Spring事务是如何应用到你的业务场景中的？
2020-09-14 23:08
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初衷
日常开发中经常用到@Transaction注解，那你知道它是怎么应用到你的业务代码中的吗？本篇文章将从以下两个方面阐述Spring事务实现原理：
解析并加载事务配置：本质上是解析xml文件将标签加载成 BeanDefinition 对象；
生成事务代理对象并运行：本质上是Spring AOP在事务这块的应用，将业务Bean替换成事务代理对象（JdkDynamicAopProxy：JDK代理，CglibAopProxy：CGLIB代理）；
本文使用的源码版本是Spring 4.3.18.RELEASE，使用的是XML开启事务。
关键类
这里列出几个核心类，提前留个印象，后面会讲解什么时候调用
XmlBeanDefinitionReader 加载XML定义的Bean入口
TxNamespaceHandler 解析XML中的事务标签： advice、annotation-driven等；
InfrastructureAdvisorAutoProxyCreator 该类实现了 BeanPostProcessor 接口（可以在Bean初始化后进行替换），是生成事务代理类并替换的关键类；
AnnotationTransactionAttributeSource 用来解析业务方法使用注解 @Transaction 上的配置，提供给 TransactionInterceptor 使用；
TransactionInterceptor 事务拦截类：真正处理事务的类，开启、回滚事务，可以理解成切面中的通知：做什么；
BeanFactoryTransactionAttributeSourceAdvisor 实现了Advisor接口，可以理解为切面：切点(TransactionAttributeSourcePointcut) + 通知(TransactionInterceptor)。
示例代码：
public class TransactionService {
@Transactional(rollbackFor = Throwable.class) public void testTransaction(){
System.out.println("方法逻辑");
}
} ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("config.xml");
TransactionService transactionService = context.getBean(TransactionService.class);
transactionService.testTransaction();<?xml version="1.0" encoding="UTF-8"?><beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:context="http://www.springframework.org/schema/context" xmlns:jee="http://www.springframework.org/schema/jee" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:task="http://www.springframework.org/schema/task" xsi:schemaLocation=" http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-4.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task-4.0.xsd"> <bean id="transactionService" class="com.yangwq.spring.transaction.TransactionService"/> <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="driverClassName" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://127.0.0.1:3306/blog"/> <property name="username" value="root"/> <property name="password" value="11"/> </bean> <!-- 定义事务管理器 --> <bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean> <!--使用注释事务 --> <tx:annotation-driven/></beans>
1. 解析并加载事务配置
加载的入口(同时也是Spring容器加载的核心代码)：
synchronized (this.startupShutdownMonitor) { // Prepare this context for refreshing.
prepareRefresh(); // 重点，这里是Spring初始化默认的容器，在这一步会通过解析配置文件将定义的bean转换为 BeanDefinition，
// 保存在 beanDefinitionMap 中
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory(); // Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory); try { // Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory); // Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory); // Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory); // Initialize message source for this context.
initMessageSource(); // Initialize event multicaster for this context.
initApplicationEventMulticaster(); // Initialize other special beans in specific context subclasses.
onRefresh(); // Check for listener beans and register them.
registerListeners(); // 重点，初始化所有非懒加载bean的方法，也可以理解为根据特定规则将 BeanDefinition 转成 Bean对象的方法
finishBeanFactoryInitialization(beanFactory); // Last step: publish corresponding event.
finishRefresh();
} catch (BeansException ex) { if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " + "cancelling refresh attempt: " + ex);
} // Destroy already created singletons to avoid dangling resources.
destroyBeans(); // Reset 'active' flag.
cancelRefresh(ex); // Propagate exception to caller.
