spring系列10-spring源码分析之事务管理器

1.开启事务配置

开启事务管理器分为三步：

• 注册事务管理器

	@Bean
	public PlatformTransactionManager platformTransactionManager() throws PropertyVetoException{
		return new DataSourceTransactionManager(dataSource());
	}

• 开启事务管理
  用@EnableTransactionManagement注解
• 在需要事务管理的方法上加上@Transactional

2.源码分析

看到@EnableTransactionManagement是不是感觉有点熟悉，在分析AOP源码时，开启AOP使用@EnableAspectJAutoProxy。所以@EnableTransactionManagement注解中肯定是向IOC容器中注册了bean。

//向容器注册了TransactionManagementConfigurationSelector
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

	/**
	 * Indicate whether subclass-based (CGLIB) proxies are to be created ({@code true}) as
	 * opposed to standard Java interface-based proxies ({@code false}). The default is
	 * {@code false}. <strong>Applicable only if {@link #mode()} is set to
	 * {@link AdviceMode#PROXY}</strong>.
	 * <p>Note that setting this attribute to {@code true} will affect <em>all</em>
	 * Spring-managed beans requiring proxying, not just those marked with
	 * {@code @Transactional}. For example, other beans marked with Spring's
	 * {@code @Async} annotation will be upgraded to subclass proxying at the same
	 * time. This approach has no negative impact in practice unless one is explicitly
	 * expecting one type of proxy vs another, e.g. in tests.
	 */
	boolean proxyTargetClass() default false;

	/**
	 * Indicate how transactional advice should be applied.
	 * <p><b>The default is {@link AdviceMode#PROXY}.</b>
	 * Please note that proxy mode allows for interception of calls through the proxy
	 * only. Local calls within the same class cannot get intercepted that way; an
	 * {@link Transactional} annotation on such a method within a local call will be
	 * ignored since Spring's interceptor does not even kick in for such a runtime
	 * scenario. For a more advanced mode of interception, consider switching this to
	 * {@link AdviceMode#ASPECTJ}.
	 */
	AdviceMode mode() default AdviceMode.PROXY;

	/**
	 * Indicate the ordering of the execution of the transaction advisor
	 * when multiple advices are applied at a specific joinpoint.
	 * <p>The default is {@link Ordered#LOWEST_PRECEDENCE}.
	 */
	int order() default Ordered.LOWEST_PRECEDENCE;

}

TransactionManagementConfigurationSelector实现了ImportSelector，也是用来向容器注册bean。
	protected String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
			case PROXY:
				//如果是PROXY模式，向容器注册AutoProxyRegistrar和ProxyTransactionManagementConfiguration
				return new String[] {AutoProxyRegistrar.class.getName(),
						ProxyTransactionManagementConfiguration.class.getName()};
			case ASPECTJ:
				//如果使用ASPECTJ，向容器注册AspectJTransactionManagementConfiguration
				return new String[] {determineTransactionAspectClass()};
			default:
				return null;
		}
	}

默认的模式是PROXY。这里我们分析下AutoProxyRegistrar和ProxyTransactionManagementConfiguration两个类。

2.1AutoProxyRegistrar

AutoProxyRegistrar又实现了ImportBeanDefinitionRegistrar接口，这个同样是向容器注册bean的。

public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
		boolean candidateFound = false;
		Set<String> annTypes = importingClassMetadata.getAnnotationTypes();
		for (String annType : annTypes) {
			AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
			if (candidate == null) {
				continue;
			}
			Object mode = candidate.get("mode");
			Object proxyTargetClass = candidate.get("proxyTargetClass");
			if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
					Boolean.class == proxyTargetClass.getClass()) {
				candidateFound = true;
				if (mode == AdviceMode.PROXY) {
					//默认模式是PROXY
					AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
					if ((Boolean) proxyTargetClass) {
						AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
						return;
					}
				}
			}
		}
		if (!candidateFound && logger.isInfoEnabled()) {
			String name = getClass().getSimpleName();
			logger.info(String.format("%s was imported but no annotations were found " +
					"having both 'mode' and 'proxyTargetClass' attributes of type " +
					"AdviceMode and boolean respectively. This means that auto proxy " +
					"creator registration and configuration may not have occurred as " +
					"intended, and components may not be proxied as expected. Check to " +
					"ensure that %s has been @Import'ed on the same class where these " +
					"annotations are declared; otherwise remove the import of %s " +
					"altogether.", name, name, name));
		}
	}
	

