• IOC
  • IOC 是一個容器
  • 容器啟動的時候建立所有單例項物件
  • 我們可以直接從容器中獲取到這個物件
• Spring IOC
  • IOC 容器的啟動過程？啟動期間都做了什麼（什麼時候建立所有單例項bean）
  • IOC 是如何建立這些單例項 bean，並如何管理的，到底儲存在了那裡？

ClassPathXMLApplicationContext

public ClassPathXmlApplicationContext(String[] configLocations,
        boolean refresh, ApplicationContext parent) throws BeansException {
    super(parent);
    setConfigLocations(configLocations);
    if (refresh) {
        refresh(); // 所有單例項 bean 建立完成
    }
}

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

AbstractApplicationContext

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

        // Tell the subclass to refresh the internal bean factory.
        // Spring 解析 XML 配置檔案，將要建立的所有 bean 的配置資訊儲存起來
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // 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.
            // 用來支援國際化功能的
            initMessageSource();

            // Initialize event multicaster for this context.
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            // 留給子類的
            onRefresh();

            // Check for listener beans and register them.
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            // 初始化所有單例項 bean
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            finishRefresh();
            }

            catch (BeansException ex) {
            // Destroy already created singletons to avoid dangling resources.
            destroyBeans();

            // Reset 'active' flag.
            cancelRefresh(ex);

            // Propagate exception to caller.
            throw ex;
        }
    }
}

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    // Initialize conversion service for this context.
    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));
    }

    // Initialize LoadTimeWeaverAware beans early
    // to allow for registering their transformers early.
    String[] weaverAwareNames = beanFactory
            .getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
    for (String weaverAwareName : weaverAwareNames) {
        getBean(weaverAwareName);
    }

    // Stop using the temporary ClassLoader for type matching.
    beanFactory.setTempClassLoader(null);

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

    // Instantiate all remaining (non-lazy-init) singletons.
    // 初始化所有單例項 bean
    beanFactory.preInstantiateSingletons();
}
 

DefaultListableBeanFactory

@Override
public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in " + this);
    }
    // 所有要建立的 bean 的 id
    List<String> beanNames;
    synchronized (this.beanDefinitionMap) {
        // Iterate over a copy to allow for init methods which in turn register new bean definitions.
        // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
        beanNames = new ArrayList<String>(this.beanDefinitionNames);
    }
    // 按順序建立 bean
    for (String beanName : beanNames) {
        // 根據 beanId 獲取到 bean 的定義資訊
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        // 如果 bean (不是抽象的 && bean 是單例的 && 不是懶載入的)
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            // 如果是工廠 bean(實現了 FactoryBean 介面的 bean)
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                boolean isEagerInit;
                if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                    isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
                            @Override
                            public Boolean run() {
                                return ((SmartFactoryBean<?>) factory).isEagerInit();
                            }
                        }, getAccessControlContext());
                } else {
                    isEagerInit = (factory instanceof SmartFactoryBean &&
                            ((SmartFactoryBean<?>) factory).isEagerInit());
                }
                if (isEagerInit) {
                    getBean(beanName);
                }
            } else {
                getBean(beanName); // core！！！
            }
        }
    }
}
 

AbstractBeanFactory

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

protected <T> T doGetBean(final String name, final Class<T> requiredType
    , final Object[] args, boolean typeCheckOnly) throws BeansException {

    final String beanName = transformedBeanName(name);
    Object bean;

    // 先從已經註冊的所有單例項 bean 中看有沒有這個 bean (第一次肯定沒有)
    // Eagerly check singleton cache for manually registered singletons.
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.debug("Returning eagerly cached instance of singleton bean '" + beanName
                    + "' that is not fully initialized yet - a consequence of a circular reference");
            } else {
                logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we're already creating this bean instance:
        // We're assumably within a circular reference.
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            if (args != null) {
                // Delegation to parent with explicit args.
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }

        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            // 拿到建立當前 bean 之前需要提前建立的 bean → depends-on 屬性
            String[] dependsOn = mbd.getDependsOn();
            // 如果有，就迴圈建立
            if (dependsOn != null) {
                for (String dependsOnBean : dependsOn) {
                    if (isDependent(beanName, dependsOnBean)) {
                        throw new BeanCreationException("Circular depends-on relationship between '" +
                        beanName + "' and '" + dependsOnBean + "'");
                    }
                    registerDependentBean(dependsOnBean, beanName);
                    getBean(dependsOnBean); // 建立
                }
            }

