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

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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