registerAutoProxyCreatorIfNecessary方法

public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
		return registerAutoProxyCreatorIfNecessary(registry, null);
	}
	
		public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry,
			@Nullable Object source) {

		return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
	}

最后向容器注册了InfrastructureAdvisorAutoProxyCreator,这个类就是AOP的创建代理对象的核心类。如果你看过我AOP的文章可能有点疑惑，我们再分析AOP源码的时候，发现启动AOP注册的是AnnotationAwareAspectJAutoProxyCreator。其实这两个类只是不同的实现，本质上还是使用动态代理封装对象。
InfrastructureAdvisorAutoProxyCreator继承自AbstractAdvisorAutoProxyCreator类。AnnotationAwareAspectJAutoProxyCreator类继承AspectJAwareAdvisorAutoProxyCreator类，而AspectJAwareAdvisorAutoProxyCreator同样继承自AbstractAdvisorAutoProxyCreator类。到这里可以知道事务管理本质上也是使用了AOP，对方法进行拦截，对对象进行封装。对AOP不明白的可以看我分析AOP的文章。这里出现了很多向IOC注册bean的类，这个我在之前的文章也说过，有兴趣的可以看下。

2.2ProxyTransactionManagementConfiguration

public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)

 //事务增强器(Advisor)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource());
		advisor.setAdvice(transactionInterceptor());
		if (this.enableTx != null) {
			advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		}
		return advisor;
	}

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	//@Transactional注解的解析类；负责解析事务方法上@Transactional中的各个参数配置
	public TransactionAttributeSource transactionAttributeSource() {
		return new AnnotationTransactionAttributeSource();
	}

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	//拦截器，实际是一个MethodInterceptor
	//		在目标方法执行的时候；执行拦截器链；
	public TransactionInterceptor transactionInterceptor() {
		TransactionInterceptor interceptor = new TransactionInterceptor();
		interceptor.setTransactionAttributeSource(transactionAttributeSource());
		if (this.txManager != null) {
			interceptor.setTransactionManager(this.txManager);
		}
		return interceptor;
	}

}

2.3 事务动态代理

既然事务也是AOP动态代理，前一篇我们也分析了AOP源码，这里我们直接看下核心部分。
前面AOP的时候有个canApply方法判断增强器是否需要在当前类增强，我们直接从这里开始分析。

public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
		Assert.notNull(pc, "Pointcut must not be null");
		if (!pc.getClassFilter().matches(targetClass)) {
			return false;
		}

		MethodMatcher methodMatcher = pc.getMethodMatcher();
		if (methodMatcher == MethodMatcher.TRUE) {
			// No need to iterate the methods if we're matching any method anyway...
			return true;
		}

		IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
		if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
			introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
		}

		Set<Class<?>> classes = new LinkedHashSet<>();
		if (!Proxy.isProxyClass(targetClass)) {
			classes.add(ClassUtils.getUserClass(targetClass));
		}
		classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));

		for (Class<?> clazz : classes) {
			Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
			for (Method method : methods) {
				if (introductionAwareMethodMatcher != null ?
						introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
						methodMatcher.matches(method, targetClass)) {//判断方法是否需要增强
					return true;
				}
			}
		}

		return false;
	}

是不是很熟悉，我们直接看判断方法是否需要增强的部分。这里的methodMatcher.matches(method, targetClass)调用的是TransactionAttributeSourcePointcut类的matches方法。
（1）matches方法

	public boolean matches(Method method, @Nullable Class<?> targetClass) {
		if (targetClass != null && TransactionalProxy.class.isAssignableFrom(targetClass)) {
			return false;
		}
		TransactionAttributeSource tas = getTransactionAttributeSource();
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}