throw ex;
} finally { // Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}事务配置的解析在上面的obtainFreshBeanFactory，由于我们使用的是ClassPathXmlApplicationContext 作为容器，它的解析类为：XmlBeanDefinitionReader；核心方法：
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null"); if (logger.isInfoEnabled()) {
logger.info("Loading XML bean definitions from " + encodedResource.getResource());
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get(); if (currentResources == null) {
currentResources = new HashSet<EncodedResource>(4); this.resourcesCurrentlyBeingLoaded.set(currentResources);
} if (!currentResources.add(encodedResource)) { throw new BeanDefinitionStoreException( "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
} try { // 读取 config.xml 文件
InputStream inputStream = encodedResource.getResource().getInputStream(); try {
InputSource inputSource = new InputSource(inputStream); if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
} // 开始加载 config.xml 文件中定义的bean，这里只是加载成BeanDefinition，初始化在另外的方法
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
} finally {
inputStream.close();
}
} catch (IOException ex) { throw new BeanDefinitionStoreException( "IOException parsing XML document from " + encodedResource.getResource(), ex);
} finally {
currentResources.remove(encodedResource); if (currentResources.isEmpty()) { this.resourcesCurrentlyBeingLoaded.remove();
}
}
}doLoadBeanDefinitions 具体解析的方法是在org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#doRegisterBeanDefinitions 中进行，该方法如下
protected void doRegisterBeanDefinitions(Element root) { // Any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate; this.delegate = createDelegate(getReaderContext(), root, parent); if (this.delegate.isDefaultNamespace(root)) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE); if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS); if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) { if (logger.isInfoEnabled()) {
logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec + "] not matching: " + getReaderContext().getResource());
} return;
}
}
}
preProcessXml(root); // 真正解析config.xml文件
parseBeanDefinitions(root, this.delegate);
postProcessXml(root); this.delegate = parent;
}
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) { // 判断根节点是不是默认的节点，实际通过判断根节点的 namespaceURI 属性是不是 http://www.springframework.org/schema/beans ，
// 我们这里的配置文件声明的正好是这个属性，所以返回的是true，注意bean标签也是默认标签
if (delegate.isDefaultNamespace(root)) { // 获取子节点
NodeList nl = root.getChildNodes(); for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i); // 判断子节点是不是标签
if (node instanceof Element) {
Element ele = (Element) node; // bean标签的解析
if (delegate.isDefaultNamespace(ele)) {
parseDefaultElement(ele, delegate);
} // 重点：tx标签在这里进行
else {
delegate.parseCustomElement(ele);
}
}
}
} else {
delegate.parseCustomElement(root);
}
} // 解析非bean的标签
public BeanDefinition parseCustomElement(Element ele) { return parseCustomElement(ele, null);
} // 解析非默认标签方法
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
String namespaceUri = getNamespaceURI(ele); // 这里是Spring使用了策略模式解析配置，通过namespaceUri匹配对应的处理类，也就是上面的关键类 TxNamespaceHandler，
// 这里的resolve运用了懒加载，在获取映射关系handlerMappings属性为null时会从META-INF/spring.handlers 下面获取映射关系，
// 由于IDEA debug的时候默认会调用getHandlerMappings方法，所以使用debug的时候会发现一开始就有值
NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri); if (handler == null) {
error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele); return null;
} // 获取到处理类后，让处理类开始解析配置
return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}TxNamespaceHandler 最终交给类 AnnotationDrivenBeanDefinitionParser 进行解析事务配置；
public BeanDefinition parse(Element element, ParserContext parserContext) { // 注册一个事务工厂事件，允许我们自定义监听事务的提交等操作
registerTransactionalEventListenerFactory(parserContext);
String mode = element.getAttribute("mode"); // 如果使用的是 aspectj 方式，走这个方法
if ("aspectj".equals(mode)) { // mode="aspectj"
registerTransactionAspect(element, parserContext);
} else { // 默认使用的是代理模式
// mode="proxy"
AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
} return null;
}
/**
* Inner class to just introduce an AOP framework dependency when actually in proxy mode.
