【Spring系列】Spring事务

我们通过Debug Spring IOC的启动流程来看Spring的事务管理。

第一阶段：初始化BeanPostProcessor: AnnotationAwareAspectJAutoProxyCreator

先进入Spring启动的核心流程AbstractApplicationContext的refresh方法，

定位到第二步：初始化BeanFactory

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

        // 第二步：初始化BeanFactory, 加载工程的xml配置文件定义的Bean
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

		// 第三步：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();

			// 第十一步：实例化@Service
			finishBeanFactoryInitialization(beanFactory);

			// Last step: publish corresponding event.
			finishRefresh();
		} catch (BeansException ex) {
			// 省略
		} finally {
			// 省略
		}
	}
}

进入第二步，看源码：

@Override
protected final void refreshBeanFactory() throws BeansException {
	if (hasBeanFactory()) {
		destroyBeans();
		closeBeanFactory();
	}
	try {
        // 创建BeanFactory
		DefaultListableBeanFactory beanFactory = createBeanFactory();
		beanFactory.setSerializationId(getId());
		customizeBeanFactory(beanFactory);
        // 解析并加载xml中的Bean
		loadBeanDefinitions(beanFactory);
		synchronized (this.beanFactoryMonitor) {
		    this.beanFactory = beanFactory;
		}
	} catch (IOException ex) {
		// 省略
	}
}

在loadBeanDefinitions方法加载xml配置的时候，如果配置了解析xml文件的时候，NamespaceHandlerSupport的parse方法查找解析器：

@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
	return findParserForElement(element, parserContext).parse(element, parserContext);
}

找到的解析器为：AspectJAutoProxyBeanDefinitionParser，开始解析：

@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
    // 会获取到AnnotationAwareAspectJAutoProxyCreator BeanDefinition注册到BeanFactory的beanDefinitionMap中去
	AopNamespaceUtils.registerAspectJAnnotationAutoProxyCreatorIfNecessary(parserContext, element);
	extendBeanDefinition(element, parserContext);
	return null;
}

通过上面分析可知，在Spring启动初始化BeanFactory加载xml配置的时候会把AnnotationAwareAspectJAutoProxyCreator放到BeanFactory的beanDefinitionMap中。

第二阶段：初始化Service, 创建代理

进入Spring启动的第十一步：实例化

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // 省略...

    beanFactory.setTempClassLoader(null);

    // Allow for caching all bean definition metadata, not expecting further changes.
    beanFactory.freezeConfiguration();

    // 实例化
    beanFactory.preInstantiateSingletons();
}

preInstantiateSingletons方法getBean实例化：

@Override
public void preInstantiateSingletons() throws BeansException {
    // ...
	
    List beanNames = new ArrayList(this.beanDefinitionNames);

    // Trigger initialization of all non-lazy singleton beans...
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
	    	    // 省略...
                if (isEagerInit) {
                    getBean(beanName);
                }
            } else {
                getBean(beanName);
            }
        }
    }
    // 省略
}

getBean方法：

@Override
public Object getBean(String name) throws BeansException {
	return doGetBean(name, null, null, false);
}

protected  T doGetBean(final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
    // 省略。。。

    // 第一次获取不到sharedInstance
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        // 省略。。。
    } else {    

        // 核心步骤： bean instance.
        if (mbd.isSingleton()) {
            sharedInstance = getSingleton(beanName, new ObjectFactory