IOC（Inversion of Control）极谊，即控制反轉(zhuǎn)什荣，是面向?qū)ο缶幊讨械囊环N設(shè)計(jì)原則，可以用來減低代碼之間的耦合度怀酷。在Spring中，IOC意味著對(duì)象和資源的創(chuàng)建和獲取統(tǒng)一交給了容器嗜闻，由容器控制對(duì)象的生命周期和對(duì)象間的依賴關(guān)系蜕依。

而DI（Dependency Injection），即依賴注入琉雳，是實(shí)現(xiàn)IOC的一種手段样眠。DI意味著組件之間依賴關(guān)系由容器在運(yùn)行期決定，形象的說翠肘，即由容器動(dòng)態(tài)的將某個(gè)依賴關(guān)系注入到組件之中檐束。

要理解Spring IOC的實(shí)現(xiàn)，首先需要了解下面兩個(gè)接口：BeanFactory和ApplicationContext束倍。

1. BeanFactory

BeanFactory是Spring中的Bean工廠， 用于Spring中Bean的生命周期管理绪妹。它的主要接口如下：

public interface BeanFactory {
    /*
     * 四個(gè)不同形式的getBean方法甥桂，獲取實(shí)例
     */
    Object getBean(String name) throws BeansException;

    <T> T getBean(String name, Class<T> requiredType) throws BeansException;

    <T> T getBean(Class<T> requiredType) throws BeansException;

    Object getBean(String name, Object... args) throws BeansException;

    boolean containsBean(String name); // 是否存在

    boolean isSingleton(String name) throws NoSuchBeanDefinitionException;// 是否為單例

    boolean isPrototype(String name) throws NoSuchBeanDefinitionException;// 是否為原型（多實(shí)例）

    boolean isTypeMatch(String name, Class<?> targetType)
            throws NoSuchBeanDefinitionException;// 名稱、類型是否匹配

    Class<?> getType(String name) throws NoSuchBeanDefinitionException; // 獲取類型

    String[] getAliases(String name);// 根據(jù)實(shí)例的名字獲取實(shí)例的別名
    
    //......
}

2. ApplicationContext

ApplicationContext是BeanFactory的子接口邮旷，在擁有所有BeanFactory功能的基礎(chǔ)上黄选，它還繼承了一些其他接口：

ApplicationContext.jpg

因此，ApplicationContext擁有一些高階功能婶肩，例如：

• HierarchicalBeanFactory 是一個(gè)具有層級(jí)關(guān)系的 BeanFactory办陷，擁有屬性 parentBeanFactory。ListableBeanFactory 實(shí)現(xiàn)了枚舉方法可以列舉出當(dāng)前 BeanFactory 中所有的 bean 對(duì)象而不必根據(jù) name 一個(gè)一個(gè)的獲取律歼。
• MessageSource接口提供國際化功能
• ApplicationEventPublisher提供事件發(fā)布訂閱功能
• ResourceLoader提供資源加載功能
• EnvironmentCapable用于獲取 Environment 的功能

ApplicationContext常用實(shí)現(xiàn)類包括：ClassPathXMLApplicationContext民镜， FileSystemXmlApplicationContext，AnnotationConfigApplicationContext和WebApplicationContext苗膝。

3. 容器入口

我們以ClassPathXmlApplicationContext為例殃恒，下面是其構(gòu)造函數(shù)：

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
            throws BeansException {

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

Spring首先讀取Config文件位置，然后調(diào)用refresh方法進(jìn)行BeanFactory的構(gòu)建辱揭。

4. 容器構(gòu)建流程

下面是AbstractApplicationContext中refresh方法的實(shí)現(xiàn)：

public void refresh() throws BeansException, IllegalStateException {
  // refresh過程只能一個(gè)線程處理离唐，不允許并發(fā)執(zhí)行
  synchronized (this.startupShutdownMonitor) {
    prepareRefresh();
    ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
    prepareBeanFactory(beanFactory);
    try {
      postProcessBeanFactory(beanFactory);
      invokeBeanFactoryPostProcessors(beanFactory);
      registerBeanPostProcessors(beanFactory);
      initMessageSource();
      initApplicationEventMulticaster();
      onRefresh();
      registerListeners();
      finishBeanFactoryInitialization(beanFactory);
      finishRefresh();
    }
    catch (BeansException ex) {
      if (logger.isWarnEnabled()) {
        logger.warn("Exception encountered during context initialization - " +
            "cancelling refresh attempt: " + ex);
      }
      destroyBeans();
      cancelRefresh(ex);
      throw ex;
    }
    finally {
      resetCommonCaches();
    }
  }
}