*/
private static class AopAutoProxyConfigurer { public static void configureAutoProxyCreator(Element element, ParserContext parserContext) { // 重点，注册 InfrastructureAdvisorAutoProxyCreator 的 BeanDefinition，这个是个关键类，代理业务bean的操作都靠它了
AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);
String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME; if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
Object eleSource = parserContext.extractSource(element); // 重点，注册一个全局的 TransactionAttributeSource 的BeanDefinition ,用于解析 @Transaction 定义的元数据
RootBeanDefinition sourceDef = new RootBeanDefinition( "org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
sourceDef.setSource(eleSource);
sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef); // 重点，注册 TransactionInterceptor 的BeanDefinition，这个就是事务真正执行的类，可以理解为是事务切面上的通知
RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
interceptorDef.setSource(eleSource);
interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
registerTransactionManager(element, interceptorDef);
interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef); // 重点，注册 BeanFactoryTransactionAttributeSourceAdvisor 的BeanDefinition，可以理解为事务的切面，
// 包含了切点(TransactionAttributeSourcePointcut) + 通知(TransactionInterceptor)，保存在 ProxyFactory 中，用于生成代理对象
RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);
advisorDef.setSource(eleSource);
advisorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
advisorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
advisorDef.getPropertyValues().add("adviceBeanName", interceptorName); if (element.hasAttribute("order")) {
advisorDef.getPropertyValues().add("order", element.getAttribute("order"));
}
parserContext.getRegistry().registerBeanDefinition(txAdvisorBeanName, advisorDef);
CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), eleSource);
compositeDef.addNestedComponent(new BeanComponentDefinition(sourceDef, sourceName));
compositeDef.addNestedComponent(new BeanComponentDefinition(interceptorDef, interceptorName));
compositeDef.addNestedComponent(new BeanComponentDefinition(advisorDef, txAdvisorBeanName));
parserContext.registerComponent(compositeDef);
}
}
}到这里，事务配置的加载就结束了，上面的步骤只是将事务的关键类变成 BeanDefinition，实际上还没有生成Bean对象的，下面我们看下BeanDefinition 是如何变成Bean的（这一个步骤不是Spring事务独有的，是所有BeanDefinition 的共同逻辑），然后是如何生成业务bean代理对象并替换业务bean的。
2. 生成事务代理对象
入口在 1.解析并加载事务配置 列举的finishBeanFactoryInitialization(beanFactory) 方法中：
/**
* Finish the initialization of this context's bean factory,
* initializing all remaining singleton beans.
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) { // Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
} // Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(new StringValueResolver() { @Override
public String resolveStringValue(String strVal) { return getEnvironment().resolvePlaceholders(strVal);
}
});
} // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false); for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
} // Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null); // Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration(); // 重点，开始初始化所有非懒加载的bean
beanFactory.preInstantiateSingletons();
}beanFactory.preInstantiateSingletons() 具体方法如下：
public void preInstantiateSingletons() throws BeansException { if (this.logger.isDebugEnabled()) { this.logger.debug("Pre-instantiating singletons in " + this);
} // Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.将所有注册的BeanDefinition 名称保存到起来
List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames); // Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) { // 获取 BeanDefinition
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName); // bean定义不是抽象、懒加载而且是单例bean
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) { // 工厂bean特有逻辑，这不是我们的关注点，我们关注点是else的 getBean(beanName);
if (isFactoryBean(beanName)) { final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName); boolean isEagerInit; if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() { @Override
public Boolean run() { return ((SmartFactoryBean<?>) factory).isEagerInit();
}
}, getAccessControlContext());
} else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
} if (isEagerInit) {
getBean(beanName);
}
} else { // 重点，将 BeanDefinition 创建成bean的核心方法，同时也是执行 BeanPostProcessor 的入口
getBean(beanName);
}
}
} // Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName); if (singletonInstance instanceof SmartInitializingSingleton) { final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance; if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override
public Object run() {
smartSingleton.afterSingletonsInstantiated(); return null;
}
}, getAccessControlContext());
} else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}getBean(beanName) 方法比较复杂，这里只列举出关键点：AbstractAutowireCapableBeanFactory 的 initializeBean方法，所有bean的创建都绕不开这个方法，这个方法是执行实现了 BeanPostProcessor 的bean的逻辑：
