Spring应用上下文生命周期-技术圈

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作者 | Gouden

来源 | urlify.cn/QrYzia

Spring应用上下文生命周期整体分成四个阶段

ConfigurableApplicationContext#refresh，加载或者刷新持久化配置
ConfigurableApplicationContext#start，启动应用上下文
ConfigurableApplicationContext#stop，停止应用上下文
ConfigurableApplicationContext#close，关闭应用上下文，释放锁定资源

实际上refresh执行完成后Spring应用上下文从广义上来说已经启动了，start回调用LifeCycleProcessors的start方法，可以理解refresh处理Spring应用上下文启动需要的东西，start相当于是一个扩展，close和stop是和refresh和close类似的逆向操作。

因此Spring应用上下文生命周期重头戏是refresh和close两个方法对应的实现。

refresh-刷新应用上下文

整个refresh阶段代码如下所示:

从代码可以看出，refresh整体分为以下几个阶段:

准备刷新阶段: prepareRefresh
BeanFactory创建阶段: obtainFreshBeanFactory
BeanFactory准备阶段: prepareBeanFactory
BeanFactory后置处理阶段: postProcessBeanFactory、invokeBeanFactoryPostProcessors
BeanFactory注册BeanPostProcessor阶段: registerBeanPostProcessors
初始化内建Bean: initMessageSource、initApplicationEventMulticaster
Spring应用上下文刷新阶段: onRefresh
Spring事件监听器注册阶段: registerListener
BeanFactory初始化完成阶段: finishBeanFactoryInitialization
Spring应用上下文刷新完成阶段: finishRefresh

准备刷新阶段

prepareRefresh方法注释如下所示，可以看出，这个方法主要做了三件事来准备上下文刷新:

设置启动时间
设置激活标
执行任何初始化属性来源

protected void prepareRefresh() {
        //设置启动时间、激活标
        this.startupDate = System.currentTimeMillis();
        this.closed.set(false);
        this.active.set(true);

        if (logger.isDebugEnabled()) {
            if (logger.isTraceEnabled()) {
                logger.trace("Refreshing " + this);
            }
            else {
                logger.debug("Refreshing " + getDisplayName());
            }
        }

        //初始化属性源，扩展用
        initPropertySources();

        //校验必要属性
        getEnvironment().validateRequiredProperties();

        //创建存储早期applicationListeners容器
        if (this.earlyApplicationListeners == null) {
            this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
        }
        else {
            this.applicationListeners.clear();
            this.applicationListeners.addAll(this.earlyApplicationListeners);
        }

        //创建存储早期applicationEvents容器，存储早期Spring Application事件，用于后面applicationEventMulticaster发布事件用
        this.earlyApplicationEvents = new LinkedHashSet<>();
    }

BeanFactory创建阶段

在AbstractRefreshableApplicationContext的refreshBeanFactory实现里，会对这个应用上下文的底层BeanFactory做一个刷新，如果之前有BeanFactory，会先停止，再初始化一个新的BeanFactory

protected final void refreshBeanFactory() throws BeansException {     //如果有BeanFactory，先关掉这个BeanFactory  
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {        //创建新的BeanFactory
            DefaultListableBeanFactory beanFactory = createBeanFactory();        //设置BeanFactory的id
            beanFactory.setSerializationId(getId());        //设置是否允许BeanDefinition覆盖、是否允许循环依赖
            customizeBeanFactory(beanFactory);        //加载BeanDefinition
            loadBeanDefinitions(beanFactory);        //设置新的BeanFactory为当前应用上下文IoC容器
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

BeanFactory准备阶段

配置容器上下文特征，例如上下文的ClassLoader和后置处理器。

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        //设置容器使用的类加载器: ClassLoader，默认是线程上下文类设置的加载器
        beanFactory.setBeanClassLoader(getClassLoader());     //设置Bean表达式处理器，指定bean definition中的表达是值的解析策略
        beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));     //添加PropertyEditorRegistrar实现，存储ResourceLoader和PropertyResolver
        beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

        //添加BeanPostProcessor实现，用于处理ApplicationContext的Aware回调
        beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));     //忽略指定接口作为依赖注入接口
        beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
        beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
        beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
        beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

        //注册ResolvableDependency，这部分可以被依赖注入但不能被依赖查找
        beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
        beanFactory.registerResolvableDependency(ResourceLoader.class, this);
        beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
        beanFactory.registerResolvableDependency(ApplicationContext.class, this);

        //注册ApplicationListener探测器，如果一个Bean实现了ApplicationListener接口会被当作ApplicationListener注册到Publisher
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

        //注册动态织入后置处理器
        if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            // Set a temporary ClassLoader for type matching.
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }

        //注册环境Bean单例对象
        if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
        }
        if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
        }
        if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
            beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
        }
    }

BeanFactory后置处理阶段

postProcessBeanFactory和invokeBeanFactoryPostProcessors都是Spring提供的扩展方式。

postProcessBeanFactory在AbstractApplicationContext里没有实际实现，postProcessBeanFactory注释如下:

可以看出这时Spring应用上下文内部BeanFactory已经标准初始化完成，这时所有Bean的BeanDefinition已经被加载进来，但是还没有被实例化，这时允许继承类注册特殊的BeanPostProcessors等内容。

invokeBeanFactoryPostProcessors方法注释如下:

这个方法会实例化并且执行所有被注册的BeanFactoryPostProcessor的Bean，如果给定显示顺序，按显示顺序执行，invokeBeanFactoryPostProcessors代码如下:

