Spring源码解析(34)之Spring事务回滚流程

一、前言

在上一个篇章我们主要介绍了Spring事务的运行流程，也带着一步步debug看了整个事务的运行流程，但是还是欠缺了Spring事务的回滚的流程。

在上篇也主要介绍了Spring事务的传播特性，这里还是要看一下Spring事务的传播特性，因为不同的事务传播特性在回滚时会用不同的操作。

接下来将看下在不同的事务传播特性下，Spring事务回滚是如何去操作的，下面调试代码。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/aop https://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd">
    <context:property-placeholder location="classpath:dbconfig.properties"></context:property-placeholder>
    <bean id="dataSource" class="com.alibaba.druid.pool.DruidDataSource">
        <property name="username" value="${jdbc.username}"></property>
        <property name="password" value="${jdbc.password}"></property>
        <property name="url" value="${jdbc.url}"></property>
        <property name="driverClassName" value="${jdbc.driverClassName}"></property>
    </bean>
    <bean id="jdbcTemplate" class="org.springframework.jdbc.core.JdbcTemplate" >
        <constructor-arg name="dataSource" ref="dataSource"></constructor-arg>
    </bean>
    <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
        <property name="dataSource" ref="dataSource"></property>
    </bean>
    <bean id="bookService" class="com.mashibing.tx.xml.service.BookService">
        <property name="bookDao" ref="bookDao"></property>
    </bean>
    <bean id="bookDao" class="com.mashibing.tx.xml.dao.BookDao">
        <property name="jdbcTemplate" ref="jdbcTemplate"></property>
    </bean>
    <aop:config>
        <aop:pointcut id="txPoint" expression="execution(* com.mashibing.tx.xml.*.*.*(..))"/>
        <aop:advisor advice-ref="myAdvice" pointcut-ref="txPoint"></aop:advisor>
    </aop:config>
    <tx:advice id="myAdvice" transaction-manager="transactionManager">
        <tx:attributes>
            <tx:method name="checkout" propagation="REQUIRED" />
            <tx:method name="updateStock" propagation="REQUIRED" />
        </tx:attributes>
    </tx:advice>
</beans>

业务代码以及启动类代码如下：

public class TxTest {
    public static void main(String[] args) throws SQLException {
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY,"d:\\code");
        ApplicationContext context = new ClassPathXmlApplicationContext("tx.xml");
        BookService bookService = context.getBean("bookService", BookService.class);
        bookService.checkout("zhangsan",1);
    }
}


public class BookService {

    BookDao bookDao;

    public BookDao getBookDao() {
        return bookDao;
    }

    public void setBookDao(BookDao bookDao) {
        this.bookDao = bookDao;
    }

    /**
     * 结账：传入哪个用户买了哪本书
     * @param username
     * @param id
     */
    public void checkout(String username,int id){

        try {
            bookDao.updateStock(id);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

public class BookDao {

    JdbcTemplate jdbcTemplate;

    public JdbcTemplate getJdbcTemplate() {
        return jdbcTemplate;
    }

    public void setJdbcTemplate(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }

    /**
     * 减去某个用户的余额
     * @param userName
     * @param price
     */
    public void updateBalance(String userName,int price){
        String sql = "update account set balance=balance-? where username=?";
        jdbcTemplate.update(sql,price,userName);
    }

    /**
     * 按照图书的id来获取图书的价格
     * @param id
     * @return
     */
    public int getPrice(int id){
        String sql = "select price from book where id=?";
        return jdbcTemplate.queryForObject(sql,Integer.class,id);
    }

    /**
     * 减库存，减去某本书的库存
     * @param id
     */
    public void updateStock(int id){
        String sql = "update book_stock set stock=stock-1 where id=?";
        jdbcTemplate.update(sql,id);
        for (int i = 1 ;i>=0 ;i--)
            System.out.println(10/i);
    }
}

我们外层事务跟内层事务的隔离级别都是required，然后在内层事务中制造出来一个异常，看下Spring事务是如何处理的。

二、Spring 事务回滚源码分析

复习一下，我们在事务解析的时候注入了以下对象：

然后我们执行业务方法的时候，首先会从cglibInvocationInterceptor进去，然后生产拦截器链，然后通过cglibMethodInvocation去执行链接器链路，最后就会执行到TransactionInterceptor中。

我们直接断点打到TransactionInterceptor中。

在这里就会去创建一个事务，具体的逻辑可以去看一下上一篇我们介绍的事务源码流程，这里我们直接跳过，然后来执行创建对应的里层事务。

此时就到执行创建里面那层也就是执行updateStock时候需要去创建事务了，我们继续往里跟。

protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
			@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {

