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接着上一章节的内容，我们来分析当new一个FileSystemXmlApplicationContext对象的时候，spring到底做了那些事。FileSystemXmlApplicationContext类的内容主要是定义了若干重载的构造方法，核心构造方法如下：
/** * Create a new FileSystemXmlApplicationContext with the given parent, * loading the definitions from the given XML files. * * loading all bean definitions and creating all singletons. * Alternatively, call refresh manually after further configuring the context. * */ public FileSystemXmlApplicationContext( String[] configLocations, boolean refresh, @Nullable ApplicationContext parent) throws BeansException { super(parent); setConfigLocations(configLocations); if (refresh) { refresh(); } }
从方法说明可以看出，在这个构造方法里加载所有bean定义并创建bean单例实例。其中的refresh()方法就是IOC容器初始化的入口，refresh()方法位AbstractApplicationContext类中，这是一个抽象类，它实现了ApplicationContext的基础功能，这里使用了模版方法模式，给实现它的子类提供了统一的模板：
@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.告诉子类刷新内部bean工厂 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(); } } }
refresh()方法里列出了IOC容器初始化的步骤，第一个方法是初始化准备，这里只是设置启动日期和活动标识以及执行属性源的初始化。我们重点看第二个方法obtainFreshBeanFactory()，它告诉子类刷新内部bean工厂，返回了一个ConfigurableListableBeanFactory，跟踪这个方法：
/** * Tell the subclass to refresh the internal bean factory. * @return the fresh BeanFactory instance * @see #refreshBeanFactory() * @see #getBeanFactory() */ protected ConfigurableListableBeanFactory obtainFreshBeanFactory() { refreshBeanFactory(); ConfigurableListableBeanFactory beanFactory = getBeanFactory(); if (logger.isDebugEnabled()) { logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory); } return beanFactory; }
protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;
/** * Return the internal bean factory of this application context. * Can be used to access specific functionality of the underlying factory. * */ ConfigurableListableBeanFactory getBeanFactory() throws IllegalStateException;
obtainFreshBeanFactory()方法的第一行调用了refreshBeanFactory()方法，这是一个抽象方法，由它的子类来实现，方法的第二行调用了getBeanFactory()，这是在其父接口中定义的一个空方法。抽象方法refreshBeanFactory()在其子类子类AbstractRefreshableApplicationContext中实现：
/** * This implementation performs an actual refresh of this context's underlying * bean factory, shutting down the previous bean factory (if any) and * initializing a fresh bean factory for the next phase of the context's lifecycle. * * 此实现执行该上下文的底层bean工厂的实际刷新，关闭以前的bean工厂（如果有的话）， * 并为上下文生命周期的下一阶段初始化一个新的bean工厂 */ @Override protected final void refreshBeanFactory() throws BeansException { if (hasBeanFactory()) { destroyBeans(); closeBeanFactory(); } try { DefaultListableBeanFactory beanFactory = createBeanFactory(); beanFactory.setSerializationId(getId()); customizeBeanFactory(beanFactory); loadBeanDefinitions(beanFactory); synchronized (this.beanFactoryMonitor) { this.beanFactory = beanFactory; } } catch (IOException ex) { throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex); } }
这个方法被final关键字修饰，也就是说不可以被重写，IOC容器的初始化就是在这个方法中完成的。第一步先判断有没有现有的工厂，有的话就销毁掉，然后创建一个默认的工厂，也就是DefaultListableBeanFactory ，接下来两行代码是设置bean工厂的一些属性，注意看loadBeanDefinitions(beanFactory)这行，当创建了一个默认的bean工厂后，加载bean定义，这跟我们上一章节使用原始方式初始化bean工厂类似。从这里不难看出，FileSystemXmlApplicationContext的构造方法中其实已经包含了我们上一章节中原始的初始化过程。接下来我们跟踪一下loadBeanDefinitions(beanFactory)的实现，这个方法是由AbstractXmlApplicationContext抽象类实现的：
/** * Loads the bean definitions via an XmlBeanDefinitionReader.装载bean定义通过XmlBeanDefinitionReader * */ @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); }
方法的第一行首先定义了一个Reader，这个Reader就是用来读取xml配置文件的，最后一行就是真正载入bean定义的实现过程，代码如下：
/** * Load the bean definitions with the given XmlBeanDefinitionReader. * */ protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException { Resource[] configResources = getConfigResources(); if (configResources != null) { reader.loadBeanDefinitions(configResources); } String[] configLocations = getConfigLocations(); if (configLocations != null) { reader.loadBeanDefinitions(configLocations); } }
上面的方法调用了XmlBeanDefinitionReader类的loadBeanDefinitions(EncodedResource encodedResource)方法：
/** * Load bean definitions from the specified XML file. * rows BeanDefinitionStoreException in case of loading or parsing errors */ public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException { Assert.notNull(encodedResource, "EncodedResource must not be null"); if (logger.isInfoEnabled()) { logger.info("Loading XML bean definitions from " + encodedResource.getResource()); } 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 { 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(); } } }
从方法说明可以看出，这个方法是从指定的xml文件中加载bean定义，try块中的代码才是载入bean定义的过程。spring将资源返回的输入流包装以后传给了doLoadBeanDefinitions()方法，我们进入这个方法，代码如下：
/** * Actually load bean definitions from the specified XML file. * */ protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException { try { Document doc = doLoadDocument(inputSource, resource); 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); } }
/** * Actually load the specified document using the configured DocumentLoader. * */ protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception { return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler, getValidationModeForResource(resource), isNamespaceAware()); }
从try块中的代码可以看出，spring使用documentLoader将资源转换成了Document资源，spring使用的documentLoader为DefaultDocumentLoader，loadDocument方法定义在此类中：
/** * Load the {@link Document} at the supplied {@link InputSource} using the standard JAXP-configured * XML parser. */ @Override public Document loadDocument(InputSource inputSource, EntityResolver entityResolver, ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception { DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware); if (logger.isDebugEnabled()) { logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]"); } DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler); return builder.parse(inputSource); }
从这里不难看出，这就是我们非常熟悉的DOM解析xml了，可以想象spring是根据XSD文件规定的格式解析了xml文件的各节点及属性。我们再来回头看看registerBeanDefinitions(doc, resource)方法，
/** * Register the bean definitions contained in the given DOM document. * Called by {@code loadBeanDefinitions}. * */ public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException { BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader(); int countBefore = getRegistry().getBeanDefinitionCount(); documentReader.registerBeanDefinitions(doc, createReaderContext(resource)); return getRegistry().getBeanDefinitionCount() - countBefore; }
方法说明很明确的告诉我们，这个方法是注册给定的DOM文档中包含的bean定义。到这里思路就很明确了，spring将包装的输入流解析成DOM文档，然后将DOM中包含的bean定义信息注册到IOC容器持有的Map<String，BeanDefinition>对象中。只要我们的IOC容器持有了bean定义，就能正确的生产bean实例。
通过阅读源码，我们分析了Spring IOC的实现原理。有些实现细节并没有去深究，更重要的是去理解它的核心思想和实现思路。