spring源码浅析之ioc

一、什么是IOC

    维基百科上说到：2004年Martin Fowler 提出了“控制反转的”概念，他得出的结论是：依赖对象的获得被反转了。后来为这个创造了一个更好的名字：依赖注入(IOC = Inversion of Control).简单的解释是：系统的运作是通过两个或多个类的合作来实现业务逻辑，这使得每个对象都需要与其合作的对象的引用（依赖关系），这个依赖 对象以前是通过自身实现去获得，现在通过一个容器统一的管理这些依赖关系，从而获得这种依赖的这种实现方式，我们可以成为IOC。

二、Ioc容器系列的设计和实现

spring ioc容器主要有2个系列的容器，一个是实现BeanFactory接口的简单系列容器，一个是ApplicationContext上下文，它作为容器的高级形态存在。

三、Ioc的初始化过程

3.1BeanFactory初始化过程

BeanFactory初始化主要分4步：

1,.创建容器配置文件的抽象资源Resource

2.,创建容器BeanFactory

3.创建资源的读取器BeanDefinitionReader

4.从定位的资源位置读取资源

下面以XmlBeanFactory源码分析

3.2ApplicationContext初始化过程

3.2.1ApplicationContext初始化过程主要包含3个过程：

 1.Resource定位

 2.BeanDefinition载入

 3.Ioc容器注册BeanDefinition

3.2.2下面以FileSystemXmlApplicationContext为例

3.2.2.1FileSystemXmlApplicationContext的继承体系

 通过下图可以看到FileSystemXmlApplicationContext父类AbstractRefreshableConfigApplicationContext的getConfigLocations()方法在初始化过程中栈信息可以了解到ApplicationContext的定位过程

AbstractApplicationContext的refresh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.
                        //在子类启动refreshBeanFactory的地方
			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.
                                //设置BeanFactory的后处理
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
                                //调用BeanFactory的后处理，这些后处理器是在bean定义中向容器注册的
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
                                //注册bean的后处理器，在bean的创建过程中调用
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
                                //消息源上下文进行初始化
				initMessageSource();

				// Initialize event multicaster for this context.
                                //初始化上下文事件
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
                                //初始化其他bean
				onRefresh();

				// Check for listener beans and register them.
                                //检查监听bean,并向容器注册这些bean
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				//初始化所有non-lazy-init的单例bean
                                finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
                                //发布容器事件
				finishRefresh();
			}

			catch (BeansException ex) {
				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;
			}
		}
	}

AbstractXmlApplicationContext的loadBeanDefinitions方法可以得知BeanDefinition载入是通过BeanDefinitionReader载入的

@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		beanDefinitionReader.setEnvironment(this.getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		initBeanDefinitionReader(beanDefinitionReader);
		loadBeanDefinitions(beanDefinitionReader);
	}

 XmlBeanDefinitionReader的registerBeanDefinitions方法

@SuppressWarnings("deprecation")
	public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
                //解析xml
		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
		documentReader.setEnvironment(getEnvironment());
		int countBefore = getRegistry().getBeanDefinitionCount();
                //具体的解析过程在registerBeanDefinitions
		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
		return getRegistry().getBeanDefinitionCount() - countBefore;
	}

 DefaultBeanDefinitionDocumentReader的registerBeanDefinitions方法

@Override
	public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
		this.readerContext = readerContext;
		logger.debug("Loading bean definitions");
		Element root = doc.getDocumentElement();
		doRegisterBeanDefinitions(root);
	}

  DefaultBeanDefinitionDocumentReader的doRegisterBeanDefinitions方法

/**
	 * Register each bean definition within the given root {@code <beans/>} element.
	 */
	protected void doRegisterBeanDefinitions(Element root) {
		// Any nested <beans> elements will cause recursion in this method. In
		// order to propagate and preserve <beans> default-* attributes correctly,
		// keep track of the current (parent) delegate, which may be null. Create
		// the new (child) delegate with a reference to the parent for fallback purposes,
		// then ultimately reset this.delegate back to its original (parent) reference.
		// this behavior emulates a stack of delegates without actually necessitating one.
		BeanDefinitionParserDelegate parent = this.delegate;
		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)) {
					return;
				}
			}
		}

