【spring】事务底层的实现流程

从源码层面分析，spring是如何支持事务的！ 事务简单介绍

• 本文源码基于spring-framework-5.3.10。
• 事务是基于AOP的机制进行实现的！

Spring事务基本执行原理

• 一个Bean在执行Bean的创建生命周期时，会经过InfrastructureAdvisorAutoProxyCreator的初始化后的方法，会判断当前当前Bean对象是否和BeanFactoryTransactionAttributeSourceAdvisor匹配，匹配逻辑为判断该Bean的类上是否存在@Transactional注解，或者类中的某个方法上是否存在@Transactional注解，如果存在则表示该Bean需要进行动态代理产生一个代理对象作为Bean对象。

Spring事务基本执行流程

• 利用所配置的PlatformTransactionManager事务管理器新建一个数据库连接。
• 修改数据库连接的autocommit为false。
• 执行MethodInvocation.proceed()方法，简单理解就是执行业务方法，其中就会执行sql。
• 如果没有抛异常，则提交。
• 如果抛了异常，则回滚。

源码执行流程

• 加了@Transactional注解的类，或者类中拥有@Transactional注解的方法，都会生成代理对象作为bean。
• 代理对象执行方法时。
• 获取当前正在执行的方法上的@Transactional注解的信息TransactionAttribute。
• 查看@Transactional注解上是否指定了TransactionManager，如果没有指定，则默认获取TransactionManager类型的bean作为TransactionManager。
• 对于TransactionManager有一个限制，必须是PlatformTransactionManager。
• 生成一个joinpointIdentification，作为事务的名字。
• 开始创建事务。
• 创建事务成功后执行业务方法。
• 如果执行业务方法出现异常，则会进行回滚，然后执行完finally中的方法后再将异常抛出。
• 如果执行业务方法没有出现异常，那么则会执行完finally中的方法后再进行提交。

创建事务源码流程

• 得到一个TransactionStatus对象、
• 将PlatformTransactionManager、TransactionAttribute、TransactionStatus构造成为一个TransactionInfo对象，并返回TransactionInfo对象。

回滚事务源码流程

• 判断当前异常是否需要回滚。不需要回滚直接走提价的流程。
• 触发同步器的beforeCompletion()。
• 调用数据库连接对象的rollback()。
• 触发同步器的afterCompletion()。
• 判断是否有事务挂起。
• 如果有则把挂起的事务重新设置到TransactionSynchronizationManager中去，并执行同步器的resume()方法。

提交事务源码流程

• 触发同步器的beforeCommit。
• 触发同步器的beforeCompletion()。
• 调用数据库连接对象的commit()。
• 触发同步器的afterCommit。
• 触发同步器的afterCompletion()。
• 判断是否有事务挂起。
• 如果有则把挂起的事务重新设置到TransactionSynchronizationManager中去，并执行同步器的resume()方法。

创建TransactionStatus的源码流程

• 调用AbstractPlatformTransactionManager类中的getTransaction(txAttr)方法，实际上这个方法就是真正去开启事务的方法。
• 调用DataSourceTransactionManager中的doGetTransaction()得到一个事务对象，得到的事务对象中可能持有也可能没有持有数据库连接对象。不同船舶机制下，是否持有事务，逻辑不同！

传播机制 含义 之前方法持有事务的逻辑 之前方法未持有事务的逻辑 REQUIRED(TransactionDefinition.PROPAGATION_REQUIRED) 支持当前事务，如果没有事务会创建一个新的事务 在当前事务运行 创建一个新的事务 SUPPORTS(TransactionDefinition.PROPAGATION_SUPPORTS) 支持当前事务，如果没有事务的话以非事务方式执行 在当前事务运行 使用非事务的方式运行 MANDATORY(TransactionDefinition.PROPAGATION_MANDATORY) 支持当前事务，如果没有事务抛出异常 在当前事务运行 抛异常 REQUIRES_NEW(TransactionDefinition.PROPAGATION_REQUIRES_NEW) 创建一个新的事务并挂起当前事务 创建一个新的事务并挂起当前事务 创建一个新的事务 NOT_SUPPORTED(TransactionDefinition.PROPAGATION_NOT_SUPPORTED) 以非事务方式执行，如果当前存在事务则将当前事务挂起 挂起当前事务 使用非事务的方式运行 NEVER(TransactionDefinition.PROPAGATION_NEVER) 以非事务方式进行，如果存在事务则抛出异常 抛异常 使用非事务的方式运行 NESTED(TransactionDefinition.PROPAGATION_NESTED) 如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则创建一个新事务 利用数据库连接对象，设置一个savepoint，比如mysql就支持，在一个事务中，可以在某个位置设置一个savepoint，后续可以只回滚到某个savepoint 创建一个新的事务 doBegin源码流程

