Spring Ioc 之 Bean的加载（三）：各个 scope 的 Bean 创建

// Create bean instance.
	//创建单例Bean
	if (mbd.isSingleton()) {
	//这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
	sharedInstance = getSingleton(beanName, () -> {
		try {
			//创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
			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.
			//显式地从容器单例模式Bean缓存中清除实例对象
			destroySingleton(beanName);
			throw ex;
		}
	});
	//获取给定Bean的实例对象
	bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
	}

// DefaultSingletonBeanRegistry.java

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(beanName, "Bean name must not be null");
//全局加锁
synchronized (this.singletonObjects) {
	// 从缓存中获取单例bean
	// 因为 singleton 模式其实就是复用已经创建的 bean 所以这步骤必须检查
	Object singletonObject = this.singletonObjects.get(beanName);
	if (singletonObject == null) {
		//是否正在销毁该bean
		if (this.singletonsCurrentlyInDestruction) {
			throw new BeanCreationNotAllowedException(beanName,
					"Singleton bean creation not allowed while singletons of this factory are in destruction " +
					"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
		}
		if (logger.isDebugEnabled()) {
			logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
		}
		// 加载前置处理
		beforeSingletonCreation(beanName);
		boolean newSingleton = false;
		boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
		if (recordSuppressedExceptions) {
			this.suppressedExceptions = new LinkedHashSet<>();
		}
		try {
			// 初始化 bean
			// 这个过程其实是调用 createBean() 方法
			singletonObject = singletonFactory.getObject();
			newSingleton = true;
		}
		catch (IllegalStateException ex) {
			// Has the singleton object implicitly appeared in the meantime ->
			// if yes, proceed with it since the exception indicates that state.
			singletonObject = this.singletonObjects.get(beanName);
			if (singletonObject == null) {
				throw ex;
			}
		}
		catch (BeanCreationException ex) {
			if (recordSuppressedExceptions) {
				for (Exception suppressedException : this.suppressedExceptions) {
					ex.addRelatedCause(suppressedException);
				}
			}
			throw ex;
		}
		finally {
			if (recordSuppressedExceptions) {
				this.suppressedExceptions = null;
			}
			//后置处理
			afterSingletonCreation(beanName);
		}
		if (newSingleton) {
			//加入缓存中
			addSingleton(beanName, singletonObject);
		}
	}
	return singletonObject;
	}
	}

// 用于添加标志，当前 bean 正处于创建中
	protected void beforeSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
			//添加失败，抛出异常
			throw new BeanCurrentlyInCreationException(beanName);
		}
	}

// 用于移除标记，当前 Bean 不处于创建中
	protected void afterSingletonCreation(String beanName) {
		if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
			//移除失败，抛出异常
			throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
		}
	}

protected void addSingleton(String beanName, Object singletonObject) {
	synchronized (this.singletonObjects) {
		this.singletonObjects.put(beanName, singletonObject);
		this.singletonFactories.remove(beanName);
		this.earlySingletonObjects.remove(beanName);
		this.registeredSingletons.add(beanName);
	    }
	}

        //创建多例Bean
	else if (mbd.isPrototype()) {
		// It's a prototype -> create a new instance.
		//原型模式(Prototype)是每次都会创建一个新的对象
		Object prototypeInstance = null;
		try {
			//加载前置处理，默认的功能是注册当前创建的原型对象
			beforePrototypeCreation(beanName);
			//创建指定Bean对象实例
			prototypeInstance = createBean(beanName, mbd, args);
		}
		finally {
			//加载后置处理，默认的功能告诉IOC容器指定Bean的原型对象不再创建
			afterPrototypeCreation(beanName);
		}
		//获取给定Bean的实例对象
		bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
	}

//要创建的Bean既不是Singleton也不是Prototype
	//如：request、session、application等生命周期
	else {
		String scopeName = mbd.getScope();
		final Scope scope = this.scopes.get(scopeName);
		//Bean定义资源中没有配置生命周期范围，则Bean定义不合法
		if (scope == null) {
			throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
		}
		try {
			//这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
			Object scopedInstance = scope.get(beanName, () -> {
				//前置处理
				beforePrototypeCreation(beanName);
				try {
					return createBean(beanName, mbd, args);
				}
				finally {
					//后置处理
					afterPrototypeCreation(beanName);
				}
			});
			//获取给定Bean的实例对象
			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);
		}
	}

Object get(String name, ObjectFactory<?> objectFactory);

//SimpleThreadScope.java

private final ThreadLocal<Map<String, Object>> threadScope =
			new NamedThreadLocal<Map<String, Object>>("SimpleThreadScope") {
				@Override
				protected Map<String, Object> initialValue() {
					return new HashMap<>();
				}
			};


	//获取bean的实例
	@Override
	public Object get(String name, ObjectFactory<?> objectFactory) {
		// 获取 scope 缓存
		Map<String, Object> scope = this.threadScope.get();
		Object scopedObject = scope.get(name);
		if (scopedObject == null) {
			scopedObject = objectFactory.getObject();
			// 加入缓存
			scope.put(name, scopedObject);
		}
		return scopedObject;
	}

总结