		preProcessXml(root);
		parseBeanDefinitions(root, this.delegate);
		postProcessXml(root);

		this.delegate = parent;
	}

/**
	 * Parse the elements at the root level in the document:
	 * "import", "alias", "bean".
	 * @param root the DOM root element of the document
	 */
	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)) {
                                                //处理默认<bean>标签
						parseDefaultElement(ele, delegate);
					}
					else {
                                                //处理自定义标签 (i.e.  AOP,CONTENT,JDBC)
                                                //自定义标签分4步
                                                //1、提供一个xsd文件，负责对xml的标签<datasource>进行校验
                                                //2、定义一个BeanDefinitionParser负责解析xml,并将必要的信息放入spring中
                                                //3、定义个NamespaceHandler,由sping框架的调用入口。这也是我们自定义xml解析的入口
                                                //4、配置schema和handler
						delegate.parseCustomElement(ele);
					}
				}
			}
		}
		else {
			delegate.parseCustomElement(root);
		}
	}

/**
	 * Process the given bean element, parsing the bean definition
	 * and registering it with the registry.
	 */
	protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
		if (bdHolder != null) {
			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
			try {
				// Register the final decorated instance.
                                //向ioc容器注册解析的BeanDefinition
				BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error("Failed to register bean definition with name '" +
						bdHolder.getBeanName() + "'", ele, ex);
			}
			// Send registration event.
			getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
		}
	}

 

 DefaultListableBeanFactory的registerBeanDefinition方法

@Override
	public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {

		Assert.hasText(beanName, "Bean name must not be empty");
		Assert.notNull(beanDefinition, "BeanDefinition must not be null");

		if (beanDefinition instanceof AbstractBeanDefinition) {
			try {
				((AbstractBeanDefinition) beanDefinition).validate();
			}
			catch (BeanDefinitionValidationException ex) {
				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Validation of bean definition failed", ex);
			}
		}

		BeanDefinition oldBeanDefinition;

		oldBeanDefinition = this.beanDefinitionMap.get(beanName);
		if (oldBeanDefinition != null) {
                        //如果设置为不允许覆盖时，注册相同名称的bean将抛出异常
			if (!isAllowBeanDefinitionOverriding()) {
				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
						"': There is already [" + oldBeanDefinition + "] bound.");
			}
			else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
				// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
				if (this.logger.isWarnEnabled()) {
					this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
							"' with a framework-generated bean definition: replacing [" +
							oldBeanDefinition + "] with [" + beanDefinition + "]");
				}
			}
			else {
				if (this.logger.isInfoEnabled()) {
					this.logger.info("Overriding bean definition for bean '" + beanName +
							"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
				}
			}
		}
		else {
                       //正常BeanDefinition的注册过程
			this.beanDefinitionNames.add(beanName);
			this.manualSingletonNames.remove(beanName);
			this.frozenBeanDefinitionNames = null;
		}
		this.beanDefinitionMap.put(beanName, beanDefinition);

		if (oldBeanDefinition != null || containsSingleton(beanName)) {
			resetBeanDefinition(beanName);
		}
	}

四、Ioc容器的依赖注入

ApplicationContext依赖的BeanFactory默认是DefaultListableBeanFactory

 下面以DefaultListableBeanFactory的基类AbstractBeanFactory的getBean实现为入口进一步了解依赖注入过程

@SuppressWarnings("unchecked")
	protected <T> T doGetBean(
			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

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

		// Eagerly check singleton cache for manually registered singletons.
                //先从缓存中取得单例的bean
		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.
                        //检查definition 是否存在，没有从双亲取，依次类推
			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.
                                //获取当前bean依赖bean,出发getBean的递归调用
				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()) {
                                       //通过createBean方法创建单例
					sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
						@Override
						public Object getObject() throws BeansException {
							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 '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
							@Override
							public Object getObject() throws BeansException {
								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 && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
			try {
				return getTypeConverter().convertIfNecessary(bean, requiredType);
			}
			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;
	}

 

 本文spring为4.1.6版本