• 如果事务对象中没有持有数据库连接对象，那么则调用DataSource获取一个数据库连接对象，并设置到事务对象中去
• 设置当前数据库连接的隔离级别。
• 设置数据库连接的autoCommit为false。
• 设置数据库连接的timeout。
• 把获得的数据库连接对象通过TransactionSynchronizationManager设置到当前线程的ThreadLocal中。

同步器使用方式

TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {
        @Override
        public void afterCommit() {
            System.out.println("after commit...");
        }
}

开启事务的注解@EnableTransactionManagement源码分析

/**
 * 这个注解导入了TransactionManagementConfigurationSelector类
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

	boolean proxyTargetClass() default false;

	AdviceMode mode() default AdviceMode.PROXY;

	int order() default Ordered.LOWEST_PRECEDENCE;
}

导入的TransactionManagementConfigurationSelector源码分析

/**
 * 在调用process方法的时候会调用到这里
 */
protected String[] selectImports(AdviceMode adviceMode) {
	switch (adviceMode) {
		case PROXY:
			// 默认是PROXY。往Spring容器中添加了两个Bean：AutoProxyRegistrar、ProxyTransactionManagementConfiguration
			return new String[] {AutoProxyRegistrar.class.getName(),
					ProxyTransactionManagementConfiguration.class.getName()};
		case ASPECTJ:
			// 表示不用动态代理技术，用ASPECTJ技术，比较麻烦了
			return new String[] {determineTransactionAspectClass()};
		default:
			return null;
	}
}

AutoProxyRegistrar的Bean源码分析

/**
 * 这个类实现了ImportBeanDefinitionRegistrar，他在启动的时候会调用registerBeanDefinitions方法。
 * 最核心的逻辑是往Spring容器中注册了一个InfrastructureAdvisorAutoProxyCreator的Bean。
 * InfrastructureAdvisorAutoProxyCreator继承了AbstractAdvisorAutoProxyCreator，所以这个类的主要作用就是开启自动代理的作用，也就是一个BeanPostProcessor，会在初始化后步骤中去寻找Advisor类型的Bean，并判断当前某个Bean是否有匹配的Advisor，是否需要利用动态代理产生一个代理对象。
 */
public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {
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) {
					// 注册InfrastructureAdvisorAutoProxyCreator，才可以Bean进行AOP
					AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
					if ((Boolean) proxyTargetClass) {
						// 设置InfrastructureAdvisorAutoProxyCreator的proxyTargetClass为true
						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));
		}
	}
}

导入的ProxyTransactionManagementConfiguration源码分析

/**
 * ProxyTransactionManagementConfiguration是一个配置类，它又定义了另外三个bean：
 * BeanFactoryTransactionAttributeSourceAdvisor：一个Advisor
 * AnnotationTransactionAttributeSource：相当于BeanFactoryTransactionAttributeSourceAdvisor中的Pointcut
 * TransactionInterceptor：相当于BeanFactoryTransactionAttributeSourceAdvisor中的Advice
 */
@Configuration(proxyBeanMethods = false)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

	// 定义一个Advisor的Bean
	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor(
			TransactionAttributeSource transactionAttributeSource, TransactionInterceptor transactionInterceptor) {

		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource);
		advisor.setAdvice(transactionInterceptor);
		if (this.enableTx != null) {
			advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		}
		return advisor;
	}

	// 定义Advisor的Pointcut
	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionAttributeSource transactionAttributeSource() {
		// AnnotationTransactionAttributeSource中定义了一个Pointcut
		// 并且AnnotationTransactionAttributeSource可以用来解析@Transactional注解，并得到一个RuleBasedTransactionAttribute对象
		return new AnnotationTransactionAttributeSource();
	}

	// 定义Advisor的Advice
	// 开启事务、回滚、提交都在这个TransactionInterceptor中
	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionInterceptor transactionInterceptor(TransactionAttributeSource transactionAttributeSource) {
		TransactionInterceptor interceptor = new TransactionInterceptor();
		interceptor.setTransactionAttributeSource(transactionAttributeSource);
		if (this.txManager != null) {
			interceptor.setTransactionManager(this.txManager);
		}
		return interceptor;
	}

