務
?? ?TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

?? ?Object retVal;
?? ?try {
?? ??? ?// This is an around advice: Invoke the next interceptor in the chain.
?? ??? ?// This will normally result in a target object being invoked.
?? ??? ?retVal = invocation.proceedWithInvocation();
?? ?} catch (Throwable ex) {
?? ??? ?// target invocation exception
?? ??? ?//事務回滾
?? ??? ?completeTransactionAfterThrowing(txInfo, ex);
?? ??? ?throw ex;
?? ?} finally {
?? ??? ?cleanupTransactionInfo(txInfo);
?? ?}

?? ?if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
?? ??? ?// Set rollback-only in case of Vavr failure matching our rollback rules...
?? ??? ?TransactionStatus status = txInfo.getTransactionStatus();
?? ??? ?if (status != null && txAttr != null) {
?? ??? ??? ?retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
?? ??? ?}
?? ?}

?? ?//事務提交
?? ?commitTransactionAfterReturning(txInfo);
?? ?return retVal;
}

public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
?? ?// Use defaults if no transaction definition given.
?? ?TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

?? ?//重點看.. DataSourceTransactionObject拿到對象
?? ?Object transaction = doGetTransaction();
?? ?boolean debugEnabled = logger.isDebugEnabled();

?? ?//第一次進來connectionHolder為空的, 所以不存在事務
?? ?if (isExistingTransaction(transaction)) {
?? ??? ?// Existing transaction found -> check propagation behavior to find out how to behave.
?? ??? ?//如果不是第一次進來, 則會走這個邏輯
?? ??? ?return handleExistingTransaction(def, transaction, debugEnabled);
?? ?}

?? ?// Check definition settings for new transaction.
?? ?if (def.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
?? ??? ?throw new InvalidTimeoutException("Invalid transaction timeout", def.getTimeout());
?? ?}

?? ?// No existing transaction found -> check propagation behavior to find out how to proceed.
?? ?if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
?? ??? ?throw new IllegalTransactionStateException(
?? ??? ??? ??? ?"No existing transaction found for transaction marked with propagation 'mandatory'");
?? ?}
?? ?//第一次進來大部分會走這里（傳播屬性是 Required ｜ Requested New ｜ Nested）
?? ?else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
?? ??? ??? ?def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
?? ??? ??? ?def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
?? ??? ?//先掛起
?? ??? ?SuspendedResourcesHolder suspendedResources = suspend(null);
?? ??? ?if (debugEnabled) {
?? ??? ??? ?logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
?? ??? ?}
?? ??? ?try {
?? ??? ??? ?//開啟事務
?? ??? ??? ?return startTransaction(def, transaction, debugEnabled, suspendedResources);
?? ??? ?} catch (RuntimeException | Error ex) {
?? ??? ??? ?resume(null, suspendedResources);
?? ??? ??? ?throw ex;
?? ??? ?}
?? ?} else {
?? ??? ?// Create "empty" transaction: no actual transaction, but potentially synchronization.
?? ??? ?if (def.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
?? ??? ??? ?logger.warn("Custom isolation level specified but no actual transaction initiated; " +
?? ??? ??? ??? ??? ?"isolation level will effectively be ignored: " + def);
?? ??? ?}
?? ??? ?boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
?? ??? ?return prepareTransactionStatus(def, null, true, newSynchronization, debugEnabled, null);
?? ?}
}

private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {
?? ?boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
?? ?//創(chuàng)建一個新的事務狀態(tài), 注意這里的newTransaction 屬性為true
?? ?DefaultTransactionStatus status = newTransactionStatus(
?? ??? ??? ?definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
?? ?//開啟事務
?? ?doBegin(transaction, definition);
?? ?//開啟事務后, 改變事務狀態(tài)
?? ?prepareSynchronization(status, definition);
?? ?return status;
}

//如果第二次進來還是PROPAFGATION_REQUIRED， 走這里, newTransation為false
return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);

