# [七] Bean的实例化-循环依赖

@Service
public class CircularRefA {
    CircularRefA(){
        System.out.println("Instance of CircularRefA ======");
    }
    @Autowired
    private CircularRefB circularRefB;
}

@Service
public class CircularRefB {
    CircularRefB(){
        System.out.println("Instance of CircularRefB  ======");
    }
    @Autowired
    private CircularRefA circularRefA;
}

public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
		// We must check each value to see whether it requires a runtime reference
		// to another bean to be resolved.
		if (value instanceof RuntimeBeanReference) {
			RuntimeBeanReference ref = (RuntimeBeanReference) value;
			return resolveReference(argName, ref);
		}
}

private Object resolveReference(Object argName, RuntimeBeanReference ref) {
		// 触发getBean 实例化
		bean = this.beanFactory.getParentBeanFactory().getBean(refName);
	}

		//寻找当前正在实例化的bean中有@Autowired注解的构造函数
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
			//如果ctors不为空，就说明构造函数上有@Autowired注解
			return autowireConstructor(beanName, mbd, ctors, args);
		}

if (instanceCandidate instanceof Class) {
		//在这里真正拿到了构造函数中依赖的实例
		instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
			}

public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
			throws BeansException {
		// 将引用类型实例化
		return beanFactory.getBean(beanName);
	}

try {
	... 
	
	//注册bean销毁时的类DisposableBeanAdapter
	registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
	throw new BeanCreationException(
		mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}

protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {
		AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null);
		if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {
			// 如果是单例 bean
			if (mbd.isSingleton()) {
				// 对这个bean注册一个销毁的Adapter对象，对给定的bean执行销毁工作
				// 过滤了 DestructionAwareBeanPostProcessor 类型的接口
				registerDisposableBean(beanName,
						new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
			}
			else {
				// 拿到bean的作用域
				Scope scope = this.scopes.get(mbd.getScope());
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + mbd.getScope() + "'");
				}
				scope.registerDestructionCallback(beanName,
						new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));
			}
		}
	}

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
    public ContextLoaderListener() {
    }

    public ContextLoaderListener(WebApplicationContext context) {
        super(context);
    }

    public void contextInitialized(ServletContextEvent event) {
        this.initWebApplicationContext(event.getServletContext());
    }
	//  servlet中的销毁方法
    public void contextDestroyed(ServletContextEvent event) {
       // 该方法最终会调用Spring的DisposableBeanAdapter类的destroy()
        this.closeWebApplicationContext(event.getServletContext());
        ContextCleanupListener.cleanupAttributes(event.getServletContext());
    }
}