spring事务详解（三）源码详解
16lz
2021-04-14
系列目录
spring事务详解（一）初探事务
spring事务详解（二）简单样例
spring事务详解（三）源码详解
spring事务详解（四）测试验证
spring事务详解（五）总结提高
一、引子
在Spring中，事务有两种实现方式：
- 编程式事务管理： 编程式事务管理使用TransactionTemplate可实现更细粒度的事务控制。
- 申明式事务管理： 基于Spring AOP实现。其本质是对方法前后进行拦截，然后在目标方法开始之前创建或者加入一个事务，在执行完目标方法之后根据执行情况提交或者回滚事务。
申明式事务管理不需要***代码，通过@Transactional就可以进行事务操作，更快捷而且简单（尤其是配合spring boot自动配置，可以说是精简至极！），且大部分业务都可以满足，推荐使用。
其实不管是编程式事务还是申明式事务，最终调用的底层核心代码是一致的。本章分别从编程式、申明式入手，再进入核心源码贯穿式讲解。
二、事务源码
2.1 编程式事务TransactionTemplate
编程式事务，Spring已经给我们提供好了模板类TransactionTemplate，可以很方便的使用，如下图：
TransactionTemplate全路径名是：org.springframework.transaction.support.TransactionTemplate。看包名也知道了这是spring对事务的模板类。（spring动不动就是各种Template...），看下类图先：
一看，哟西，实现了TransactionOperations、InitializingBean这2个接口（熟悉spring源码的知道这个InitializingBean又是老套路），我们来看下接口源码如下：
1 public interface TransactionOperations { 2 3 /** 4 * Execute the action specified by the given callback object within a transaction. 5 * <p>Allows for returning a result object created within the transaction, that is, 6 * a domain object or a collection of domain objects. A RuntimeException thrown 7 * by the callback is treated as a fatal exception that enforces a rollback. 8 * Such an exception gets propagated to the caller of the template. 9 * @param action the callback object that specifies the transactional action 10 * @return a result object returned by the callback, or {@code null} if none 11 * @throws TransactionException in case of initialization, rollback, or system errors 12 * @throws RuntimeException if thrown by the TransactionCallback 13 */ 14 <T> T execute(TransactionCallback<T> action) throws TransactionException; 15 16 } 17 18 public interface InitializingBean { 19 20 /** 21 * Invoked by a BeanFactory after it has set all bean properties supplied 22 * (and satisfied BeanFactoryAware and ApplicationContextAware). 23 * <p>This method allows the bean instance to perform initialization only 24 * possible when all bean properties have been set and to throw an 25 * exception in the event of misconfiguration. 26 * @throws Exception in the event of misconfiguration (such 27 * as failure to set an essential property) or if initialization fails. 28 */ 29 void afterPropertiesSet() throws Exception; 30 31 }如上图，TransactionOperations这个接口用来执行事务的回调方法，InitializingBean这个是典型的spring bean初始化流程中（飞机票：Spring IOC（四）总结升华篇）的预留接口，专用用来在bean属性加载完毕时执行的方法。
回到正题，TransactionTemplate的2个接口的impl方法做了什么？
1 @Override 2 public void afterPropertiesSet() { 3 if (this.transactionManager == null) { 4 throw new IllegalArgumentException("Property 'transactionManager' is required"); 5 } 6 } 7 8 9 @Override 10 public <T> T execute(TransactionCallback<T> action) throws TransactionException { // 内部封装好的事务管理器 11 if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager) { 12 return ((CallbackPreferringPlatformTransactionManager) this.transactionManager).execute(this, action); 13 }// 需要手动获取事务，执行方法，提交事务的管理器 14 else {// 1.获取事务状态 15 TransactionStatus status = this.transactionManager.getTransaction(this); 16 T result; 17 try {// 2.执行业务逻辑 18 result = action.doInTransaction(status); 19 } 20 catch (RuntimeException ex) { 21 // 应用运行时异常 -> 回滚 22 rollbackOnException(status, ex); 23 throw ex; 24 } 25 catch (Error err) { 26 // Error异常 -> 回滚 27 rollbackOnException(status, err); 28 throw err; 29 } 30 catch (Throwable ex) { 31 // 未知异常 -> 回滚 32 rollbackOnException(status, ex); 33 throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception"); 34 }// 3.事务提交 35 this.transactionManager.commit(status); 36 return result; 37 } 38 }如上图所示，实际上afterPropertiesSet只是校验了事务管理器不为空，execute()才是核心方法，execute主要步骤：
1.getTransaction()获取事务，源码见3.3.1
2.doInTransaction()执行业务逻辑，这里就是用户自定义的业务代码。如果是没有返回值的，就是doInTransactionWithoutResult()。
3.commit()事务提交：调用AbstractPlatformTransactionManager的commit，rollbackOnException()异常回滚：调用AbstractPlatformTransactionManager的rollback()，事务提交回滚，源码见3.3.3
2.2 申明式事务@Transactional
1.AOP相关概念
申明式事务使用的是spring AOP，即面向切面编程。（什么❓你不知道什么是AOP...一句话概括就是：把业务代码中重复代码做成一个切面，提取出来，并定义哪些方法需要执行这个切面。其它的自行百度吧...）AOP核心概念如下：
- 通知（Advice）:定义了切面(各处业务代码中都需要的逻辑提炼成的一个切面)做什么what+when何时使用。例如：前置通知Before、后置通知After、返回通知After-returning、异常通知After-throwing、环绕通知Around.
