Java中Spring-IOC容器是什么

這篇文章將為大家詳細講解有關Java中Spring-IOC容器是什么，小編覺得挺實用的，因此分享給大家做個參考，希望大家閱讀完這篇文章后可以有所收獲。

Spring IoC容器對Bean定義資源的載入是從refresh()函數開始的，refresh()是一個模板方法，refresh()方法的作用是：在創建IoC容器前，如果已經有容器存在，則需要把已有的容器銷毀和關閉，以保證在refresh之后使用的是新建立起來的IoC容器。refresh的作用類似于對IoC容器的重啟，在新建立好的容器中對容器進行初始化，對Bean定義資源進行載入

public void refresh() throws BeansException, IllegalStateException {
       Object var1 = this.startupShutdownMonitor;
       synchronized(this.startupShutdownMonitor) {
           //調用容器準備刷新的方法，獲取容器的當時時間，同時給容器設置同步標識
           this.prepareRefresh();
           //告訴子類啟動refreshBeanFactory()方法，Bean定義資源文件的載入從  
           //子類的refreshBeanFactory()方法啟動 
           //這個bean的載入過程 包括對xml的解析和加載為BeanDefinitions 都是從this.obtainFreshBeanFactory()這里進入
           ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
           /*這之后的代碼都是注冊容器的信息源和生命周期事件*/
           //為BeanFactory配置容器特性，例如類加載器、事件處理器等
           this.prepareBeanFactory(beanFactory);

           try {
               //為容器的某些子類指定特殊的BeanPost事件處理器
               this.postProcessBeanFactory(beanFactory);
               //調用所有注冊的BeanFactoryPostProcessor的Bean
               this.invokeBeanFactoryPostProcessors(beanFactory);
               //為BeanFactory注冊BeanPost事件處理器.  
               //BeanPostProcessor是Bean后置處理器，用于監聽容器觸發的事件 
               this.registerBeanPostProcessors(beanFactory);
               //初始化信息源，和國際化相關.
               this.initMessageSource();
               //初始化容器事件傳播器.
               this.initApplicationEventMulticaster();
               //調用子類的某些特殊Bean初始化方法
               this.onRefresh();
               //為事件傳播器注冊事件監聽器. 
               this.registerListeners();
               //初始化所有剩余的單態Bean.
               this.finishBeanFactoryInitialization(beanFactory);
               //初始化容器的生命周期事件處理器，并發布容器的生命周期事件
               this.finishRefresh();
           } catch (BeansException var9) {
               if (this.logger.isWarnEnabled()) {
                   this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
               }
               //銷毀以創建的單態Bean
               this.destroyBeans();
               //取消refresh操作，重置容器的同步標識.
               this.cancelRefresh(var9);
               throw var9;
           } finally {
               this.resetCommonCaches();
           }

       }
   }

AbstractRefreshableApplicationContext 類：
AbstractRefreshableApplicationContext中只定義了抽象的loadBeanDefinitions方法，容器真正調用的是其子類AbstractXmlApplicationContext對該方法的實現

protected final void refreshBeanFactory() throws BeansException {
        //如果已經創建了BeanFactory，則銷毀并關閉BeanFactory
        if (this.hasBeanFactory()) {
            this.destroyBeans();
            this.closeBeanFactory();
        }

        try {
             //創建了一個IOC容器
            DefaultListableBeanFactory beanFactory = this.createBeanFactory();
            beanFactory.setSerializationId(this.getId());
            /對IoC容器進行定制化，如設置啟動參數，開啟注解的自動裝配等
            this.customizeBeanFactory(beanFactory);
            //調用載入Bean定義的方法，主要這里又使用了一個委派模式，在當前類中只定義了抽象的loadBeanDefinitions方法，具體的實現調用子類容器
            this.loadBeanDefinitions(beanFactory);
            Object var2 = this.beanFactoryMonitor;
            synchronized(this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        } catch (IOException var5) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + this.getDisplayName(), var5);
        }
    }