		// If no name specified, apply method identification as transaction name.
		// 如果没有名称指定则使用方法唯一标识，并使用DelegatingTransactionAttribute封装txAttr
		if (txAttr != null && txAttr.getName() == null) {
			txAttr = new DelegatingTransactionAttribute(txAttr) {
				@Override
				public String getName() {
					return joinpointIdentification;
				}
			};
		}

		TransactionStatus status = null;
		if (txAttr != null) {
			if (tm != null) {
				// 获取TransactionStatus事务状态信息
				status = tm.getTransaction(txAttr);
			}
			else {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
							"] because no transaction manager has been configured");
				}
			}
		}
		// 根据指定的属性与status准备一个TransactionInfo，
		return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
	}

继续往下跟getTransaction方法。

@Override
	public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
			throws TransactionException {

		// Use defaults if no transaction definition given.
		// 如果没有事务定义信息则使用默认的事务管理器定义信息
		TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

		// 获取事务
		Object transaction = doGetTransaction();
		boolean debugEnabled = logger.isDebugEnabled();

		// 判断当前线程是否存在事务，判断依据为当前线程记录的连接不为空且连接中的transactionActive属性不为空
		if (isExistingTransaction(transaction)) {
			// Existing transaction found -> check propagation behavior to find out how to behave.
			// 当前线程已经存在事务
			return handleExistingTransaction(def, transaction, debugEnabled);
		}

		// Check definition settings for new transaction.
		// 事务超时设置验证
		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.
		// 如果当前线程不存在事务，但是PropagationBehavior却被声明为PROPAGATION_MANDATORY抛出异常
		if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}
		// PROPAGATION_REQUIRED，PROPAGATION_REQUIRES_NEW，PROPAGATION_NESTED都需要新建事务
		else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
				def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
				def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			//没有当前事务的话，REQUIRED，REQUIRES_NEW，NESTED挂起的是空事务，然后创建一个新事务
			SuspendedResourcesHolder suspendedResources = suspend(null);
			if (debugEnabled) {
				logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
			}
			try {
				return startTransaction(def, transaction, debugEnabled, suspendedResources);
			}
			catch (RuntimeException | Error ex) {
				// 恢复挂起的事务
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			// Create "empty" transaction: no actual transaction, but potentially synchronization.
			// 创建一个空的事务
			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);
		}
	}

继续跟下doGetTransaction方法。

	protected Object doGetTransaction() {
		// 创建一个数据源事务对象
		DataSourceTransactionObject txObject = new DataSourceTransactionObject();
		// 是否允许当前事务设置保持点
		txObject.setSavepointAllowed(isNestedTransactionAllowed());
		/**
		 * TransactionSynchronizationManager 事务同步管理器对象(该类中都是局部线程变量)
		 * 用来保存当前事务的信息,我们第一次从这里去线程变量中获取 事务连接持有器对象 通过数据源为key去获取
		 * 由于第一次进来开始事务 我们的事务同步管理器中没有被存放.所以此时获取出来的conHolder为null
		 */
		ConnectionHolder conHolder =
				(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
		// 非新创建连接则写false
		txObject.setConnectionHolder(conHolder, false);
		// 返回事务对象
		return txObject;
	}

可以看到他这里是从spring的事务同步管理器中去获取一个链接，在上一个事务中，我们已经往Spring事务同步管理器中放入了一个连接，所以这个时候我们是能够获取得到连接的。

然后此时我们是存在事务的，因为外层事务已经存在，所以我们就会走存在事务的方法。

private TransactionStatus handleExistingTransaction(
			TransactionDefinition definition, Object transaction, boolean debugEnabled)
			throws TransactionException {

		/**
		 * 判断当前的事务行为是不是PROPAGATION_NEVER的
		 * 表示为不支持事务,但是当前又存在一个事务,所以抛出异常
		 */
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
			throw new IllegalTransactionStateException(
					"Existing transaction found for transaction marked with propagation 'never'");
		}

		/**
		 * 判断当前的事务属性不支持事务,PROPAGATION_NOT_SUPPORTED,所以需要先挂起已经存在的事务
		 */
		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);
			// 创建一个新的非事务状态(保存了上一个存在事务状态的属性)
			return prepareTransactionStatus(
					definition, null, false, newSynchronization, debugEnabled, suspendedResources);
		}