- 连接点（Joint point）：程序执行过程中能够插入切面的点，一般有多个。比如调用方式时、抛出异常时。
- 切点（Pointcut）:切点定义了连接点，切点包含多个连接点,即where哪里使用通知.通常指定类+方法 或者 正则表达式来匹配 类和方法名称。
- 切面（Aspect）:切面=通知+切点，即when+where+what何时何地做什么。
- 引入（Introduction）:允许我们向现有的类添加新方法或属性。
- 织入（Weaving）:织入是把切面应用到目标对象并创建新的代理对象的过程。
2.申明式事务
申明式事务整体调用过程，可以抽出2条线：
1.使用代理模式，生成代理增强类。
2.根据代理事务管理配置类，配置事务的织入，在业务方法前后进行环绕增强，增加一些事务的相关操作。例如获取事务属性、提交事务、回滚事务。
过程如下图：
申明式事务使用@Transactional这种注解的方式，那么我们就从springboot 容器启动时的自动配置载入（spring boot容器启动详解）开始看。在/META-INF/spring.factories中配置文件中查找，如下图：
载入2个关于事务的自动配置类：
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,
jta咱们就不看了，看一下TransactionAutoConfiguration这个自动配置类：
1 @Configuration 2 @ConditionalOnClass(PlatformTransactionManager.class) 3 @AutoConfigureAfter({ JtaAutoConfiguration.class, HibernateJpaAutoConfiguration.class, 4 DataSourceTransactionManagerAutoConfiguration.class, 5 Neo4jDataAutoConfiguration.class }) 6 @EnableConfigurationProperties(TransactionProperties.class) 7 public class TransactionAutoConfiguration { 8 9 @Bean10 @ConditionalOnMissingBean11 public TransactionManagerCustomizers platformTransactionManagerCustomizers(12 ObjectProvider<List<PlatformTransactionManagerCustomizer<?>>> customizers) {13 return new TransactionManagerCustomizers(customizers.getIfAvailable());14 }15 16 @Configuration17 @ConditionalOnSingleCandidate(PlatformTransactionManager.class)18 public static class TransactionTemplateConfiguration {19 20 private final PlatformTransactionManager transactionManager;21 22 public TransactionTemplateConfiguration(23 PlatformTransactionManager transactionManager) {24 this.transactionManager = transactionManager;25 }26 27 @Bean28 @ConditionalOnMissingBean29 public TransactionTemplate transactionTemplate() {30 return new TransactionTemplate(this.transactionManager);31 }32 }33 34 @Configuration35 @ConditionalOnBean(PlatformTransactionManager.class)36 @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)37 public static class EnableTransactionManagementConfiguration {38 39 @Configuration40 @EnableTransactionManagement(proxyTargetClass = false)41 @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)42 public static class JdkDynamicAutoProxyConfiguration {43 44 }45 46 @Configuration47 @EnableTransactionManagement(proxyTargetClass = true)48 @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)49 public static class CglibAutoProxyConfiguration {50 51 }52 53 }54 55 }TransactionAutoConfiguration这个类主要看：
1.2个类注解
@ConditionalOnClass(PlatformTransactionManager.class)即类路径下包含PlatformTransactionManager这个类时这个自动配置生效，这个类是spring事务的核心包，肯定引入了。
@AutoConfigureAfter({ JtaAutoConfiguration.class, HibernateJpaAutoConfiguration.class, DataSourceTransactionManagerAutoConfiguration.class, Neo4jDataAutoConfiguration.class })，这个配置在括号中的4个配置类后才生效。
2. 2个内部类
TransactionTemplateConfiguration事务模板配置类：
@ConditionalOnSingleCandidate(PlatformTransactionManager.class)当能够唯一确定一个PlatformTransactionManager bean时才生效。
@ConditionalOnMissingBean如果没有定义TransactionTemplate bean生成一个。
EnableTransactionManagementConfiguration开启事务管理器配置类：
@ConditionalOnBean(PlatformTransactionManager.class)当存在PlatformTransactionManager bean时生效。
@ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)当没有自定义抽象事务管理器配置类时才生效。（即用户自定义抽象事务管理器配置类会优先，如果没有，就用这个默认事务管理器配置类）
EnableTransactionManagementConfiguration支持2种代理方式：
- 1.JdkDynamicAutoProxyConfiguration：
@EnableTransactionManagement(proxyTargetClass = false)，即proxyTargetClass = false表示是JDK动态代理支持的是：面向接口代理。
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "false", matchIfMissing = false)，即spring.aop.proxy-target-class=false时生效，且没有这个配置不生效。
- 2.CglibAutoProxyConfiguration：
@EnableTransactionManagement(proxyTargetClass = true)，即proxyTargetClass = true标识Cglib代理支持的是子类继承代理。
@ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)，即spring.aop.proxy-target-class=true时生效，且没有这个配置默认生效。
注意了，默认没有配置，走的Cglib代理。说明@Transactional注解支持直接加在类上。
好吧，看了这么多配置类，终于到了@EnableTransactionManagement这个注解了。
1 @Target(ElementType.TYPE) 2 @Retention(RetentionPolicy.RUNTIME) 3 @Documented 4 @Import(TransactionManagementConfigurationSelector.class) 5 public @interface EnableTransactionManagement { 6 7 //proxyTargetClass = false表示是JDK动态代理支持接口代理。true表示是Cglib代理支持子类继承代理。 8 boolean proxyTargetClass() default false; 9 10 //事务通知模式(切面织入方式)，默认代理模式（同一个类中方法互相调用拦截器不会生效），可以选择增强型AspectJ11 AdviceMode mode() default AdviceMode.PROXY;12 13 //连接点上有多个通知时，排序，默认最低。值越大优先级越低。14 int order() default Ordered.LOWEST_PRECEDENCE;15 16 }重点看类注解@Import(TransactionManagementConfigurationSelector.class)
TransactionManagementConfigurationSelector类图如下：
如上图所示，TransactionManagementConfigurationSelector继承自AdviceModeImportSelector实现了ImportSelector接口。
1 public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> { 2 3 /** 4 * {@inheritDoc} 5 * @return {@link ProxyTransactionManagementConfiguration} or 6 * {@code AspectJTransactionManagementConfiguration} for {@code PROXY} and 7 * {@code ASPECTJ} values of {@link EnableTransactionManagement#mode()}, respectively 8 */ 9 @Override10 protected String[] selectImports(AdviceMode adviceMode) {11 switch (adviceMode) {12 case PROXY:13 return new String[] {AutoProxyRegistrar.class.getName(), ProxyTransactionManagementConfiguration.class.getName()};14 case ASPECTJ:15 return new String[] {TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME};16 default:17 return null;18 }19 }20 21 }如上图，最终会执行selectImports方法导入需要加载的类，我们只看proxy模式下，载入了AutoProxyRegistrar、ProxyTransactionManagementConfiguration2个类。
- AutoProxyRegistrar：给容器中注册一个 InfrastructureAdvisorAutoProxyCreator 组件；利用后置处理器机制在对象创建以后，包装对象，返回一个代理对象（增强器），代理对象执行方法利用拦截器链进行调用；
- ProxyTransactionManagementConfiguration：就是一个配置类，定义了事务增强器。
AutoProxyRegistrar
先看AutoProxyRegistrar实现了ImportBeanDefinitionRegistrar接口，复写registerBeanDefinitions方法，源码如下：
1 public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { 2 boolean candidateFound = false; 3 Set<String> annoTypes = importingClassMetadata.getAnnotationTypes(); 4 for (String annoType : annoTypes) { 5 AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType); 6 if (candidate == null) { 7 continue; 8 } 9 Object mode = candidate.get("mode");10 Object proxyTargetClass = candidate.get("proxyTargetClass");11 if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&12 Boolean.class == proxyTargetClass.getClass()) {13 candidateFound = true;14 if (mode == AdviceMode.PROXY) {//代理模式15 AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);16 if ((Boolean) proxyTargetClass) {//如果是CGLOB子类代理模式17 AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);18 return;19 }20 }21 }22 }23 if (!candidateFound) {24 String name = getClass().getSimpleName();25 logger.warn(String.format("%s was imported but no annotations were found " +26 "having both 'mode' and 'proxyTargetClass' attributes of type " +27 "AdviceMode and boolean respectively. This means that auto proxy " +28 "creator registration and configuration may not have occurred as " +29 "intended, and components may not be proxied as expected. Check to " +30 "ensure that %s has been @Import'ed on the same class where these " +31 "annotations are declared; otherwise remove the import of %s " +32 "altogether.", name, name, name));33 }34 }代理模式：AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
最终调用的是：registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);基础构建增强自动代理构造器
1 private static BeanDefinition registerOrEscalateApcAsRequired(Class<?> cls, BeanDefinitionRegistry registry, Object source) { 2 Assert.notNull(registry, "BeanDefinitionRegistry must not be null"); //如果当前注册器包含internalAutoProxyCreator 3 if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {//org.springframework.aop.config.internalAutoProxyCreator内部自动代理构造器 4 BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME); 5 if (!cls.getName().equals(apcDefinition.getBeanClassName())) {//如果当前类不是internalAutoProxyCreator 6 int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName()); 7 int requiredPriority = findPriorityForClass(cls); 8 if (currentPriority < requiredPriority) {//如果下标大于已存在的内部自动代理构造器，index越小，优先级越高,InfrastructureAdvisorAutoProxyCreator index=0,requiredPriority最小，不进入 9 apcDefinition.setBeanClassName(cls.getName());10 }11 }12 return null;//直接返回13 }//如果当前注册器不包含internalAutoProxyCreator，则把当前类作为根定义14 RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);15 beanDefinition.setSource(source);16 beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);//优先级最高17 beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);18 registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);19 return beanDefinition;20 }如上图，APC_PRIORITY_LIST列表如下图：
1 /** 2 * Stores the auto proxy creator classes in escalation order. 3 */ 4 private static final List<Class<?>> APC_PRIORITY_LIST = new ArrayList<Class<?>>(); 5 6 /** 7 * 优先级上升list 8 */ 9 static {10 APC_PRIORITY_LIST.add(InfrastructureAdvisorAutoProxyCreator.class);11 APC_PRIORITY_LIST.add(AspectJAwareAdvisorAutoProxyCreator.class);12 APC_PRIORITY_LIST.add(AnnotationAwareAspectJAutoProxyCreator.class);13 }如上图，由于InfrastructureAdvisorAutoProxyCreator这个类在list中第一个index=0,requiredPriority最小，不进入，所以没有重置beanClassName，啥都没做，返回null.
