Spring源碼:IOC原理解析(二)
版權聲明:本文為博主原創文章,轉載請註明出處,歡迎交流學習!
接著上一章節的內容,我們來分析當new一個FileSystemXmlApplicationContext對象的時候,spring到底做了那些事。FileSystemXmlApplicationContext類的內容主要是定義了若幹重載的構造方法,核心構造方法如下:
/** * 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(); } }
從方法說明可以看出,在這個構造方法裏加載所有bean定義並創建bean單例實例。其中的refresh()方法就是IOC容器初始化的入口,refresh()方法位AbstractApplicationContext類中,這是一個抽象類,它實現了ApplicationContext的基礎功能,這裏使用了模版方法模式,給實現它的子類提供了統一的模板:
@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.告訴子類刷新內部bean工廠 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()方法裏列出了IOC容器初始化的步驟,第一個方法是初始化準備,這裏只是設置啟動日期和活動標識以及執行屬性源的初始化。我們重點看第二個方法obtainFreshBeanFactory(),它告訴子類刷新內部bean工廠,返回了一個ConfigurableListableBeanFactory,跟蹤這個方法:
/** * 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()方法的第一行調用了refreshBeanFactory()方法,這是一個抽象方法,由它的子類來實現,方法的第二行調用了getBeanFactory(),這是在其父接口中定義的一個空方法。抽象方法refreshBeanFactory()在其子類子類AbstractRefreshableApplicationContext中實現:
/** * 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. * * 此實現執行該上下文的底層bean工廠的實際刷新,關閉以前的bean工廠(如果有的話), * 並為上下文生命周期的下一階段初始化一個新的bean工廠 */ @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); } }
這個方法被final關鍵字修飾,也就是說不可以被重寫,IOC容器的初始化就是在這個方法中完成的。第一步先判斷有沒有現有的工廠,有的話就銷毀掉,然後創建一個默認的工廠,也就是DefaultListableBeanFactory ,接下來兩行代碼是設置bean工廠的一些屬性,註意看loadBeanDefinitions(beanFactory)這行,當創建了一個默認的bean工廠後,加載bean定義,這跟我們上一章節使用原始方式初始化bean工廠類似。從這裏不難看出,FileSystemXmlApplicationContext的構造方法中其實已經包含了我們上一章節中原始的初始化過程。接下來我們跟蹤一下loadBeanDefinitions(beanFactory)的實現,這個方法是由AbstractXmlApplicationContext抽象類實現的:
/** * Loads the bean definitions via an XmlBeanDefinitionReader.裝載bean定義通過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義了一個Reader,這個Reader就是用來讀取xml配置文件的,最後一行就是真正載入bean定義的實現過程,代碼如下:
/** * 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); } }
上面的方法調用了XmlBeanDefinitionReader類的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(); } } }
從方法說明可以看出,這個方法是從指定的xml文件中加載bean定義,try塊中的代碼才是載入bean定義的過程。spring將資源返回的輸入流包裝以後傳給了doLoadBeanDefinitions()方法,我們進入這個方法,代碼如下:
/** * 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()); }
從try塊中的代碼可以看出,spring使用documentLoader將資源轉換成了Document資源,spring使用的documentLoader為DefaultDocumentLoader,loadDocument方法定義在此類中:
/** * 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); }
從這裏不難看出,這就是我們非常熟悉的DOM解析xml了,可以想象spring是根據XSD文件規定的格式解析了xml文件的各節點及屬性。我們再來回頭看看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; }
方法說明很明確的告訴我們,這個方法是註冊給定的DOM文檔中包含的bean定義。到這裏思路就很明確了,spring將包裝的輸入流解析成DOM文檔,然後將DOM中包含的bean定義信息註冊到IOC容器持有的Map<String,BeanDefinition>對象中。只要我們的IOC容器持有了bean定義,就能正確的生產bean實例。
通過閱讀源碼,我們分析了Spring IOC的實現原理。有些實現細節並沒有去深究,更重要的是去理解它的核心思想和實現思路。
Spring源碼:IOC原理解析(二)