AbstractXmlApplicationContext 類：

 protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        //創建XmlBeanDefinitionReader，即創建Bean讀取器，并通過回調設置到容器中去，容器使用該讀取器讀取Bean定義資源  
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
        beanDefinitionReader.setEnvironment(this.getEnvironment());
         //為Bean讀取器設置Spring資源加載器，AbstractXmlApplicationContext的  
        //祖先父類AbstractApplicationContext繼承DefaultResourceLoader，因此，容器本身也是一個資源加載器
        beanDefinitionReader.setResourceLoader(this);
        //為Bean讀取器設置SAX xml解析器 
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
        //當Bean讀取器讀取Bean定義的Xml資源文件時，啟用Xml的校驗機制  
        this.initBeanDefinitionReader(beanDefinitionReader);
        //Bean讀取器真正實現加載的方法
        this.loadBeanDefinitions(beanDefinitionReader);
    }
    
    protected void initBeanDefinitionReader(XmlBeanDefinitionReader reader) {
        reader.setValidating(this.validating);
    }

    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        Resource[] configResources = this.getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }

        String[] configLocations = this.getConfigLocations();
        if (configLocations != null) {
            reader.loadBeanDefinitions(configLocations);
        }

    }

    @Nullable
    protected Resource[] getConfigResources() {
        return null;
    }

在其抽象父類AbstractBeanDefinitionReader中定義了載入過程

 public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
        //獲取resourceLoader
        ResourceLoader resourceLoader = this.getResourceLoader();
        if (resourceLoader == null) {
            throw new BeanDefinitionStoreException("Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
        } else {
            int loadCount;
            if (!(resourceLoader instanceof ResourcePatternResolver)) {
                //將指定位置的Bean定義資源文件解析為Spring IoC容器封裝的資源  
                //加載多個指定位置的Bean定義資源文件 完成具體的資源定位的工作               
                Resource resource = resourceLoader.getResource(location);
                //加載資源 開始我們的第二步操作 轉換為BeanDefinition對象
                loadCount = this.loadBeanDefinitions((Resource)resource);
                if (actualResources != null) {
                    actualResources.add(resource);
                }

                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
                }

                return loadCount;
            } else {
                try {
                   //將指定位置的Bean定義資源文件解析為Spring IoC容器封裝的資源  
                   //加載單個指定位置的Bean定義資源文件  
                    Resource[] resources = ((ResourcePatternResolver)resourceLoader).getResources(location);
                    //加載資源 開始我們的第二步操作 轉換為BeanDefinition對象
                    loadCount = this.loadBeanDefinitions(resources);
                    if (actualResources != null) {
                        Resource[] var6 = resources;
                        int var7 = resources.length;

                        for(int var8 = 0; var8 < var7; ++var8) {
                            Resource resource = var6[var8];
                            actualResources.add(resource);
                        }
                    }

                    if (this.logger.isDebugEnabled()) {
                        this.logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                    }

                    return loadCount;
                } catch (IOException var10) {
                    throw new BeanDefinitionStoreException("Could not resolve bean definition resource pattern [" + location + "]", var10);
                }
            }
        }
    }

具體加載資源的方法
首先，調用資源加載器的獲取資源方法resourceLoader.getResource(location)，獲取到要加載的資源。
其次，真正執行加載功能是其子類XmlBeanDefinitionReader的loadBeanDefinitions方法。
資源定位到這里就結束 最終返回的是一個Resource的對象來進行BeanDefinition的載入。在定位完成后,為BeanDefinition的載入創客I/O條件，
但是具體的載入還沒有開始載入。

public Resource getResource(String location) {
        Assert.notNull(location, "Location must not be null");
        Iterator var2 = this.protocolResolvers.iterator();

        Resource resource;
        do {
            if (!var2.hasNext()) {
               //處理/開頭的定位
                if (location.startsWith("/")) {
                    return this.getResourceByPath(location);
                }
                //帶有classpath標識的Resource
                if (location.startsWith("classpath:")) {
                    return new ClassPathResource(location.substring("classpath:".length()), this.getClassLoader());
                }

                try {
                //處理URL標識的Resource定位
                    URL url = new URL(location);
                    return (Resource)(ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
                } catch (MalformedURLException var5) {
                    //如果不存classpath、Url、/標志的 就教給他處理
                    return this.getResourceByPath(location);
                }
            }

            ProtocolResolver protocolResolver = (ProtocolResolver)var2.next();
            resource = protocolResolver.resolve(location, this);
        } while(resource == null);

        return resource;
    }

protected Resource getResourceByPath(String path) {    
   if (path != null && path.startsWith("/")) {    
        path = path.substring(1);    
    }  
    //這里使用文件系統資源對象來定義bean 文件
    return new FileSystemResource(path);  
}