那么增强代理类何时生成呢？
InfrastructureAdvisorAutoProxyCreator类图如下：
如上图所示，看2个核心方法：InstantiationAwareBeanPostProcessor接口的postProcessBeforeInstantiation实例化前+BeanPostProcessor接口的postProcessAfterInitialization初始化后。关于spring bean生命周期飞机票：Spring IOC（四）总结升华篇
1 @Override 2 public Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException { 3 Object cacheKey = getCacheKey(beanClass, beanName); 4 5 if (beanName == null || !this.targetSourcedBeans.contains(beanName)) { 6 if (this.advisedBeans.containsKey(cacheKey)) {//如果已经存在直接返回 7 return null; 8 }//是否基础构件（基础构建不需要代理）：Advice、Pointcut、Advisor、AopInfrastructureBean这四类都算基础构建 9 if (isInfrastructureClass(beanClass) || shouldSkip(beanClass, beanName)) {10 this.advisedBeans.put(cacheKey, Boolean.FALSE);//添加进advisedBeans ConcurrentHashMap<k=Object,v=Boolean>标记是否需要增强实现，这里基础构建bean不需要代理，都置为false，供后面postProcessAfterInitialization实例化后使用。11 return null;12 }13 }14 15 // TargetSource是spring aop预留给我们用户自定义实例化的接口，如果存在TargetSource就不会默认实例化，而是按照用户自定义的方式实例化，咱们没有定义，不进入18 if (beanName != null) {19 TargetSource targetSource = getCustomTargetSource(beanClass, beanName);20 if (targetSource != null) {21 this.targetSourcedBeans.add(beanName);22 Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(beanClass, beanName, targetSource);23 Object proxy = createProxy(beanClass, beanName, specificInterceptors, targetSource);24 this.proxyTypes.put(cacheKey, proxy.getClass());25 return proxy;26 }27 }28 29 return null;30 }通过追踪，由于InfrastructureAdvisorAutoProxyCreator是基础构建类，
advisedBeans.put(cacheKey, Boolean.FALSE)
添加进advisedBeans ConcurrentHashMap<k=Object,v=Boolean>标记是否需要增强实现，这里基础构建bean不需要代理，都置为false，供后面postProcessAfterInitialization实例化后使用。
我们再看postProcessAfterInitialization源码如下：
1 @Override 2 public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { 3 if (bean != null) { 4 Object cacheKey = getCacheKey(bean.getClass(), beanName); 5 if (!this.earlyProxyReferences.contains(cacheKey)) { 6 return wrapIfNecessary(bean, beanName, cacheKey); 7 } 8 } 9 return bean;10 }11 12 protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) { // 如果是用户自定义获取实例，不需要增强处理，直接返回13 if (beanName != null && this.targetSourcedBeans.contains(beanName)) {14 return bean;15 }// 查询map缓存，标记过false,不需要增强直接返回16 if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {17 return bean;18 }// 判断一遍springAOP基础构建类，标记过false,不需要增强直接返回19 if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {20 this.advisedBeans.put(cacheKey, Boolean.FALSE);21 return bean;22 }23 24 // 获取增强List<Advisor> advisors25 Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null); // 如果存在增强26 if (specificInterceptors != DO_NOT_PROXY) {27 this.advisedBeans.put(cacheKey, Boolean.TRUE);// 标记增强为TRUE,表示需要增强实现 // 生成增强代理类28 Object proxy = createProxy(29 bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));30 this.proxyTypes.put(cacheKey, proxy.getClass());31 return proxy;32 }33 // 如果不存在增强，标记false,作为缓存，再次进入提高效率，第16行利用缓存先校验34 this.advisedBeans.put(cacheKey, Boolean.FALSE);35 return bean;36 }下面看核心方法createProxy如下：
1 protected Object createProxy( 2 Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) { 3 // 如果是ConfigurableListableBeanFactory接口（咱们DefaultListableBeanFactory就是该接口的实现类）则，暴露目标类 4 if (this.beanFactory instanceof ConfigurableListableBeanFactory) { //给beanFactory->beanDefinition定义一个属性：k=AutoProxyUtils.originalTargetClass,v=需要被代理的bean class 5 AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass); 6 } 7 8 ProxyFactory proxyFactory = new ProxyFactory(); 9 proxyFactory.copyFrom(this);10 //如果不是代理目标类11 if (!proxyFactory.isProxyTargetClass()) {//如果beanFactory定义了代理目标类（CGLIB）12 if (shouldProxyTargetClass(beanClass, beanName)) {13 proxyFactory.setProxyTargetClass(true);//代理工厂设置代理目标类14 }15 else {//否则设置代理接口（JDK）16 evaluateProxyInterfaces(beanClass, proxyFactory);17 }18 }19 //把拦截器包装成增强（通知）20 Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);21 proxyFactory.addAdvisors(advisors);//设置进代理工厂22 proxyFactory.setTargetSource(targetSource);23 customizeProxyFactory(proxyFactory);//空方法，留给子类拓展用，典型的spring的风格，喜欢处处留后路24 //用于控制代理工厂是否还允许再次添加通知，默认为false（表示不允许）25 proxyFactory.setFrozen(this.freezeProxy);26 if (advisorsPreFiltered()) {//默认false，上面已经前置过滤了匹配的增强Advisor27 proxyFactory.setPreFiltered(true);28 }29 //代理工厂获取代理对象的核心方法30 return proxyFactory.getProxy(getProxyClassLoader());31 }最终我们生成的是CGLIB代理类.到此为止我们分析完了代理类的构造过程。
ProxyTransactionManagementConfiguration
下面来看ProxyTransactionManagementConfiguration：
1 @Configuration 2 public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration { 3 4 @Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME) 5 @Role(BeanDefinition.ROLE_INFRASTRUCTURE)//定义事务增强器 6 public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() { 7 BeanFactoryTransactionAttributeSourceAdvisor j = new BeanFactoryTransactionAttributeSourceAdvisor(); 8 advisor.setTransactionAttributeSource(transactionAttributeSource()); 9 advisor.setAdvice(transactionInterceptor());10 advisor.setOrder(this.enableTx.<Integer>getNumber("order"));11 return advisor;12 }13 14 @Bean15 @Role(BeanDefinition.ROLE_INFRASTRUCTURE)//定义基于注解的事务属性资源16 public TransactionAttributeSource transactionAttributeSource() {17 return new AnnotationTransactionAttributeSource();18 }19 20 @Bean21 @Role(BeanDefinition.ROLE_INFRASTRUCTURE)//定义事务拦截器22 public TransactionInterceptor transactionInterceptor() {23 TransactionInterceptor interceptor = new TransactionInterceptor();24 interceptor.setTransactionAttributeSource(transactionAttributeSource());25 if (this.txManager != null) {26 interceptor.setTransactionManager(this.txManager);27 }28 return interceptor;29 }30 31 }核心方法：transactionAdvisor()事务织入
定义了一个advisor，设置事务属性、设置事务拦截器TransactionInterceptor、设置顺序。核心就是事务拦截器TransactionInterceptor。
TransactionInterceptor使用通用的spring事务基础架构实现“声明式事务”，继承自TransactionAspectSupport类（该类包含与Spring的底层事务API的集成），实现了MethodInterceptor接口。spring类图如下：
事务拦截器的拦截功能就是依靠实现了MethodInterceptor接口，熟悉spring的同学肯定很熟悉MethodInterceptor了，这个是spring的方法拦截器，主要看invoke方法：
