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声明式事务（Transactional）的工作原理_声明式事物 原理

作者：羊村懒王 | 2024-05-02 19:31:25

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声明式事物 原理

声明式事务（Transactional）的工作原理

Spring Boot通过起步依赖和自动配置简化了开发环境的配置，节约了开发成本。但是作为一名技术人员，如果不了解一下底层的实现原理，未免太过肤浅。

声明式事物我们一直在用，但是除了@Transactional注解的使用方式，具体底层是如何支撑的，实现原理是什么你真的知道吗？

1. 前置条件（pre-condition）

要实现Transaction的自动配置，需要执行以下自动配置路径
DataSourceAutoConfiguration -> DataSourceTransactionManagerAutoConfiguration -> TransactionAutoConfiguration

DataSourceAutoConfiguration可以配置数据源、数据库连接池等信息。

DataSourceTransactionManagerAutoConfiguration配置了数据源事物管理器（DataSourceTransactionManager），DataSourceTransactionManager的类图如下，它实现了PlatformTransactionManager接口。
在这里插入图片描述

2. Transaction自动配置（TransactionAutoConfiguration）

Transaction自动配置实现了一下功能：

  1. 个性化PlatformTransactionManager，如上述的DataSourceTransactionManager
  2. 基于PlatformTransactionManager创建TransactionTemplate
  3. 通过AOP代理实现事务管理，默认配置下，SpringBoot默认启用的是Cglib事务代理。

org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration.EnableTransactionManagementConfiguration

@Configuration
@ConditionalOnClass(PlatformTransactionManager.class)
@AutoConfigureAfter({ JtaAutoConfiguration.class, HibernateJpaAutoConfiguration.class,
		DataSourceTransactionManagerAutoConfiguration.class,
		Neo4jDataAutoConfiguration.class })
@EnableConfigurationProperties(TransactionProperties.class)
public class TransactionAutoConfiguration {

	@Bean
	@ConditionalOnMissingBean
	public TransactionManagerCustomizers platformTransactionManagerCustomizers(
			ObjectProvider<PlatformTransactionManagerCustomizer<?>> customizers) {
		return new TransactionManagerCustomizers(
				customizers.orderedStream().collect(Collectors.toList()));
	}

	@Configuration
	@ConditionalOnSingleCandidate(PlatformTransactionManager.class)
	public static class TransactionTemplateConfiguration {

		private final PlatformTransactionManager transactionManager;

		public TransactionTemplateConfiguration(
				PlatformTransactionManager transactionManager) {
			this.transactionManager = transactionManager;
		}

		@Bean
		@ConditionalOnMissingBean
		public TransactionTemplate transactionTemplate() {
			return new TransactionTemplate(this.transactionManager);
		}

	}

	@Configuration
	@ConditionalOnBean(PlatformTransactionManager.class)
	@ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
	public static class EnableTransactionManagementConfiguration {

		@Configuration
		@EnableTransactionManagement(proxyTargetClass = false)
		@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
				havingValue = "false", matchIfMissing = false)
		public static class JdkDynamicAutoProxyConfiguration {

		}

		@Configuration
		@EnableTransactionManagement(proxyTargetClass = true)
		@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class",
				havingValue = "true", matchIfMissing = true)
		public static class CglibAutoProxyConfiguration {

		}
	}
}

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通过AOP代理实现事务管理通过注解@EnableTransactionManagement实现。
首先，它通过Import注解引入了TransactionManagementConfigurationSelector，它是一个ImportSelector实现。
在代理模式下他会返回 AutoProxyRegistrar 和 ProxyTransactionManagementConfiguration。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

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public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {
	/**
	 * Returns {@link ProxyTransactionManagementConfiguration} or
	 * {@code AspectJ(Jta)TransactionManagementConfiguration} for {@code PROXY}
	 * and {@code ASPECTJ} values of {@link EnableTransactionManagement#mode()},
	 * respectively.
	 */
	@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;
		}
	}

	private String determineTransactionAspectClass() {
		return (ClassUtils.isPresent("javax.transaction.Transactional", getClass().getClassLoader()) ?
				TransactionManagementConfigUtils.JTA_TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME :
				TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME);
	}
}

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2.1 AutoProxyRegistrar -> InfrastructureAdvisorAutoProxyCreator

这是一个ImportBeanDefinitionRegistrar类，它引入了一个类型为InfrastructureAdvisorAutoProxyCreator的BeanDefinition，InfrastructureAdvisorAutoProxyCreator是BeanPostProcessor 的实现类，通过它可以在Bean实例化前后创建AOP代理类。

2.1.1 postProcessBeforeInstantiation

如果指定了个性化TargetSource，会在实例化之前创建代理类。
org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#postProcessBeforeInstantiation

@Override
public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) {
	Object cacheKey = getCacheKey(beanClass, beanName);

	if (!StringUtils.hasLength(beanName) || !this.targetSourcedBeans.contains(beanName)) {
		if (this.advisedBeans.containsKey(cacheKey)) {
			return null;
		}
		if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {
			this.advisedBeans.put(cacheKey, Boolean.FALSE);
			return null;
		}
	}

	// Create proxy here if we have a custom TargetSource.
	// Suppresses unnecessary default instantiation of the target bean:
	// The TargetSource will handle target instances in a custom fashion.
	TargetSource targetSource = getCustomTargetSource(beanClass, beanName);
	if (targetSource != null) {
		if (StringUtils.hasLength(beanName)) {
			this.targetSourcedBeans.add(beanName);
		}
		Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);
		Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);
		this.proxyTypes.put(cacheKey, proxy.getClass());
		return proxy;
	}

	return null;
}

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2.1.2 postProcessAfterInitialization