BeanDefinition的載入分成兩部分,首先通過XML的解析器得到document對象，
但這些document對象沒有按照spring的Bean規則進行解析。在完成XML解析后，才是按照Spring的Bean規則進行解析，這個解析的過程是在documentReader實現。

 public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
        //獲取resourceLoader
        ResourceLoader resourceLoader = this.getResourceLoader();
        if (resourceLoader == null) {
            throw new BeanDefinitionStoreException("Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
        } else {
            int loadCount;
            if (!(resourceLoader instanceof ResourcePatternResolver)) {
                //將指定位置的Bean定義資源文件解析為Spring IoC容器封裝的資源  
                //加載多個指定位置的Bean定義資源文件 完成具體的資源定位的工作               
                Resource resource = resourceLoader.getResource(location);
 
                //加載資源 開始我們的第二步操作 轉換為BeanDefinition對象
                loadCount = this.loadBeanDefinitions((Resource)resource);
                if (actualResources != null) {
                    actualResources.add(resource);
                }

                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
                }

                return loadCount;
            } else {
                try {
                   //將指定位置的Bean定義資源文件解析為Spring IoC容器封裝的資源  
                   //加載單個指定位置的Bean定義資源文件  
                    Resource[] resources = ((ResourcePatternResolver)resourceLoader).getResources(location);
                    //加載資源 開始我們的第二步操作 轉換為BeanDefinition對象
                    loadCount = this.loadBeanDefinitions(resources);
                    if (actualResources != null) {
                        Resource[] var6 = resources;
                        int var7 = resources.length;

                        for(int var8 = 0; var8 < var7; ++var8) {
                            Resource resource = var6[var8];
                            actualResources.add(resource);
                        }
                    }

                    if (this.logger.isDebugEnabled()) {
                        this.logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                    }

                    return loadCount;
                } catch (IOException var10) {
                    throw new BeanDefinitionStoreException("Could not resolve bean definition resource pattern [" + location + "]", var10);
                }
            }
        }
    }

xml文件轉換為document對象 再解析BeanDefinition
這里是載入XML形式Bean定義資源文件方法

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if(this.logger.isInfoEnabled()) {
            this.logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }

        Set<EncodedResource> currentResources = (Set)this.resourcesCurrentlyBeingLoaded.get();
        if(currentResources == null) {
            currentResources = new HashSet(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }

        if(!((Set)currentResources).add(encodedResource)) {
            throw new BeanDefinitionStoreException("Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        } else {
            int var5;
            try {
                //將資源文件轉為InputStream的IO流
                InputStream inputStream = encodedResource.getResource().getInputStream();

                try {
                    //從InputStream中得到XML的解析源
                    InputSource inputSource = new InputSource(inputStream);
                    if(encodedResource.getEncoding() != null) {
                        inputSource.setEncoding(encodedResource.getEncoding());
                    }
                     //這里是具體的讀取過程
                    var5 = this.doLoadBeanDefinitions(inputSource, encodedResource.getResource());
                } finally {
                    //IO流的關閉
                    inputStream.close();
                }
            } catch (IOException var15) {
                throw new BeanDefinitionStoreException("IOException parsing XML document from " + encodedResource.getResource(), var15);
            } finally {
                ((Set)currentResources).remove(encodedResource);
                if(((Set)currentResources).isEmpty()) {
                    this.resourcesCurrentlyBeingLoaded.remove();
                }

            }

            return var5;
        }
    }

    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException {
        try {
            //@1將XML文件轉換為DOM對象，解析過程由documentLoader實現
            Document doc = this.doLoadDocument(inputSource, resource);
            //@2這里是啟動對Bean定義解析的詳細過程，該解析過程會用到Spring的Bean配置規則
            return this.registerBeanDefinitions(doc, resource);
        } catch (BeanDefinitionStoreException var4) {
            throw var4;
        } catch (SAXParseException var5) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(), "Line " + var5.getLineNumber() + " in XML document from " + resource + " is invalid", var5);
        } catch (SAXException var6) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(), "XML document from " + resource + " is invalid", var6);
        } catch (ParserConfigurationException var7) {
            throw new BeanDefinitionStoreException(resource.getDescription(), "Parser configuration exception parsing XML from " + resource, var7);
        } catch (IOException var8) {
            throw new BeanDefinitionStoreException(resource.getDescription(), "IOException parsing XML document from " + resource, var8);
        } catch (Throwable var9) {
            throw new BeanDefinitionStoreException(resource.getDescription(), "Unexpected exception parsing XML document from " + resource, var9);
        }
    }

