Spring事务源码分析总结

Spring事务是我们日常工作中经常使用的一项技术，Spring提供了编程、注解、aop切面三种方式供我们使用Spring事务，其中编程式事务因为对代码入侵较大所以不被推荐使用，注解和aop切面的方式可以基于需求自行选择，我们以注解的方式为例来分析Spring事务的原理和源码实现。

//配置事务管理器
<tx:annotation-driven transaction-manager="transactionManager"/>
 <bean id="transactionManager" 
         class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
     <property name="dataSource" ref="dataSource"/>
 </bean>

注意:在需要开启事务的方法上加上@Transactional注解即可，这里需要注意的是，当tx:annotation-driven/标签在不指定transaction-manager属性的时候，会默认寻找id固定名为transactionManager的bean作为事务管理器，如果没有id为transactionManager的bean并且在使用@Transactional注解时也没有指定value（事务管理器），程序就会报错

TxNamespaceHandler

@Override
public void init() {
    //对tx:advice标签进行解析
        registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
    //对annotation-driven标签进行解析  
    registerBeanDefinitionParser("annotation-driven", new         AnnotationDrivenBeanDefinitionParser());
        registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
    }

AnnotationDrivenBeanDefinitionParser.parse()

    public BeanDefinition parse(Element element, ParserContext parserContext) {
        registerTransactionalEventListenerFactory(parserContext);
        //对<tx:annotation-driven/> 标签的mode属性进行判断
        String mode = element.getAttribute("mode");
        if ("aspectj".equals(mode)) {
            // mode="aspectj"  提供对aspectj方式进行事务的支持
            registerTransactionAspect(element, parserContext);
        }
        else {
            // mode="proxy"
            AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
        }
        return null;
    }

AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);

public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
        //1.表示<tx:annoation-driven/>标签只会被解析一次,只有第一次才剩下  
        AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);
​
            String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME;
            if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
                Object eleSource = parserContext.extractSource(element);
​
                // Create the TransactionAttributeSource definition.
               //2.创建AnnotationTransactionAttributeSource
                RootBeanDefinition sourceDef = new RootBeanDefinition(
                        "org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
                sourceDef.setSource(eleSource);
                sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);
​
                // Create the TransactionInterceptor definition.
                //3.创建TransactionInterceptor
                RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
                interceptorDef.setSource(eleSource);
                interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                //注册事务管理器
                registerTransactionManager(element, interceptorDef);
 
                interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);
​
                // Create the TransactionAttributeSourceAdvisor definition.
                //4.创建BeanFactoryTransactionAttributeSourceAdvisor,并把前2个添加到属性中
                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);
            }

上面注册的三个Bean支持了整个事务的功能

registerAutoProxyCreatorIfNecessary()

    public static void registerAutoProxyCreatorIfNecessary(
            ParserContext parserContext, Element sourceElement) {
        //注册InfrastructureAdvisorAutoProxyCreator
        BeanDefinition beanDefinition = AopConfigUtils.registerAutoProxyCreatorIfNecessary(
                parserContext.getRegistry(), parserContext.extractSource(sourceElement));
        //设置proxy-target-class和expose-proxy的配置
        useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
        registerComponentIfNecessary(beanDefinition, parserContext);
    }

这两个方法的主要目的是注册InfrastructureAdvisorAutoProxyCreator ,这个类间接实现了BeanPostProcessor接口，我们知道，Spring会保证所有bean在实例化的时候都会调用其postProcessAfterInitialization方法，我们可以使用这个方法包装和改变bean，而真正实现这个方法是在其父类AbstractAutoProxyCreator类中：

//实例化userService的Bean就会调用此方法

postProcessAfterInitialization()

    @Override
    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) throws BeansException {
        if (bean != null) {
            //根据给定的Bean的class和name构建出key
            Object cacheKey = getCacheKey(bean.getClass(), beanName);
            if (!this.earlyProxyReferences.contains(cacheKey)) {
                return wrapIfNecessary(bean, beanName, cacheKey);
            }
        }
        return bean;
    }

wrapIfNecessary()

    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
        if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
            return bean;
        }
        if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
            return bean;
        }
        if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return bean;
        }
​
        // Create proxy if we have advice.
        //获取指定Bean对应的增强器
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
        if (specificInterceptors != DO_NOT_PROXY) {
            this.advisedBeans.put(cacheKey, Boolean.TRUE);
            //根据对应的增强器创建代理对象
            Object proxy = createProxy(
                    bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
            this.proxyTypes.put(cacheKey, proxy.getClass());
            return proxy;
        }
​
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }

registerTransactionManager()

    private static void registerTransactionManager(Element element, BeanDefinition def) {
        def.getPropertyValues().add("transactionManagerBeanName",
                TxNamespaceHandler.getTransactionManagerName(element));
    }

getTransactionManagerName(element));

tx:annotation-driven/不指定属性的时候,默认去找id固定名为transactionManager的bean作为事务管理器

tatic final String TRANSACTION_MANAGER_ATTRIBUTE = "transaction-manager";
 
	static final String DEFAULT_TRANSACTION_MANAGER_BEAN_NAME = "transactionManager";
 
	static String getTransactionManagerName(Element element) {
		return (element.hasAttribute(TRANSACTION_MANAGER_ATTRIBUTE) ?
				element.getAttribute(TRANSACTION_MANAGER_ATTRIBUTE) : DEFAULT_TRANSACTION_MANAGER_BEAN_NAME);
	}