1.故事前因

在分析mybatis源码时一直带的疑问，一直以为事务是在SqlSessionTemplate#SqlSessionInterceptor#invoke完成的，直到断点才发现并不简单！

  • 在没有事务注解时连接数据库都是SimpleExecutor#prepareStatement中getConnection连接的，这里何来的事务一说，下一步都是直接查询了
  • 然后在有事务注解的时候，经过断点发现这个连接早就创建好了，那么是何时创建的？事务又是在哪里开启的呢？
    今天来一探究竟！！！

2.首先再回顾一下aop原理

  • 大家都知道首先向容器注册一个事务管理类，同时扫描所有的@Transactional注解并生成代理类。然后再执行加了事务的代码是会先走动态代理的代码。

2.1 首先找到bean初始化之后的回调AbstractAutowireCapableBeanFactory

@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor processor : getBeanPostProcessors()) {//将原始类进行代理包装Object current = processor.postProcessAfterInitialization(result, beanName);if (current == null) {return result;}result = current;}return result;
}

2.2 来到AOP处理类AbstractAutoProxyCreator#wrapIfNecessary

  • wrapIfNecessary中找到这段代码
// Create proxy if we have advice. 如果有advice就创建代理
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
if (specificInterceptors != DO_NOT_PROXY) {this.advisedBeans.put(cacheKey, Boolean.TRUE);//满足添加就创建代理Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));this.proxyTypes.put(cacheKey, proxy.getClass());return proxy;
}//getAdvicesAndAdvisorsForBean -> findEligibleAdvisors -> findCandidateAdvisors
//-> BeanFactoryAdvisorRetrievalHelper#findAdvisorBeans
public List<Advisor> findAdvisorBeans() {// Determine list of advisor bean names, if not cached already.String[] advisorNames = this.cachedAdvisorBeanNames;if (advisorNames == null) {// Do not initialize FactoryBeans here: We need to leave all regular beans// uninitialized to let the auto-proxy creator apply to them!//找到Advisor实现类的beanName数组advisorNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(this.beanFactory, Advisor.class, true, false);this.cachedAdvisorBeanNames = advisorNames;}if (advisorNames.length == 0) {return new ArrayList<>();}List<Advisor> advisors = new ArrayList<>();for (String name : advisorNames) {//。。。省略部分代码//通过beanName找到所有的Advisor实现类beanadvisors.add(this.beanFactory.getBean(name, Advisor.class));//。。。省略部分代码     return advisors;
}

2.3 AOP的调用阶段

  • 上述内容会包装成一个config最终由JdkDynamicAopProxy或者CglibAopProxy生成动代理，代理invoke类自然就是这两个类，本内容大概分析一下JdkDynamicAopProxy，在该类找到invoke类
//找到这个代码 将advised的实现类根据满足规则 最终组成一条调用链路
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);//封装成MethodInvocation
MethodInvocation invocation =new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
// Proceed to the joinpoint through the interceptor chain.
retVal = invocation.proceed();//ReflectiveMethodInvocation 递归调用代理类
public Object proceed() throws Throwable {// We start with an index of -1 and increment early.if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {//链路调用完成后 最终调用原始类return invokeJoinpoint();}Object interceptorOrInterceptionAdvice =this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {// Evaluate dynamic method matcher here: static part will already have// been evaluated and found to match.InterceptorAndDynamicMethodMatcher dm =(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {//调用满足条件的切面类return dm.interceptor.invoke(this);}else {// Dynamic matching failed.// Skip this interceptor and invoke the next in the chain.//递归return proceed();}}else {// It's an interceptor, so we just invoke it: The pointcut will have// been evaluated statically before this object was constructed.return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);}
}

3.事务的源码分析

3.1 事务的初始化类ProxyTransactionManagementConfiguration

//加了Configuration 能被spring扫描到
@Configuration(proxyBeanMethods = false)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {//该类实现了Advisor beanName为org.springframework.transaction.config.internalTransactionAdvisor//也就是上面的切面扫描类会扫描到该类@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)@Role(BeanDefinition.ROLE_INFRASTRUCTURE)public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor(TransactionAttributeSource transactionAttributeSource, TransactionInterceptor transactionInterceptor) {BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();//校验规则 都是由下面的bean实现初始化的advisor.setTransactionAttributeSource(transactionAttributeSource);//链路类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(TransactionAttributeSource transactionAttributeSource) {//链路执行类为TransactionInterceptorTransactionInterceptor interceptor = new TransactionInterceptor();interceptor.setTransactionAttributeSource(transactionAttributeSource);if (this.txManager != null) {interceptor.setTransactionManager(this.txManager);}return interceptor;}
}
  • 简单分析下AnnotationTransactionAttributeSource
public AnnotationTransactionAttributeSource(boolean publicMethodsOnly) {this.publicMethodsOnly = publicMethodsOnly;if (jta12Present || ejb3Present) {this.annotationParsers = new LinkedHashSet<>(4);this.annotationParsers.add(new SpringTransactionAnnotationParser());if (jta12Present) {this.annotationParsers.add(new JtaTransactionAnnotationParser());}if (ejb3Present) {this.annotationParsers.add(new Ejb3TransactionAnnotationParser());}}else {this.annotationParsers = Collections.singleton(new SpringTransactionAnnotationParser());}
}//都会添加该类SpringTransactionAnnotationParser 只贴这个方法了 根据这个判断是否生成事务的代理类
@Override
public boolean isCandidateClass(Class<?> targetClass) {return AnnotationUtils.isCandidateClass(targetClass, Transactional.class);
}