@1

//使用標準的JAXP將載入的Bean定義資源轉換成document對象
 public Document loadDocument(InputSource inputSource, EntityResolver entityResolver, ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {
        //創建文件解析器工廠 
        DocumentBuilderFactory factory = this.createDocumentBuilderFactory(validationMode, namespaceAware);
        if(logger.isDebugEnabled()) {
            logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
        }
        //創建文檔解析器  
        DocumentBuilder builder = this.createDocumentBuilder(factory, entityResolver, errorHandler);
        //解析Spring的Bean定義資源
        return builder.parse(inputSource);
    }

    protected DocumentBuilderFactory createDocumentBuilderFactory(int validationMode, boolean namespaceAware) throws ParserConfigurationException {
     //創建文檔解析工廠
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        factory.setNamespaceAware(namespaceAware);
        if(validationMode != 0) {
           //設置解析XML的校驗 
            factory.setValidating(true);
            if(validationMode == 3) {
                factory.setNamespaceAware(true);

                try {
                    factory.setAttribute("http://java.sun.com/xml/jaxp/properties/schemaLanguage", "http://www.w3.org/2001/XMLSchema");
                } catch (IllegalArgumentException var6) {
                    ParserConfigurationException pcex = new ParserConfigurationException("Unable to validate using XSD: Your JAXP provider [" + factory + "] does not support XML Schema. Are you running on Java 1.4 with Apache Crimson? Upgrade to Apache Xerces (or Java 1.5) for full XSD support.");
                    pcex.initCause(var6);
                    throw pcex;
                }
            }
        }

        return factory;
    }

Spring IoC容器根據定位的Bean定義資源文件，將其加載讀入并轉換成為Document對象過程完成。

下面是 Spring IoC容器將載入的Bean定義資源文件轉換為Document對象之后，是如何將其解析為Spring IoC管理的Bean對象并將其注冊到容器中的
@2

 //按照Spring的Bean語義要求將Bean定義資源解析并轉換為容器內部數據結構
 public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        /得到BeanDefinitionDocumentReader來對xml格式的BeanDefinition解析 
        BeanDefinitionDocumentReader documentReader = this.createBeanDefinitionDocumentReader();
        //獲得容器中注冊的Bean數量
        int countBefore = this.getRegistry().getBeanDefinitionCount();
        //解析過程入口，這里使用了委派模式，BeanDefinitionDocumentReader只是個接口
        //具體的解析實現過程有實現類DefaultBeanDefinitionDocumentReader完成
        //@1
        documentReader.registerBeanDefinitions(doc, this.createReaderContext(resource));
        /統計解析的Bean數量 
        return this.getRegistry().getBeanDefinitionCount() - countBefore;
    }
    //創建BeanDefinitionDocumentReader對象，解析Document對象
    protected BeanDefinitionDocumentReader createBeanDefinitionDocumentReader() {
        return (BeanDefinitionDocumentReader)BeanDefinitionDocumentReader.class.cast(BeanUtils.instantiateClass(this.documentReaderClass));
    }

DefaultBeanDefinitionDocumentReader 解析document對象按照spring Bean的定義規則轉換為BeanDefinition
@1

 public class DefaultBeanDefinitionDocumentReader implements BeanDefinitionDocumentReader {
    public static final String BEAN_ELEMENT = "bean";
    public static final String NESTED_BEANS_ELEMENT = "beans";
    public static final String ALIAS_ELEMENT = "alias";
    public static final String NAME_ATTRIBUTE = "name";
    public static final String ALIAS_ATTRIBUTE = "alias";
    public static final String IMPORT_ELEMENT = "import";
    public static final String RESOURCE_ATTRIBUTE = "resource";
    public static final String PROFILE_ATTRIBUTE = "profile";
    protected final Log logger = LogFactory.getLog(this.getClass());
    private XmlReaderContext readerContext;
    private BeanDefinitionParserDelegate delegate;

    public DefaultBeanDefinitionDocumentReader() {
    }
    //根據Spring DTD對Bean的定義規則解析Bean定義Document對象 
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        //獲得XML描述符 
        this.readerContext = readerContext;
        this.logger.debug("Loading bean definitions");
        //獲得Document的根元素 
        Element root = doc.getDocumentElement();
        this.doRegisterBeanDefinitions(root);
    }

