spring是如何实现声明式事务的

前言

今天我们来讲讲spring的声明式事务。

开始

说到声明式事务，我们现在回顾一下事务这个概念，什么是事务呢，事务指的是逻辑上的⼀组操作，组成这组操作的各个单元，要么全部成功，要么全部不成功。从而确保了数据的准确与安全。事务有着四大特性（ACID），分别是原子性（Atomicity）原⼦性是指事务是⼀个不可分割的⼯作单位，事务中的操作要么都发⽣，要么都不发⽣。

⼀致性（Consistency）事务必须使数据库从⼀个⼀致性状态变换到另外⼀个⼀致性状态。

隔离性（Isolation）事务的隔离性是多个⽤户并发访问数据库时，数据库为每⼀个⽤户开启的事务，每个事务不能被其他事务的操作数据所⼲扰，多个并发事务之间要相互隔离。

持久性（Durability） 持久性是指⼀个事务⼀旦被提交，它对数据库中数据的改变就是永久性的，接下来即使数据库发⽣故障

也不应该对其有任何影响。

在spring中，一共有两种方式可以实现事务控制，分别是编程式事务和声明式事务。编程式事务指的是在代码中添加事务控制代码，而声明式事务指的是利用xml或者注解的形式来配置控制事务，下面就以纯注解配置声明式事务为例进行剖析。

spring开启声明式事务的注解是@EnableTransactionManagement，讲到这里首先要明白一点，spring的事务管理器管理事务其实就是利用aop的方式，通过创建动态代理加上拦截，实现的事务管理。在spring的配置类中加上这个注解，就支持了声明式事务，那么spring是怎么通过这么一个注解就可以支持事务的呢，我们来看代码。首先我们看到，在这个注解上，import了一个selector

@Import(TransactionManagementConfigurationSelector.class)

我们看这个selector类中的这么一段代码

@Override protected String[] selectImports(AdviceMode adviceMode) { switch (adviceMode) { case PROXY: return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()}; case ASPECTJ: return new String[] {determineTransactionAspectClass()}; default: return null; } }

这段代码中，引入了AutoProxyRegistrar和ProxyTransactionManagementConfiguration这两个类，我们先来看AutoProxyRegistrar这个类，这个类中有一段这样的代码

if (mode == AdviceMode.PROXY) { //重要的是这句代码 AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry); if ((Boolean) proxyTargetClass) { AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry); return; }}//我们进到这个方法中@Nullable public static BeanDefinition registerAutoProxyCreatorIfNecessary( BeanDefinitionRegistry registry, @Nullable Object source) {//可以看到引入了InfrastructureAdvisorAutoProxyCreator这个类，那么这个类又是什么呢 return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source); }//先看一下public class InfrastructureAdvisorAutoProxyCreator extends AbstractAdvisorAutoProxyCreator { @Nullable private ConfigurableListableBeanFactory beanFactory; @Override protected void initBeanFactory(ConfigurableListableBeanFactory beanFactory) { super.initBeanFactory(beanFactory); this.beanFactory = beanFactory; } @Override protected boolean isEligibleAdvisorBean(String beanName) { return (this.beanFactory != null && this.beanFactory.containsBeanDefinition(beanName) && this.beanFactory.getBeanDefinition(beanName).getRole() == BeanDefinition.ROLE_INFRASTRUCTURE); }}

看一下继承结构图

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可以看到这个方法间接继承于SmartInstantiationAwareBeanPostProcessor，最终继承于BeanPostProcessor，这说明InfrastructureAdvisorAutoProxyCreator类是一个后置处理器，并且跟 spring AOP 开启@EnableAspectJAutoProxy 时注册的AnnotationAwareAspectJProxyCreator实现的是同⼀个接口，这也对应了我之前所说声明式事务是springAOP思想的一种应用。

然后我们回过头来再看ProxyTransactionManagementConfiguration这个类，我们看到其中有一个事务增强器，一个属性解析器和是一个事务拦截器

@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME) @Role(BeanDefinition.ROLE_INFRASTRUCTURE) public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor( TransactionAttributeSource transactionAttributeSource, TransactionInterceptor transactionInterceptor) { // 事务增强器 BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor(); // 注入属性解析器 advisor.setTransactionAttributeSource(transactionAttributeSource); // 注入事务拦截器 advisor.setAdvice(transactionInterceptor); if (this.enableTx != null) { advisor.setOrder(this.enableTx.<Integer>getNumber('order')); } return advisor; } @Bean @Role(BeanDefinition.ROLE_INFRASTRUCTURE) // 属性解析器 public TransactionAttributeSource transactionAttributeSource() { return new AnnotationTransactionAttributeSource(); } @Bean @Role(BeanDefinition.ROLE_INFRASTRUCTURE) // 事务拦截器 public TransactionInterceptor transactionInterceptor(TransactionAttributeSource transactionAttributeSource) { TransactionInterceptor interceptor = new TransactionInterceptor(); interceptor.setTransactionAttributeSource(transactionAttributeSource); if (this.txManager != null) { interceptor.setTransactionManager(this.txManager); } return interceptor; }

