我們先看AbstractBeanFactory.getBean方法，這個方法通過bean名稱類型等信息獲取類實例，如果實例不存在則生產。

    //---------------------------------------------------------------------
    // Implementation of BeanFactory interface
    //---------------------------------------------------------------------

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

關鍵方法doGetBean解讀如下

    /**
     * Return an instance, which may be shared or independent, of the specified bean.
     * 返回一個共享或獨立實例（單例或者多實例）
     * @param name the name of the bean to retrieve
     * bean名稱
     * @param requiredType the required type of the bean to retrieve
     * 期望返回的bean類型
     * 
 
@param args arguments to use when creating a bean instance using explicit arguments
     * (only applied when creating a new instance as opposed to retrieving an existing one)
     * 實例初始化參數（構造函數參數）
     * @param typeCheckOnly whether the instance is obtained for a type check,
     * not for actual use
     * 標誌是否僅僅檢查類型
     * 
 
@return an instance of the bean
     * 返回bean實例
     * @throws BeansException if the bean could not be created
     */
    @SuppressWarnings("unchecked")
    protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
            @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

        // bean名稱轉換，包括別名轉換和&前綴處理（&前綴用於工廠類）
        final String beanName = transformedBeanName(name);
        Object bean;

        // Eagerly check singleton cache for manually registered singletons.
        // 單例模式下優先使用已經創建的實例，已經在創建中則等待
        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.
            // 防止類環形依賴創建時出現死鎖
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // Check if bean definition exists in this factory.
            // 檢查父工廠是否存在這個類實例
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // Not found -> check parent.
                String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
                else 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);
                }
            }

            // 實例創建前上鎖
            if (!typeCheckOnly) {
                markBeanAsCreated(beanName);
            }

            try {
                // 獲取bean配置，這裏會把配置轉化為RootBeanDefinition
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                // 檢查配置是否為抽象類
                checkMergedBeanDefinition(mbd, beanName, args);

                // Guarantee initialization of beans that the current bean depends on.
                // 優先加載依賴類
                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);
                    }
                }

                // Create bean instance.
                // 創建單例
                if (mbd.isSingleton()) {
                    sharedInstance = getSingleton(beanName, () -> {
                        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;
                        }
                    });
                    // 1、 如果有其他類依賴此類，需要進行通知
                    // 2、 如果此類是工廠類，一般需要調用工廠方法返回類實例，而不是直接返回工廠類
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }
                // 創建多實例
                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 = 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 name ‘" + scopeName + "‘");
                    }
                    try {
                        Object scopedInstance = scope.get(beanName, () -> {
                            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.
        // 如果傳入了類型，則會進行類型轉換
        if (requiredType != null && !requiredType.isInstance(bean)) {
            try {
                T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
                if (convertedBean == null) {
                    throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
                }
                return convertedBean;
            }
            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;
    }

關鍵代碼getSingleton下一講繼續研究。

Spring源碼閱讀（二）