public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {
	// ...
	// 主方法进入，调用invokeWithinTransaction方法
	public Object invoke(final MethodInvocation invocation) throws Throwable {
		// Work out the target class: may be {@code null}.
		// The TransactionAttributeSource should be passed the target class
		// as well as the method, which may be from an interface.
		Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

		// Adapt to TransactionAspectSupport's invokeWithinTransaction...
		return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
			public Object proceedWithInvocation() throws Throwable {
				return invocation.proceed();
			}
		});
	}
	/**
	 * General delegate for around-advice-based subclasses, delegating to several other template
	 * methods on this class. Able to handle {@link CallbackPreferringPlatformTransactionManager}
	 * as well as regular {@link PlatformTransactionManager} implementations.
	 * @param method the Method being invoked
	 * @param targetClass the target class that we're invoking the method on
	 * @param invocation the callback to use for proceeding with the target invocation
	 * @return the return value of the method, if any
	 * @throws Throwable propagated from the target invocation
	 */
	// 总共步 1. createTransactionIfNecessary；2. completeTransactionAfterThrowing；3. cleanupTransactionInfo；4. commitTransactionAfterReturning
	// 代码中的日志记录了抛出异常Participating transaction failed - marking existing transaction as rollback-only的过程
	protected Object invokeWithinTransaction(Method method, Class targetClass, final InvocationCallback invocation)
			throws Throwable {

		// If the transaction attribute is null, the method is non-transactional.
		final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
		// 全局的事务处理类，子事务、主事务共用一个，里边有globalRollbackOnParticipationFailure标识
		final PlatformTransactionManager tm = determineTransactionManager(txAttr);
		final String joinpointIdentification = methodIdentification(method, targetClass);

		if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
			// Standard transaction demarcation with getTransaction and commit/rollback calls.
			// 如果需要的话，就创建事务，事务放入TransactionStatus
			// 如果有事务，从当前事务中初始化子事务TransactionStatus，跟当前事务用一个ConnectionHolder，里边有rollbackOnly标识
			// ------------- create new transaction start ----------------
			// Creating new transaction with name[*impl#**]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT; ''
			// Acquired Connection [A] for JDBC transaction
			// Switching JDBC Connection [A] to manual commit
			// --- 实际业务方法中用到数据库连接，并操作数据库
			// --- Creating a new SqlSession
			// --- Registering transaction synchronization for SqlSession [27375710]
			// --- JDBC Connection [A] will be managed by Spring
			// --- Using Connection [A]
			// --- Preparing: some sql
			// --- Releasing transactional SqlSession [27375710]
			// ------------- create child transaction start （主事务嵌套调用其他服务的子事务，子事务非新建）---------------- 
			// Participating in existing transaction
			// --- 实际业务方法中用到数据库连接，并操作数据库
			// --- Fetched SqlSession [ 27375710] from current transaction
			// --- Using Connection [A]
			// --- Preparing: some sql
			// --- Releasing transactional SqlSession [27375710]
			TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
			Object retVal = null;
			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
				// 判断是否设置了rollbackExcepiton，如果没有设置，则回滚 ‘ex instanceof RuntimeException || ex instanceof Error’ 异常
				// 如果回滚异常，调用AbstractPlatformTransactionManager.processRollback
				// 有异常，根据子
				// Participating transaction failed - marking existing transaction as rollback-only
				// Setting JDBC transaction [A] rollback-only
				completeTransactionAfterThrowing(txInfo, ex);
				throw ex;
			}
			finally {
				// 如果子事务嵌入到主事务，还原事务上下文到主事务
				cleanupTransactionInfo(txInfo);
			}
			
			// Global transaction is marked as rollback-only but transactional code requested commit
			// Initiating transaction rollback
			// Rolling back JDBC transaction on Connection [A]
			// Transaction synchronization rolling back SqlSession [27375710]
			// Transaction synchronization closing SqlSession [27375710]
			// Releasing JDBC Connection [A] after transaction
			// Returning JDBC Connection to DataSource
			// ** throw UnexpectedRollbackException("Transaction rolled back because it has been marked as rollback-only")
			commitTransactionAfterReturning(txInfo);
			return retVal;
		}
		else {
			// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
			try {
				Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr,
						new TransactionCallback<Object>() {
							public Object doInTransaction(TransactionStatus status) {
								TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
								try {
									return invocation.proceedWithInvocation();
								}
								catch (Throwable ex) {
									if (txAttr.rollbackOn(ex)) {
										// A RuntimeException: will lead to a rollback.
										if (ex instanceof RuntimeException) {
											throw (RuntimeException) ex;
										}
										else {
											throw new ThrowableHolderException(ex);
										}
									}
									else {
										// A normal return value: will lead to a commit.
										return new ThrowableHolder(ex);
									}
								}
								finally {
									cleanupTransactionInfo(txInfo);
								}
							}
						});

				// Check result: It might indicate a Throwable to rethrow.
				if (result instanceof ThrowableHolder) {
					throw ((ThrowableHolder) result).getThrowable();
				}
				else {
					return result;
				}
			}
			catch (ThrowableHolderException ex) {
				throw ex.getCause();
			}
		}
	}
	// ...
}

