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上节回顾

上一节，我们分析了Spring IoC容器是如何去初始化一个BeanFactory的，同时也分析了BeanFactory是如何去加载spring.xml中的的bean。本质上就是BeanFactory是如何去初始化BeanDefinnitionMap的的过程。接下来，我们将进入IoC源码的另一个核心部分——创建并初始化Bean。

Spring介绍

Spring(http://spring.io/)是一个轻量级的Java 开发框架，同时也是轻量级的IoC和AOP的容器框架，主要是针对JavaBean的生命周期进行管理的轻量级容器，可以单独使用，也可以和Struts框架，MyBatis框架等组合使用。

IoC介绍

IoC是什么

Ioc—Inversion of Control，即“控制反转”，不是什么技术，而是一种设计思想。在Java开发中，Ioc意味着将你设计好的对象交给容器控制，而不是传统的在你的对象内部直接控制。如何理解好Ioc呢？理解好Ioc的关键是要明确“谁控制谁，控制什么，为何是反转（有反转就应该有正转了），哪些方面反转了”，那我们来深入分析一下：

　　●谁控制谁，控制什么：传统Java SE程序设计，我们直接在对象内部通过new进行创建对象，是程序主动去创建依赖对象；而IoC是有专门一个容器来创建这些对象，即由Ioc容器来控制对象的创建；谁控制谁？当然是IoC 容器控制了对象；控制什么？那就是主要控制了外部资源获取（不只是对象包括比如文件等）。

　　●为何是反转，哪些方面反转了：有反转就有正转，传统应用程序是由我们自己在对象中主动控制去直接获取依赖对象，也就是正转；而反转则是由容器来帮忙创建及注入依赖对象；为何是反转？因为由容器帮我们查找及注入依赖对象，对象只是被动的接受依赖对象，所以是反转；哪些方面反转了？依赖对象的获取被反转了。

IoC能做什么

IoC 不是一种技术，只是一种思想，一个重要的面向对象编程的法则，它能指导我们如何设计出松耦合、更优良的程序。传统应用程序都是由我们在类内部主动创建依赖对象，从而导致类与类之间高耦合，难于测试；有了IoC容器后，把创建和查找依赖对象的控制权交给了容器，由容器进行注入组合对象，所以对象与对象之间是松散耦合，这样也方便测试，利于功能复用，更重要的是使得程序的整个体系结构变得非常灵活。

其实IoC对编程带来的最大改变不是从代码上，而是从思想上，发生了“主从换位”的变化。应用程序原本是老大，要获取什么资源都是主动出击，但是在IoC/DI思想中，应用程序就变成被动的了，被动的等待IoC容器来创建并注入它所需要的资源了。

IoC很好的体现了面向对象设计法则之一—— 好莱坞法则：“别找我们，我们找你”；即由IoC容器帮对象找相应的依赖对象并注入，而不是由对象主动去找。

那么，IoC容器到底是如何从初始化完成的BeanFactory中对Bean进行创建并初始化的呢？接下来我们就一探究竟。

源码解析

准备工作

首先写一个Spring的配置文件spring.xml，为了方便测试，这里面就只有一个名为test的bean。

编写程序入口代码，可以直接打断点进行调试。

ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
Test bean = context.getBean("test", Test.class);

开始解析

开始源码解析，紧接着上一节，首先进入AbstractApplicationContext.java的refresh方法，这一节我们重点来看里面的invokeBeanFactoryPostProcessors方法。

@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

进入invokeBeanFactoryPostProcessors方法

	/**
	 * Instantiate and invoke all registered BeanFactoryPostProcessor beans,
	 * respecting explicit order if given.
	 * Must be called before singleton instantiation.
	 */
	protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

		// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
		// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
		if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}
	}

打开PostProcessorRegistrationDelegate类中的invokeBeanFactoryPostProcessors方法，可以看到，这个方法里有很多内容，这里我们只分析最关键的部分。从本质上来说，该方法就是去执行BeanFactoryPostProcessor这个接口中的方法去的，上面代码注释也清楚的写到如果想先执行BeanFactoryPostProcessor这个接口的方法，必须先去实例化实现这个接口的Bean,也就是getBean这个方法。

	public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List beanFactoryPostProcessors) {

		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
		Set processedBeans = new HashSet<>();

		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
			List regularPostProcessors = new LinkedList<>();
			List registryPostProcessors =
					new LinkedList<>();

			for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
				if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
					BeanDefinitionRegistryPostProcessor registryPostProcessor =
							(BeanDefinitionRegistryPostProcessor) postProcessor;
					registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
					registryPostProcessors.add(registryPostProcessor);
				}
				else {
					regularPostProcessors.add(postProcessor);
				}
			}