    protected final XmlReaderContext getReaderContext() {
        return this.readerContext;
    }

    protected Object extractSource(Element ele) {
        return this.getReaderContext().extractSource(ele);
    }
    //具體的解析過程由BeanDefinitionParserDelegate實現，  
    //BeanDefinitionParserDelegate中定義了Spring Bean定義XML文件的各種元素
    protected void doRegisterBeanDefinitions(Element root) {
        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = this.createDelegate(this.getReaderContext(), root, parent);
        if(this.delegate.isDefaultNamespace(root)) {
            String profileSpec = root.getAttribute("profile");
            if(StringUtils.hasText(profileSpec)) {
                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(profileSpec, ",; ");
                if(!this.getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                    if(this.logger.isInfoEnabled()) {
                        this.logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec + "] not matching: " + this.getReaderContext().getResource());
                    }

                    return;
                }
            }
        }
        //在解析Bean定義之前，進行自定義的解析，增強解析過程的可擴展性
        this.preProcessXml(root);
        //從Document的根元素開始進行Bean定義的Document對象
        this.parseBeanDefinitions(root, this.delegate);
        //在解析Bean定義之后，進行自定義的解析，增加解析過程的可擴展性
        this.postProcessXml(root);
        this.delegate = parent;
    }
    //創建BeanDefinitionParserDelegate，用于完成真正的解析過程
    protected BeanDefinitionParserDelegate createDelegate(XmlReaderContext readerContext, Element root, BeanDefinitionParserDelegate parentDelegate) {
        BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
        //BeanDefinitionParserDelegate初始化Document根元素
        delegate.initDefaults(root, parentDelegate);
        return delegate;
    }
    //使用Spring的Bean規則從Document的根元素開始進行Bean定義的Document對象
    protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
         //Bean定義的Document對象使用了Spring默認的XML命名空間
        if(delegate.isDefaultNamespace(root)) {
           //獲取Bean定義的Document對象根元素的所有子節點
            NodeList nl = root.getChildNodes();

            for(int i = 0; i < nl.getLength(); ++i) {
                Node node = nl.item(i);
                //獲得Document節點是XML元素節點
                if(node instanceof Element) {
                    Element ele = (Element)node;
                    //Bean定義的Document的元素節點使用的是Spring默認的XML命名空間
                    if(delegate.isDefaultNamespace(ele)) {
                        //使用Spring的Bean規則解析元素節點
                        this.parseDefaultElement(ele, delegate);
                    } else {
                       //沒有使用Spring默認的XML命名空間，則使用用戶自定義的解//析規則解析元素節點
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        } else {
            //Document的根節點沒有使用Spring默認的命名空間，則使用用戶自定義的  
           //解析規則解析Document根節點
            delegate.parseCustomElement(root);
        }

    }

    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        //如果元素節點是<Import>導入元素，進行導入解析  
        if(delegate.nodeNameEquals(ele, "import")) {
            this.importBeanDefinitionResource(ele);
            
        } else if(delegate.nodeNameEquals(ele, "alias")) {
        //如果元素節點是<Alias>別名元素，進行別名解析 
            this.processAliasRegistration(ele);
        } else if(delegate.nodeNameEquals(ele, "bean")) {
        //元素節點既不是導入元素，也不是別名元素，即普通的<Bean>元素，  
        //按照Spring的Bean規則解析元素  
            this.processBeanDefinition(ele, delegate);
        } else if(delegate.nodeNameEquals(ele, "beans")) {
         //如果元素節點是<beans>元素注冊為BeanDefintion
            this.doRegisterBeanDefinitions(ele);
        }

    }
    //解析<Import>導入元素，從給定的導入路徑加載Bean定義資源到Spring IoC容器中
    protected void importBeanDefinitionResource(Element ele) {
        /獲取給定的導入元素的location屬性 
        String location = ele.getAttribute("resource");
        if(!StringUtils.hasText(location)) {
            this.getReaderContext().error("Resource location must not be empty", ele);
        } else {
            //獲取location的值 
            location = this.getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);
            Set<Resource> actualResources = new LinkedHashSet(4);
            //標識給定的導入元素的location是否是絕對路徑
            boolean absoluteLocation = false;

            try {
                absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
            } catch (URISyntaxException var11) {
                ;
            }