所有的Bean实例化后都会执行postProcessAfterInitialization方法；如果需要则创建代理类。

/**
 * Create a proxy with the configured interceptors if the bean is
 * identified as one to proxy by the subclass.
 * @see #getAdvicesAndAdvisorsForBean
 */
@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
	if (bean != null) {
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (this.earlyProxyReferences.remove(cacheKey) != bean) {
			return wrapIfNecessary(bean, beanName, cacheKey);
		}
	}
	return bean;
}

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判断是否需要创建代理类的逻辑：
具体代码可参考org.springframework.aop.support.AopUtils#canApply(org.springframework.aop.Pointcut, java.lang.Class<?>, boolean)

  1. 获取所有的类型为 Advisor 的Bean Definition 列表，并获取 Bean 实例；
  2. 如果 Advisor 是 IntroductionAdvisor 的子类，通过 ClassFilter 匹配是否需要代理类，如果匹配返回 TRUE；
  3. 如果 Advisor 是 PointcutAdvisor 的子类，通过 ClassFilter 进行匹配，如果不匹配直接返回 FALSE；
  4. 获取目标类以及其所有接口类，校验所有的方法是否满足 MethodMatcher 匹配，只要有满足匹配的方法返回 TRUE;

支持事务的Advisor类为：BeanFactoryTransactionAttributeSourceAdvisor，它是PointcutAdvisor的子类，其注入的切点类为
org.springframework.transaction.interceptor.TransactionAttributeSourcePointcut。

创建代理类的逻辑：新建ProxyFactory对象，设置属性，通过getProxy方法获取代理类。
org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator#createProxy

protected Object createProxy(Class<?> beanClass, @Nullable String beanName,
	@Nullable Object[] specificInterceptors, TargetSource targetSource) {

	if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
		AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
	}

	ProxyFactory proxyFactory = new ProxyFactory();
	proxyFactory.copyFrom(this);

	if (!proxyFactory.isProxyTargetClass()) {
		if (shouldProxyTargetClass(beanClass, beanName)) {
			proxyFactory.setProxyTargetClass(true);
		}
		else {
			evaluateProxyInterfaces(beanClass, proxyFactory);
		}
	}

	Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
	proxyFactory.addAdvisors(advisors);
	proxyFactory.setTargetSource(targetSource);
	customizeProxyFactory(proxyFactory);

	proxyFactory.setFrozen(this.freezeProxy);
	if (advisorsPreFiltered()) {
		proxyFactory.setPreFiltered(true);
	}

	return proxyFactory.getProxy(getProxyClassLoader());
}

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创建AopProxy，这里最终创建的是ObjenesisCglibAopProxy，然后调用AopProxy的getProxy方法获取代理类。

@Override
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
	if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
		Class<?> targetClass = config.getTargetClass();
		if (targetClass == null) {
			throw new AopConfigException("TargetSource cannot determine target class: " +
					"Either an interface or a target is required for proxy creation.");
		}
		if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
			return new JdkDynamicAopProxy(config);
		}
		return new ObjenesisCglibAopProxy(config);
	}
	else {
		return new JdkDynamicAopProxy(config);
	}
}

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2.2 注册Advisor -> ProxyTransactionManagementConfiguration

该Configuration类注册了一个类型为BeanFactoryTransactionAttributeSourceAdvisor的BeanDefinition。

该Advisor的主要作用是为所有基于@Transactional注解的方法创建AOP代理，以便增加事务控制，事务控制通过TransactionInterceptor来实现。

@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource());
		advisor.setAdvice(transactionInterceptor());
		if (this.enableTx != null) {
			advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		}
		return advisor;
	}

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionAttributeSource transactionAttributeSource() {
		return new AnnotationTransactionAttributeSource();
	}

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionInterceptor transactionInterceptor() {
		TransactionInterceptor interceptor = new TransactionInterceptor();
		interceptor.setTransactionAttributeSource(transactionAttributeSource());
		if (this.txManager != null) {
			interceptor.setTransactionManager(this.txManager);
		}
		return interceptor;
	}
}

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3. 代理类执行事务的逻辑 - TransactionInterceptor

org.springframework.transaction.interceptor.TransactionAspectSupport#invokeWithinTransaction

@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.
	TransactionAttributeSource tas = getTransactionAttributeSource();
	final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
	final PlatformTransactionManager tm = determineTransactionManager(txAttr);
	final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

	if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
		// Standard transaction demarcation with getTransaction and commit/rollback calls.
		TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);

		Object retVal;
		try {
			// This is an around advice: Invoke the next interceptor in the chain.
			// This will normally result in a target object being invoked.
			retVal = invocation.proceedWithInvocation();
		}
		catch (Throwable ex) {
			// target invocation exception
			completeTransactionAfterThrowing(txInfo, ex);
			throw ex;
		}
		finally {
			cleanupTransactionInfo(txInfo);
		}
		commitTransactionAfterReturning(txInfo);
		return retVal;
	}

	else {
		final ThrowableHolder throwableHolder = new ThrowableHolder();

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

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4. 事务同步管理

Spring开始事务后会将连接信息保存到ThreadLocal内以备后续处理使用，以此来保证事务的一致性。整个过程通过类TransactionSynchronizationManager来实现。

org.springframework.jdbc.datasource.DataSourceTransactionManager#doBegin

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

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

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

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

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

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

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

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

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

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参考

spring源码阅读–@Transactional实现原理
深入理解Spring系列之十二：@Transactional是如何工作的

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