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {  
	 // ...
	 // 事务拦截器主入口代码，调用父类的invokeWithinTransaction，
    public Object invoke(final MethodInvocation invocation) throws Throwable {
        // Work out the target class: may be {@code null}.
        // The TransactionAttributeSource should be passed the target class
        // as well as the method, which may be from an interface.
        Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
        // Adapt to TransactionAspectSupport's invokeWithinTransaction...
        return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
            public Object proceedWithInvocation() throws Throwable {
                return invocation.proceed();
            }
        });
    } 
	// ...
} 

@SuppressWarnings("serial")
public abstract class AbstractPlatformTransactionManager implements PlatformTransactionManager, Serializable {  

	/**
	 * Set whether to globally mark an existing transaction as rollback-only
	 * after a participating transaction failed.
	 * <p>Default is "true": If a participating transaction (e.g. with
	 * PROPAGATION_REQUIRES or PROPAGATION_SUPPORTS encountering an existing
	 * transaction) fails, the transaction will be globally marked as rollback-only.
	 * The only possible outcome of such a transaction is a rollback: The
	 * transaction originator <i>cannot</i> make the transaction commit anymore.
	 * <p>Switch this to "false" to let the transaction originator make the rollback
	 * decision. If a participating transaction fails with an exception, the caller
	 * can still decide to continue with a different path within the transaction.
	 * However, note that this will only work as long as all participating resources
	 * are capable of continuing towards a transaction commit even after a data access
	 * failure: This is generally not the case for a Hibernate Session, for example;
	 * neither is it for a sequence of JDBC insert/update/delete operations.
	 * <p><b>Note:</b>This flag only applies to an explicit rollback attempt for a
	 * subtransaction, typically caused by an exception thrown by a data access operation
	 * (where TransactionInterceptor will trigger a {@code PlatformTransactionManager.rollback()}
	 * call according to a rollback rule). If the flag is off, the caller can handle the exception
	 * and decide on a rollback, independent of the rollback rules of the subtransaction.
	 * This flag does, however, <i>not</i> apply to explicit {@code setRollbackOnly}
	 * calls on a {@code TransactionStatus}, which will always cause an eventual
	 * global rollback (as it might not throw an exception after the rollback-only call).
	 * <p>The recommended solution for handling failure of a subtransaction
	 * is a "nested transaction", where the global transaction can be rolled
	 * back to a savepoint taken at the beginning of the subtransaction.
	 * PROPAGATION_NESTED provides exactly those semantics; however, it will
	 * only work when nested transaction support is available. This is the case
	 * with DataSourceTransactionManager, but not with JtaTransactionManager.
	 * @see #setNestedTransactionAllowed
	 * @see org.springframework.transaction.jta.JtaTransactionManager
	 */
	// 注释已经说的很明确了，要改变该值，请用PROPAGATION_NESTED，不过必须得支持JDBC3.0
	public final void setGlobalRollbackOnParticipationFailure(boolean globalRollbackOnParticipationFailure) {
		this.globalRollbackOnParticipationFailure = globalRollbackOnParticipationFailure;
	}

	/**
	 * This implementation of commit handles participating in existing
	 * transactions and programmatic rollback requests.
	 * Delegates to {@code isRollbackOnly}, {@code doCommit}
	 * and {@code rollback}.
	 * @see org.springframework.transaction.TransactionStatus#isRollbackOnly()
	 * @see #doCommit
	 * @see #rollback
	 */
	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;
		// 如果子事务异常，全局回滚标识会被设置为rollbackOnly，回滚主事务
		if (defStatus.isLocalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Transactional code has requested rollback");
			}
			processRollback(defStatus);
			return;
		}
		// 如果子事务异常，TransactionManager的globalRollbackOnParticipationFailure为true，且事务jdbc连接的connectionHold为rollbackOnly，等主事务回滚成功后再抛出异常！
		if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
			}
			processRollback(defStatus);
			// Throw UnexpectedRollbackException only at outermost transaction boundary
			// or if explicitly asked to.
			if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
				throw new UnexpectedRollbackException(
						"Transaction rolled back because it has been marked as rollback-only");
			}
			return;
		}

		processCommit(defStatus);
	}
	
    /**
     * Process an actual rollback.
     * The completed flag has already been checked.
     * @param status object representing the transaction
     * @throws TransactionException in case of rollback failure
     */
    private void processRollback(DefaultTransactionStatus status) {
        try {
            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 if (status.hasTransaction()) {
                    if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
                        if (status.isDebug()) {
                            logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
                        }
                        // Setting JDBC transaction [connetion name] rollback-only，标记连接拥有者rollbackOnly=true(ResourceHolderSupport.rollbackOnly)
                        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");
                }
            }
            catch (RuntimeException ex) {
                triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
                throw ex;
            }
            catch (Error err) {
                triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
                throw err;
            }
            triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
        }
        finally {
            cleanupAfterCompletion(status);
        }
    }
}