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) { if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override
public Object run() {
invokeAwareMethods(beanName, bean); return null;
}
}, getAccessControlContext());
} else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean; if (mbd == null || !mbd.isSynthetic()) { // 执行 BeanPostProcessor 的 postProcessBeforeInitialization 方法
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
} try {
invokeInitMethods(beanName, wrappedBean, mbd);
} catch (Throwable ex) { throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
} if (mbd == null || !mbd.isSynthetic()) { // 重点，InfrastructureAdvisorAutoProxyCreator 实现了BeanPostProcessor 接口，所以这里会调用 InfrastructureAdvisorAutoProxyCreator 的postProcessBeforeInitialization 方法，
// wrappedBean 是生成代理后对象，wrappedBean 会替换掉传入的bean，如果当前bean需要事务，wrappedBean就是被JDK或者CGLIB代理后的bean
wrappedBean = postProcessAfterInitialization(wrappedBean, beanName);
} return wrappedBean;
}InfrastructureAdvisorAutoProxyCreator 的 postProcessAfterInitialization 定义在父类：AbstractAutoProxyCreator，AbstractAutoProxyCreator这个类同时是Spring AOP处理的入口，并不是说事务处理独有的，具体实现如下：
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { if (bean != null) {
Object cacheKey = getCacheKey(bean.getClass(), beanName); if (!this.earlyProxyReferences.contains(cacheKey)) { // 重点，AOP生成代理对象的入口，这里也会生成事务代理对象
return wrapIfNecessary(bean, beanName, cacheKey);
}
} return bean;
}
// 从Spring容器中获取所有切面bean，并判断是否在切面中，如果是的话生成一个代理对象，怎么代理由具体的切面定义
protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) { if (beanName != null && this.targetSourcedBeans.contains(beanName)) { return bean;
} if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) { return bean;
} // 这里就是用来判断当前bean 能不能代理
if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) { this.advisedBeans.put(cacheKey, Boolean.FALSE); return bean;
} // 首先查找切面Bean，对于本篇文章就是我们上面定义的 BeanFactoryTransactionAttributeSourceAdvisor 的 BeanDefinition，注意:实际应用场景肯定不仅只有一个切面的。
// spring现在只有BeanDefinition，这个方法先通过获取切面 beanNames ，然后再调用上面的 getBean 方法生成 BeanFactoryTransactionAttributeSourceAdvisor bean对象
Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null); // 需要代理，生成代理对象
if (specificInterceptors != DO_NOT_PROXY) { // 加入缓存中
this.advisedBeans.put(cacheKey, Boolean.TRUE); // 真正生成代理的地方
Object proxy = createProxy(
bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean)); this.proxyTypes.put(cacheKey, proxy.getClass()); // 返回代理后的bean
return proxy;
} this.advisedBeans.put(cacheKey, Boolean.FALSE); // 不代理，返回原始bean
return bean;
}
// 获取切面bean,先通过获取切面BeanNames ，然后通过获取bean
protected Object[] getAdvicesAndAdvisorsForBean(Class<?> beanClass, String beanName, TargetSource targetSource) {
List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName); if (advisors.isEmpty()) { return DO_NOT_PROXY;
} return advisors.toArray();
}
protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) { // 这个方法就是获取切面bean，首先从容器中获取实现了 Advisor 的beanNames，然后通过beanName再获取Bean，
// 对于本篇文章只定义了一个事务切面，所以这里返回的结果是 BeanFactoryTransactionAttributeSourceAdvisor Bean对象
List<Advisor> candidateAdvisors = findCandidateAdvisors(); // 判断当前bean是否能被切面应用，判断规则在下面有说明
List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
extendAdvisors(eligibleAdvisors); if (!eligibleAdvisors.isEmpty()) {
eligibleAdvisors = sortAdvisors(eligibleAdvisors);
} return eligibleAdvisors;
}
// findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName) 具体实现逻辑在org.springframework.aop.support.AopUtils#findAdvisorsThatCanApply
public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) { if (candidateAdvisors.isEmpty()) { return candidateAdvisors;
}
List<Advisor> eligibleAdvisors = new LinkedList<Advisor>(); for (Advisor candidate : candidateAdvisors) { if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
eligibleAdvisors.add(candidate);
}
} boolean hasIntroductions = !eligibleAdvisors.isEmpty(); for (Advisor candidate : candidateAdvisors) { if (candidate instanceof IntroductionAdvisor) { // already processed
continue;
} if (canApply(candidate, clazz, hasIntroductions)) {
eligibleAdvisors.add(candidate);
}
} return eligibleAdvisors;
}
// 请注意，这个方法是AOP通用判断是否能应用切面的方法，不是事务处理独有的
public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
Assert.notNull(pc, "Pointcut must not be null"); // 首先判断类能不能匹配上，匹配不上就返回false， BeanFactoryTransactionAttributeSourceAdvisor 的切点类：
// TransactionAttributeSourcePointcut，默认ClassFilter 返回true，所以这里不会进if
if (!pc.getClassFilter().matches(targetClass)) { return false;
} // 然后判断方法匹配，TransactionAttributeSourcePointcut 通过继承关系可以看到不是 MethodMatcher.TRUE，所以不会进if
MethodMatcher methodMatcher = pc.getMethodMatcher(); if (methodMatcher == MethodMatcher.TRUE) { // No need to iterate the methods if we're matching any method anyway...
