spring IOC

基本实现代码：

public class TestMain {
	public static void main(String[] args) throws IOException {
		//资源解析器
		ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
		//获取路径下的配置
		Resource resource = resolver.getResource("classpath:bean.xml");
		//创建bean工厂
		DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
		//定义xml bean解析器
		XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(beanFactory);
		//装载bean
		reader.loadBeanDefinitions(resource);
		//获取bean
		Car car = (Car) beanFactory.getBean("car");
		System.out.println(car.getBrand());
		//注意，和ApplicationContext不同，BeanFactory在首次调用getBean方法时才去创建bean的实例
	}
}
上面的步奏都是创建对象实例，加载资源
我们重点关注加载bean和读取bean的方法。首先是loadBeanDefinitions方法，源码如下：
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
	return loadBeanDefinitions(new EncodedResource(resource));
}
继续跟进：
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
	//断言资源不能为空
	Assert.notNull(encodedResource, "EncodedResource must not be null");
	if (logger.isInfoEnabled()) {
		logger.info("Loading XML bean definitions from " + encodedResource.getResource());
	}

	//当前已经加载的资源放入一个set缓存
	Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
	if (currentResources == null) {
		currentResources = new HashSet<>(4);
		this.resourcesCurrentlyBeingLoaded.set(currentResources);
	}
	if (!currentResources.add(encodedResource)) {
		throw new BeanDefinitionStoreException(
				"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
	}
	try {
		//直接获取资源文件输入流
		InputStream inputStream = encodedResource.getResource().getInputStream();
		try {
			InputSource inputSource = new InputSource(inputStream);
			if (encodedResource.getEncoding() != null) {
				inputSource.setEncoding(encodedResource.getEncoding());
			}
			return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
		}
		finally {
			inputStream.close();
		}
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(
				"IOException parsing XML document from " + encodedResource.getResource(), ex);
	}
	finally {
		currentResources.remove(encodedResource);
		if (currentResources.isEmpty()) {
			this.resourcesCurrentlyBeingLoaded.remove();
		}
	}
}
//继续看doLoadBeanDefinitions方法：
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
		throws BeanDefinitionStoreException {
	try {
		//将资转成document。首先创建DocumentBuilderFactory实例，通过DocumentBuilderFactory创建DocumentBuilder，DocumentBuilder调用parse方法将InputSource解析称为Document
		Document doc = doLoadDocument(inputSource, resource);
		//注册定义的bean
		return registerBeanDefinitions(doc, resource);
	}
	catch (BeanDefinitionStoreException ex) {
		throw ex;
	}
	catch (SAXParseException ex) {
		throw new XmlBeanDefinitionStoreException(resource.getDescription(),
				"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
	}
	catch (SAXException ex) {
		throw new XmlBeanDefinitionStoreException(resource.getDescription(),
				"XML document from " + resource + " is invalid", ex);
	}
	catch (ParserConfigurationException ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"Parser configuration exception parsing XML from " + resource, ex);
	}
	catch (IOException ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"IOException parsing XML document from " + resource, ex);
	}
	catch (Throwable ex) {
		throw new BeanDefinitionStoreException(resource.getDescription(),
				"Unexpected exception parsing XML document from " + resource, ex);
	}
}
//注册方法如下：
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
	//创建documentReader
	BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
	//获取已经注册的bean的总数
	int countBefore = getRegistry().getBeanDefinitionCount();
	//注册bean
	documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
	return getRegistry().getBeanDefinitionCount() - countBefore;
}
//在看registerBeanDefinitions方法之前，我们先看看如何创建的上下文：
public XmlReaderContext createReaderContext(Resource resource) {
	return new XmlReaderContext(resource, this.problemReporter, this.eventListener,
			this.sourceExtractor, this, getNamespaceHandlerResolver());
}
//可以看到，直接new了一个XmlReaderContext接下来：
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
	this.readerContext = readerContext;
	logger.debug("Loading bean definitions");
	Element root = doc.getDocumentElement();
	doRegisterBeanDefinitions(root);
}
protected void doRegisterBeanDefinitions(Element root) {
	BeanDefinitionParserDelegate parent = this.delegate;
	//创建代理
	this.delegate = createDelegate(getReaderContext(), root, parent);

	if (this.delegate.isDefaultNamespace(root)) {
		String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
		if (StringUtils.hasText(profileSpec)) {
			String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
					profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
			if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
				if (logger.isInfoEnabled()) {
					logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
							"] not matching: " + getReaderContext().getResource());
				}
				return;
			}
		}
	}

	preProcessXml(root);//此方法不做任何操作
	//具体对每一个Element的解析
	parseBeanDefinitions(root, this.delegate);
	postProcessXml(root);//此方法也不做任何操作

	this.delegate = parent;
}
//解析bean
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
	if (delegate.isDefaultNamespace(root)) {
		NodeList nl = root.getChildNodes();
		for (int i = 0; i < nl.getLength(); i++) {
			Node node = nl.item(i);
			if (node instanceof Element) {
				Element ele = (Element) node;
				if (delegate.isDefaultNamespace(ele)) {
					parseDefaultElement(ele, delegate);
				}
				else {
					delegate.parseCustomElement(ele);
				}
			}
		}
	}
	else {
		delegate.parseCustomElement(root);
	}
}
//解析bean完成之后，会将解析出来的bean缓存入DefaultListableBeanFactory的beanDefinitionMap中

//总结一下bean的解析过程：首先通过PathMatchingResourcePatternResolver获取配置文件，调用getResource方法获取Resource --> 然后获取文件输入流InputStream-->之后初始化Document,DefaultBeanDefinitionDocumentReader解析Document，解析xml的配置元信息Element-->BeanDefinitionReaderUtils工具将解析出来的bean存入DefaultListableBeanFactory的beanDefinitionMap中。至此，初步解析完成。

下面我们看看getBean方法：
public Object getBean(String name) throws BeansException {
	return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
		@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	//首先获取beanName
	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);//首次获取sharedInstance为空，直接看else
	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.
		//如果当前实例正在被创建，则抛出异常
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// Check if bean definition exists in this factory.
		//parentBeanFactory为空 
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
			// Not found -> check parent.
			String nameToLookup = originalBeanName(name);
			if (parentBeanFactory instanceof AbstractBeanFactory) {
				return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
						nameToLookup, requiredType, args, typeCheckOnly);
			}
			else 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) {
			//标记bean被创建，并将beanName存入alreadyCreated中
			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 dep : dependsOn) {
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
					registerDependentBean(dep, beanName);
					getBean(dep);
				}
			}

			// Create bean instance.
			if (mbd.isSingleton()) {
				sharedInstance = getSingleton(beanName, () -> {
					try {
						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.
						destroySingleton(beanName);
						throw ex;
					}
				});
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

			else {
				String scopeName = mbd.getScope();
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
				try {
					Object scopedInstance = scope.get(beanName, () -> {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						}
						finally {
							afterPrototypeCreation(beanName);
						}
					});
					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);
				}
			}
		}
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && !requiredType.isInstance(bean)) {
		try {
			T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
			if (convertedBean == null) {
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
			return convertedBean;
		}
		catch (TypeMismatchException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to convert bean '" + name + "' to required type '" +
						ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}
//首先从singletonObjects这个map中获取对象，有则直接返回，没有则创建之，并缓存入