1 @Override 2 public Object invoke(final MethodInvocation invocation) throws Throwable { 3 // Work out the target class: may be {@code null}. 4 // The TransactionAttributeSource should be passed the target class 5 // as well as the method, which may be from an interface. 6 Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null); 7 8 // 调用TransactionAspectSupport的 invokeWithinTransaction方法 9 return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {10 @Override11 public Object proceedWithInvocation() throws Throwable {12 return invocation.proceed();13 }14 });15 }如上图TransactionInterceptor复写MethodInterceptor接口的invoke方法，并在invoke方法中调用了父类TransactionAspectSupport的invokeWithinTransaction()方法，源码如下：
1 protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation) 2 throws Throwable { 3 4 // 如果transaction attribute为空,该方法就是非事务（非编程式事务） 5 final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass); 6 final PlatformTransactionManager tm = determineTransactionManager(txAttr); 7 final String joinpointIdentification = methodIdentification(method, targetClass, txAttr); 8 // 标准声明式事务：如果事务属性为空 或者 非回调偏向的事务管理器 9 if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {10 // Standard transaction demarcation with getTransaction and commit/rollback calls.11 TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);12 Object retVal = null;13 try {14 // 这里就是一个环绕增强，在这个proceed前后可以自己定义增强实现15 // 方法执行16 retVal = invocation.proceedWithInvocation();17 }18 catch (Throwable ex) {19 // 根据事务定义的，该异常需要回滚就回滚，否则提交事务20 completeTransactionAfterThrowing(txInfo, ex);21 throw ex;22 }23 finally {//清空当前事务信息，重置为老的24 cleanupTransactionInfo(txInfo);25 }//返回结果之前提交事务26 commitTransactionAfterReturning(txInfo);27 return retVal;28 }29 // 编程式事务：（回调偏向）30 else {31 final ThrowableHolder throwableHolder = new ThrowableHolder();32 33 // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.34 try {35 Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr,36 new TransactionCallback<Object>() {37 @Override38 public Object doInTransaction(TransactionStatus status) {39 TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);40 try {41 return invocation.proceedWithInvocation();42 }43 catch (Throwable ex) {// 如果该异常需要回滚44 if (txAttr.rollbackOn(ex)) {45 // 如果是运行时异常返回46 if (ex instanceof RuntimeException) {47 throw (RuntimeException) ex;48 }// 如果是其它异常都抛ThrowableHolderException49 else {50 throw new ThrowableHolderException(ex);51 }52 }// 如果不需要回滚53 else {54 // 定义异常，最终就直接提交事务了55 throwableHolder.throwable = ex;56 return null;57 }58 }59 finally {//清空当前事务信息，重置为老的60 cleanupTransactionInfo(txInfo);61 }62 }63 });64 65 // 上抛异常66 if (throwableHolder.throwable != null) {67 throw throwableHolder.throwable;68 }69 return result;70 }71 catch (ThrowableHolderException ex) {72 throw ex.getCause();73 }74 catch (TransactionSystemException ex2) {75 if (throwableHolder.throwable != null) {76 logger.error("Application exception overridden by commit exception", throwableHolder.throwable);77 ex2.initApplicationException(throwableHolder.throwable);78 }79 throw ex2;80 }81 catch (Throwable ex2) {82 if (throwableHolder.throwable != null) {83 logger.error("Application exception overridden by commit exception", throwableHolder.throwable);84 }85 throw ex2;86 }87 }88 }如上图，我们主要看第一个分支，申明式事务，核心流程如下：
1.createTransactionIfNecessary():如果有必要，创建事务
2.InvocationCallback的proceedWithInvocation()：InvocationCallback是父类的内部回调接口，子类中实现该接口供父类调用，子类TransactionInterceptor中invocation.proceed()。回调方法执行
3.异常回滚completeTransactionAfterThrowing()
1.createTransactionIfNecessary():
1 protected TransactionInfo createTransactionIfNecessary( 2 PlatformTransactionManager tm, TransactionAttribute txAttr, final String joinpointIdentification) { 3 4 // 如果还没有定义名字，把连接点的ID定义成事务的名称 5 if (txAttr != null && txAttr.getName() == null) { 6 txAttr = new DelegatingTransactionAttribute(txAttr) { 7 @Override 8 public String getName() { 9 return joinpointIdentification;10 }11 };12 }13 14 TransactionStatus status = null;15 if (txAttr != null) {16 if (tm != null) {17 status = tm.getTransaction(txAttr);18 }19 else {20 if (logger.isDebugEnabled()) {21 logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +22 "] because no transaction manager has been configured");23 }24 }25 }26 return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);27 }核心就是：
1）getTransaction()，根据事务属性获取事务TransactionStatus，大道归一，都是调用PlatformTransactionManager.getTransaction()，源码见3.3.1。
2）prepareTransactionInfo(),构造一个TransactionInfo事务信息对象，绑定当前线程：ThreadLocal<TransactionInfo>。
2.invocation.proceed()回调业务方法:
最终实现类是ReflectiveMethodInvocation，类图如下：
如上图，ReflectiveMethodInvocation类实现了ProxyMethodInvocation接口，但是ProxyMethodInvocation继承了3层接口...ProxyMethodInvocation->MethodInvocation->Invocation->Joinpoint
Joinpoint：连接点接口，定义了执行接口：Object proceed() throws Throwable; 执行当前连接点，并跳到拦截器链上的下一个拦截器。
Invocation：调用接口，继承自Joinpoint，定义了获取参数接口： Object[] getArguments();是一个带参数的、可被拦截器拦截的连接点。
MethodInvocation：方法调用接口，继承自Invocation，定义了获取方法接口：Method getMethod(); 是一个带参数的可被拦截的连接点方法。
ProxyMethodInvocation：代理方法调用接口，继承自MethodInvocation，定义了获取代理对象接口：Object getProxy();是一个由代理类执行的方法调用连接点方法。
ReflectiveMethodInvocation：实现了ProxyMethodInvocation接口，自然就实现了父类接口的的所有接口。获取代理类，获取方法，获取参数，用代理类执行这个方法并且自动跳到下一个连接点。
下面看一下proceed方法源码：
1 @Override 2 public Object proceed() throws Throwable { 3 // 启动时索引为-1，唤醒连接点，后续递增 4 if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) { 5 return invokeJoinpoint(); 6 } 7 8 Object interceptorOrInterceptionAdvice = 9 this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);10 if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {11 // 这里进行动态方法匹配校验，静态的方法匹配早已经校验过了（MethodMatcher接口有两种典型：动态/静态校验）13 InterceptorAndDynamicMethodMatcher dm =14 (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;15 if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) {16 return dm.interceptor.invoke(this);17 }18 else {19 // 动态匹配失败，跳过当前拦截，进入下一个（拦截器链）21 return proceed();22 }23 }24 else {25 // 它是一个拦截器，所以我们只调用它:在构造这个对象之前，切入点将被静态地计算。27 return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);28 }29 }咱们这里最终调用的是((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);就是TransactionInterceptor事务拦截器回调 目标业务方法（addUserBalanceAndUser）。
3.completeTransactionAfterThrowing()
最终调用AbstractPlatformTransactionManager的rollback()，提交事务commitTransactionAfterReturning()最终调用AbstractPlatformTransactionManager的commit(),源码见3.3.3
总结：
可见不管是编程式事务，还是声明式事务，最终源码都是调用事务管理器的PlatformTransactionManager接口的3个方法：
- getTransaction
- commit
- rollback
下一节我们就来看看这个事务管理如何实现这3个方法。
三、事务核心源码