}

使用用到的Pointcut源码分析

public class BeanFactoryTransactionAttributeSourceAdvisor extends AbstractBeanFactoryPointcutAdvisor {

	/**
	 * pointcut就是一个new TransactionAttributeSourcePointcut对象
	 */
	private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
		@Override
		@Nullable
		protected TransactionAttributeSource getTransactionAttributeSource() {
			return transactionAttributeSource;
		}
	};

	/**
	 * 获取pointcut采用的是内部类的方式构建
	 */
	@Override
	public Pointcut getPointcut() {
		return this.pointcut;
	}

}

/**
 * 构建Pointcut用到的类
 */
abstract class TransactionAttributeSourcePointcut extends StaticMethodMatcherPointcut implements Serializable {

	// 外部构造方法调用到这里，其实就是设置new TransactionAttributeSourceClassFilter()
	// 这里面主要进行类的判断
	protected TransactionAttributeSourcePointcut() {
		setClassFilter(new TransactionAttributeSourceClassFilter());
	}

	// 判断方法是否匹配
	@Override
	public boolean matches(Method method, Class<?> targetClass) {
		// 调用外部重写的方法
		TransactionAttributeSource tas = getTransactionAttributeSource();
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}

	// 具体的匹配逻辑
	private class TransactionAttributeSourceClassFilter implements ClassFilter {

		@Override
		public boolean matches(Class<?> clazz) {
			// 事务内部的一些类，直接返回false
			if (TransactionalProxy.class.isAssignableFrom(clazz) ||
					TransactionManager.class.isAssignableFrom(clazz) ||
					PersistenceExceptionTranslator.class.isAssignableFrom(clazz)) {
				return false;
			}
			// 调用外部重写的方法
			TransactionAttributeSource tas = getTransactionAttributeSource();
			// 判断有没有@Transaction注解
			return (tas == null || tas.isCandidateClass(clazz));
		}
	}
}

/**
 * 源码位置：org.springframework.transaction.annotation.AnnotationTransactionAttributeSource.isCandidateClass(Class<?>)
 * 判断是否能成为候选者
 */
public boolean isCandidateClass(Class<?> targetClass) {
	for (TransactionAnnotationParser parser : this.annotationParsers) {
		if (parser.isCandidateClass(targetClass)) {
			return true;
		}
	}
	return false;
}

/**
 * 源码位置：org.springframework.transaction.interceptor.AbstractFallbackTransactionAttributeSource.getTransactionAttribute(Method, Class<?>)
 * 判断当前类上面是否有@Transactional注解
 */
public boolean isCandidateClass(Class<?> targetClass) {
	return AnnotationUtils.isCandidateClass(targetClass, Transactional.class);
}

/**
 * 源码位置：org.springframework.transaction.annotation.SpringTransactionAnnotationParser.isCandidateClass(Class<?>)
 * 获取方法上或者类上是否有@Transaction注解
 */
public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
	// Object类直接返回null
	if (method.getDeclaringClass() == Object.class) {
		return null;
	}

	// First, see if we have a cached value.
	// 检查缓存里的结果，缓存里存了当前类和方法是否存在Transactional注解
	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.
		// 缓存中没有，直接返回null
		if (cached == NULL_TRANSACTION_ATTRIBUTE) {
			return null;
		}
		else {
			return cached;
		}
	}
	else {
		// We need to work it out.
		// 解析。实际对象为RuleBasedTransactionAttribute
		TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
		// Put it in the cache.
		// 为空，缓存一个空的
		if (txAttr == null) {
			this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
		}
		else {
			String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
			if (txAttr instanceof DefaultTransactionAttribute) {
				DefaultTransactionAttribute dta = (DefaultTransactionAttribute) txAttr;
				dta.setDescriptor(methodIdentification);
				dta.resolveAttributeStrings(this.embeddedValueResolver);
			}
			if (logger.isTraceEnabled()) {
				logger.trace("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
			}
			// 不为空，缓存结果
			this.attributeCache.put(cacheKey, txAttr);
		}
		return txAttr;
	}
}

最终执行的Advisor:TransactionInterceptor源码分析

/**
 * 源码位置：org.springframework.transaction.interceptor.TransactionInterceptor.invoke(MethodInvocation)
 */
public Object invoke(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.
	// 获取代理类
	Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

