摘要：重要的會在后面拎出來多帶帶詳解加鎖，防止在過程中，重啟或銷毀造成不必要的問題準備此上下文以進行刷新，設置其啟動日期和，活動標志以及執行屬性源的任何初始化，校驗配置文件。以后所有的相關的操作其實是委托給這個實例來處理的。

public static void main(String[] args){ 
//配置文件來啟動一個 ApplicationContext
 ApplicationContext context = new ClassPathXmlApplicationContext("classpath:application.xml");           
 System.out.println("context 啟動成功");
//從context 中取出我們的 Bean，而不是用 new HelloServiceImpl() 這種方式
 HelloService helloService = context.getBean(HelloService.class);
// 這句將輸出: ok
 System.out.println(helloService.get();
}

public interface HelloService {
    String get();
}

public class HelloServiceImpl implements HelloService{
    @Override
    public String get() {
        return "ok";
    }
}

在resource目錄下創建application.xml

跟蹤可以發現ClassPathXmlApplicationContext繼承ApplicationContext
下面沿著代碼一步步看都干了些什么：

public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
        this(new String[] {configLocation}, true, null);
    }

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
            throws BeansException {
        //獲取資源管理器
        super(parent);
        //設置配置文件路徑，即classpath:application.xml，當然結果是個數組
        setConfigLocations(configLocations);
        //這里是我們的重點，重新創建一個AbstractApplicationContext
        if (refresh) {
            refresh();
        }
    }
//AbstractApplicationContext中實現了super(parent)
public AbstractApplicationContext() {
        this.resourcePatternResolver = getResourcePatternResolver();
    }

一些不重要的就不把代碼貼出來了，只寫方法注釋。重要的會在后面拎出來多帶帶詳解

public void refresh() throws BeansException, IllegalStateException {
        //加鎖，防止在refresh過程中，重啟或銷毀造成不必要的問題
        synchronized (this.startupShutdownMonitor) {
            // 準備此上下文以進行刷新，設置其啟動日期和，活動標志以及執行屬性源的任何初始化，校驗 xml 配置文件。
            prepareRefresh();
            // 創建beanFactory，解析配置文件為一個個beanDefinition(類似map，key是beanname，value是bean屬性（總有的有，beanname，是否懶加載，作用范圍，依賴等等）)，然后放到beanFactory中，詳細看步驟4
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
            // 設置 BeanFactory 的類加載器，添加幾個 BeanPostProcessor，手動注冊幾個特殊的 bean，詳情看步驟5
            prepareBeanFactory(beanFactory);

            try {
                //允許上下文子類在初始化的時候對beanFactory進行修改
                postProcessBeanFactory(beanFactory);

                //調用所有beanFactoryPostProcessor對beanDefinite信息進行修改，詳情見步驟6
                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();

                // Instantiate all remaining (non-lazy-init) singletons.
                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();
            }
        }
    }

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        //關閉舊的 BeanFactory (如果有)，創建新的 BeanFactory，加載 Bean 定義、注冊 Bean 等
        refreshBeanFactory();
        //返回剛剛創建的beanFactory
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {//是否已經存在beanFactory
            destroyBeans();//銷毀所有bean
            closeBeanFactory();//關閉beanFactory
        }
        try {
            //創建DefaultListableBeanFactory，為什么用這個，可以參考beanFactory繼承結構
            //ApplicationContext 繼承自 BeanFactory，但是它不應該被理解為 BeanFactory 的實現類，而是說其內部持有一個實例化的 BeanFactory（DefaultListableBeanFactory）。以后所有的 BeanFactory 相關的操作其實是委托給這個實例來處理的。
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            //序列化id
            beanFactory.setSerializationId(getId());
            //設置 BeanFactory 的兩個配置屬性：是否允許 Bean 覆蓋、是否允許循環引用
            customizeBeanFactory(beanFactory);
            //解析配置文件的封裝成beandefinite，加載到beanFactory（解析過程就不多少了，想了解的同學可以看看）
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
        if (this.allowBeanDefinitionOverriding != null) {
            //設置是否允許bean被覆蓋
            beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
        }
        if (this.allowCircularReferences != null) {
            //設置是否可以循環引用
            beanFactory.setAllowCircularReferences(this.allowCircularReferences);
        }
    }

了解完后，我們回到步驟3繼續跟代碼

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
        // 設置類加載器
        // return (this.classLoader != null ? this.classLoader : ClassUtils.getDefaultClassLoader())
        //這里是當前ApplicationContext類的類加載器
        beanFactory.setBeanClassLoader(getClassLoader());
        //設置bean的表達式解析器
        beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
        //添加注冊編輯屬性
        beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

        // 添加一個beanPostProcessor，這個挺重要
        //它會檢測該對象是否實現了xxxAware接口，并將相關的xxxAware實例注入給bean，如BeanNameAware等
        //關于xxxAware的作用可以參考這個http://www.what21.com/article/view/b_java_1484728658282.html
        beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
        // 下面幾行的意思就是，如果某個 bean 依賴于以下幾個接口的實現類，在自動裝配的時候忽略它們，
        // Spring 會通過其他方式來處理這些依賴。
        beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
        beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
        beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
        beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
        beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