            // Create bean instance.
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
                    @Override
                    public Object getObject() throws BeansException {
                        try {
                            return createBean(beanName, mbd, args);
                        }
                        catch (BeansException ex) {
                            // Explicitly remove instance from singleton cache: It might have been put there
                            // eagerly by the creation process, to allow for circular reference resolution.
                            // Also remove any beans that received a temporary reference to the bean.
                            destroySingleton(beanName);
                            throw ex;
                        }
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

            else if (mbd.isPrototype()) {
                // It's a prototype -> create a new instance.
                Object prototypeInstance = null;
                try {
                    beforePrototypeCreation(beanName);
                    prototypeInstance = createBean(beanName, mbd, args);
                }
                finally {
                    afterPrototypeCreation(beanName);
                }
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }

            else {
                String scopeName = mbd.getScope();
                final Scope scope = this.scopes.get(scopeName);
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
                }
                try {
                    Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
                        @Override
                        public Object getObject() throws BeansException {
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                afterPrototypeCreation(beanName);
                            }
                        }
                    });
                    bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
                } catch (IllegalStateException ex) {
                    throw new BeanCreationException(beanName, "Scope '" + scopeName
                        + "' is not active for the current thread; consider defining a scoped "
                        + "proxy for this bean if you intend to refer to it from a singleton", ex);
                }
            }
        } 
        catch (BeansException ex) {
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
    }

    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
        try {
            return getTypeConverter().convertIfNecessary(bean, requiredType);
        }
        catch (TypeMismatchException ex) {
            if (logger.isDebugEnabled()) {
                logger.debug("Failed to convert bean '" + name + "' to required type [" +
                ClassUtils.getQualifiedName(requiredType) + "]", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

DefaultSingletonBeanRegistry

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
    Assert.notNull(beanName, "'beanName' must not be null");
    synchronized (this.singletonObjects) {
        // 先從一個地方把這個 bean 拿出來
        /** Cache of singleton objects: bean name --> bean instance */
        Object singletonObject = this.singletonObjects.get(beanName);
        if (singletonObject == null) {
            if (this.singletonsCurrentlyInDestruction) {
                throw new BeanCreationNotAllowedException(beanName,
                "Singleton bean creation not allowed while the singletons of this factory are in destruction " +
                "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
            }
            beforeSingletonCreation(beanName);
            boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
            if (recordSuppressedExceptions) {
                this.suppressedExceptions = new LinkedHashSet<Exception>();
            }
            try {
                // bean 創建出來了！！！
                singletonObject = singletonFactory.getObject();
            } catch (BeanCreationException ex) {
                if (recordSuppressedExceptions) {
                    for (Exception suppressedException : this.suppressedExceptions) {
                        ex.addRelatedCause(suppressedException);
                    }
                }
                throw ex;
            } finally {
                if (recordSuppressedExceptions) {
                    this.suppressedExceptions = null;
                }
                afterSingletonCreation(beanName);
            }
            // 把 bean 物件加入 singletonObjects 這個 Map 中
            addSingleton(beanName, singletonObject);
        }
        return (singletonObject != NULL_OBJECT ? singletonObject : null);
    }
}

• 建立好的物件最終會儲存在一個 Map 中。IOC 容器之一：儲存單例項 bean 的 Map，名為 singletonObjects
  
• getBean 的時候，如果是單例項 bean，直接找 singletonObjects 這個 Map 要
  

BeanFactory 和 ApplicationContext 的區別？

• BeanFactory 是最底層的介面，ApplicationContext 是留給程式設計師使用的 IOC 容器介面；ApplicationContext 是 BeanFactory 的子介面。
• BeanFactory（物件建立）
  • 是 bean 工廠介面，負責建立 bean 例項
  • 容器中儲存所有單例 bean 的其實是一個 Map
• ApplicationContext（物件之間的動態裝配、管理）
  • 是 IOC 容器介面，更多的負責容器功能的實現
  • 可以基於 BeanFactory 建立好的 bean 之上，完成強大的容器功能
  • 容器從 Map 中獲取這個 bean，繼而進行 DI，AOP 等操作
• Spring 裡面最大的模式就是【工廠模式】。BeanFactory 如字面意思，就是一個 bean 工廠，根據使用者提供的配置資訊，建立 bean。

IOC 容器建立和銷燬的時機

• 專案啟動：IOC 容器建立
• 專案關閉：IOC 容器銷燬

ContextLoaderListener

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    public ContextLoaderListener() {}

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }

    @Override
    public void contextInitialized(ServletContextEvent event) {
        initWebApplicationContext(event.getServletContext());
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}

ContextLoader

/**
 * The root WebApplicationContext instance that this loader manages.
 */
private WebApplicationContext context;

public static WebApplicationContext getCurrentWebApplicationContext() {
    ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    if (ccl != null) {
        WebApplicationContext ccpt = currentContextPerThread.get(ccl);
        if (ccpt != null) {
            return ccpt;
        }
    }
    return currentContext;
}