	// Adapt to TransactionAspectSupport's invokeWithinTransaction...
	return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {
		@Override
		@Nullable
		public Object proceedWithInvocation() throws Throwable {
			// 执行后续的Interceptor，以及被代理的方法
			return invocation.proceed(); // test() sql
		}
		@Override
		public Object getTarget() {
			return invocation.getThis();
		}
		@Override
		public Object[] getArguments() {
			return invocation.getArguments();
		}
	});
}

/**
 * 具体的执行代码
 */
protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
		final InvocationCallback invocation) throws Throwable {

	// If the transaction attribute is null, the method is non-transactional.
	// TransactionAttribute就是@Transactional中的配置
	TransactionAttributeSource tas = getTransactionAttributeSource();
	// 获取@Transactional注解中的属性值
	final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);

	// 返回Spring容器中类型为TransactionManager的Bean对象。事务的开启，提交，回滚都会用到TransactionManager对象。
	final TransactionManager tm = determineTransactionManager(txAttr);

	// ReactiveTransactionManager用得少，并且它只是执行方式是响应式的，原理流程和普通的是一样的
	if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager) {
		boolean isSuspendingFunction = KotlinDetector.isSuspendingFunction(method);
		boolean hasSuspendingFlowReturnType = isSuspendingFunction &&
				COROUTINES_FLOW_CLASS_NAME.equals(new MethodParameter(method, -1).getParameterType().getName());
		if (isSuspendingFunction && !(invocation instanceof CoroutinesInvocationCallback)) {
			throw new IllegalStateException("Coroutines invocation not supported: " + method);
		}
		CoroutinesInvocationCallback corInv = (isSuspendingFunction ? (CoroutinesInvocationCallback) invocation : null);

		ReactiveTransactionSupport txSupport = this.transactionSupportCache.computeIfAbsent(method, key -> {
			Class<?> reactiveType =
					(isSuspendingFunction ? (hasSuspendingFlowReturnType ? Flux.class : Mono.class) : method.getReturnType());
			ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(reactiveType);
			if (adapter == null) {
				throw new IllegalStateException("Cannot apply reactive transaction to non-reactive return type: " +
						method.getReturnType());
			}
			return new ReactiveTransactionSupport(adapter);
		});

		InvocationCallback callback = invocation;
		if (corInv != null) {
			callback = () -> CoroutinesUtils.invokeSuspendingFunction(method, corInv.getTarget(), corInv.getArguments());
		}
		Object result = txSupport.invokeWithinTransaction(method, targetClass, callback, txAttr, (ReactiveTransactionManager) tm);
		if (corInv != null) {
			Publisher<?> pr = (Publisher<?>) result;
			return (hasSuspendingFlowReturnType ? KotlinDelegate.asFlow(pr) :
					KotlinDelegate.awaitSingleOrNull(pr, corInv.getContinuation()));
		}
		return result;
	}

	// 把tm强制转换为PlatformTransactionManager，所以我们在定义时得定义PlatformTransactionManager类型
	PlatformTransactionManager ptm = asPlatformTransactionManager(tm);

	// joinpoint的唯一标识，就是当前在执行的方法名字
	final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

	// CallbackPreferringPlatformTransactionManager表示拥有回调功能的PlatformTransactionManager，也不常用
	if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
		// Standard transaction demarcation with getTransaction and commit/rollback calls.
		// 如果有必要就创建事务，这里就涉及到事务传播机制的实现了
		// TransactionInfo表示一个逻辑事务，比如两个逻辑事务属于同一个物理事务
		TransactionInfo txInfo = createTransactionIfNecessary(ptm, 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.
			// 执行下一个Interceptor或被代理对象中的方法
			retVal = invocation.proceedWithInvocation(); //test
		}
		catch (Throwable ex) {
			// target invocation exception
			// 抛异常了，则回滚事务
			completeTransactionAfterThrowing(txInfo, ex);
			throw ex;
		}
		finally {
			cleanupTransactionInfo(txInfo);
		}

		if (retVal != null && 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 {
		Object result;
		final ThrowableHolder throwableHolder = new ThrowableHolder();

		// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
		try {
			result = ((CallbackPreferringPlatformTransactionManager) ptm).execute(txAttr, status -> {
				TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
				try {
					Object retVal = invocation.proceedWithInvocation();
					if (retVal != null && 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);
				}
			});
		}
		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;
		}

		// Check result state: It might indicate a Throwable to rethrow.
		if (throwableHolder.throwable != null) {
			throw throwableHolder.throwable;
		}
		return result;
	}
}

