嵌套事務(wù)、掛起事務(wù)，Spring 事務(wù)機制有什么奧秘？

* 
This transaction manager supports nested transactions via the JDBC 3.0* {@link java.sql.Savepoint} mechanism. The* {@link #setNestedTransactionAllowed "nestedTransactionAllowed"} flag defaults* to "true", since nested transactions will work without restrictions on JDBC* drivers that support savepoints (such as the Oracle JDBC driver).

/**   * Suspend the given transaction. Suspends transaction synchronization first,   * then delegates to the {@code doSuspend} template method.   * @param transaction the current transaction object   * (or {@code null} to just suspend active synchronizations, if any)   * @return an object that holds suspended resources   * (or {@code null} if neither transaction nor synchronization active)   * @see #doSuspend   * @see #resume   */  @Nullable  protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {    if (TransactionSynchronizationManager.isSynchronizationActive()) {      List suspendedSynchronizations = doSuspendSynchronization();      try {        Object suspendedResources = null;        if (transaction != null) {          suspendedResources = doSuspend(transaction);        }        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);??????}  }

  @Override  protected Object doSuspend(Object transaction) {    DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;    txObject.setConnectionHolder(null);    return TransactionSynchronizationManager.unbindResource(obtainDataSource());  }
  @Override  protected void doResume(@Nullable Object transaction, Object suspendedResources) {    TransactionSynchronizationManager.bindResource(obtainDataSource(), suspendedResources);  }

private static final ThreadLocal> resources =      new NamedThreadLocal<>("Transactional resources");/**   * Bind the given resource for the given key to the current thread.   * @param key the key to bind the value to (usually the resource factory)   * @param value the value to bind (usually the active resource object)   * @throws IllegalStateException if there is already a value bound to the thread   * @see ResourceTransactionManager#getResourceFactory()   */  public static void bindResource(Object key, Object value) throws IllegalStateException {    Object actualKey = TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);    Assert.notNull(value, "Value must not be null");    Map map = resources.get();    // set ThreadLocal Map if none found    if (map == null) {      map = new HashMap<>();      resources.set(map);    }    Object oldValue = map.put(actualKey, value);    // Transparently suppress a ResourceHolder that was marked as void...    if (oldValue instanceof ResourceHolder && ((ResourceHolder) oldValue).isVoid()) {      oldValue = null;????}  }