咱们看一下核心类图：
如上提所示，PlatformTransactionManager顶级接口定义了最核心的事务管理方法，下面一层是AbstractPlatformTransactionManager抽象类，实现了PlatformTransactionManager接口的方法并定义了一些抽象方法，供子类拓展。最后下面一层是2个经典事务管理器：
1.DataSourceTransactionmanager,即JDBC单数据库事务管理器，基于Connection实现，
2.JtaTransactionManager,即多数据库事务管理器（又叫做分布式事务管理器），其实现了JTA规范，使用XA协议进行两阶段提交。
我们这里只看基于JDBC connection的DataSourceTransactionmanager源码。
PlatformTransactionManager接口：
1 public interface PlatformTransactionManager {2 // 获取事务状态 3 TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException; 4 // 事务提交 5 void commit(TransactionStatus status) throws TransactionException; 6 // 事务回滚 7 void rollback(TransactionStatus status) throws TransactionException; 8 }1. getTransaction获取事务
AbstractPlatformTransactionManager实现了getTransaction()方法如下：
1 @Override 2 public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException { 3 Object transaction = doGetTransaction(); 4 5 // Cache debug flag to avoid repeated checks. 6 boolean debugEnabled = logger.isDebugEnabled(); 7 8 if (definition == null) { 9 // Use defaults if no transaction definition given. 10 definition = new DefaultTransactionDefinition(); 11 } 12 // 如果当前已经存在事务 13 if (isExistingTransaction(transaction)) { 14 // 根据不同传播机制不同处理 15 return handleExistingTransaction(definition, transaction, debugEnabled); 16 } 17 18 // 超时不能小于默认值 19 if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) { 20 throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout()); 21 } 22 23 // 当前不存在事务，传播机制=MANDATORY（支持当前事务，没事务报错），报错 24 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) { 25 throw new IllegalTransactionStateException( 26 "No existing transaction found for transaction marked with propagation 'mandatory'"); 27 }// 当前不存在事务，传播机制=REQUIRED/REQUIRED_NEW/NESTED,这三种情况，需要新开启事务，且加上事务同步 28 else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED || 29 definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW || 30 definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) { 31 SuspendedResourcesHolder suspendedResources = suspend(null); 32 if (debugEnabled) { 33 logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition); 34 } 35 try {// 是否需要新开启同步// 开启// 开启 36 boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER); 37 DefaultTransactionStatus status = newTransactionStatus( 38 definition, transaction, true, newSynchronization, debugEnabled, suspendedResources); 39 doBegin(transaction, definition);// 开启新事务 40 prepareSynchronization(status, definition);//预备同步 41 return status; 42 } 43 catch (RuntimeException ex) { 44 resume(null, suspendedResources); 45 throw ex; 46 } 47 catch (Error err) { 48 resume(null, suspendedResources); 49 throw err; 50 } 51 } 52 else { 53 // 当前不存在事务当前不存在事务，且传播机制=PROPAGATION_SUPPORTS/PROPAGATION_NOT_SUPPORTED/PROPAGATION_NEVER，这三种情况，创建“空”事务:没有实际事务，但可能是同步。警告：定义了隔离级别，但并没有真实的事务初始化，隔离级别被忽略有隔离级别但是并没有定义实际的事务初始化，有隔离级别但是并没有定义实际的事务初始化， 54 if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) { 55 logger.warn("Custom isolation level specified but no actual transaction initiated; " + 56 "isolation level will effectively be ignored: " + definition); 57 } 58 boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS); 59 return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null); 60 } 61 }如上图，源码分成了2条处理线，
1.当前已存在事务：isExistingTransaction()判断是否存在事务，存在事务handleExistingTransaction()根据不同传播机制不同处理
2.当前不存在事务: 不同传播机制不同处理
handleExistingTransaction()源码如下：
1 private TransactionStatus handleExistingTransaction( 2 TransactionDefinition definition, Object transaction, boolean debugEnabled) 3 throws TransactionException { 4 // 1.NERVER（不支持当前事务;如果当前事务存在，抛出异常）报错 5 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) { 6 throw new IllegalTransactionStateException( 7 "Existing transaction found for transaction marked with propagation 'never'"); 8 } 9 // 2.NOT_SUPPORTED（不支持当前事务，现有同步将被挂起）挂起当前事务10 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {11 if (debugEnabled) {12 logger.debug("Suspending current transaction");13 }14 Object suspendedResources = suspend(transaction);15 boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);16 return prepareTransactionStatus(17 definition, null, false, newSynchronization, debugEnabled, suspendedResources);18 }19 // 3.REQUIRES_NEW挂起当前事务，创建新事务20 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {21 if (debugEnabled) {22 logger.debug("Suspending current transaction, creating new transaction with name [" +23 definition.getName() + "]");24 }// 挂起当前事务25 SuspendedResourcesHolder suspendedResources = suspend(transaction);26 try {// 创建新事务27 boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);28 DefaultTransactionStatus status = newTransactionStatus(29 definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);30 doBegin(transaction, definition);31 prepareSynchronization(status, definition);32 return status;33 }34 catch (RuntimeException beginEx) {35 resumeAfterBeginException(transaction, suspendedResources, beginEx);36 throw beginEx;37 }38 catch (Error beginErr) {39 resumeAfterBeginException(transaction, suspendedResources, beginErr);40 throw beginErr;41 }42 }43 // 4.NESTED嵌套事务44 if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {45 if (!isNestedTransactionAllowed()) {46 throw new NestedTransactionNotSupportedException(47 "Transaction manager does not allow nested transactions by default - " +48 "specify 'nestedTransactionAllowed' property with value 'true'");49 }50 if (debugEnabled) {51 logger.debug("Creating nested transaction with name [" + definition.getName() + "]");52 }// 是否支持保存点：非JTA事务走这个分支。