/**
 * 创建事务的逻辑
 */
protected TransactionInfo createTransactionIfNecessary(@Nullable PlatformTransactionManager tm,
		@Nullable TransactionAttribute txAttr, final String joinpointIdentification) {

	// If no name specified, apply method identification as transaction name.
	// name为空，取方法名字
	if (txAttr != null && txAttr.getName() == null) {
		txAttr = new DelegatingTransactionAttribute(txAttr) {
			@Override
			public String getName() {
				return joinpointIdentification;
			}
		};
	}

	// 每个逻辑事务都会创建一个TransactionStatus，但是TransactionStatus中有一个属性代表当前逻辑事务底层的物理事务是不是新的
	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");
			}
		}
	}

	// 返回一个TransactionInfo对象，表示得到了一个事务，可能是新创建的一个事务，也可能是拿到的已有的事务
	return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}

获取（开启）事务的逻辑

/**
 * 获取事务的逻辑
 * 源码位置：org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(TransactionDefinition)
 * definition：Transaction注解的信息的对象
 */
public final TransactionStatus getTransaction(@Nullable TransactionDefinition definition)
		throws TransactionException {

	// Use defaults if no transaction definition given.
	TransactionDefinition def = (definition != null ? definition : TransactionDefinition.withDefaults());

	// 从ThreadLocal中拿到txObject对象
	Object transaction = doGetTransaction();
	boolean debugEnabled = logger.isDebugEnabled();

	// transaction.getConnectionHolder().isTransactionActive()
	// 判断是不是存在一个事务
	if (isExistingTransaction(transaction)) {
		// Existing transaction found -> check propagation behavior to find out how to behave.
		return handleExistingTransaction(def, transaction, debugEnabled);
	}

	// Check definition settings for new transaction.
	// 配置的数据库返回的超时时间小于-1，抛异常。
	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.
	// 当前不存在事务，并且配置的传播机制为PROPAGATION_MANDATORY（支持当前事务，如果没有事务抛出异常），抛异常
	if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
		throw new IllegalTransactionStateException(
				"No existing transaction found for transaction marked with propagation 'mandatory'");
	}
	// 在当前Thread中没有事务的前提下，以下三个是等价的。
	// PROPAGATION_REQUIRED :支持当前事务，如果没有事务会创建一个新的事务
	// PROPAGATION_REQUIRES_NEW ：创建一个新的事务并挂起当前事务
	// PROPAGATION_NESTED：	如果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则创建一个事务。
	else if (def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
			def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
			def.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
		// 没有事务需要挂起，不过TransactionSynchronization有可能需要挂起。直接调用TransactionSynchronizationManager.initSynchronization方法会有需要被挂起的。
		// suspendedResources表示当前线程被挂起的资源持有对象（数据库连接、TransactionSynchronization）
		SuspendedResourcesHolder suspendedResources = suspend(null);
		if (debugEnabled) {
			logger.debug("Creating new transaction with name [" + def.getName() + "]: " + def);
		}
		try {
			// 开启事务方法
			// 开启事务后，transaction中就会有数据库连接了，并且isTransactionActive为true
			// 并返回TransactionStatus对象，该对象保存了很多信息，包括被挂起的资源
			return startTransaction(def, transaction, debugEnabled, suspendedResources);
		}
		catch (RuntimeException | Error ex) {
			resume(null, suspendedResources);
			throw ex;
		}
	}
	else {
		// Create "empty" transaction: no actual transaction, but potentially synchronization.
		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);
	}
}

/**
 * 开启Transaction事务
 */
private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,
		boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {

	// 是否开启一个新的TransactionSynchronization
	boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);

	// 开启的这个事务的状态信息：
	// 事务的定义、用来保存数据库连接的对象、是否是新事务，是否是新的TransactionSynchronization
	DefaultTransactionStatus status = newTransactionStatus(
			definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);

	// 开启事务
	doBegin(transaction, definition);

	// 如果需要新开一个TransactionSynchronization，就把新创建的事务的一些状态信息设置到TransactionSynchronizationManager中
	prepareSynchronization(status, definition);
	return status;
}