		/**
		 * 当前的事务属性状态是PROPAGATION_REQUIRES_NEW表示需要新开启一个事务状态
		 */
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
			if (debugEnabled) {
				logger.debug("Suspending current transaction, creating new transaction with name [" +
						definition.getName() + "]");
			}
			// 挂起当前事务并返回挂起的资源持有器
			SuspendedResourcesHolder suspendedResources = suspend(transaction);
			try {
				// 创建一个新的非事务状态(保存了上一个存在事务状态的属性)
				return startTransaction(definition, transaction, debugEnabled, suspendedResources);
			}
			catch (RuntimeException | Error beginEx) {
				resumeAfterBeginException(transaction, suspendedResources, beginEx);
				throw beginEx;
			}
		}

		// 嵌套事务
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			// 不允许就报异常
			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() + "]");
			}
			// 嵌套事务的处理
			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.
				// 如果没有可以使用保存点的方式控制事务回滚，那么在嵌入式事务的建立初始简历保存点
				DefaultTransactionStatus status =
						prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
				// 为事务设置一个回退点
				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.
				// 有些情况是不能使用保存点操作
				return startTransaction(definition, transaction, debugEnabled, null);
			}
		}

		// 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);
	}

我们这里是required就会走这个方法，此时他就会去创建一个事务状态信息，可以看到我们传传入的 newSynchronization为true，并且isNewTransaction为false，标记这不是一个新事务，我们记得这两个标记位，继续往下跟。

然后在newTransactionStatus中又会修改一下newSynchronization的值。

	protected final DefaultTransactionStatus prepareTransactionStatus(
			TransactionDefinition definition, @Nullable Object transaction, boolean newTransaction,
			boolean newSynchronization, boolean debug, @Nullable Object suspendedResources) {

		DefaultTransactionStatus status = newTransactionStatus(
				definition, transaction, newTransaction, newSynchronization, debug, suspendedResources);
		prepareSynchronization(status, definition);
		return status;
	}

	protected DefaultTransactionStatus newTransactionStatus(
			TransactionDefinition definition, @Nullable Object transaction, boolean newTransaction,
			boolean newSynchronization, boolean debug, @Nullable Object suspendedResources) {

		// 是否需要新同步，只要有新同步且当前无同步激活事务
		boolean actualNewSynchronization = newSynchronization &&
				!TransactionSynchronizationManager.isSynchronizationActive();
		return new DefaultTransactionStatus(
				transaction, newTransaction, actualNewSynchronization,
				definition.isReadOnly(), debug, suspendedResources);
	}

	protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
		if (status.isNewSynchronization()) {
			// 绑定事务激活
			TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
			// 当前事务的隔离级别
			TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
					definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
							definition.getIsolationLevel() : null);
			// 是否为只读事务
			TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
			// 事务的名称
			TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
			TransactionSynchronizationManager.initSynchronization();
		}
	}

通过以上代码其实我们可以知道 newSynchronization就是控制我们是否要去改变Spring事务同步器里面那几个线程变量里面的值，我们知道如果内层事务是require，他就会从外层事务，所以他这里是不用去修改线程变量里面的值，并且它也不会一个新事务，所以isNewTransaction也为false。

此时我们已经创建好了一个事务，然后执行具体的业务方法之后就会抛出来一个异常，我们继续往下跟 completeTransactionAfterThrowing看下具体是如何去处理的。

protected void completeTransactionAfterThrowing(@Nullable TransactionInfo txInfo, Throwable ex) {
		if (txInfo != null && txInfo.getTransactionStatus() != null) {
			if (logger.isTraceEnabled()) {
				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
						"] after exception: " + ex);
			}
			if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
				try {
					// 进行回滚
					txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
				}
				catch (TransactionSystemException ex2) {
					logger.error("Application exception overridden by rollback exception", ex);
					ex2.initApplicationException(ex);
					throw ex2;
				}
				catch (RuntimeException | Error ex2) {
					logger.error("Application exception overridden by rollback exception", ex);
					throw ex2;
				}
			}
			else {
				// We don't roll back on this exception.
				// Will still roll back if TransactionStatus.isRollbackOnly() is true.
				try {
					txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
				}
				catch (TransactionSystemException ex2) {
					logger.error("Application exception overridden by commit exception", ex);
					ex2.initApplicationException(ex);
					throw ex2;
				}
				catch (RuntimeException | Error ex2) {
					logger.error("Application exception overridden by commit exception", ex);
					throw ex2;
				}
			}
		}
	}


public boolean rollbackOn(Throwable ex) {
		if (logger.isTraceEnabled()) {
			logger.trace("Applying rules to determine whether transaction should rollback on " + ex);
		}

		RollbackRuleAttribute winner = null;
		int deepest = Integer.MAX_VALUE;