//如果有回滾點
if (status.hasSavepoint()) {
?? ?if (status.isDebug()) {
?? ??? ?logger.debug("Releasing transaction savepoint");
?? ?}
?? ?unexpectedRollback = status.isGlobalRollbackOnly();
?? ?status.releaseHeldSavepoint();
}
//如果是新事務, 則提交事務
else if (status.isNewTransaction()) {
?? ?if (status.isDebug()) {
?? ??? ?logger.debug("Initiating transaction commit");
?? ?}
?? ?unexpectedRollback = status.isGlobalRollbackOnly();
?? ?doCommit(status);
}
else if (isFailEarlyOnGlobalRollbackOnly()) {
?? ?unexpectedRollback = status.isGlobalRollbackOnly();
}

private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
?? ?try {
?? ??? ?boolean unexpectedRollback = unexpected;

?? ??? ?try {
?? ??? ??? ?triggerBeforeCompletion(status);

?? ??? ??? ?if (status.hasSavepoint()) {
?? ??? ??? ??? ?if (status.isDebug()) {
?? ??? ??? ??? ??? ?logger.debug("Rolling back transaction to savepoint");
?? ??? ??? ??? ?}
?? ??? ??? ??? ?status.rollbackToHeldSavepoint();
?? ??? ??? ?} else if (status.isNewTransaction()) {
? ? ? ? if (status.isDebug()) {
?? ??? ??? ??? ??? ?logger.debug("Initiating transaction rollback");
?? ??? ??? ??? ?}
?? ??? ??? ??? ?doRollback(status);
?? ??? ??? ?} else {
?? ??? ??? ??? ?// Participating in larger transaction
?? ??? ??? ??? ?if (status.hasTransaction()) {
?? ??? ??? ??? ??? ?if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
?? ??? ??? ??? ??? ??? ?if (status.isDebug()) {
?? ??? ??? ??? ??? ??? ??? ?logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
?? ??? ??? ??? ??? ??? ?}
?? ??? ??? ??? ??? ??? ?doSetRollbackOnly(status);
?? ??? ??? ??? ??? ?} else {
?? ??? ??? ??? ??? ??? ?if (status.isDebug()) {
?? ??? ??? ??? ??? ??? ??? ?logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
?? ??? ??? ??? ??? ??? ?}
?? ??? ??? ??? ??? ?}
?? ??? ??? ??? ?} else {
?? ??? ??? ??? ??? ?logger.debug("Should roll back transaction but cannot - no transaction available");
?? ??? ??? ??? ?}
?? ??? ??? ??? ?// Unexpected rollback only matters here if we're asked to fail early
?? ??? ??? ??? ?if (!isFailEarlyOnGlobalRollbackOnly()) {
?? ??? ??? ??? ??? ?unexpectedRollback = false;
?? ??? ??? ??? ?}
?? ??? ??? ?}
?? ??? ?} catch (RuntimeException | Error ex) {
?? ??? ??? ?triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
?? ??? ??? ?throw ex;
?? ??? ?}

?? ??? ?triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);

?? ??? ?// Raise UnexpectedRollbackException if we had a global rollback-only marker
?? ??? ?if (unexpectedRollback) {
?? ??? ??? ?throw new UnexpectedRollbackException(
?? ??? ??? ??? ??? ?"Transaction rolled back because it has been marked as rollback-only");
?? ??? ?}
?? ?} finally {
?? ??? ?cleanupAfterCompletion(status);
?? ?}
}

public void setRollbackOnly() {
?? ?this.rollbackOnly = true;
}

public final void commit(TransactionStatus status) throws TransactionException {
?? ?if (status.isCompleted()) {
?? ??? ?throw new IllegalTransactionStateException("Transaction is already completed - do not call commit or rollback more than once per transaction");
?? ?}

?? ?DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
?? ?if (defStatus.isLocalRollbackOnly()) {
?? ??? ?if (defStatus.isDebug()) {
?? ??? ??? ?logger.debug("Transactional code has requested rollback");
?? ??? ?}
?? ??? ?processRollback(defStatus, false);
?? ??? ?return;
?? ?}

?? ?if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
?? ??? ?if (defStatus.isDebug()) {
?? ??? ??? ?logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
?? ??? ?}
?? ??? ?processRollback(defStatus, true);
?? ??? ?return;
?? ?}

?? ?//執(zhí)行事務提交
?? ?processCommit(defStatus);
}