/**
 * 开启事务！DataSource层面
 * 源码位置：org.springframework.jdbc.datasource.DataSourceTransactionManager.doBegin(Object, TransactionDefinition)
 */
protected void doBegin(Object transaction, TransactionDefinition definition) {
	DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
	Connection con = null;

	try {

		// 如果当前线程中所使用的DataSource还没有创建过数据库连接，就获取一个新的数据库连接
		if (!txObject.hasConnectionHolder() ||
				txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
			// 得到连接对象
			Connection newCon = obtainDataSource().getConnection();
			if (logger.isDebugEnabled()) {
				logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
			}
			// 设置到DataSourceTransactionObject。注意这里设置的true，表示txObject的链接一个新的
			txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
		}

		txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
		// 得到连接对象
		con = txObject.getConnectionHolder().getConnection();

		// 根据@Transactional注解中的设置，设置Connection的readOnly与隔离级别
		Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
		txObject.setPreviousIsolationLevel(previousIsolationLevel);
		txObject.setReadOnly(definition.isReadOnly());

		// 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).
		// 保证autocommit是false。autocommit为true的时候设置autocommit为false
		if (con.getAutoCommit()) {
			txObject.setMustRestoreAutoCommit(true);
			if (logger.isDebugEnabled()) {
				logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
			}
			con.setAutoCommit(false);
		}

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

		// 设置数据库连接的过期时间
		int timeout = determineTimeout(definition);
		if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
			txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
		}

		// Bind the connection holder to the thread.
		// 把新建的数据库连接设置到resources中，resources就是一个ThreadLocal<Map<Object, Object>>，事务管理器中的设置的DataSource对象为key，数据库连接对象为value
		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);
	}
}

挂起相关流程

/**
 * 挂起相关流程
 * 源码位置：org.springframework.transaction.support.AbstractPlatformTransactionManager.suspend(Object)
 */
protected final SuspendedResourcesHolder suspend(@Nullable Object transaction) throws TransactionException {
	// synchronizations是一个ThreadLocal<Set<TransactionSynchronization>>
	// 我们可以在任何地方通过TransactionSynchronizationManager给当前线程添加TransactionSynchronization，

	// 这里判断有没有开启事务。在prepareSynchronization方法中开启！
	if (TransactionSynchronizationManager.isSynchronizationActive()) {
		// 调用TransactionSynchronization的suspend方法，并清空和返回当前线程中所有的TransactionSynchronization对象
		List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization();
		try {
			Object suspendedResources = null;
			if (transaction != null) {
				// 挂起事务，把transaction中的Connection清空，并把resources中的key-value进行移除，并返回数据库连接Connection对象
				suspendedResources = doSuspend(transaction);
			}

			// 获取并清空当前线程中关于TransactionSynchronizationManager的设置
			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);

			// 将当前线程中的数据库连接对象、TransactionSynchronization对象、TransactionSynchronizationManager中的设置构造成一个对象
			// 表示被挂起的资源持有对象，持有了当前线程中的事务对象、TransactionSynchronization对象
			// suspendedResources数据库连接、suspendedSynchronizations自己定义的同步器、name事务的名称、readOnly事务是不是只读、isolationLevel事务隔离级别、wasActive切面的Active
			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;
	}
}

/**
 * 调用TransactionSynchronization的suspend方法，并清空和返回当前线程中所有的TransactionSynchronization对象
 */
private List<TransactionSynchronization> doSuspendSynchronization() {
	// 从synchronizations（一个ThreadLocal）中拿到所设置的TransactionSynchronization对象
	List<TransactionSynchronization> suspendedSynchronizations =
			TransactionSynchronizationManager.getSynchronizations();

	// 调用TransactionSynchronization对象的suspend()
	for (TransactionSynchronization synchronization : suspendedSynchronizations) {
		synchronization.suspend();
	}

	// 清空synchronizations
	TransactionSynchronizationManager.clearSynchronization();

	// 把获取到的TransactionSynchronization返回
	return suspendedSynchronizations;
}

之前存在事务的执行逻辑

/**
 * 之前存在事务的执行逻辑
 * 源码位置：org.springframework.transaction.support.AbstractPlatformTransactionManager.handleExistingTransaction(TransactionDefinition, Object, boolean)
 */
private TransactionStatus handleExistingTransaction(
		TransactionDefinition definition, Object transaction, boolean debugEnabled)
		throws TransactionException {

	// PROPAGATION_NEVER：以非事务方式进行，如果存在事务则抛出异常
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
		throw new IllegalTransactionStateException(
				"Existing transaction found for transaction marked with propagation 'never'");
	}