主要的流程圖如下：

ApplicationContext refresh.png

4.1 prepareRefresh

執(zhí)行一些初始化操作丙躏，包括記錄當(dāng)前時(shí)間舅世，初始化變量等嫩码。其源碼如下：

protected void prepareRefresh() {
        this.startupDate = System.currentTimeMillis();
        this.closed.set(false);
        this.active.set(true);

        if (logger.isInfoEnabled()) {
            logger.info("Refreshing " + this);
        }

        // Initialize any placeholder property sources in the context environment
        initPropertySources();

        // Validate that all properties marked as required are resolvable
        // see ConfigurablePropertyResolver#setRequiredProperties
        getEnvironment().validateRequiredProperties();

        // Allow for the collection of early ApplicationEvents,
        // to be published once the multicaster is available...
        this.earlyApplicationEvents = new LinkedHashSet<>();
}

4.2 obtainFreshBeanFactory

創(chuàng)建一個(gè)BeanFactory鳍寂。此方法會(huì)調(diào)用refreshBeanFactory方法饮怯，其源碼如下：

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
}

從源碼上看，refreshBeanFactory方法的主要流程如下：

1. 若已經(jīng)存在beanFactory则披，銷毀它
2. 創(chuàng)建新的beanFactory(new DefaultListableBeanFactory(getInternalParentBeanFactory()))
3. 通過customizeBeanFactory定制BeanFactory的屬性搂誉；
4. 加載BeanDefinition到當(dāng)前BeanFactory中（對(duì)于ClassPathXmlApplicationContext來說，會(huì)調(diào)用XmlBeanDefinitionReader讀取所有的Bean定義熟呛，封裝為BeanDefinition注冊到BeanFactory中）宽档；

ApplicationContext實(shí)現(xiàn)了BeanFactory接口，其內(nèi)部是通過聚合一個(gè)DefaultListableBeanFactory來實(shí)現(xiàn)所有的BeanFactory功能的庵朝。

下面時(shí)BeanDefinition接口的主要定義：

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
    //單例作用域
    String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON;

    //原型作用域
    String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE;

    String getParentName();
    void setParentName(String parentName);

    String getBeanClassName();
    void setBeanClassName(String beanClassName);

    String getScope();
    void setScope(String scope);

    boolean isLazyInit();
    void setLazyInit(boolean lazyInit);

    boolean isPrimary();
    void setPrimary(boolean primary);

    boolean isSingleton();
    boolean isPrototype();
    //......
}

DefaultListableBeanFactory中維護(hù)了一個(gè)Map用于存儲(chǔ)所有的BeanDefinition：

/** Map of bean definition objects, keyed by bean name */
private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);

另外吗冤，DefaultSingletonBeanRegistry（DefaultListableBeanFactory的父類）中維護(hù)了一個(gè)Map用于存儲(chǔ)所有的Bean：

/** Cache of singleton objects: bean name --> bean instance */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

4.3 prepareBeanFactory

在BeanFactory已經(jīng)創(chuàng)建成功后，Spring會(huì)調(diào)用prepareBeanFactory(beanFactory)來向已經(jīng)創(chuàng)建的BeanFactory注冊一些特定的Bean九府，包括environment等：

   // 如果沒有定義 "environment" 這個(gè) bean椎瘟，那么 Spring 會(huì) "手動(dòng)" 注冊一個(gè)
   if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
      beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
   }
   // 如果沒有定義 "systemProperties" 這個(gè) bean，那么 Spring 會(huì) "手動(dòng)" 注冊一個(gè)
   if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
      beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
   }
   // 如果沒有定義 "systemEnvironment" 這個(gè) bean侄旬，那么 Spring 會(huì) "手動(dòng)" 注冊一個(gè)
   if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
      beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
   }

另外肺蔚，還會(huì)注冊一些BeanPostProcessor，包括ApplicationContextAwareProcessor儡羔，ApplicationListenerDetector等：

   // 添加一個(gè) BeanPostProcessor宣羊，這個(gè) processor 比較簡單，
   // 實(shí)現(xiàn)了 Aware 接口的幾個(gè)特殊的 beans 在初始化的時(shí)候汰蜘，這個(gè) processor 負(fù)責(zé)回調(diào)
   beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));