return true;
} // 判断是不是引介方法匹配，什么是引介增强:为目标类追加方法，属于类的增强，
// 而 PointcutAdvisor 属于拦截目标类的方法并增强，TransactionAttributeSourcePointcut 是 PointcutAdvisor 的子类，不属于引介增强
IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null; if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
}
// 开始读取目标类的每一个方法，判断是否在切面中
Set<Class<?>> classes = new LinkedHashSet<Class<?>>(ClassUtils.getAllInterfacesForClassAsSet(targetClass));
classes.add(targetClass); for (Class<?> clazz : classes) {
Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz); for (Method method : methods) { // 对于事务而言，判断条件重点在 TransactionAttributeSourcePointcut 的 matches 方法
if ((introductionAwareMethodMatcher != null &&
introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions)) ||
methodMatcher.matches(method, targetClass)) { return true;
}
}
} return false;
} // TransactionAttributeSourcePointcut 的 matches 方法核心逻辑在 SpringTransactionAnnotationParser#parseTransactionAnnotation(java.lang.reflect.AnnotatedElement)
// 注意：只会判断 public 的方法，判断有没有 Transactional 注解；判断的顺序为：先判断方法上面有没有，再判断类上面有没有。
// 判断核心就是看方法或类上面有没有 Transactional 注解，有这个注解就解析这个注解的元数据
public TransactionAttribute parseTransactionAnnotation(AnnotatedElement ae) {
AnnotationAttributes attributes = AnnotatedElementUtils.getMergedAnnotationAttributes(ae, Transactional.class); if (attributes != null) { return parseTransactionAnnotation(attributes);
} else { return null;
}
} // 解析 Transactional 注解元数据
protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();
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"));
ArrayList<RollbackRuleAttribute> rollBackRules = new ArrayList<RollbackRuleAttribute>();
Class<?>[] rbf = attributes.getClassArray("rollbackFor"); for (Class<?> rbRule : rbf) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] rbfc = attributes.getStringArray("rollbackForClassName"); for (String rbRule : rbfc) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
Class<?>[] nrbf = attributes.getClassArray("noRollbackFor"); for (Class<?> rbRule : nrbf) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] nrbfc = attributes.getStringArray("noRollbackForClassName"); for (String rbRule : nrbfc) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
rbta.getRollbackRules().addAll(rollBackRules); return rbta;
} // 以上就是 findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName) 的逻辑
// 下面回到wrapIfNecessary方法，如果当前bean需要生成代理对象，会调用下面的这个方法
protected Object createProxy(
Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) { if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
} // 创建代理工厂，用于创建代理bean
ProxyFactory proxyFactory = new ProxyFactory();
proxyFactory.copyFrom(this); // 判断是否配置了代理目标类，配置了这个选项会全部使用cglib代理
if (!proxyFactory.isProxyTargetClass()) { if (shouldProxyTargetClass(beanClass, beanName)) {
proxyFactory.setProxyTargetClass(true);
} else {
evaluateProxyInterfaces(beanClass, proxyFactory);
}
} // 把所有的切面类放入 ProxyFactory
Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
proxyFactory.addAdvisors(advisors);
proxyFactory.setTargetSource(targetSource);
customizeProxyFactory(proxyFactory);
proxyFactory.setFrozen(this.freezeProxy); if (advisorsPreFiltered()) {
proxyFactory.setPreFiltered(true);
}
// 开始创建代理bean
return proxyFactory.getProxy(getProxyClassLoader());
}proxyFactory.getProxy(getProxyClassLoader()) 方法实现：
public Object getProxy(ClassLoader classLoader) { return createAopProxy().getProxy(classLoader);
}
// createAopProxy方法逻辑，config 其实就是上面的 proxyFactory 对象，这个对象包含了目标对象以及切面类：
public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException { // optimize：代理是否应该执行优化，一般用不上
// proxyTargetClass：这个属性为treu时，不管目标类是不是实现的接口，都使用cglib代理