		// 处理设置的回滚规则
		if (this.rollbackRules != null) {
			for (RollbackRuleAttribute rule : this.rollbackRules) {
				int depth = rule.getDepth(ex);
				if (depth >= 0 && depth < deepest) {
					deepest = depth;
					winner = rule;
				}
			}
		}

		if (logger.isTraceEnabled()) {
			logger.trace("Winning rollback rule is: " + winner);
		}

		// User superclass behavior (rollback on unchecked) if no rule matches.
		if (winner == null) {
			logger.trace("No relevant rollback rule found: applying default rules");
			return super.rollbackOn(ex);
		}

		return !(winner instanceof NoRollbackRuleAttribute);
	}

首先会判断是否判断rollbackOn的规则，如果满足则进行回滚。

	@Override
	public final void rollback(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;
		processRollback(defStatus, false);
	}

	private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
		try {
			// 意外的回滚
			boolean unexpectedRollback = unexpected;

			try {
				// 回滚完成前回调
				triggerBeforeCompletion(status);
				// 有保存点回滚到保存点
				if (status.hasSavepoint()) {
					if (status.isDebug()) {
						logger.debug("Rolling back transaction to savepoint");
					}
					status.rollbackToHeldSavepoint();
				}
				// 当前状态是一个新事务
				else if (status.isNewTransaction()) {
					if (status.isDebug()) {
						logger.debug("Initiating transaction rollback");
					}
					// 进行回滚
					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");
							}
							//设置连接要回滚标记，也就是全局回滚
							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;
			}

			// 回滚完成后回调
			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 {
			// 根据事务状态信息，完成后数据清除，和线程的私有资源解绑，重置连接自动提交，隔离级别，是否只读，释放连接，恢复挂起事务等
			cleanupAfterCompletion(status);
		}
	}

此时我们这里没有保存点并且也不是一个新事务，但是我们又存在事务，所以我们继续往下跟。

可以看到内层事务只是做一个回滚标记而已并没有做任何回滚操作。

	/**
	 * 设置会馆标记，如果既没有保存点，又不是新的事务，如果可以设置全局的回滚标记的话，就会设置。
	 * @param status the status representation of the transaction
	 */
	@Override
	protected void doSetRollbackOnly(DefaultTransactionStatus status) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
		if (status.isDebug()) {
			logger.debug("Setting JDBC transaction [" + txObject.getConnectionHolder().getConnection() +
					"] rollback-only");
		}
		txObject.setRollbackOnly();
	}

大家要注意一下，我们这里此时外层是将事务捕获了，并没有往外部抛出异常信息。

此时我们回到最外层事务中。

可以看到此时虽然异常信息虽然已经是打印了，但是要知道此时外层已经将事务吞掉并没有抛出，此时就会去执行对应的事务提交的方法，我们继续往下跟。

	protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) {
		if (txInfo != null && txInfo.getTransactionStatus() != null) {
			if (logger.isTraceEnabled()) {
				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
			}
			txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
		}
	}
/**
	 * 提交事务，就算没有异常，但是提交的时候也可能会回滚，因为有内层事务可能会标记回滚。所以这里先判断是否状态是否需要本地回滚，
	 * 也就是设置回滚标记为全局回滚，不会进行回滚，再判断是否需要全局回滚，就是真的执行回滚。但是这里如果是发现有全局回滚，还要进行提交，就会报异常
	 *
	 * 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
	 */
	@Override
	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;
		// 如果在事务链中已经被标记回滚，那么不会尝试提交事务，直接回滚
		if (defStatus.isLocalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Transactional code has requested rollback");
			}
			// 不可预期的回滚
			processRollback(defStatus, false);
			return;
		}

		// 设置了全局回滚
		if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
			if (defStatus.isDebug()) {
				logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
			}
			// 可预期的回滚，可能会报异常
			processRollback(defStatus, true);
			return;
		}

		// 处理事务提交
		processCommit(defStatus);
	}

private void processRollback(DefaultTransactionStatus status, boolean unexpected) {
		try {
			// 意外的回滚
			boolean unexpectedRollback = unexpected;

			try {
				// 回滚完成前回调
				triggerBeforeCompletion(status);
				// 有保存点回滚到保存点
				if (status.hasSavepoint()) {
					if (status.isDebug()) {
						logger.debug("Rolling back transaction to savepoint");
					}
					status.rollbackToHeldSavepoint();
				}
				// 当前状态是一个新事务
				else if (status.isNewTransaction()) {
					if (status.isDebug()) {
						logger.debug("Initiating transaction rollback");
					}
					// 进行回滚
					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");
							}
							//设置连接要回滚标记，也就是全局回滚
							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;
			}

