前言

在上一節中，我們已經瞭解過Spring對bean從XML中提取，並且以BeanDefinition型別解析註冊到Spring的DefaultListableBeanFactory中取了，那麼，接下來就應該來看下我們在Spring中獲取一個例項過程中，bean是如何載入成為我們需要的物件的了。

通過getBean方法瞭解bean的載入

在Spring中我們是通過getBean方法去獲取，那麼我們直接開啟原始碼，深入到主要的方法doGetBean中去：

protected <T> T doGetBean(
        final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
        throws BeansException {
    //獲取beanName 
    final String beanName = transformedBeanName(name);
    Object bean;

    // Eagerly check singleton cache for manually registered singletons.
    //首先檢查在快取單例中是否已經註冊過
    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 + "'");
            }
        }
        //真正返回對應的例項，之前得到的其實是工廠bean的初始狀態
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

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

        // Check if bean definition exists in this factory.
        //嘗試從parentBeanFactory中查詢
        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);
            }
        }

        //如果不僅僅是檢查型別，那麼標記此bean已經建立
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {
            //轉化為RootBeanDefinition型別
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            //要例項化的bean中是否存在依賴，如果有依賴，那麼從中遞迴例項化歷來
            String[] dependsOn = mbd.getDependsOn();
            if (dependsOn != null) {
                for (String dependsOnBean : dependsOn) {
                    if (isDependent(beanName, dependsOnBean)) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                "Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
                    }
                    registerDependentBean(dependsOnBean, beanName);
                    getBean(dependsOnBean);
                }
            }

            // Create bean instance.
            //單例模式建立bean
            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);
            }
            //原型模式建立bean
            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.
    //檢查需要的型別是否是實際的型別
    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;
}
 

以上載入的過程是非常多的，每一步都非常的細緻。主要可以分為以下幾個步驟：

• 獲取對應的beanName
• 嘗試從快取中載入單例
• bean的例項化
• 原型模式的依賴檢查
• 嘗試從parentBeanFactory中查詢（嘗試從父類中載入）
• 類中依賴的例項化
• 針對不同的scope建立bean（預設單例，getObjectForBeanInstance才是真正進行例項化的過程）
• 型別轉換