            int importCount;
            if(absoluteLocation) {
                try {
                    //使用資源讀入器加載給定路徑的Bean定義資源
                    importCount = this.getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
                    if(this.logger.isDebugEnabled()) {
                        this.logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
                    }
                } catch (BeanDefinitionStoreException var10) {
                    this.getReaderContext().error("Failed to import bean definitions from URL location [" + location + "]", ele, var10);
                }
            } else {
            //給定的導入元素的location是相對路徑  
                try {
                    Resource relativeResource = this.getReaderContext().getResource().createRelative(location);
                    if(relativeResource.exists()) {
                         //使用資源讀入器加載Bean定義資源 
                        importCount = this.getReaderContext().getReader().loadBeanDefinitions(relativeResource);
                        actualResources.add(relativeResource);
                    } else {
                        String baseLocation = this.getReaderContext().getResource().getURL().toString();
                        importCount = this.getReaderContext().getReader().loadBeanDefinitions(StringUtils.applyRelativePath(baseLocation, location), actualResources);
                    }

                    if(this.logger.isDebugEnabled()) {
                        this.logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
                    }
                } catch (IOException var8) {
                    this.getReaderContext().error("Failed to resolve current resource location", ele, var8);
                } catch (BeanDefinitionStoreException var9) {
                    this.getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]", ele, var9);
                }
            }
            //在解析完<Import>元素之后，發送容器導入其他資源處理完成事件
            Resource[] actResArray = (Resource[])actualResources.toArray(new Resource[actualResources.size()]);
            this.getReaderContext().fireImportProcessed(location, actResArray, this.extractSource(ele));
        }
    }
   //解析<Alias>別名元素，為Bean向Spring IoC容器注冊別名  
    protected void processAliasRegistration(Element ele) {
        //獲取<Alias>別名元素中name的屬性值
        String name = ele.getAttribute("name");
        //獲取<Alias>別名元素中alias的屬性值  
        String alias = ele.getAttribute("alias");
        boolean valid = true;
        if(!StringUtils.hasText(name)) {
            this.getReaderContext().error("Name must not be empty", ele);
            valid = false;
        }

        if(!StringUtils.hasText(alias)) {
            this.getReaderContext().error("Alias must not be empty", ele);
            valid = false;
        }

        if(valid) {
            try {
                //向容器的資源讀入器注冊別名
                this.getReaderContext().getRegistry().registerAlias(name, alias);
            } catch (Exception var6) {
                this.getReaderContext().error("Failed to register alias '" + alias + "' for bean with name '" + name + "'", ele, var6);
            }
            //在解析完<Alias>元素之后，發送容器別名處理完成事件
            this.getReaderContext().fireAliasRegistered(name, alias, this.extractSource(ele));
        }

    } 
    //解析Bean定義資源Document對象的普通元素
    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        // BeanDefinitionHolder是對BeanDefinition的封裝，即Bean定義的封裝類  
       //對Document對象中<Bean>元素的解析由BeanDefinitionParserDelegate實現  BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele); 
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if(bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);

            try {
                //向Spring IoC容器注冊解析得到的Bean定義，這是Bean定義向IoC容器注冊的入口
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, this.getReaderContext().getRegistry());
            } catch (BeanDefinitionStoreException var5) {
                this.getReaderContext().error("Failed to register bean definition with name '" + bdHolder.getBeanName() + "'", ele, var5);
            }
            //在完成向Spring IoC容器注冊解析得到的Bean定義之后，發送注冊事件
            this.getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
        }

    }

    protected void preProcessXml(Element root) {
    }

    protected void postProcessXml(Element root) {
    }
}

通過上述Spring IoC容器對載入的Bean定義Document解析可以看出，我們使用Spring時，在Spring配置文件中可以使用<Import>元素來導入IoC容器所需要的其他資源，Spring IoC容器在解析時會首先將指定導入的資源加載進容器中。使用<Ailas>別名時，Spring IoC容器首先將別名元素所定義的別名注冊到容器中。
對于既不是<Import>元素，又不是<Alias>元素的元素，即Spring配置文件中普通的<Bean>元素的解析由BeanDefinitionParserDelegate類的parseBeanDefinitionElement方法來實現。