先看看属性解析器

//注解解析器集合 private final Set<TransactionAnnotationParser> annotationParsers;

这是一个注解解析器的集合，可以添加多种注解解析器，在这里我们主要关注的是spring事务注解解析器SpringTransactionParser，看一下相关代码

protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) { RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute(); // 对应Transaction注解的相关属性 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')); List<RollbackRuleAttribute> rollbackRules = new ArrayList<>(); for (Class<?> rbRule : attributes.getClassArray('rollbackFor')) { rollbackRules.add(new RollbackRuleAttribute(rbRule)); } for (String rbRule : attributes.getStringArray('rollbackForClassName')) { rollbackRules.add(new RollbackRuleAttribute(rbRule)); } for (Class<?> rbRule : attributes.getClassArray('noRollbackFor')) { rollbackRules.add(new NoRollbackRuleAttribute(rbRule)); } for (String rbRule : attributes.getStringArray('noRollbackForClassName')) { rollbackRules.add(new NoRollbackRuleAttribute(rbRule)); } rbta.setRollbackRules(rollbackRules); return rbta; }

可以看到这段代码中的Enum和ClassArray其实正是@Transaction注解中的相关属性，这个属性解析器的作用之一就是用来解析@Transaction注解中的属性看完了属性解析器，我们接下来看事务拦截器TransactionInterceptor，其中重要的是这段代码

@Override @Nullable 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. Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null); // Adapt to TransactionAspectSupport’s invokeWithinTransaction... // 增加事务支持 return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed); }

然后我们进到这个方法里面

@Nullable protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass, final InvocationCallback invocation) throws Throwable { // If the transaction attribute is null, the method is non-transactional. // 获取属性解析器，在配置类ProxyTransactionManagementConfiguration配置时加入 TransactionAttributeSource tas = getTransactionAttributeSource(); final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null); final TransactionManager tm = determineTransactionManager(txAttr); if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager) { ReactiveTransactionSupport txSupport = this.transactionSupportCache.computeIfAbsent(method, key -> { if (KotlinDetector.isKotlinType(method.getDeclaringClass()) && KotlinDelegate.isSuspend(method)) { throw new TransactionUsageException( 'Unsupported annotated transaction on suspending function detected: ' + method + '. Use TransactionalOperator.transactional extensions instead.'); } ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(method.getReturnType()); if (adapter == null) { throw new IllegalStateException('Cannot apply reactive transaction to non-reactive return type: ' + method.getReturnType()); } return new ReactiveTransactionSupport(adapter); }); return txSupport.invokeWithinTransaction( method, targetClass, invocation, txAttr, (ReactiveTransactionManager) tm); } // 获取事务管理器 PlatformTransactionManager ptm = asPlatformTransactionManager(tm); final String joinpointIdentification = methodIdentification(method, targetClass, txAttr); if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) { // Standard transaction demarcation with getTransaction and commit/rollback calls. 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. retVal = invocation.proceedWithInvocation(); } catch (Throwable ex) { // target invocation exception // 目标方法抛异常，会执行回滚的操作 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); } } // 目标方法正常运行，会执行commitTransactionAfterReturning，执行事务提交操作 commitTransactionAfterReturning(txInfo); return retVal; } else { final ThrowableHolder throwableHolder = new ThrowableHolder(); // It’s a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in. try { Object result = ((CallbackPreferringPlatformTransactionManager) ptm).execute(txAttr, status -> { TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status); try { 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; } } }总结

总体来说，spring实现声明式事务的过程是这样的

@EnableTransactionManagement 注解，通过@import引⼊了TransactionManagementConfigurationSelector类，它的selectImports⽅法导⼊了另外两个类：AutoProxyRegistrar和ProxyTransactionManagementConfiguration AutoProxyRegistrar类中方法registerBeanDefinitions中，通过 AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry)引⼊InfrastructureAdvisorAutoProxyCreator，是一个后置处理器类 ProxyTransactionManagementConfiguration 是⼀个添加了@Configuration注解的配置类，注册了事务增强器（注⼊属性解析器、事务拦截器）AnnotationTransactionAttributeSource和TransactionInterceptor，AnnotationTransactionAttributeSource内部持有了⼀个解析器集合 Set annotationParsers，具体使⽤的是SpringTransactionAnnotationParser解析器，用来解析@Transactional的事务属性，事务拦截器TransactionInterceptor实现了MethodInterceptor接⼝，该通用拦截会在产⽣代理对象之前和aop增强合并，最终⼀起影响到代理对象，TransactionInterceptor的invoke⽅法中invokeWithinTransaction会触发原有业务逻辑调用（增强事务）

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