			// Do not initialize FactoryBeans here: We need to leave all regular beans
			// uninitialized to let the bean factory post-processors apply to them!
			// Separate between BeanDefinitionRegistryPostProcessors that implement
			// PriorityOrdered, Ordered, and the rest.
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);

			// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
			List priorityOrderedPostProcessors = new ArrayList<>();
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
			registryPostProcessors.addAll(priorityOrderedPostProcessors);
			invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);

			// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
			postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			List orderedPostProcessors = new ArrayList<>();
			for (String ppName : postProcessorNames) {
				if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
					orderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			sortPostProcessors(beanFactory, orderedPostProcessors);
			registryPostProcessors.addAll(orderedPostProcessors);
			invokeBeanDefinitionRegistryPostProcessors(orderedPostProcessors, registry);

			// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
			boolean reiterate = true;
			while (reiterate) {
				reiterate = false;
				postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
				for (String ppName : postProcessorNames) {
					if (!processedBeans.contains(ppName)) {
						BeanDefinitionRegistryPostProcessor pp = beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class);
						registryPostProcessors.add(pp);
						processedBeans.add(ppName);
						pp.postProcessBeanDefinitionRegistry(registry);
						reiterate = true;
					}
				}
			}

			// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
			invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
			invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
		}

		else {
			// Invoke factory processors registered with the context instance.
			invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
		}

		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let the bean factory post-processors apply to them!
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

		// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
		List priorityOrderedPostProcessors = new ArrayList<>();
		List orderedPostProcessorNames = new ArrayList<>();
		List nonOrderedPostProcessorNames = new ArrayList<>();
		for (String ppName : postProcessorNames) {
			if (processedBeans.contains(ppName)) {
				// skip - already processed in first phase above
			}
			else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
			}
			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
				orderedPostProcessorNames.add(ppName);
			}
			else {
				nonOrderedPostProcessorNames.add(ppName);
			}
		}

		// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
		sortPostProcessors(beanFactory, priorityOrderedPostProcessors);
		invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

		// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
		List orderedPostProcessors = new ArrayList<>();
		for (String postProcessorName : orderedPostProcessorNames) {
			orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		sortPostProcessors(beanFactory, orderedPostProcessors);
		invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

		// Finally, invoke all other BeanFactoryPostProcessors.
		List nonOrderedPostProcessors = new ArrayList<>();
		for (String postProcessorName : nonOrderedPostProcessorNames) {
			nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

		// Clear cached merged bean definitions since the post-processors might have
		// modified the original metadata, e.g. replacing placeholders in values...
		beanFactory.clearMetadataCache();
	}

接下来进入AbstractBeanFactory.java类中的doGetBean方法，这个方法的具体实现可以分为三个部分：

第一部分，首先先去singleton缓存中去找实例。由于我们例子中没有把我们的bean手动放入singletonObjects这个Map里面去，所以这里肯定没找到。

第二部分，然后是去获取该BeanFactory父Factory，希望从这些Factory中获取，如果该Beanfactory有父类，则希望用父类去实例化该bean，类似于JVM类加载的双亲委派机制。由于我们例子中的的Beanfactory为null，所以暂不讨论这种情况。

第三部分，这一部分是我们关注的重点，这里我们将这一大部分再分为三个小的部分来进行分析：

先将目前的bean标记为的正在创建
再获取根据beanName得到对应bean在beanfactory中的beanDefinitionMap的BeanDefinition（上一节初始化beanFactory时存入的），然后去获取这个bean依赖的bean。如果依赖的bean还没有创建，则先创建依赖的bean，进行递归调用（这就是依赖注入Dependence Injection）。如果找不到依赖，则忽略。
最后如果是单例（Spring默认是单例），则调用createBean()这个方法进行Bean的创建。

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * @param name the name of the bean to retrieve
	 * @param requiredType the required type of the bean to retrieve
	 * @param args arguments to use when creating a bean instance using explicit arguments
	 * (only applied when creating a new instance as opposed to retrieving an existing one)
	 * @param typeCheckOnly whether the instance is obtained for a type check,
	 * not for actual use
	 * @return an instance of the bean
	 * @throws BeansException if the bean could not be created
	 */
	@SuppressWarnings("unchecked")
	protected  T doGetBean(
			final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

		final String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isDebugEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				if (args != null) {
					// Delegation to parent with explicit args.
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}

			try {
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dependsOnBean : dependsOn) {
						if (isDependent(beanName, dependsOnBean)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
						}
						registerDependentBean(dependsOnBean, beanName);
						getBean(dependsOnBean);
					}
				}

				// Create bean instance.
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, new ObjectFactory

Spring IoC源码解析（二）——Bean的创建和初始化