（2）getTransactionAttribute方法

public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
		if (method.getDeclaringClass() == Object.class) {
			return null;
		}

		// First, see if we have a cached value.
		Object cacheKey = getCacheKey(method, targetClass);
		TransactionAttribute cached = this.attributeCache.get(cacheKey);
		if (cached != null) {
			// Value will either be canonical value indicating there is no transaction attribute,
			// or an actual transaction attribute.
			if (cached == NULL_TRANSACTION_ATTRIBUTE) {
				return null;
			}
			else {
				return cached;
			}
		}
		else {
			// We need to work it out.
			//方法匹配的判断
			TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
			// Put it in the cache.
			if (txAttr == null) {
				this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
			}
			else {
				String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
				if (txAttr instanceof DefaultTransactionAttribute) {
					((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
				}
				if (logger.isTraceEnabled()) {
					logger.trace("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
				}
				this.attributeCache.put(cacheKey, txAttr);
			}
			return txAttr;
		}
	}

（3）computeTransactionAttribute方法

protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
		// Don't allow no-public methods as required.
		if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
			return null;
		}

		// The method may be on an interface, but we need attributes from the target class.
		// If the target class is null, the method will be unchanged.
		Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);

		// First try is the method in the target class.
		TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
		if (txAttr != null) {
			return txAttr;
		}

		// Second try is the transaction attribute on the target class.
		txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
		if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
			return txAttr;
		}

		if (specificMethod != method) {
			// Fallback is to look at the original method.
			txAttr = findTransactionAttribute(method);
			if (txAttr != null) {
				return txAttr;
			}
			// Last fallback is the class of the original method.
			txAttr = findTransactionAttribute(method.getDeclaringClass());
			if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
				return txAttr;
			}
		}

		return null;
	}

（4）findTransactionAttribute

	protected TransactionAttribute findTransactionAttribute(Method method) {
		return determineTransactionAttribute(method);
	}

继续往下看
（5）determineTransactionAttribute

	protected TransactionAttribute determineTransactionAttribute(AnnotatedElement ae) {
		for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
			TransactionAttribute attr = annotationParser.parseTransactionAnnotation(ae);
			if (attr != null) {
				return attr;
			}
		}
		return null;
	}

我们看下parseTransactionAnnotation，这个是解析事务的方法
（6）parseTransactionAnnotation
这个方法调用的是SpringTransactionAnnotationParser的方法。

	public TransactionAttribute parseTransactionAnnotation(AnnotatedElement ae) {
		AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
				ae, Transactional.class, false, false);
		if (attributes != null) {
			return parseTransactionAnnotation(attributes);
		}
		else {
			return null;
		}
	}

我们可以看到接续判断是否包含@Transactional属性。
（7）parseTransactionAnnotation
这个方法就是用来解析属性的。

protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
		RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();
		Propagation propagation = attributes.getEnum("propagation");
		rbta.setPropagationBehavior(propagation.value());
		Isolation isolation = attributes.getEnum("isolation");
		rbta.setIsolationLevel(isolation.value());
		rbta.setTimeout(attributes.getNumber("timeout").intValue());
		rbta.setReadOnly(attributes.getBoolean("readOnly"));
		rbta.setQualifier(attributes.getString("value"));
		ArrayList<RollbackRuleAttribute> rollBackRules = new ArrayList<>();
		Class<?>[] rbf = attributes.getClassArray("rollbackFor");
		for (Class<?> rbRule : rbf) {
			RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
			rollBackRules.add(rule);
		}
		String[] rbfc = attributes.getStringArray("rollbackForClassName");
		for (String rbRule : rbfc) {
			RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
			rollBackRules.add(rule);
		}
		Class<?>[] nrbf = attributes.getClassArray("noRollbackFor");
		for (Class<?> rbRule : nrbf) {
			NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
			rollBackRules.add(rule);
		}
		String[] nrbfc = attributes.getStringArray("noRollbackForClassName");
		for (String rbRule : nrbfc) {
			NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
			rollBackRules.add(rule);
		}
		rbta.getRollbackRules().addAll(rollBackRules);
		return rbta;
	}