   // 這個(gè) BeanPostProcessor 也很簡單段只，在 bean 實(shí)例化后，如果是 ApplicationListener 的子類鉴扫，
   // 那么將其添加到 listener 列表中赞枕，可以理解成：注冊事件監(jiān)聽器
   beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

4.3 postProcessBeanFactory

在BeanFactory基本完成后，Spring調(diào)用postProcessBeanFactory(beanFactory)來進(jìn)行后續(xù)的針對(duì)BeanFactory的操作坪创。AbstractApplicationContext中的默認(rèn)實(shí)現(xiàn)為空炕婶。而其子類AnnotationConfigEmbeddedWebApplicationContext對(duì)應(yīng)的postProcessBeanFactory方法則會(huì)掃描basePackage下的bean并注冊到BeanFactory中。

4.4 invokeBeanFactoryPostProcessors

在BeanFactory創(chuàng)建完畢后莱预，Spring調(diào)用invokeBeanFactoryPostProcessors(beanFactory)來運(yùn)行所有已經(jīng)注冊的BeanFactoryPostProcessor柠掂，從而進(jìn)行修改BeanFactory或修改BeanFactory中已經(jīng)存在的BeanDefinition。

public interface BeanFactoryPostProcessor {

    /**
     * Modify the application context's internal bean factory after its standard
     * initialization. All bean definitions will have been loaded, but no beans
     * will have been instantiated yet. This allows for overriding or adding
     * properties even to eager-initializing beans.
     * @param beanFactory the bean factory used by the application context
     * @throws org.springframework.beans.BeansException in case of errors
     */
    void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;
}

常見的BeanFactoryPostProcessor包括ConfigurationClassPostProcessor依沮，PropertyPlaceholderConfigurer等涯贞。

ConfigurationClassPostProcessor用于讀取所有已經(jīng)注冊的Configuration類型的Bean，然后注入Configuration類中定義的Bean危喉，最后判斷該Configuration是否擁有ComponentScan注解宋渔，如果擁有則掃描basePackage下的所有bean并注冊到BeanFactory中。

如果spring配置文件中包含了<context:annotation-config/> 或 <context:component-scan/>辜限，spring則會(huì)自動(dòng)啟用ConfigurationClassPostProcessor皇拣。

invokeBeanFactoryPostProcessors(beanFactory)方法內(nèi)部首先對(duì)所有的BeanFactoryPostProcessor進(jìn)行排序，然后依次調(diào)用。

4.5 registerBeanPostProcessors

registerBeanPostProcessors方法會(huì)對(duì)所有BeanFactory內(nèi)的BeanPostProcessor類型的Bean進(jìn)行排序氧急，然后調(diào)用registerBeanPostProcessors注冊這些BeanPostProcessor颗胡，最終在createBean時(shí)會(huì)依次調(diào)用這些BeanPostProcessor來customize最終生成的Bean。BeanPostProcessor和BeanFactoryPostProcessor的區(qū)別在于吩坝，BeanPostProcessor用于實(shí)例化bean后再customize bean毒姨，而BeanFactoryPostProcessor則用于BeanFactory創(chuàng)建完成后customize BeanFactory。

BeanPostProcessor接口如下：

public interface BeanPostProcessor {
    // inti方法調(diào)用之前的操作
    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;
    // init方法調(diào)用之后的操作
    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
}

BeanPostProcessor調(diào)用時(shí)bean已經(jīng)被實(shí)例化钉寝，并且所有該注入的屬性都已經(jīng)被注入手素，是一個(gè)完整的Bean。下面是postProcessBeforeInitialization和postProcessAfterInitialization的調(diào)用時(shí)間：

1. 在Bean實(shí)例化完成后調(diào)用BeanPostProcessor.postProcessBeforeInitialization
2. 如果Bean實(shí)現(xiàn)了InitializingBean接口瘩蚪，則調(diào)用InitializingBean.afterPropertiesSet方法
3. 如果Bean中配置了init-method，則調(diào)用init-method
4. 調(diào)用BeanPostProcessor.postProcessAfterInitialization