// hasNoUserSuppliedProxyInterfaces：是否只使用了Spring支持的代理接口，如果用户自定义了代理接口不能进行cglib代理
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.");
} // 如果目标类是接口或者已经被jdk代理过了，使用jdk代理
if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) { return new JdkDynamicAopProxy(config);
} // 其他情况使用CGLIB代理
return new ObjenesisCglibAopProxy(config);
} // 使用JDK代理
else { return new JdkDynamicAopProxy(config);
}
} // ObjenesisCglibAopProxy 的 getProxy(classLoader) 方法，cglib是使用Enhancer创建代理对象的 ：
public Object getProxy(ClassLoader classLoader) { if (logger.isDebugEnabled()) {
logger.debug("Creating CGLIB proxy: target source is " + this.advised.getTargetSource());
} try { // 获取目标类
Class<?> rootClass = this.advised.getTargetClass();
Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy"); // 代理类的父类，默认是目标类
Class<?> proxySuperClass = rootClass; // 如果目标类以及是cglib代理类，
if (ClassUtils.isCglibProxyClass(rootClass)) { // 取cglib代理类的父类作为代理类的父类
proxySuperClass = rootClass.getSuperclass(); // 获取代理类的接口追加到当前类的接口集合中
Class<?>[] additionalInterfaces = rootClass.getInterfaces(); for (Class<?> additionalInterface : additionalInterfaces) { this.advised.addInterface(additionalInterface);
}
} // 验证目标类是否能被代理，仅仅是打印日志，不做其他处理
validateClassIfNecessary(proxySuperClass, classLoader); // 使用Enhancer 来构造cglib代理对象
Enhancer enhancer = createEnhancer(); if (classLoader != null) {
enhancer.setClassLoader(classLoader); if (classLoader instanceof SmartClassLoader &&
((SmartClassLoader) classLoader).isCla***eloadable(proxySuperClass)) {
enhancer.setUseCache(false);
}
}
enhancer.setSuperclass(proxySuperClass);
enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader)); // 重点，设置回调的类，很重要的一个类 DynamicAdvisedInterceptor，这个类就是应用AOP 通知的地方，对于本篇文章就是应用TransactionInterceptor
Callback[] callbacks = getCallbacks(rootClass);
Class<?>[] types = new Class<?>[callbacks.length]; for (int x = 0; x < types.length; x++) {
types[x] = callbacks[x].getClass();
} // fixedInterceptorMap only populated at this point, after getCallbacks call above
enhancer.setCallbackFilter(new ProxyCallbackFilter( this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
enhancer.setCallbackTypes(types); // 创建代理类的字节码，并创建实例，实例设置回调
return createProxyClassAndInstance(enhancer, callbacks);
} catch (CodeGenerationException ex) { throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() + ": Common causes of this problem include using a final class or a non-visible class",
ex);
} catch (IllegalArgumentException ex) { throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() + ": Common causes of this problem include using a final class or a non-visible class",
ex);
} catch (Throwable ex) { // TargetSource.getTarget() failed
throw new AopConfigException("Unexpected AOP exception", ex);
}
}
// JdkDynamicAopProxy 的 getProxy(classLoader) 方法：public Object getProxy(ClassLoader classLoader) { if (logger.isDebugEnabled()) {
logger.debug("Creating JDK dynamic proxy: target source is " + this.advised.getTargetSource());
}
Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
findDefinedEqualsAndHashCodeMethods(proxiedInterfaces); return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
}通过2. 生成事务代理对象，此时创建的bean就是以及被JDK或者CGLIB代理的类，这一步是Spring AOP通用的处理逻辑，那具体是怎么运行的呢？
JDK动态代理运行：
// 本质是调用org.springframework.aop.framework.JdkDynamicAopProxy#invoke方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
MethodInvocation invocation;
Object oldProxy = null; boolean setProxyContext = false;
TargetSource targetSource = this.advised.targetSource;
Class<?> targetClass = null;
Object target = null; try { if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) { // The target does not implement the equals(Object) method itself.
