【spring】事务底层的实现流程

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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