【Spring Framework】Spring源码Bean创建及依赖注入的实现

除去lazy-init属性来控制对bean的预实例化，通常依赖注入都是通过用户第一次向IOC容器索要Bean时触发的。
初始环境,定义一个Spring容器，容器中有个注解注入的类：
启动类：

public class SpringDemoMain {

    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:conf/application.xml");
    }
}

将要注入到spring容器中的类：

@Component("userServer1")
public class UserServiceImpl implements UserService {
    @Override
    public int count() {
        return 1;
    }
}

所以getBean方法是依赖注入的起点，启动main方法时，在ClassPathXmlApplicationContext中refresh初始化时，即调用AbstractApplicationContext中的refresh方法：

// 实例化所有剩余的(非惰性初始化)单例。
// 此方法会初始化用户自定义的bean注入到Spring容器中，可自行通过断点查看过程
finishBeanFactoryInitialization(beanFactory);

然后进入方法调用：

// 实例化所有剩余的(非惰性初始化)单例。
beanFactory.preInstantiateSingletons();

跟着继续调用DefaultListableBeanFactory的preInstantiateSingletons()方法：

@Override
public void preInstantiateSingletons() throws BeansException {
	if (logger.isTraceEnabled()) {
		logger.trace("Pre-instantiating singletons in " + this);
	}

	// 所有的bean
	List beanNames = new ArrayList<>(this.beanDefinitionNames);

	// 触发所有non-lazy 单例bean的初始化….
	for (String beanName : beanNames) {
		RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
		if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
			if (isFactoryBean(beanName)) {
				Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
				if (bean instanceof FactoryBean) {
					final FactoryBean factory = (FactoryBean) bean;
					boolean isEagerInit;
					if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
						isEagerInit = AccessController.doPrivileged((PrivilegedAction)
										((SmartFactoryBean) factory)::isEagerInit,
								getAccessControlContext());
					}
					else {
						isEagerInit = (factory instanceof SmartFactoryBean &&
								((SmartFactoryBean) factory).isEagerInit());
					}
					if (isEagerInit) {
						getBean(beanName);
					}
				}
			}
			else {
				getBean(beanName);
			}
		}
	}

	// 为所有适用的bean触发初始化后回调…
	for (String beanName : beanNames) {
		Object singletonInstance = getSingleton(beanName);
		if (singletonInstance instanceof SmartInitializingSingleton) {
			final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
			if (System.getSecurityManager() != null) {
				AccessController.doPrivileged((PrivilegedAction