return equals(args[0]);
} else if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) { // The target does not implement the hashCode() method itself.
return hashCode();
} else if (method.getDeclaringClass() == DecoratingProxy.class) { // There is only getDecoratedClass() declared -> dispatch to proxy config.
return AopProxyUtils.ultimateTargetClass(this.advised);
} else if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
method.getDeclaringClass().isAssignableFrom(Advised.class)) { // Service invocations on ProxyConfig with the proxy config...
return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
}
Object retVal; if (this.advised.exposeProxy) { // Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
} // May be null. Get as late as possible to minimize the time we "own" the target,
// in case it comes from a pool.
target = targetSource.getTarget(); if (target != null) {
targetClass = target.getClass();
} // 获取方法的拦截器链，对于本篇文章就是关键类：BeanFactoryTransactionAttributeSourceAdvisor
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass); // Check whether we have any advice. If we don't, we can fallback on direct
// reflective invocation of the target, and avoid creating a MethodInvocation.
if (chain.isEmpty()) { // We can skip creating a MethodInvocation: just invoke the target directly
// Note that the final invoker must be an InvokerInterceptor so we know it does
// nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = AopUtils.invokeJoinpointUsingReflection(target, method, argsToUse);
} else { // 将所有参数：代理对象，目标对象，方法，参数，拦截器链封装到一个ReflectiveMethodInvocation对象中
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain); // 然后调用ReflectiveMethodInvocation的proceed方法，会执行拦截器
retVal = invocation.proceed();
} // Massage return value if necessary.
Class<?> returnType = method.getReturnType(); if (retVal != null && retVal == target &&
returnType != Object.class && returnType.isInstance(proxy) &&
!RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) { // Special case: it returned "this" and the return type of the method
// is type-compatible. Note that we can't help if the target sets
// a reference to itself in another returned object.
retVal = proxy;
} else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) { throw new AopInvocationException( "Null return value from advice does not match primitive return type for: " + method);
} return retVal;
} finally { if (target != null && !targetSource.isStatic()) { // Must have come from TargetSource.
targetSource.releaseTarget(target);
} if (setProxyContext) { // Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
}org.springframework.aop.framework.ReflectiveMethodInvocation#proceed 方法：
public Object proceed() throws Throwable { // 所有拦截器调用完成，一般情况是没有匹配到任意的拦截器，这里会执行目标类本身的方法
if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) { return invokeJoinpoint();
}
// 获取拦截器链中的第一个拦截器
Object interceptorOrInterceptionAdvice = this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex); // 通知器或通知是动态匹配方法拦截器类型，对于本篇文章，interceptorOrInterceptionAdvice 是TransactionInterceptor对象，
// 所有走的是else，直接执行TransactionInterceptor的invoke方法
if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) { // 动态匹配方法拦截器
InterceptorAndDynamicMethodMatcher dm =
(InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice; // 匹配成功就执行对应的拦截器
if (dm.methodMatcher.matches(this.method, this.targetClass, this.arguments)) { return dm.interceptor.invoke(this);
} else { // 如果不匹配,就跳过此拦截器，递归执行下一个拦截器
return proceed();
}
} else { // 如果是一个interceptor，直接调用这个interceptor对应的方法
return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
}
}CGLIB代理运行：
// 其实就是创建代理对象时设置的回调类DynamicAdvisedInterceptor
private static class DynamicAdvisedInterceptor implements MethodInterceptor, Serializable { private final AdvisedSupport advised; public DynamicAdvisedInterceptor(AdvisedSupport advised) { this.advised = advised;
} // 主要是这个方法执行拦截，处理逻辑大致和JDK动态代理差不多，都是获取拦截器链，
// 然后构建ReflectiveMethodInvocation的子类CglibMethodInvocation对象，
// 执行ReflectiveMethodInvocation的proceed方法
@Override
public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
Object oldProxy = null; boolean setProxyContext = false;
Class<?> targetClass = null;
Object target = null; try { if (this.advised.exposeProxy) { // Make invocation available if necessary.