至此，整个AOP的流程已经事务类的初始化以及全部分析完了，接下来继续分析链路中的事务类调用

3.2 事务代理类TransactionInterceptor

@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...//执行return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {@Override@Nullablepublic Object proceedWithInvocation() throws Throwable {return invocation.proceed();}@Overridepublic Object getTarget() {return invocation.getThis();}@Overridepublic Object[] getArguments() {return invocation.getArguments();}});
}

3.3 事务管理类TransactionAspectSupport#invokeWithinTransaction

if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {// Standard transaction demarcation with getTransaction and commit/rollback calls.//创建一个事务 主要分析这里 ##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.//业务执行 这里如果无其他切面正常会到SqlSessionTemplateretVal = invocation.proceedWithInvocation();}catch (Throwable ex) {// target invocation exception//事务异常切面completeTransactionAfterThrowing(txInfo, ex);throw ex;}finally {//清除事务相关cleanupTransactionInfo(txInfo);}if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {// Set rollback-only in case of Vavr failure matching our rollback rules...TransactionStatus status = txInfo.getTransactionStatus();if (status != null && txAttr != null) {retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);}}//提交或者回滚事务commitTransactionAfterReturning(txInfo);return retVal;
}

3.4 事务入口AbstractPlatformTransactionManager

  • createTransactionIfNecessary -> AbstractPlatformTransactionManager#getTransaction -> startTransaction
private TransactionStatus startTransaction(TransactionDefinition definition, Object transaction,boolean debugEnabled, @Nullable SuspendedResourcesHolder suspendedResources) {boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);DefaultTransactionStatus status = newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);// ###doBegin(transaction, definition);prepareSynchronization(status, definition);return status;
}

3.5 事务开启DataSourceTransactionManager#doBegin

@Override
protected void doBegin(Object transaction, TransactionDefinition definition) {DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;Connection con = null;try {if (!txObject.hasConnectionHolder() ||txObject.getConnectionHolder().isSynchronizedWithTransaction()) {//根据数据源插件获取一个连接  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);txObject.setReadOnly(definition.isReadOnly());// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,// so we don't want to do it unnecessarily (for example if we've explicitly// configured the connection pool to set it already).//如果是自动提交的if (con.getAutoCommit()) {//设置必须有序提交txObject.setMustRestoreAutoCommit(true);if (logger.isDebugEnabled()) {logger.debug("Switching JDBC Connection [" + con + "] to manual commit");}//同时关闭自动提交con.setAutoCommit(false);}//开始事务的入口 stmt.executeUpdate("SET TRANSACTION READ ONLY"); ###prepareTransactionalConnection(con, definition);//设置事务的活跃状态txObject.getConnectionHolder().setTransactionActive(true);//。。。省略后续代码
}//prepareTransactionalConnection
protected void prepareTransactionalConnection(Connection con, TransactionDefinition definition)throws SQLException {if (isEnforceReadOnly() && definition.isReadOnly()) {try (Statement stmt = con.createStatement()) {//直接执行sql 事务开始后 取决于数据库隔离级别 默认是行锁stmt.executeUpdate("SET TRANSACTION READ ONLY");}}
}

3.6 事务之rollback

  • TransactionAspectSupport#commitTransactionAfterReturning -> AbstractPlatformTransactionManager#commit -> processRollback - > doRollback -> DataSourceTransactionManager#con.rollback()
  • ConnectionImpl#rollback
private void rollbackNoChecks() throws SQLException {synchronized (getConnectionMutex()) {if (this.useLocalTransactionState.getValue()) {if (!this.session.getServerSession().inTransactionOnServer()) {return; // effectively a no-op}}//执行sqlthis.session.execSQL(null, "rollback", -1, null, false, this.nullStatementResultSetFactory, null, false);}}

3.7 事务之commit

  • TransactionAspectSupport#commitTransactionAfterReturning ->AbstractPlatformTransactionManager#commit -> processCommit- > doCommit -> DataSourceTransactionManager#con.commit()
  • ConnectionImpl#commit
//执行sql
this.session.execSQL(null, "commit", -1, null, false, this.nullStatementResultSetFactory, null, false);

4 总结

1.事务是根据Spring的AOP机制去完成的
2.AOP生是根据类生成代理类的，所以事务代码块执行时使用this.addMoney(事务块代码)是无效的，只能重新获取该代理类执行事务块代码才会生效
3.事务是每次处理完都会被回收掉的。

以上就是本章的全部内容了。

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