        // 下面是處理幾個特殊的bean注入相應的值
        // 
        beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
        beanFactory.registerResolvableDependency(ResourceLoader.class, this);
        beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
        beanFactory.registerResolvableDependency(ApplicationContext.class, this);

        // 添加一個beanPostProcessor，在bean被實例化后，如果是ApplicationListener的子類，則加到listeners中
        beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

        // 如果bean是LoadTimeWeaver，則織入（在運行時織入，是和AOP相似，只是時機不一樣）
        if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            // 設置一個臨時的類加載器
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }

        // 手動注入一些系統環境、配置所需要的bean
        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());
        }
    }
我們回到步驟3繼續

//實例化并調用所有已注冊的BeanFactoryPostProcessor bean
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
        //getBeanFactoryPostProcessors()：獲取所有的beanFactoryPostProcessor
        PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

        // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
        // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
        if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
            beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
            beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
        }
    }

public static void invokeBeanFactoryPostProcessors(
            ConfigurableListableBeanFactory beanFactory, List beanFactoryPostProcessors) {

        // 記錄已經遍歷的beanFactoryPostProcessor
        Set processedBeans = new HashSet();
        if (beanFactory instanceof BeanDefinitionRegistry) {
            BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
            List regularPostProcessors = new LinkedList();
            List registryPostProcessors =
                    new LinkedList();

            for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
                //BeanDefinitionRegistryPostProcessor的子類放入registryPostProcessors中，其他放在regularPostProcessors中
                if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                    BeanDefinitionRegistryPostProcessor registryPostProcessor =
                            (BeanDefinitionRegistryPostProcessor) postProcessor;
                    registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
                    registryPostProcessors.add(registryPostProcessor);
                }
                else {
                    regularPostProcessors.add(postProcessor);
                }
            }

            // Do not initialize FactoryBeans here: We need to leave all regular beans
            // uninitialized to let the bean factory post-processors apply to them!
            // Separate between BeanDefinitionRegistryPostProcessors that implement
            // PriorityOrdered, Ordered, and the rest.
            //獲取beanFactory中所有BeanDefinitionRegistryPostProcessor
            String[] postProcessorNames =
                    beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);

            // 調用實現PriorityOrdered的BeanDefinitionRegistryPostProcessors
            List priorityOrderedPostProcessors = new ArrayList();
            for (String ppName : postProcessorNames) {
                if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                    priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
            registryPostProcessors.addAll(priorityOrderedPostProcessors);
            invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);

            // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
            postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
            List orderedPostProcessors = new ArrayList();
            for (String ppName : postProcessorNames) {
                if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
                    orderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                    processedBeans.add(ppName);
                }
            }
            sortPostProcessors(beanFactory, orderedPostProcessors);
            registryPostProcessors.addAll(orderedPostProcessors);
            invokeBeanDefinitionRegistryPostProcessors(orderedPostProcessors, registry);

            // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
            boolean reiterate = true;
            while (reiterate) {
                reiterate = false;
                postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
                for (String ppName : postProcessorNames) {
                    if (!processedBeans.contains(ppName)) {
                        BeanDefinitionRegistryPostProcessor pp = beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class);
                        registryPostProcessors.add(pp);
                        processedBeans.add(ppName);
                        pp.postProcessBeanDefinitionRegistry(registry);
                        reiterate = true;
                    }
                }
            }

            // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
            invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
            invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
        }

        else {
            // Invoke factory processors registered with the context instance.
            invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
        }

        // Do not initialize FactoryBeans here: We need to leave all regular beans
        // uninitialized to let the bean factory post-processors apply to them!
        String[] postProcessorNames =
                beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

        // Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
        // Ordered, and the rest.
        List priorityOrderedPostProcessors = new ArrayList();
        List orderedPostProcessorNames = new ArrayList();
        List nonOrderedPostProcessorNames = new ArrayList();
        for (String ppName : postProcessorNames) {
            if (processedBeans.contains(ppName)) {
                // skip - already processed in first phase above
            }
            else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
            }
            else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
                orderedPostProcessorNames.add(ppName);
            }
            else {
                nonOrderedPostProcessorNames.add(ppName);
            }
        }

        // First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
        sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
        invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

        // Next, invoke the BeanFactoryPostProcessors that implement Ordered.
        List orderedPostProcessors = new ArrayList();
        for (String postProcessorName : orderedPostProcessorNames) {
            orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        sortPostProcessors(beanFactory, orderedPostProcessors);
        invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

        // Finally, invoke all other BeanFactoryPostProcessors.
        List nonOrderedPostProcessors = new ArrayList();
        for (String postProcessorName : nonOrderedPostProcessorNames) {
            nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
        }
        invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

        // Clear cached merged bean definitions since the post-processors might have
        // modified the original metadata, e.g. replacing placeholders in values...
        beanFactory.clearMetadataCache();
    }

未完繼續