InitializingBean接口的定義如下：

public interface InitializingBean {

    /**
     * Invoked by a BeanFactory after it has set all bean properties supplied
     * (and satisfied BeanFactoryAware and ApplicationContextAware).
     * <p>This method allows the bean instance to perform initialization only
     * possible when all bean properties have been set and to throw an
     * exception in the event of misconfiguration.
     * @throws Exception in the event of misconfiguration (such
     * as failure to set an essential property) or if initialization fails.
     */
    void afterPropertiesSet() throws Exception;

}

常見的BeanPostProcessor包括ApplicationContextAwareProcessor稿黍，ApplicationListenerDetector疹瘦，AutowiredAnnotationBeanPostProcessor等。

ApplicationContextAwareProcessor用于調(diào)用Bean的Aware的接口（包括ApplicationContextAware巡球，MessageSourceAware言沐，ApplicationEventPublisherAware等）。例如如果Bean實(shí)現(xiàn)了ApplicationContextAware接口酣栈，在Bean實(shí)例化結(jié)束后险胰，會(huì)調(diào)用ApplicationContextAware接口的setApplicationContext(ApplicationContext applicationContext)方法來cutomize實(shí)例化后的Bean。

ApplicationListenerDetector用于如果Bean實(shí)現(xiàn)了ApplicationListener接口矿筝，那么它會(huì)將這個(gè)Bean加入ApplicationEventMulticaster的listener列表中起便。

AutowiredAnnotationBeanPostProcessor用于注入所有使用了@Autowired注解的屬性。

4.6 initMessageSource

MessageSource是Spring提供國際化的接口窖维。ApplicationContext實(shí)現(xiàn)了MessageSource接口榆综，其內(nèi)部是通過聚合一個(gè)MessageSource對(duì)象來實(shí)現(xiàn)的。initMessageSource方法主要是為了初始化ApplicationContext內(nèi)部的MessageSource對(duì)象铸史。如果BeanFactory中存在一個(gè)MessageSource Bean鼻疮，那么直接將這個(gè)Bean賦值給ApplicationContext內(nèi)部的MessageSource對(duì)象。如果沒有琳轿，則會(huì)構(gòu)造一個(gè)new DelegatingMessageSource()對(duì)象判沟。

4.7 initApplicationEventMulticaster

initApplicationEventMulticaster顧名思義，是為了初始化ApplicationEventMulticaster崭篡。ApplicationContext實(shí)現(xiàn)了ApplicationEventPublisher接口挪哄，從而具備了事件發(fā)布功能。其內(nèi)部是通過聚合一個(gè)ApplicationEventMulticaster實(shí)現(xiàn)的琉闪。下面是初始化代碼：

protected void initApplicationEventMulticaster() {
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
            this.applicationEventMulticaster =
                    beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
            }
        }
        else {
            this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
            beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
                        APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
                        "': using default [" + this.applicationEventMulticaster + "]");
            }
        }
    }

和initMessageSource方法類似中燥，如果BeanFactory中存在一個(gè)名為applicationEventMulticaster的對(duì)象，則將其賦值給內(nèi)部的this.applicationEventMulticaster塘偎。否則疗涉，直接通過new SimpleApplicationEventMulticaster(beanFactory)實(shí)例化一個(gè)ApplicationEventMulticaster拿霉。之后ApplicationContext所有事件發(fā)布功能都會(huì)轉(zhuǎn)交給內(nèi)部的applicationEventMulticaster對(duì)象。

ApplicationEventMulticaster主要負(fù)責(zé)事件的發(fā)布和監(jiān)聽者的注冊咱扣。主要接口如下：

// 注冊監(jiān)聽者
void addApplicationListener(ApplicationListener<?> listener);
//發(fā)布事件
void multicastEvent(ApplicationEvent event);
//......

ApplicationEventMulticaster的實(shí)現(xiàn)類內(nèi)部擁有一個(gè)Set<ApplicationListener<?>> applicationListeners監(jiān)聽者集合绽淘。在掃描出所有實(shí)現(xiàn)了ApplicationListener接口的類后，通過以下方法創(chuàng)建一個(gè)ApplicationListener<?>對(duì)象闹伪，然后添加到Set<ApplicationListener<?>> applicationListeners監(jiān)聽者集合中沪铭。

public ApplicationListener<?> createApplicationListener(String beanName, Class<?> type, Method method) {
   return new ApplicationListenerMethodAdapter(beanName, type, method);
}

4.8 onRefresh

擴(kuò)展方法，由子類實(shí)現(xiàn)用于初始化其他的特殊Bean偏瓤。AbstractApplicationContext默認(rèn)實(shí)現(xiàn)為空杀怠。