下面是針對對Bean元素的解析過程

    //解析Bean定義資源文件中的<Bean>元素，這個方法中主要處理<Bean>元素的id，name
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
        //獲取id屬性
        String id = ele.getAttribute("id");
        //獲取name屬性
        String nameAttr = ele.getAttribute("name");
        List<String> aliases = new ArrayList();
        //將Bean元素的name屬性全部放到alias屬性里面
        if(StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, ",; ");
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;
        if(!StringUtils.hasText(id) && !aliases.isEmpty()) {
            beanName = (String)aliases.remove(0);
            if(this.logger.isDebugEnabled()) {
                this.logger.debug("No XML 'id' specified - using '" + beanName + "' as bean name and " + aliases + " as aliases");
            }
        }

        if(containingBean == null) {
            //檢查bean元素的id和name是否是唯一的 
            this.checkNameUniqueness(beanName, aliases, ele);
        }
        //詳細對<Bean>元素中配置的Bean定義進行解析的地方
        AbstractBeanDefinition beanDefinition = this.parseBeanDefinitionElement(ele, beanName, containingBean);
        if(beanDefinition != null) {
            if(!StringUtils.hasText(beanName)) {
                try {
                    if(containingBean != null) {
                        //判斷如果沒有id和name屬性時候是否包含子元素的<Bean>
                        beanName = BeanDefinitionReaderUtils.generateBeanName(beanDefinition, this.readerContext.getRegistry(), true);
                    } else {
                        //如果<Bean>元素中沒有配置id、別名或者name，且包含了子//<Bean>元素，為解析的Bean使用別名向IoC容器注冊  
                        beanName = this.readerContext.generateBeanName(beanDefinition);
                        String beanClassName = beanDefinition.getBeanClassName();
                        if(beanClassName != null && beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() && !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                            aliases.add(beanClassName);
                        }
                    }

                    if(this.logger.isDebugEnabled()) {
                        this.logger.debug("Neither XML 'id' nor 'name' specified - using generated bean name [" + beanName + "]");
                    }
                } catch (Exception var9) {
                    this.error(var9.getMessage(), ele);
                    return null;
                }
            }

            String[] aliasesArray = StringUtils.toStringArray(aliases);
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        } else {
            return null;
        }
    }
    //詳細解析bean元素的地方
    public AbstractBeanDefinition parseBeanDefinitionElement(Element ele, String beanName, BeanDefinition containingBean) {
        this.parseState.push(new BeanEntry(beanName));
        String className = null;
        //是否包含class屬性
        if(ele.hasAttribute("class")) {
            //獲取class屬性值 不做實例化，bean的實例化是在第一次獲取bean的時候完成，這里只獲取class的值
            className = ele.getAttribute("class").trim();
        }

        try {
            String parent = null;
            //如果<Bean>元素中配置了parent屬性，則獲取parent屬性的值
            if(ele.hasAttribute("parent")) {
                parent = ele.getAttribute("parent");
            }
           //根據<Bean>元素配置的class名稱和parent屬性值創建BeanDefinition  
            //為載入Bean定義信息做準備 
            AbstractBeanDefinition bd = this.createBeanDefinition(className, parent);
            //對當前的<Bean>元素中配置的一些屬性進行解析和設置，如配置的單態(singleton)屬性等
            this.parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
            //為<Bean>元素解析的Bean設置description信息 bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, "description"));
            //對<Bean>元素的meta(元信息)屬性解析
            this.parseMetaElements(ele, bd);
            //對<Bean>元素的lookup-method屬性解析
            this.parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            //對<Bean>元素的replaced-method屬性解析
            this.parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
            //解析<Bean>元素的構造方法設置
            this.parseConstructorArgElements(ele, bd);
             //解析<Bean>元素的<property>設置
            this.parsePropertyElements(ele, bd);
             //解析<Bean>元素的qualifier屬性
            this.parseQualifierElements(ele, bd);
            bd.setResource(this.readerContext.getResource());
            bd.setSource(this.extractSource(ele));
            AbstractBeanDefinition var7 = bd;
            return var7;
        } catch (ClassNotFoundException var13) {
            this.error("Bean class [" + className + "] not found", ele, var13);
        } catch (NoClassDefFoundError var14) {
            this.error("Class that bean class [" + className + "] depends on not found", ele, var14);
        } catch (Throwable var15) {
            this.error("Unexpected failure during bean definition parsing", ele, var15);
        } finally {
            this.parseState.pop();
        }

        return null;
    }

通過對上述源碼的分析，就會明白我們在Spring配置文件中<Bean>元素的中配置的屬性就是通過該方法解析和設置到Bean中去的。
注意：在解析<Bean>元素過程中沒有創建和實例化Bean對象，只是創建了Bean對象的定義類BeanDefinition，將<Bean>元素中的配置信息設置到BeanDefinition中作為記錄，當依賴注入時才使用這些記錄信息創建和實例化具體的Bean對象。
上面方法中一些對一些配置如元信息(meta)、qualifier等的解析，我們在Spring中配置時使用的也不多，我們在使用Spring的<Bean>元素時，配置最多的是<property>屬性。