2.4 事务的拦截

前面AOP的封装和需要拦截的增强器也分析了，这里分析下方法调用的逻辑，和AOP类似，最后方法调用肯定是用了拦截器，这里就是在前面看到的TransactionInterceptor的invoke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, invocation::proceed);
	}

（1）invokeWithinTransaction
这里调用的是TransactionAspectSupport类的invokeWithinTransaction方法。

protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
											 final InvocationCallback invocation) throws Throwable {

		// If the transaction attribute is null, the method is non-transactional.
		TransactionAttributeSource tas = getTransactionAttributeSource();
		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
		final PlatformTransactionManager tm = determineTransactionManager(txAttr);
		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

		if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
			// Standard transaction demarcation with getTransaction and commit/rollback calls.
			//开启事务
			TransactionInfo txInfo = createTransactionIfNecessary(tm, 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
				//开启抛出异常后，回滚事务
				completeTransactionAfterThrowing(txInfo, ex);
				throw ex;
			} finally {
				cleanupTransactionInfo(txInfo);
			}

			if (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);
				}
			}
			//提交事务
			commitTransactionAfterReturning(txInfo);
			return retVal;
		} else {
			final ThrowableHolder throwableHolder = new ThrowableHolder();

			// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
			try {
				Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
					TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
					try {
					//执行被拦截的方法
						Object retVal = invocation.proceedWithInvocation();
						if (vavrPresent && VavrDelegate.isVavrTry(retVal)) {
							// Set rollback-only in case of Vavr failure matching our rollback rules...
							retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
						}
						return retVal;
					} 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.
							throwableHolder.throwable = ex;
							return null;
						}
					} finally {
						cleanupTransactionInfo(txInfo);
					}
				});

				// Check result state: It might indicate a Throwable to rethrow.
				if (throwableHolder.throwable != null) {
					throw throwableHolder.throwable;
				}
				return result;
			} catch (ThrowableHolderException ex) {
				throw ex.getCause();
			} catch (TransactionSystemException ex2) {
				if (throwableHolder.throwable != null) {
					logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
					ex2.initApplicationException(throwableHolder.throwable);
				}
				throw ex2;
			} catch (Throwable ex2) {
				if (throwableHolder.throwable != null) {
					logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
				}
				throw ex2;
			}
		}
	}

（2）createTransactionIfNecessary

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

		// If no name specified, apply method identification as transaction name.
		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) {
			//开启事务
				status = tm.getTransaction(txAttr);
			}
			else {
				if (logger.isDebugEnabled()) {
					logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
							"] because no transaction manager has been configured");
				}
			}
		}
		return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
	}

（3）getTransaction
这里调用的是AbstractPlatformTransactionManager类的方法。

public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition) throws TransactionException {
		Object transaction = doGetTransaction();//子类实现

		// Cache debug flag to avoid repeated checks.
		boolean debugEnabled = logger.isDebugEnabled();

		if (definition == null) {
			// Use defaults if no transaction definition given.
			definition = new DefaultTransactionDefinition();
		}

		if (isExistingTransaction(transaction)) {
			// Existing transaction found -> check propagation behavior to find out how to behave.
			return handleExistingTransaction(definition, transaction, debugEnabled);
		}

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

		// No existing transaction found -> check propagation behavior to find out how to proceed.
		if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
			throw new IllegalTransactionStateException(
					"No existing transaction found for transaction marked with propagation 'mandatory'");
		}
		else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
				definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
			SuspendedResourcesHolder suspendedResources = suspend(null);
			if (debugEnabled) {
				logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
			}
			try {
				boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
				DefaultTransactionStatus status = newTransactionStatus(
						definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
				//开启事务
				doBegin(transaction, definition);
				prepareSynchronization(status, definition);
				return status;
			}
			catch (RuntimeException | Error ex) {
				resume(null, suspendedResources);
				throw ex;
			}
		}
		else {
			// Create "empty" transaction: no actual transaction, but potentially synchronization.
			if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
				logger.warn("Custom isolation level specified but no actual transaction initiated; " +
						"isolation level will effectively be ignored: " + definition);
			}
			boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
			return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
		}
	}