oldProxy = AopContext.setCurrentProxy(proxy);
setProxyContext = true;
} // May be null. Get as late as possible to minimize the time we
// "own" the target, in case it comes from a pool...
target = getTarget(); if (target != null) {
targetClass = target.getClass();
} // 获取方法的拦截器链，对于本篇文章就是关键类：BeanFactoryTransactionAttributeSourceAdvisor
List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
Object retVal; // Check whether we only have one InvokerInterceptor: that is,
// no real advice, but just reflective invocation of the target.
if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) { // We can skip creating a MethodInvocation: just invoke the target directly.
// Note that the final invoker must be an InvokerInterceptor, so we know
// it does nothing but a reflective operation on the target, and no hot
// swapping or fancy proxying.
Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
retVal = methodProxy.invoke(target, argsToUse);
} else { // 将参数封装成CglibMethodInvocation对象并执行proceed方法，CglibMethodInvocation 其实是ReflectiveMethodInvocation的子类，可以理解为ReflectiveMethodInvocation是模板类，CglibMethodInvocation通过重写了部分方法，proceed是两种代理处理的共同逻辑
retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
}
retVal = proce***eturnType(proxy, target, method, retVal); return retVal;
} finally { if (target != null) {
releaseTarget(target);
} if (setProxyContext) { // Restore old proxy.
AopContext.setCurrentProxy(oldProxy);
}
}
} @Override
public boolean equals(Object other) { return (this == other ||
(other instanceof DynamicAdvisedInterceptor && this.advised.equals(((DynamicAdvisedInterceptor) other).advised)));
} /**
* CGLIB uses this to drive proxy creation.
*/
@Override
public int hashCode() { return this.advised.hashCode();
} protected Object getTarget() throws Exception { return this.advised.getTargetSource().getTarget();
} protected void releaseTarget(Object target) throws Exception { this.advised.getTargetSource().releaseTarget(target);
}
}事务的最终执行类 TransactionInterceptor 的invoke 方法：
public Object invoke(final 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...
return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() { @Override
public Object proceedWithInvocation() throws Throwable { // 事务执行完毕后调用链继续向下执行
return invocation.proceed();
}
});
} protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
throws Throwable { // 获取当前方法的事务属性
final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass); // 获取事务管理器
final PlatformTransactionManager tm = determineTransactionManager(txAttr); // 方法名
final String joinpointIdentification = methodIdentification(method, targetClass, txAttr); // 如果方法没有事务或者事务管理器不属于CallbackPreferringPlatformTransactionManager，CallbackPreferringPlatformTransactionManager需要回调函数来实现事务流程，而我们常用的DataSourceTransactionManager就不是CallbackPreferringPlatformTransactionManager
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) { // 创建TransactionInfo事务对象，事务的管理都是通过TransactionInfo对象来完成，这里创建事务会使用到Spring的事务隔离级别，具体的逻辑可以自行查看
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal = null; try { // 拦截器链继续向下执行
retVal = invocation.proceedWithInvocation();
} catch (Throwable ex) { // 抛出异常时提交或者回滚事务
completeTransactionAfterThrowing(txInfo, ex); throw ex;
} finally { // 重置TransactionInfo 的 ThreadLocal
cleanupTransactionInfo(txInfo);
} // 提交/回滚事务
commitTransactionAfterReturning(txInfo); return retVal;
} // else使用的是CallbackPreferringPlatformTransactionManager，
else { // .......略
}
}总结
以上就是事务应用到业务场景中的原理，可以简单理解：Spring事务是在Spring AOP的基础上开发的，关注关键类：TransactionInterceptor 的实现就行了，不管是JDK动态代理还是CGLIB代理都是要用到这个类去提交/回滚事务的。如有错误欢迎指出。
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