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {     //处理BeanFactoryPostProcessors，这里细节挺多的，要按一定顺序执行BeanDefinitionRegistryPostProcessor#postProcessBeanDefinitionRegistry和BeanFactoryPostProcessor#postProcessBeanFactory
        PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

        //添加LoadTimeWeaverAwareProcessor
        if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }
    }

BeanFactory注册BeanPostProcessor

registerBeanPostProcessors方法注释如下:

这个方法的作用就是实例化并且注册所有BeanPostProcessor的Bean，会按给定顺序注册。

这里注册的顺序是:

先注册实现PriorityOrdered的BeanPostProcessor的Bean
再注册实现Ordered的BeanPostProcessor的Bean
然后注册普通的BeanPostProcessor的Bean
后面注册MergedBeanDefinitionPostProcess的Bean
最后注册ApplicationListenerDetector的Bean

初始化内建Bean

初始化内建Bean会初始化两个Bean: MessageSource和ApplicationEventMulticaster。

这两个实现比较类似，都是判断当前BeanFactory(不判断父BeanFactory)是否包含对应Bean，如果不包含，就创建并且用registerSingleton方法注册到BeanFactory。

Spring应用上下文刷新阶段

onRefresh方法注释如下所示:

onRefresh方法是个可以被子类覆盖的模板方法，可以在实例化单例前初始化特殊的Bean。

Spring事件监听器注册阶段

registerListeners方法注释如下所示:

registerListeners方法会把实现ApplicationListener的Bean和非Bean对象注册到ApplicationEventMulticaster。

registerListeners方法代码如下:

protected void registerListeners() {
        //注册非Bean的ApplicationListener实现类的对象
        for (ApplicationListener listener : getApplicationListeners()) {
            getApplicationEventMulticaster().addApplicationListener(listener);
        }

        //注册ApplicationListener实现类对应的Bean
        String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
        for (String listenerBeanName : listenerBeanNames) {
            getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
        }

        //发送早期ApplicationEvent事件
        Set earlyEventsToProcess = this.earlyApplicationEvents;
        this.earlyApplicationEvents = null;
        if (earlyEventsToProcess != null) {
            for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
                getApplicationEventMulticaster().multicastEvent(earlyEvent);
            }
        }
    }

BeanFactory初始化完成阶段

finishBeanFactoryInitialization方法注释如下所示:

这个方法会结束Spring应用上下文的BeanFactory的初始化，初始化所有剩余的单例Bean。

finishBeanFactoryInitialization方法代码如下所示:

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        //如果BeanFactory有ConversionService对象，关联到BeanFactory
        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));
        }

        //添加StringValueResolver对象
        if (!beanFactory.hasEmbeddedValueResolver()) {
            beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
        }

        //依赖查找LoadTimeWeaverAware的Bean
        String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
        for (String weaverAwareName : weaverAwareNames) {
            getBean(weaverAwareName);
        }

        //BeanFactory临时ClassLoader置为null
        beanFactory.setTempClassLoader(null);

        //BeanFactory冻结配置
        beanFactory.freezeConfiguration();

        //实例化所有不是懒加载的单例对象
        beanFactory.preInstantiateSingletons();
    }

Spring应用上下文刷新完成阶段

finishRefresh方法注释如下所示:

finishRefresh方法结束Spring应用上下文刷新，调用LifecycleProcessor#onRefresh方法并且发送ContextRefreshedEvent。

finishRefresh方法代码如下所示:

protected void finishRefresh() {
        //清除ResourceLoader缓存
        clearResourceCaches();

        //初始化LifecycleProcessor对象
        initLifecycleProcessor();

        //调用LifecycleProcessor#onRefresh方法
        getLifecycleProcessor().onRefresh();

        //发布Spring应用上下文已刷新事件
        publishEvent(new ContextRefreshedEvent(this));

        //向MBeanServer托管Live Bean
        LiveBeansView.registerApplicationContext(this);
    }

start-启动应用上下文

start方法代码如下所示:

public void start() {     //启动LifecycleProcessor
        getLifecycleProcessor().start();     //发布应用上下文启动事件
        publishEvent(new ContextStartedEvent(this));
    }

stop-停止应用上下文

stop方法代码如下所示:

public void stop() {     //停止LifecycleProcessor
        getLifecycleProcessor().stop();     //发布应用上下文停止事件
        publishEvent(new ContextStoppedEvent(this));
    }

close-关闭应用上下文

protected void doClose() {
        //检查激活标
        if (this.active.get() && this.closed.compareAndSet(false, true)) {
            if (logger.isDebugEnabled()) {
                logger.debug("Closing " + this);
            }
        //Live Bean注销托管
            LiveBeansView.unregisterApplicationContext(this);

            try {
                //发布Spring应用上下文已关闭事件
                publishEvent(new ContextClosedEvent(this));
            }
            catch (Throwable ex) {
                logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", ex);
            }

            //关闭LifecycleProcessor
            if (this.lifecycleProcessor != null) {
                try {
                    this.lifecycleProcessor.onClose();
                }
                catch (Throwable ex) {
                    logger.warn("Exception thrown from LifecycleProcessor on context close", ex);
                }
            }

            //销毁Spring Bean
            destroyBeans();

            //关闭BeanFactory
            closeBeanFactory();

            //回调onClose
            onClose();

            //重置本地监听者
            if (this.earlyApplicationListeners != null) {
                this.applicationListeners.clear();
                this.applicationListeners.addAll(this.earlyApplicationListeners);
            }

            //激活标设置为没激活
            this.active.set(false);
        }
    }

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