private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)throws TransactionException {//不允许有事务 ， 直接异常if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {throw new IllegalTransactionStateException("Existing transaction found for transaction marked with propagation 'never'");}//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");}//挂起当前事务Object suspendedResources = suspend(transaction);boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);//修改事务状态信息 ， 把事务的一些信息存储到当前线程中 ， ThreadLocal中return prepareTransactionStatus(definition, null, false, newSynchronization, debugEnabled, suspendedResources);}if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {if (debugEnabled) {logger.debug("Suspending current transaction, creating new transaction with name [" +definition.getName() + "]");}//挂起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;}}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() + "]");}//默认是可以嵌套事务的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);//创建回滚点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;}}// 省略boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null); }这里面也是对每个传播属性的判断 ， 先看PROPAGATION_REQUIRES_NEW的处理 ， 因为该属性要求每次调用都开启一个新的事务 ， 所以首先会将当前事务挂起 ， 怎么挂起呢？
protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {if (TransactionSynchronizationManager.isSynchronizationActive()) {List suspendedSynchronizations = doSuspendSynchronization();try {Object suspendedResources = null;//第一次进来 ， 肯定为null的if (transaction != null) {//吧connectionHolder设置为空suspendedResources = doSuspend(transaction);}//做数据还原操作String name = TransactionSynchronizationManager.getCurrentTransactionName();TransactionSynchronizationManager.setCurrentTransactionName(null);boolean readOnly = TransactionSynchronizationManager.isCurrentTransactionReadOnly();TransactionSynchronizationManager.setCurrentTransactionReadOnly(false);Integer isolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(null);boolean wasActive = TransactionSynchronizationManager.isActualTransactionActive();TransactionSynchronizationManager.setActualTransactionActive(false);return new SuspendedResourcesHolder(suspendedResources, suspendedSynchronizations, name, readOnly, isolationLevel, wasActive);}catch (RuntimeException | Error ex) {// doSuspend failed - original transaction is still active...doResumeSynchronization(suspendedSynchronizations);throw ex;}}else if (transaction != null) {// Transaction active but no synchronization active.Object suspendedResources = doSuspend(transaction);return new SuspendedResourcesHolder(suspendedResources);}else {// Neither transaction nor synchronization active.return null;} } protected Object doSuspend(Object transaction) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;txObject.setConnectionHolder(null);//解除绑定关系 ，return TransactionSynchronizationManager.unbindResource(obtainDataSource()); }