spring容器加载分析 四Bean实例化

spring中按照Bean的scop可以被定义为两种模式：prototype（多例）和singleton（单例），按照Bean的实例化方式可以被定义为预加载和懒加载（@Lazy）。

prototype（多例）和懒加载（@Lazy）的Bean被延迟到使用时才进行实例化，而非延迟加载的单例Bean则在容器刷新（refresh）时通过finishBeanFactoryInitialization方法进行实例化，只是实例化的时机和处理循环依赖的方式不同，这里只分析非延迟单例Bean的实例化过程。

Bean实例化的方法是在AbstractApplicationContext类中通过doGetBean方法进行的：

protected  T doGetBean(
        final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
        throws BeansException {
    
    // 提取对应的beanName，如果是工厂Bean则加前缀&
    final String beanName = transformedBeanName(name);
    Object bean;
    // 尝试从缓存或者singletonFactories中的ObjectFactory中获取  
    Object sharedInstance = getSingleton(beanName);// step 1
    if (sharedInstance != null && args == null) {
        // 返回对应的实例
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);//step 2
    }else {
        // 只有在单例的情况才会尝试解决循环依赖
        if (isPrototypeCurrentlyInCreation(beanName)) { // step 3
            throw new BeanCurrentlyInCreationException(beanName);
        }
        // Check if bean definition exists in this factory.
        BeanFactory parentBeanFactory = getParentBeanFactory();
        // 如果beanDefinitionMap中，也就是所有已加载的类中不包含beanName则尝试从parentBeanFactory中检查
        if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
            // Not found -> check parent.
            String nameToLookup = originalBeanName(name);
            if (args != null) {
                // 递归查找
                return (T) parentBeanFactory.getBean(nameToLookup, args);
            }
            else {
                // No args -> delegate to standard getBean method.
                return parentBeanFactory.getBean(nameToLookup, requiredType);
            }
        }
        // 不仅仅是做类型检查，而是创建BEAN则记录
        if (!typeCheckOnly) {
            markBeanAsCreated(beanName);
        }

        try {// 将各种类型的BeanDefinition转换成RootBeanDefinition
            //  如果指定的BEAN为子BEAN的话，则合并父BEAN的属性
            final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
            checkMergedBeanDefinition(mbd, beanName, args);

            // Guarantee initialization of beans that the current bean depends on.
            String[] dependsOn = mbd.getDependsOn();
            // 如果存在强制依赖，则递归实例化依赖的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
                    registerDependentBean(dep, beanName);
                    // 实例化依赖的Bean
                    getBean(dep);
                }
            }

            // 实例化依赖的bean后则进行实例化
            // 单例模式创建
            if (mbd.isSingleton()) {
                sharedInstance = getSingleton(beanName, new ObjectFactory