AbstractPlatformTransactionManager默认是true，JtaTransactionManager复写了该方法false，DataSourceTransactionmanager没有复写，还是true,53 if (useSavepointForNestedTransaction()) { 54 // Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.55 DefaultTransactionStatus status =56 prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);57 status.createAndHoldSavepoint();// 创建保存点58 return status;59 }60 else {61 // JTA事务走这个分支，创建新事务62 boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);63 DefaultTransactionStatus status = newTransactionStatus(64 definition, transaction, true, newSynchronization, debugEnabled, null);65 doBegin(transaction, definition);66 prepareSynchronization(status, definition);67 return status;68 }69 }70 71 72 if (debugEnabled) {73 logger.debug("Participating in existing transaction");74 }75 if (isValidateExistingTransaction()) {76 if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {77 Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();78 if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {79 Constants isoConstants = DefaultTransactionDefinition.constants;80 throw new IllegalTransactionStateException("Participating transaction with definition [" +81 definition + "] specifies isolation level which is incompatible with existing transaction: " +82 (currentIsolationLevel != null ?83 isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :84 "(unknown)"));85 }86 }87 if (!definition.isReadOnly()) {88 if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {89 throw new IllegalTransactionStateException("Participating transaction with definition [" +90 definition + "] is not marked as read-only but existing transaction is");91 }92 }93 }// 到这里PROPAGATION_SUPPORTS 或 PROPAGATION_REQUIRED或PROPAGATION_MANDATORY，存在事务加入事务即可，prepareTransactionStatus第三个参数就是是否需要新事务。false代表不需要新事物94 boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);95 return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);96 }如上图，当前线程已存在事务情况下，新的不同隔离级别处理情况：
1.NERVER：不支持当前事务;如果当前事务存在，抛出异常:"Existing transaction found for transaction marked with propagation 'never'"
2.NOT_SUPPORTED：不支持当前事务，现有同步将被挂起:suspend()
3.REQUIRES_NEW挂起当前事务，创建新事务:
1)suspend()
2)doBegin()
4.NESTED嵌套事务
1）非JTA事务：createAndHoldSavepoint()创建JDBC3.0保存点，不需要同步
2) JTA事务：开启新事务，doBegin()+prepareSynchronization()需要同步
这里有几个核心方法：挂起当前事务suspend()、开启新事务doBegin()。
suspend()源码如下：
1 protected final SuspendedResourcesHolder suspend(Object transaction) throws TransactionException { 2 if (TransactionSynchronizationManager.isSynchronizationActive()) {// 1.当前存在同步， 3 List<TransactionSynchronization> suspendedSynchronizations = doSuspendSynchronization(); 4 try { 5 Object suspendedResources = null; 6 if (transaction != null) {// 事务不为空，挂起事务 7 suspendedResources = doSuspend(transaction); 8 }// 解除绑定当前事务各种属性：名称、只读、隔离级别、是否是真实的事务. 9 String name = TransactionSynchronizationManager.getCurrentTransactionName();10 TransactionSynchronizationManager.setCurrentTransactionName(null);11 boolean readOnly = TransactionSynchronizationManager.isCurrentTransactionReadOnly();12 TransactionSynchronizationManager.setCurrentTransactionReadOnly(false);13 Integer isolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();14 TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(null);15 boolean wasActive = TransactionSynchronizationManager.isActualTransactionActive();16 TransactionSynchronizationManager.setActualTransactionActive(false);17 return new SuspendedResourcesHolder(18 suspendedResources, suspendedSynchronizations, name, readOnly, isolationLevel, wasActive);19 }20 catch (RuntimeException ex) {21 // doSuspend failed - original transaction is still active...22 doResumeSynchronization(suspendedSynchronizations);23 throw ex;24 }25 catch (Error err) {26 // doSuspend failed - original transaction is still active...27 doResumeSynchronization(suspendedSynchronizations);28 throw err;29 }30 }// 2.没有同步但，事务不为空，挂起事务31 else if (transaction != null) {32 // Transaction active but no synchronization active.33 Object suspendedResources = doSuspend(transaction);34 return new SuspendedResourcesHolder(suspendedResources);35 }// 2.没有同步但，事务为空，什么都不用做36 else {37 // Neither transaction nor synchronization active.38 return null;39 }40 }doSuspend(),挂起事务，AbstractPlatformTransactionManager抽象类doSuspend()会报错：不支持挂起，如果具体事务执行器支持就复写doSuspend()，DataSourceTransactionManager实现如下：
1 @Override2 protected Object doSuspend(Object transaction) {3 DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;4 txObject.setConnectionHolder(null);5 return TransactionSynchronizationManager.unbindResource(this.dataSource);6 }挂起DataSourceTransactionManager事务的核心操作就是：
1.把当前事务的connectionHolder数据库连接持有者清空。
2.当前线程解绑datasource.其实就是ThreadLocal移除对应变量（TransactionSynchronizationManager类中定义的private static final ThreadLocal<Map<Object, Object>> resources = new NamedThreadLocal<Map<Object, Object>>("Transactional resources");）
TransactionSynchronizationManager事务同步管理器，该类维护了多个线程本地变量ThreadLocal，如下图：
1 public abstract class TransactionSynchronizationManager { 2 3 private static final Log logger = LogFactory.getLog(TransactionSynchronizationManager.class); 4 // 事务资源：map<k,v> 两种数据对。1.会话工厂和会话k=SqlsessionFactory v=SqlSessionHolder 2.数据源和连接k=DataSource v=ConnectionHolder 5 private static final ThreadLocal<Map<Object, Object>> resources = 6 new NamedThreadLocal<Map<Object, Object>>("Transactional resources"); 7 // 事务同步 8 private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = 9 new NamedThreadLocal<Set<TransactionSynchronization>>("Transaction synchronizations"); 10 // 当前事务名称 11 private static final ThreadLocal<String> currentTransactionName = 12 new NamedThreadLocal<String>("Current transaction name"); 13 // 当前事务的只读属性 14 private static final ThreadLocal<Boolean> currentTransactionReadOnly = 15 new NamedThreadLocal<Boolean>("Current transaction read-only status"); 16 // 当前事务的隔离级别 17 private static final ThreadLocal<Integer> currentTransactionIsolationLevel = 18 new NamedThreadLocal<Integer>("Current transaction isolation level"); 19 // 是否存在事务 20 private static final ThreadLocal<Boolean> actualTransactionActive = 21 new NamedThreadLocal<Boolean>("Actual transaction active"); 22 。。。 23 }doBegin()源码如下：