	// PROPAGATION_NOT_SUPPORTED：以非事务方式执行，如果当前存在事务则将当前事务挂起
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
		if (debugEnabled) {
			logger.debug("Suspending current transaction");
		}
		// 把当前事务挂起，其中就会把数据库连接对象从ThreadLocal中移除
		Object suspendedResources = suspend(transaction);
		boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
		return prepareTransactionStatus(
				definition, null, false, newSynchronization, debugEnabled, suspendedResources);
	}

	// PROPAGATION_REQUIRES_NEW:创建一个新的事务并挂起当前事务
	if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
		if (debugEnabled) {
			logger.debug("Suspending current transaction, creating new transaction with name [" +
					definition.getName() + "]");
		}
		// 调用挂起的逻辑
		SuspendedResourcesHolder suspendedResources = suspend(transaction);

		// 开启新事务的逻辑
		try {
			return startTransaction(definition, transaction, debugEnabled, suspendedResources);
		}
		catch (RuntimeException | Error beginEx) {
			resumeAfterBeginException(transaction, suspendedResources, beginEx);
			throw beginEx;
		}
	}

	// PROPAGATION_NESTED：果当前存在事务，则在嵌套事务内执行。如果当前没有事务，则创建一个事务。
	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() + "]");
		}
		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);
			// 创建一个savepoint
			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.
			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");
			}
		}
	}

	// 如果依然是Propagation.REQUIRED
	boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
	return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}

事务的提交逻辑

/**
 * 事务的提交
 */
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());
	}
}

/**
 * 事务提交的准备逻辑
 */
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;

	// 可以通过TransactionAspectSupport.currentTransactionStatus().setRollbackOnly();来设置
	// 事务本来是可以要提交的，但是可以强制回滚。比如报错后更有好的提示。
	if (defStatus.isLocalRollbackOnly()) {
		if (defStatus.isDebug()) {
			logger.debug("Transactional code has requested rollback");
		}
		processRollback(defStatus, false);
		return;
	}

	// 判断此事务在之前是否设置了需要回滚，跟globalRollbackOnParticipationFailure有关
	if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
		if (defStatus.isDebug()) {
			logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
		}
		processRollback(defStatus, true);
		return;
	}

	// 提交
	processCommit(defStatus);
}

/**
 * 提交外部逻辑
 */
private void processCommit(DefaultTransactionStatus status) throws TransactionException {
	try {
		boolean beforeCompletionInvoked = false;

		try {
			boolean unexpectedRollback = false;

			// 空方法，无任何子类的实现
			prepareForCommit(status);
			// 调用同步器提交前的逻辑（回滚的时候不调用这个）
			triggerBeforeCommit(status);
			// 调用同步器完成前的逻辑
			triggerBeforeCompletion(status);
			beforeCompletionInvoked = true;

			if (status.hasSavepoint()) {
				if (status.isDebug()) {
					logger.debug("Releasing transaction savepoint");
				}
				unexpectedRollback = status.isGlobalRollbackOnly();
				status.releaseHeldSavepoint();
			}
			// 新的事务，直接调用提交
			else if (status.isNewTransaction()) {
				if (status.isDebug()) {
					logger.debug("Initiating transaction commit");
				}
				unexpectedRollback = status.isGlobalRollbackOnly();
				// 调用提交方法
				doCommit(status);
			}
			else if (isFailEarlyOnGlobalRollbackOnly()) {
				unexpectedRollback = status.isGlobalRollbackOnly();
			}

			// Throw UnexpectedRollbackException if we have a global rollback-only
			// marker but still didn't get a corresponding exception from commit.
			if (unexpectedRollback) {
				throw new UnexpectedRollbackException(
						"Transaction silently rolled back because it has been marked as rollback-only");
			}
		}
		catch (UnexpectedRollbackException ex) {
			// can only be caused by doCommit
			triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
			throw ex;
		}
		catch (TransactionException ex) {
			// can only be caused by doCommit
			if (isRollbackOnCommitFailure()) {
				doRollbackOnCommitException(status, ex);
			}
			else {
				triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
			}
			throw ex;
		}
		catch (RuntimeException | Error ex) {
			if (!beforeCompletionInvoked) {
				triggerBeforeCompletion(status);
			}
			doRollbackOnCommitException(status, ex);
			throw ex;
		}

		// Trigger afterCommit callbacks, with an exception thrown there
		// propagated to callers but the transaction still considered as committed.
		try {
			// 调用同步器提交后的逻辑
			triggerAfterCommit(status);
		}
		finally {
			// 调用同步器完成后的逻辑
			triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
		}

