源碼解析 DefaultListableBeanFactory

DefaultListableBeanFactory，是 GenericApplicationContext 和 AbstractRefreshableApplicationContext 的(bean工廠)默認策略，而這兩者是所有 xxxApplicationContext 的基類，因此 DefaultListableBeanFactory 可以說是 ApplicationContext 的標準配置，非常具有學習價值

• 值得一提的是，Spring 3.1 推薦的做法是採用 DefaultListableBeanFactory + XmlBeanDefinitionReader（不要將兩者捆綁起來），前者用於管理Bean工廠，而後者可讀取多種多樣的XML資源，具有高度可配置性，這種模式得到大量使用，例如：
  
  • XmlWebApplicationContext 採用了 DefaultListableBeanFactory + XmlBeanDefinitionReader，後者專門用於讀取XML配置
  • AnnotationConfigApplicationContext 採用了 DefaultListableBeanFactory + AnnotatedBeanDefinitionReader，後者專門用於讀取註解配置

繼承體系

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1.查看getBean(String name)源碼

DefaultListableBeanFactory 的 getBean(String name) 方法是由其父類 AbstractBeanFactory 實現的，查看其源碼

@Override
public Object getBean(String name) throws BeansException {
    //第二個參數：bean的類型
    //第三個參數：僅用於實例創建過程所採用的參數
    //第四個參數：貌似是一種優化措施
    return doGetBean(name, null, null, false);
}

//......

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

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

    //快速檢查singleton實例的緩存
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            //判斷該singleton實例是否處於“正在創建”
            if (isSingletonCurrentlyInCreation(beanName)) {
                //是的話，說明這是“循環引用”現象，返回的是提前創建的未完全初始化的singleton實例
                logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                        "' that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                //否的話，說明singleton實例已完成初始化並緩存
                logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        //bean instance -> 返回它本身
        //FactoryBean -> 創建一個Object，並返回
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        //判斷該prototype實例是否處於“正在創建”(在當前線程)
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        //獲取父Bean工廠(父容器)
        BeanFactory parentBeanFactory = getParentBeanFactory();
        //判斷bean definition是否存在
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            //不存在 -> 遞歸查找父Bean工廠(父容器)
            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);
            }
        }

        //標記該bean爲“已創建”，可用於後續的重複創建實例優化緩存效果
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            //如果目標是child bean definition，呢麼會遞歸融合parent bean definition的所有屬性
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            //BeanDefinition校驗(比如Bean不能是Abstract)
            checkMergedBeanDefinition(mbd, beanName, args);

            //優先初始化當前Bean所依賴的其他Bean
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dep : dependsOn) {
                    if (isDependent(beanName, dep)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
                    }
                    registerDependentBean(dep, beanName);
                    getBean(dep);
                }
            }

            //獲取singleton實例，必要時創建一個新的並註冊(比如延遲加載的場景)
            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 instance -> 返回它本身
                //FactoryBean -> 創建一個Object，並返回
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }
            //創建一個新的prototype實例
            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 instance -> 返回它本身
                //FactoryBean -> 創建一個Object，並返回
                bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
            }
            //其他作用域(request、session、ServletContext等等)
            else {
                //獲取Bean的作用域
                String scopeName = mbd.getScope();
                final Scope scope = this.scopes.get(scopeName);
                if (scope == null) {
                    throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
                }
                try {
                    //從目標作用域的底層存儲機制中獲取該實例，必要時創建一個新的
                    //----比如 request.getAttribute("")就是“request”作用域的底層存儲機制
                    //注：如果類似於singleton(長作用域)依賴一個request(短作用域)的場景，建議採用“Spring作用域代理”
                    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 instance -> 返回它本身
                    //FactoryBean -> 創建一個Object，並返回
                    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;
        }
    }

    //類型檢查
    if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
        //如果類型不符，嘗試進行“類型轉換”
        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;
}