1 @Override 2 protected void doBegin(Object transaction, TransactionDefinition definition) { 3 DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction; 4 Connection con = null; 5 6 try {// 如果事务还没有connection或者connection在事务同步状态，重置新的connectionHolder 7 if (!txObject.hasConnectionHolder() || 8 txObject.getConnectionHolder().isSynchronizedWithTransaction()) { 9 Connection newCon = this.dataSource.getConnection();10 if (logger.isDebugEnabled()) {11 logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");12 }// 重置新的connectionHolder13 txObject.setConnectionHolder(new ConnectionHolder(newCon), true);14 }15 //设置新的连接为事务同步中16 txObject.getConnectionHolder().setSynchronizedWithTransaction(true);17 con = txObject.getConnectionHolder().getConnection();18 //conn设置事务隔离级别,只读19 Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);20 txObject.setPreviousIsolationLevel(previousIsolationLevel);//DataSourceTransactionObject设置事务隔离级别21 22 // 如果是自动提交切换到手动提交23 // so we don't want to do it unnecessarily (for example if we've explicitly24 // configured the connection pool to set it already).25 if (con.getAutoCommit()) {26 txObject.setMustRestoreAutoCommit(true);27 if (logger.isDebugEnabled()) {28 logger.debug("Switching JDBC Connection [" + con + "] to manual commit");29 }30 con.setAutoCommit(false);31 }32 // 如果只读，执行sql设置事务只读33 prepareTransactionalConnection(con, definition);34 txObject.getConnectionHolder().setTransactionActive(true);// 设置connection持有者的事务开启状态35 36 int timeout = determineTimeout(definition);37 if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {38 txObject.getConnectionHolder().setTimeoutInSeconds(timeout);// 设置超时秒数39 }40 41 // 绑定connection持有者到当前线程42 if (txObject.isNewConnectionHolder()) {43 TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());44 }45 }46 47 catch (Throwable ex) {48 if (txObject.isNewConnectionHolder()) {49 DataSourceUtils.releaseConnection(con, this.dataSource);50 txObject.setConnectionHolder(null, false);51 }52 throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);53 }54 }如上图，开启新事务的准备工作doBegin()的核心操作就是：
1.DataSourceTransactionObject“数据源事务对象”，设置ConnectionHolder，再给ConnectionHolder设置各种属性：自动提交、超时、事务开启、隔离级别。
2.给当前线程绑定一个线程本地变量，key=DataSource数据源 v=ConnectionHolder数据库连接。
2. commit提交事务
一、讲解源码之前先看一下资源管理类：
SqlSessionSynchronization是SqlSessionUtils的一个内部类，继承自TransactionSynchronizationAdapter抽象类，实现了事务同步接口TransactionSynchronization。
类图如下：
TransactionSynchronization接口定义了事务操作时的对应资源的（JDBC事务那么就是SqlSessionSynchronization）管理方法：
1 // 挂起事务 2 void suspend(); 3 // 唤醒事务 4 void resume(); 5 6 void flush(); 7 8 // 提交事务前 9 void beforeCommit(boolean readOnly);10 11 // 提交事务完成前12 void beforeCompletion();13 14 // 提交事务后15 void afterCommit();16 17 // 提交事务完成后18 void afterCompletion(int status);
后续很多都是使用这些接口管理事务。
二、 commit提交事务
AbstractPlatformTransactionManager的commit源码如下：
1 @Override 2 public final void commit(TransactionStatus status) throws TransactionException { 3 if (status.isCompleted()) {// 如果事务已完结，报错无法再次提交 4 throw new IllegalTransactionStateException( 5 "Transaction is already completed - do not call commit or rollback more than once per transaction"); 6 } 7 8 DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status; 9 if (defStatus.isLocalRollbackOnly()) {// 如果事务明确标记为回滚，10 if (defStatus.isDebug()) {11 logger.debug("Transactional code has requested rollback");12 }13 proce***ollback(defStatus);//执行回滚14 return;15 }//如果不需要全局回滚时提交 且 全局回滚16 if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {17 if (defStatus.isDebug()) {18 logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");19 }//执行回滚20 proce***ollback(defStatus);21 // 仅在最外层事务边界（新事务）或显式地请求时抛出“未期望的回滚异常”23 if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {24 throw new UnexpectedRollbackException(25 "Transaction rolled back because it has been marked as rollback-only");26 }27 return;28 }29 // 执行提交事务30 processCommit(defStatus);31 }如上图，各种判断：
- 1.如果事务明确标记为本地回滚，-》执行回滚
- 2.如果不需要全局回滚时提交 且 全局回滚-》执行回滚
- 3.提交事务，核心方法processCommit()
processCommit如下：
1 private void processCommit(DefaultTransactionStatus status) throws TransactionException { 2 try { 3 boolean beforeCompletionInvoked = false; 4 try {//3个前置操作 5 prepareForCommit(status); 6 triggerBeforeCommit(status); 7 triggerBeforeCompletion(status); 8 beforeCompletionInvoked = true;//3个前置操作已调用 9 boolean globalRollbackOnly = false;//新事务 或 全局回滚失败10 if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {11 globalRollbackOnly = status.isGlobalRollbackOnly();12 }//1.有保存点，即嵌套事务13 if (status.hasSavepoint()) {14 if (status.isDebug()) {15 logger.debug("Releasing transaction savepoint");16 }//释放保存点17 status.releaseHeldSavepoint();18 }//2.新事务19 else if (status.isNewTransaction()) {20 if (status.isDebug()) {21 logger.debug("Initiating transaction commit");22 }//调用事务处理器提交事务23 doCommit(status);24 }25 // 3.非新事务，且全局回滚失败，但是提交时没有得到异常，抛出异常27 if (globalRollbackOnly) {28 throw new UnexpectedRollbackException(29 "Transaction silently rolled back because it has been marked as rollback-only");30 }31 }32 catch (UnexpectedRollbackException ex) {33 // 触发完成后事务同步，状态为回滚34 triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);35 throw ex;36 }// 事务异常37 catch (TransactionException ex) {38 // 提交失败回滚39 if (isRollbackOnCommitFailure()) {40 doRollbackOnCommitException(status, ex);41 }// 触发完成后回调，事务同步状态为未知42 else {43 triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);44 }45 throw ex;46 }// 运行时异常47 catch (RuntimeException ex) { // 如果3个前置步骤未完成，调用前置的最后一步操作48 if (!beforeCompletionInvoked) {49 triggerBeforeCompletion(status);50 }// 提交异常回滚51 doRollbackOnCommitException(status, ex);52 throw ex;53 }// 其它异常54 catch (Error err) { // 如果3个前置步骤未完成，调用前置的最后一步操作55 if (!beforeCompletionInvoked) {56 triggerBeforeCompletion(status);57 }// 提交异常回滚58 doRollbackOnCommitException(status, err);59 throw err;60 }61 62 // Trigger afterCommit callbacks, with an exception thrown there63 // propagated to callers but the transaction still considered as committed.64 try {65 triggerAfterCommit(status);66 }67 finally {68 triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);69 }70 71 }72 finally {73 cleanupAfterCompletion(status);74 }75 }如上图，commit事务时，有6个核心操作，分别是3个前置操作，3个后置操作，如下：
1.prepareForCommit(status);源码是空的，没有拓展目前。
2.triggerBeforeCommit(status); 提交前触发操作
1 protected final void triggerBeforeCommit(DefaultTransactionStatus status) {2 if (status.isNewSynchronization()) {3 if (status.isDebug()) {4 logger.trace("Triggering beforeCommit synchronization");5 }6 TransactionSynchronizationUtils.triggerBeforeCommit(status.isReadOnly());7 }8 }triggerBeforeCommit源码如下：
1 public static void triggerBeforeCommit(boolean readOnly) {2 for (TransactionSynchronization synchronization : TransactionSynchronizationManager.getSynchronizations()) {3 synchronization.beforeCommit(readOnly);4 }5 }如上图，TransactionSynchronizationManager类定义了多个ThreadLocal（线程本地变量），其中一个用以保存当前线程的事务同步：