4.9 registerListeners

注冊事件監(jiān)聽器。將BeanFactory中存在的ApplicationListener類型的Bean注冊到ApplicationContext內(nèi)部的ApplicationEventMulticaster中厅克。

4.10 finishBeanFactoryInitialization

通過調(diào)用beanFactory.preInstantiateSingletons()實(shí)例化所有非懶加載的單例Bean赔退。beanFactory.preInstantiateSingletons()則會(huì)遍歷所有的Bean并調(diào)用getBean(beanName)方法。

下面是getBean(beanName)方法的流程：

1. 首先通過getSingleton(beanName)從signleton緩存池中獲取Bean
2. 如果獲取不到证舟，則根據(jù)當(dāng)前的beanfactory獲取父一級(jí)的beanfactory硕旗，然后逐級(jí)遞歸的查找我們需要的bean
3. 獲取仍然不到則根據(jù)final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName)拿到BeanDefinition，然后通過createBean(beanName, mbd, args)進(jìn)行Bean的創(chuàng)建女责。
4. createBean(beanName, mbd, args)方法調(diào)用doCreateBean(beanName, mbdToUse, args)方法來創(chuàng)建Bean漆枚。
5. doCreateBean(beanName, mbdToUse, args)方法主要分為四步：
   (1) instanceWrapper = createBeanInstance(beanName, mbd, args) 通過反射創(chuàng)建出一個(gè)Bean實(shí)例
   (2)applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName) 調(diào)用所有的MergedBeanDefinitionPostProcessor來customize創(chuàng)建出的Bean。
   (3) populateBean(beanName, mbd, instanceWrapper) 填充所有的屬性
   (4) exposedObject = initializeBean(beanName, exposedObject, mbd) 調(diào)用applyBeanPostProcessorsBeforeInitialization（即BeanPostProcessor.beforeInitialization）抵知，invokeInitMethods（即配置文件中配置的init-method）和applyBeanPostProcessorsAfterInitialization（即BeanPostProcessor.afterInitialization）

上文中我們提到的AutowiredAnnotationBeanPostProcessor是一種BeanPostProcessor（然而它的beforeInitialization和afterInitialization都為空）墙基，它還實(shí)現(xiàn)了MergedBeanDefinitionPostProcessor接口。因此上述第（2）步會(huì)調(diào)用AutowiredAnnotationBeanPostProcessor.postProcessMergedBeanDefinition方法刷喜，然而這個(gè)方法并不會(huì)直接向Bean中注入所有的@Autowired屬性碘橘。下面是這個(gè)方法的實(shí)現(xiàn)：

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class<?> beanType, String beanName) {
    if (beanType != null) {
        InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);
        metadata.checkConfigMembers(beanDefinition);
    }
}

AutowiredAnnotationBeanPostProcessor.postProcessMergedBeanDefinition的作用僅僅是緩存此Bean的所有@Autowired屬性信息。

在執(zhí)行第（3）步populateBean(beanName, mbd, instanceWrapper) 時(shí)吱肌，會(huì)調(diào)用其AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues方法痘拆，從而完成了所有@Autowired屬性的注入：

@Override
    public PropertyValues postProcessPropertyValues(
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeanCreationException {

        InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
        try {
            metadata.inject(bean, beanName, pvs);
        }
        catch (BeanCreationException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
        }
        return pvs;
    }

更多Bean初始化流程，請參考：https://www.shangyang.me/2017/04/01/spring-core-container-sourcecode-analysis-beans-instantiating-process/

4.11 finishRefresh

執(zhí)行refresh收尾工作氮墨。包括初始化LifecycleProcessor纺蛆，發(fā)布ContextRefreshedEvent事件等。其源碼如下：

protected void finishRefresh() {
        // Clear context-level resource caches (such as ASM metadata from scanning).
        clearResourceCaches();

        // Initialize lifecycle processor for this context.
        initLifecycleProcessor();

        // Propagate refresh to lifecycle processor first.
        getLifecycleProcessor().onRefresh();

        // Publish the final event.
        publishEvent(new ContextRefreshedEvent(this));

        // Participate in LiveBeansView MBean, if active.
        LiveBeansView.registerApplicationContext(this);
}

參考文章：

• https://www.shangyang.me/2017/04/01/spring-core-container-sourcecode-analysis-beans-instantiating-process/