			// 回滚完成后回调
			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 {
			// 根据事务状态信息，完成后数据清除，和线程的私有资源解绑，重置连接自动提交，隔离级别，是否只读，释放连接，恢复挂起事务等
			cleanupAfterCompletion(status);
		}
	}
	/**
	 * 真正回滚的处理方法，也就是获取JDBC连接，然后回滚
	 * @param status the status representation of the transaction
	 */
	@Override
	protected void doRollback(DefaultTransactionStatus status) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
		Connection con = txObject.getConnectionHolder().getConnection();
		if (status.isDebug()) {
			logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
		}
		try {
			// jdbc的回滚
			con.rollback();
		}
		catch (SQLException ex) {
			throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
		}
	}

可以看到虽然外层事务已经将异常捕获掉没有抛出异常信息，但是内层事务已经标记了一个全局的事务回滚标记，所以在提交事务中，就会回滚事务。

这样子就可以印证，Spring外层事务如果是required，内层事务也是required，如果程序正常执行，那么内层事务不会提交，在外部事务中统一进行事务提交，如果内层事务，或者外层事务中出现异常情况，那么会在外层事务的处理中统一进行异常回滚。

可以很简单的理解，因为内层是required，所以他就会用外层的事务，如果内层方法有异常，就会标记一个全局的事务回滚标记，并且在外层事务中统一提交。

三、Spring其他事物传播特性事物回滚流程

3.1内层 required_new，外层required

上面我们已经看了外内层事务是required，如果此时我们的内层事务如果改成required_new会是什么效果呢？

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context https://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/aop https://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd">
    <context:property-placeholder location="classpath:dbconfig.properties"></context:property-placeholder>
    <bean id="dataSource" class="com.alibaba.druid.pool.DruidDataSource">
        <property name="username" value="${jdbc.username}"></property>
        <property name="password" value="${jdbc.password}"></property>
        <property name="url" value="${jdbc.url}"></property>
        <property name="driverClassName" value="${jdbc.driverClassName}"></property>
    </bean>
    <bean id="jdbcTemplate" class="org.springframework.jdbc.core.JdbcTemplate" >
        <constructor-arg name="dataSource" ref="dataSource"></constructor-arg>
    </bean>
    <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
        <property name="dataSource" ref="dataSource"></property>
    </bean>
    <bean id="bookService" class="com.mashibing.tx.xml.service.BookService">
        <property name="bookDao" ref="bookDao"></property>
    </bean>
    <bean id="bookDao" class="com.mashibing.tx.xml.dao.BookDao">
        <property name="jdbcTemplate" ref="jdbcTemplate"></property>
    </bean>
    <aop:config>
        <aop:pointcut id="txPoint" expression="execution(* com.mashibing.tx.xml.*.*.*(..))"/>
        <aop:advisor advice-ref="myAdvice" pointcut-ref="txPoint"></aop:advisor>
    </aop:config>
    <tx:advice id="myAdvice" transaction-manager="transactionManager">
        <tx:attributes>
            <tx:method name="checkout" propagation="REQUIRED" />
            <tx:method name="updateStock" propagation="REQUIRES_NEW" />
        </tx:attributes>
    </tx:advice>
</beans>

我们此时断点也是直接打在创建内层事务中。

此时可以看到内层事务状态两个标记位都是true，表示这是一个新事务，然后我们接下来看下他的事务回滚操作。

可以看到内层事务如果存在异常了，那么他直接在内层事务中进行回滚。我们断点在跳回外层事务中。

我们在外层事务中把事务捕获掉了，并没有抛出异常，所以外层事务是会正常的提交，我们继续往下跟。

可以看到我们的外层事务并没有标记有全局事务回滚，所以这里的事务是会正常提交的。

所以外层如果是required，内层是requreid_new，那么如果外层方法中存在事务，内层方法在运行的时候会挂起外层事务并开启一个新的事务，如果程序正常执行，则内层方法优先事务提交，然后外层方法再提交；如果内层方法中存在异常，内层事务会优先回滚，外层方法事务也会回滚，如果外层方法中存在异常，那么内层事务正常正常提交，而外层方法会进行回滚操作。

3.2 内层NOT_SUPPORTED，外层required

事务传播特性如下：

    <tx:advice id="myAdvice" transaction-manager="transactionManager">
        <tx:attributes>
            <tx:method name="checkout" propagation="REQUIRED" />
            <tx:method name="updateStock" propagation="NOT_SUPPORTED" />
        </tx:attributes>
    </tx:advice>

他在创建第二个事务的时候如下：