	}
	finally {
		// 恢复被挂起的资源到当前线程中
		cleanupAfterCompletion(status);
	}
}

/**
 * 提交的核心逻辑，直接调用Connection的提交方法
 */
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 {
		con.commit();
	}
	catch (SQLException ex) {
		throw translateException("JDBC commit", ex);
	}
}

/**
 * 恢复被挂起的资源到当前线程中
 */
private void cleanupAfterCompletion(DefaultTransactionStatus status) {
	status.setCompleted();
	if (status.isNewSynchronization()) {
		TransactionSynchronizationManager.clear();
	}
	// 判断当前事务执行的方法，是不是创建这个事务的方法
	if (status.isNewTransaction()) {
		// 这里会去关闭数据库连接
		doCleanupAfterCompletion(status.getTransaction());
	}

	// 恢复被挂起的资源到当前线程中
	if (status.getSuspendedResources() != null) {
		if (status.isDebug()) {
			logger.debug("Resuming suspended transaction after completion of inner transaction");
		}
		Object transaction = (status.hasTransaction() ? status.getTransaction() : null);
		// 恢复
		resume(transaction, (SuspendedResourcesHolder) status.getSuspendedResources());
	}
}

回滚事务的逻辑

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);
		}

		// transactionAttribute的实现类为RuleBasedTransactionAttribute，父类为DefaultTransactionAttribute
		// 判断配置的rollBackFor的异常信息
		if (txInfo.transactionAttribute != null && txInfo.transactionAttribute.rollbackOn(ex)) {
			try {
				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;
			}
		}
	}
}

/**
 * 判断回滚条件是否满足
 * 源码位置：org.springframework.transaction.interceptor.RuleBasedTransactionAttribute.rollbackOn(Throwable)
 */
public boolean rollbackOn(Throwable ex) {
	RollbackRuleAttribute winner = null;
	int deepest = Integer.MAX_VALUE;

	if (this.rollbackRules != null) {
		// 遍历所有的RollbackRuleAttribute，判断现在抛出的异常ex是否匹配RollbackRuleAttribute中指定的异常类型的子类或本身
		for (RollbackRuleAttribute rule : this.rollbackRules) {
			int depth = rule.getDepth(ex);
			if (depth >= 0 && depth < deepest) {
				deepest = depth;
				winner = rule;
			}
		}
	}

	// User superclass behavior (rollback on unchecked) if no rule matches.
	// 没有匹配的规则，调用父类判断是不是运行时异常
	if (winner == null) {
		return super.rollbackOn(ex);
	}

	// ex所匹配的RollbackRuleAttribute，可能是NoRollbackRuleAttribute，如果是匹配的NoRollbackRuleAttribute，那就表示现在这个异常ex不用回滚
	return !(winner instanceof NoRollbackRuleAttribute);
}

/**
 * 回滚前的准备逻辑
 * 源码位置：org.springframework.transaction.support.AbstractPlatformTransactionManager.rollback(TransactionStatus)
 */
public final void rollback(TransactionStatus status) throws TransactionException {
	// 不完整的，没有执行完抛异常
	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);
}

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

		try {
			// 只会触发完成前的同步器逻辑
			triggerBeforeCompletion(status);

			// 比如mysql中的savepoint
			if (status.hasSavepoint()) {
				if (status.isDebug()) {
					logger.debug("Rolling back transaction to savepoint");
				}
				// 回滚到上一个savepoint位置
				status.rollbackToHeldSavepoint();
			}
			else if (status.isNewTransaction()) {
				if (status.isDebug()) {
					logger.debug("Initiating transaction rollback");
				}
				// 如果当前执行的方法是新开了一个事务，那么就直接回滚
				doRollback(status);
			}
			else {
				// Participating in larger transaction
				// 如果当前执行的方法，是公用了一个已存在的事务，而当前执行的方法抛了异常，则要判断整个事务到底要不要回滚，看具体配置
				if (status.hasTransaction()) {

					// 如果一个事务中有两个方法，第二个方法抛异常了，那么第二个方法就相当于执行失败需要回滚，如果globalRollbackOnParticipationFailure为true，那么第一个方法在没有抛异常的情况下也要回滚
					if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
						if (status.isDebug()) {
							logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
						}
						// 直接将rollbackOnly设置到ConnectionHolder中去，表示整个事务的sql都要回滚
						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;
		}

		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 {
		cleanupAfterCompletion(status);
	}
}

结束语

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