調用BeanDefinitionReaderUtils的registerBeanDefinition方法向IoC容器注冊解析的Bean

 protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if(bdHolder != null) {
            //通過對document對象的解析和封裝返回一個BeanDefinitionHolder
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);

            try {
                //通過這個holder來注冊bean對象
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, this.getReaderContext().getRegistry());
            } catch (BeanDefinitionStoreException var5) {
                this.getReaderContext().error("Failed to register bean definition with name '" + bdHolder.getBeanName() + "'", ele, var5);
            }

            this.getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
        }

    }

當調用BeanDefinitionReaderUtils向IoC容器注冊解析的BeanDefinition時，真正完成注冊功能的是DefaultListableBeanFactory。

    public static void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) throws BeanDefinitionStoreException {
        String beanName = definitionHolder.getBeanName();
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
        String[] aliases = definitionHolder.getAliases();
        if(aliases != null) {
            String[] var4 = aliases;
            int var5 = aliases.length;

            for(int var6 = 0; var6 < var5; ++var6) {
                String alias = var4[var6];
                registry.registerAlias(beanName, alias);
            }
        }

    }

DefaultListableBeanFactory registerBeanDefinition方法

  //存儲注冊的俄BeanDefinition
 private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap(256);
 //向IoC容器注冊解析的BeanDefiniton
 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");
        //校驗解析的BeanDefiniton
        if(beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition)beanDefinition).validate();
            } catch (BeanDefinitionValidationException var9) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName, "Validation of bean definition failed", var9);
            }
        }

        BeanDefinition oldBeanDefinition = (BeanDefinition)this.beanDefinitionMap.get(beanName);
        if(oldBeanDefinition != null) {
            if(!this.isAllowBeanDefinitionOverriding()) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName, "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName + "': There is already [" + oldBeanDefinition + "] bound.");
            }

            if(oldBeanDefinition.getRole() < beanDefinition.getRole()) {
                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(!beanDefinition.equals(oldBeanDefinition)) {
                if(this.logger.isInfoEnabled()) {
                    this.logger.info("Overriding bean definition for bean '" + beanName + "' with a different definition: replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            } else if(this.logger.isDebugEnabled()) {
                this.logger.debug("Overriding bean definition for bean '" + beanName + "' with an equivalent definition: replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
            }

            this.beanDefinitionMap.put(beanName, beanDefinition);
        } else {
            if(this.hasBeanCreationStarted()) {
                Map var4 = this.beanDefinitionMap;
                //注冊的過程中需要線程同步，以保證數據的一致性 
                synchronized(this.beanDefinitionMap) {
                    //把bean存放到map中
                    this.beanDefinitionMap.put(beanName, beanDefinition);
                    List<String> updatedDefinitions = new ArrayList(this.beanDefinitionNames.size() + 1);
                    updatedDefinitions.addAll(this.beanDefinitionNames);
                    updatedDefinitions.add(beanName);
                    this.beanDefinitionNames = updatedDefinitions;
                    if(this.manualSingletonNames.contains(beanName)) {
                        Set<String> updatedSingletons = new LinkedHashSet(this.manualSingletonNames);
                        updatedSingletons.remove(beanName);
                        this.manualSingletonNames = updatedSingletons;
                    }
                }
            } else {
                this.beanDefinitionMap.put(beanName, beanDefinition);
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }

            this.frozenBeanDefinitionNames = null;
        }

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

    }

至此，Bean定義資源文件中配置的Bean被解析過后，已經注冊到IoC容器中，被容器管理起來，真正完成了IoC容器初始化所做的全部工作。現  在IoC容器中已經建立了整個Bean的配置信息，這些BeanDefinition信息已經可以使用，并且可以被檢索，IoC容器的作用就是對這些注冊的Bean定義信息進行處理和維護。這些的注冊的Bean定義信息是IoC容器控制反轉的基礎，正是有了這些注冊的數據，容器才可以進行依賴注入。

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