（4）doGetTransaction、doBegin
这个方法是需要子类实现的，我们在前面定义的事务管理器是DataSourceTransactionManager。看下doGetTransaction和doBegin方法。

	protected Object doGetTransaction() {
		//获得事务
		DataSourceTransactionObject txObject = new DataSourceTransactionObject();
		txObject.setSavepointAllowed(isNestedTransactionAllowed());
		ConnectionHolder conHolder =
				(ConnectionHolder) TransactionSynchronizationManager.getResource(obtainDataSource());
		txObject.setConnectionHolder(conHolder, false);
		return txObject;
	}

	protected void doBegin(Object transaction, TransactionDefinition definition) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
		Connection con = null;

		try {
			if (!txObject.hasConnectionHolder() ||
					txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
				Connection newCon = obtainDataSource().getConnection();
				if (logger.isDebugEnabled()) {
					logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
				}
				txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
			}

			txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
			con = txObject.getConnectionHolder().getConnection();

			Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
			txObject.setPreviousIsolationLevel(previousIsolationLevel);

			// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
			// so we don't want to do it unnecessarily (for example if we've explicitly
			// configured the connection pool to set it already).
			if (con.getAutoCommit()) {
				txObject.setMustRestoreAutoCommit(true);
				if (logger.isDebugEnabled()) {
					logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
				}
				//设置自动提交
				con.setAutoCommit(false);
			}

			prepareTransactionalConnection(con, definition);
			txObject.getConnectionHolder().setTransactionActive(true);

			int timeout = determineTimeout(definition);
			if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
				txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
			}

			// Bind the connection holder to the thread.
			if (txObject.isNewConnectionHolder()) {
				TransactionSynchronizationManager.bindResource(obtainDataSource(), txObject.getConnectionHolder());
			}
		}

		catch (Throwable ex) {
			if (txObject.isNewConnectionHolder()) {
				DataSourceUtils.releaseConnection(con, obtainDataSource());
				txObject.setConnectionHolder(null, false);
			}
			throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
		}
	}

（5）completeTransactionAfterThrowing
我们会过来再看下抛出异常的回滚方法。

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 {
					//事务回滚
					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;
				}
			}
		}
	}

这里的回滚调用的也是我们定义事务管理器的方法。

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 {
			//事务回滚
			con.rollback();
		}
		catch (SQLException ex) {
			throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
		}
	}

（6）commitTransactionAfterReturning
再看下事务提交方法。

	protected void commitTransactionAfterReturning(@Nullable TransactionInfo txInfo) {
		if (txInfo != null && txInfo.getTransactionStatus() != null) {
			if (logger.isTraceEnabled()) {
				logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
			}
			//事务提交
			txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
		}
	}

这里的回滚提交的也是我们定义事务管理器的方法。

	protected void doCommit(DefaultTransactionStatus status) {
		DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
		Connection con = txObject.getConnectionHolder().getConnection();
		if (status.isDebug()) {
			logger.debug("Committing JDBC transaction on Connection [" + con + "]");
		}
		try {
			//事务提交
			con.commit();
		}
		catch (SQLException ex) {
			throw new TransactionSystemException("Could not commit JDBC transaction", ex);
		}
	}

至此事务分析结束。
总结：
其实spring中的事务也是AOP动态代理，利用增强器对方法进行增强。事务的提交、回滚都是调用的我们定义的事务管理器。
这里把所有和AOP拦截的代码去掉，和我们自己进行简单的事务控制一样。分为以下几步：

• 获得事务GetTransaction
• 开启事务doBegin
• 事务提交commit
• 如果方法抛出异常，事务回滚rollback