5 bean的加载

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

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

    //提取对应的beanName
    String beanName = transformedBeanName(name);
    Object bean;

    // Eagerly check singleton cache for manually registered singletons.
    /**
		 * 检查缓存中或者实例工厂中是否有对应的实例
		 * 为什么首先会使用这段代码呢，
		 * 因为在创建单例bean的时候会存在依赖注入的情况，而在创建依赖的时候为了避免循环依赖
		 * Spring创建bean的原则是不等bean创建完成就会创建bean的ObjectFactory提早曝光
		 * 也就是将ObjectFactory加入到缓存中，一旦下一个bean创建需要依赖上个bean则直接使用ObjectFactory
		 */
    //直接尝试从缓存获取或者singletonFactories中的ObjectFactory中获取
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isTraceEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.trace("Returning eagerly cached instance of singleton bean ‘" + beanName +
                             "‘ that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.trace("Returning cached instance of singleton bean ‘" + beanName + "‘");
            }
        }
        //返回对应的实例，有时候存在诸如BeanFacotry的情况并不是直接返回实例本身而是返回指定方法的实例
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else {
        // Fail if we‘re already creating this bean instance:
        // We‘re assumably within a circular reference.
        //只有在单例情况下才会尝试解决循环依赖，原型模式情况下，如果存在
        //A中有B属性，B中有A属性，那么依赖注入的时候，就会产生A中还未创建完成的时候
        //因为对于B的创建再次返回创建A，造成循环依赖，也就是下面这种情况。
        //是否正在创建（检测scope=property的循环依赖）
        if (isPrototypeCurrentlyInCreation(beanName)) {
            throw new BeanCurrentlyInCreationException(beanName);
        }

        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        //如果beanDefinitionMap中也就是所有已经加载的类中不包括beanName则尝试从
        //parentBeanFactory中检测
        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.
                //递归到BeanFactory中寻找
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else if (requiredType != null) {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
            else {
                return (T) parentBeanFactory.getBean(nameToLookup);
            }
        }

        //如果不是仅仅做类型检查那么创建bean，这里要进行记录
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
            .tag("beanName", name);
        try {
            if (requiredType != null) {
                beanCreation.tag("beanType", requiredType::toString);
            }
            //将存储XML配置文件的GernericBeanDefinition转换为RootBeandefinition，如果指定
            //BeanName是子bean的话同时会合并父类的相关属性
            RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            String[] dependsOn = mbd.getDependsOn();
            //若存在依赖则需要递归实例化依赖的bean
            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 + "‘");
                    }
                    //缓存依赖调用
                    registerDependentBean(dep, beanName);
                    try {
                        getBean(dep);
                    }
                    catch (NoSuchBeanDefinitionException ex) {
                        throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                                        "‘" + beanName + "‘ depends on missing bean ‘" + dep + "‘", ex);
                    }
                }
            }

            // Create bean instance.
            //实例化依赖的bean后便可以实例化mba本身了
            //singleton模式的创建
            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;
                    }
                });
                bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
            }

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

            else {
                //指定scope上的实例化bean
                String scopeName = mbd.getScope();
                if (!StringUtils.hasLength(scopeName)) {
                    throw new IllegalStateException("No scope name defined for bean ′" + beanName + "‘");
                }
                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 ScopeNotActiveException(beanName, scopeName, ex);
                }
            }
        }
        catch (BeansException ex) {
            beanCreation.tag("exception", ex.getClass().toString());
            beanCreation.tag("message", String.valueOf(ex.getMessage()));
            cleanupAfterBeanCreationFailure(beanName);
            throw ex;
        }
        finally {
            beanCreation.end();
        }
    }

    // Check if required type matches the type of the actual bean instance.
    //检查需要的类型是否符合bean的实际类型
    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.isTraceEnabled()) {
                logger.trace("Failed to convert bean ‘" + name + "‘ to required type ‘" +
                             ClassUtils.getQualifiedName(requiredType) + "‘", ex);
            }
            throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
        }
    }
    return (T) bean;
}

• 去除FactoryBean的修饰符，也就是如果name="&aa"，那么会去除&
• 取指定的alias所表示的最终beanName，例如别名A指向别名B，别名B指向别名C的Bean，那么返回C。