private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations = new NamedThreadLocal<Set<TransactionSynchronization>>("Transaction synchronizations");遍历事务同步器，把每个事务同步器都执行“提交前”操作，比如咱们用的jdbc事务，那么最终就是SqlSessionUtils.beforeCommit()->this.holder.getSqlSession().commit();提交会话。
3.triggerBeforeCompletion(status);完成前触发操作，如果是jdbc事务，那么最终就是
SqlSessionUtils.beforeCompletion->
TransactionSynchronizationManager.unbindResource(sessionFactory); 解绑当前线程的会话工厂
this.holder.getSqlSession().close();关闭会话。
4.triggerAfterCommit(status);提交事务后触发操作。TransactionSynchronizationUtils.triggerAfterCommit();->TransactionSynchronizationUtils.invokeAfterCommit，如下：
1 public static void invokeAfterCommit(List<TransactionSynchronization> synchronizations) {2 if (synchronizations != null) {3 for (TransactionSynchronization synchronization : synchronizations) {4 synchronization.afterCommit();5 }6 }7 }好吧，一顿找，最后在TransactionSynchronizationAdapter中复写过，并且是空的....SqlSessionSynchronization继承了TransactionSynchronizationAdapter但是没有复写这个方法。
5. triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
TransactionSynchronizationUtils.TransactionSynchronizationUtils.invokeAfterCompletion,如下：
1 public static void invokeAfterCompletion(List<TransactionSynchronization> synchronizations, int completionStatus) { 2 if (synchronizations != null) { 3 for (TransactionSynchronization synchronization : synchronizations) { 4 try { 5 synchronization.afterCompletion(completionStatus); 6 } 7 catch (Throwable tsex) { 8 logger.error("TransactionSynchronization.afterCompletion threw exception", tsex); 9 }10 }11 }12 }afterCompletion：对于JDBC事务来说，最终：
1）如果会话任然活着，关闭会话，
2）重置各种属性：SQL会话同步器（SqlSessionSynchronization）的SQL会话持有者（SqlSessionHolder）的referenceCount引用计数、synchronizedWithTransaction同步事务、rollbackOnly只回滚、deadline超时时间点。
6.cleanupAfterCompletion(status);
1）设置事务状态为已完成。
2) 如果是新的事务同步，解绑当前线程绑定的数据库资源，重置数据库连接
3）如果存在挂起的事务（嵌套事务），唤醒挂起的老事务的各种资源：数据库资源、同步器。
1 private void cleanupAfterCompletion(DefaultTransactionStatus status) { 2 status.setCompleted();//设置事务状态完成 //如果是新的同步，清空当前线程绑定的除了资源外的全部线程本地变量：包括事务同步器、事务名称、只读属性、隔离级别、真实的事务激活状态 3 if (status.isNewSynchronization()) { 4 TransactionSynchronizationManager.clear(); 5 }//如果是新的事务同步 6 if (status.isNewTransaction()) { 7 doCleanupAfterCompletion(status.getTransaction()); 8 }//如果存在挂起的资源 9 if (status.getSuspendedResources() != null) {10 if (status.isDebug()) {11 logger.debug("Resuming suspended transaction after completion of inner transaction");12 }//唤醒挂起的事务和资源（重新绑定之前挂起的数据库资源，唤醒同步器，注册同步器到TransactionSynchronizationManager）13 resume(status.getTransaction(), (SuspendedResourcesHolder) status.getSuspendedResources());14 }15 }对于DataSourceTransactionManager，doCleanupAfterCompletion源码如下：
1 protected void doCleanupAfterCompletion(Object transaction) { 2 DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction; 3 4 // 如果是最新的连接持有者，解绑当前线程绑定的<数据库资源，ConnectionHolder> 5 if (txObject.isNewConnectionHolder()) { 6 TransactionSynchronizationManager.unbindResource(this.dataSource); 7 } 8 9 // 重置数据库连接（隔离级别、只读）10 Connection con = txObject.getConnectionHolder().getConnection();11 try {12 if (txObject.isMustRestoreAutoCommit()) {13 con.setAutoCommit(true);14 }15 DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());16 }17 catch (Throwable ex) {18 logger.debug("Could not reset JDBC Connection after transaction", ex);19 }20 21 if (txObject.isNewConnectionHolder()) {22 if (logger.isDebugEnabled()) {23 logger.debug("Releasing JDBC Connection [" + con + "] after transaction");24 }// 资源引用计数-1，关闭数据库连接25 DataSourceUtils.releaseConnection(con, this.dataSource);26 }27 // 重置连接持有者的全部属性28 txObject.getConnectionHolder().clear();29 }
3. rollback回滚事务
AbstractPlatformTransactionManager中rollback源码如下：
1 public final void rollback(TransactionStatus status) throws TransactionException {2 if (status.isCompleted()) {3 throw new IllegalTransactionStateException(4 "Transaction is already completed - do not call commit or rollback more than once per transaction");5 }6 7 DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;8 proce***ollback(defStatus);9 }proce***ollback源码如下：
1 private void proce***ollback(DefaultTransactionStatus status) { 2 try { 3 try {// 解绑当前线程绑定的会话工厂，并关闭会话 4 triggerBeforeCompletion(status); 5 if (status.hasSavepoint()) {// 1.如果有保存点，即嵌套式事务 6 if (status.isDebug()) { 7 logger.debug("Rolling back transaction to savepoint"); 8 }//回滚到保存点 9 status.rollbackToHeldSavepoint();10 }//2.如果就是一个简单事务11 else if (status.isNewTransaction()) {12 if (status.isDebug()) {13 logger.debug("Initiating transaction rollback");14 }//回滚核心方法15 doRollback(status);16 }//3.当前存在事务且没有保存点，即加入当前事务的17 else if (status.hasTransaction()) {//如果已经标记为回滚 或 当加入事务失败时全局回滚（默认true）18 if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {19 if (status.isDebug()) {//debug时会打印：加入事务失败-标记已存在事务为回滚20 logger.debug("Participating transaction failed - marking existing transaction as rollback-only");21 }//设置当前connectionHolder：当加入一个已存在事务时回滚22 doSetRollbackOnly(status);23 }24 else {25 if (status.isDebug()) {26 logger.debug("Participating transaction failed - letting transaction originator decide on rollback");27 }28 }29 }30 else {31 logger.debug("Should roll back transaction but cannot - no transaction available");32 }33 }34 catch (RuntimeException ex) {//关闭会话，重置SqlSessionHolder属性35 triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);36 throw ex;37 }38 catch (Error err) {39 triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);40 throw err;41 }42 triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);43 }44 finally {、、解绑当前线程45 cleanupAfterCompletion(status);46 }47 }如上图，有几个公共方法和提交事务时一致，就不再重复。
这里主要看doRollback，DataSourceTransactionManager的doRollback()源码如下：
1 protected void doRollback(DefaultTransactionStatus status) { 2 DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction(); 3 Connection con = txObject.getConnectionHolder().getConnection(); 4 if (status.isDebug()) { 5 logger.debug("Rolling back JDBC transaction on Connection [" + con + "]"); 6 } 7 try { 8 con.rollback(); 9 }10 catch (SQLException ex) {11 throw new TransactionSystemException("Could not roll back JDBC transaction", ex);12 }13 }好吧，一点不复杂，就是Connection的rollback.
四、时序图
特地整理了时序图（简单的新事务，没有画出保存点等情况）如下：
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