Spring源码解析之一 ------ 默认标签的解析注册(IOC的第一步)

最新推荐文章于 2024-07-07 18:01:17 发布

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#spring #源码分析 #依赖注入 #IOC #Bean的解析注册

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要看Spring bean加载的过程，最好的是Debug跟着流程走，有所侧重的看。首先需要的搭建本地环境和简单的项目工程，搭建的方法我放在这篇博客里。

http://blog.youkuaiyun.com/weililansehudiefei/article/details/73744017

我也是看了不少博客，Debug了很多次才终于理解整个过程。第一次看的时候，别心急，几天看完第一次都没关系。

我相信这篇文章，能让你看懂bean加载。

一、schema、xsd的认识

我们在xml里面配置bean的时候，一定会在xml的开始有类似如下的定义

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                        http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
                        http://www.springframework.org/schema/context
                        http://www.springframework.org/schema/context/spring-context-3.1.xsd
                        http://www.springframework.org/schema/tx
                        http://www.springframework.org/schema/tx/spring-tx.xsd">


    <bean id="userId" name="userName" class="com.heitian.ssm.model.User"></bean>
</beans>

可以看到整个xml是被<beans></beans>标签给封起来的，在<beans>里面定义了schcema。它由URI + LOCATION构成。比如上面的文件中，

http://www.springframework.org/schema/beans
                        http://www.springframework.org/schema/beans/spring-beans-3.1.xsd

前面的http://www.springframeword.org/shema/beans表示URI，后面的表示xsd文件的地址，xsd文件是描述相应的标签的。比如这里的spring-beans-3.1.xsd就是描述bean这个标签的定义和组成。具体的讲解会在下一部分，实现自定义标签时候讲解。现在知道就行。当然spring并不是直接就去xsd文件的链接去下载相应的xsd，而是先去本地找。在本地的spring.shemas文件中，有对应的本地地址关系

http\://www.springframework.org/schema/beans/spring-beans-2.0.xsd=org/springframework/beans/factory/xml/spring-beans-2.0.xsd
http\://www.springframework.org/schema/beans/spring-beans-2.5.xsd=org/springframework/beans/factory/xml/spring-beans-2.5.xsd
http\://www.springframework.org/schema/beans/spring-beans-3.0.xsd=org/springframework/beans/factory/xml/spring-beans-3.0.xsd
http\://www.springframework.org/schema/beans/spring-beans-3.1.xsd=org/springframework/beans/factory/xml/spring-beans-3.1.xsd
http\://www.springframework.org/schema/beans/spring-beans-3.2.xsd=org/springframework/beans/factory/xml/spring-beans-3.2.xsd
http\://www.springframework.org/schema/beans/spring-beans-4.0.xsd=org/springframework/beans/factory/xml/spring-beans-4.0.xsd
http\://www.springframework.org/schema/beans/spring-beans-4.1.xsd=org/springframework/beans/factory/xml/spring-beans-4.1.xsd
http\://www.springframework.org/schema/beans/spring-beans-4.2.xsd=org/springframework/beans/factory/xml/spring-beans-4.2.xsd
http\://www.springframework.org/schema/beans/spring-beans.xsd=org/springframework/beans/factory/xml/spring-beans-4.2.xsd
http\://www.springframework.org/schema/tool/spring-tool-2.0.xsd=org/springframework/beans/factory/xml/spring-tool-2.0.xsd
http\://www.springframework.org/schema/tool/spring-tool-2.5.xsd=org/springframework/beans/factory/xml/spring-tool-2.5.xsd
http\://www.springframework.org/schema/tool/spring-tool-3.0.xsd=org/springframework/beans/factory/xml/spring-tool-3.0.xsd
http\://www.springframework.org/schema/tool/spring-tool-3.1.xsd=org/springframework/beans/factory/xml/spring-tool-3.1.xsd
http\://www.springframework.org/schema/tool/spring-tool-3.2.xsd=org/springframework/beans/factory/xml/spring-tool-3.2.xsd
http\://www.springframework.org/schema/tool/spring-tool-4.0.xsd=org/springframework/beans/factory/xml/spring-tool-4.0.xsd
http\://www.springframework.org/schema/tool/spring-tool-4.1.xsd=org/springframework/beans/factory/xml/spring-tool-4.1.xsd
http\://www.springframework.org/schema/tool/spring-tool-4.2.xsd=org/springframework/beans/factory/xml/spring-tool-4.2.xsd
http\://www.springframework.org/schema/tool/spring-tool.xsd=org/springframework/beans/factory/xml/spring-tool-4.2.xsd
http\://www.springframework.org/schema/util/spring-util-2.0.xsd=org/springframework/beans/factory/xml/spring-util-2.0.xsd
http\://www.springframework.org/schema/util/spring-util-2.5.xsd=org/springframework/beans/factory/xml/spring-util-2.5.xsd
http\://www.springframework.org/schema/util/spring-util-3.0.xsd=org/springframework/beans/factory/xml/spring-util-3.0.xsd
http\://www.springframework.org/schema/util/spring-util-3.1.xsd=org/springframework/beans/factory/xml/spring-util-3.1.xsd
http\://www.springframework.org/schema/util/spring-util-3.2.xsd=org/springframework/beans/factory/xml/spring-util-3.2.xsd
http\://www.springframework.org/schema/util/spring-util-4.0.xsd=org/springframework/beans/factory/xml/spring-util-4.0.xsd
http\://www.springframework.org/schema/util/spring-util-4.1.xsd=org/springframework/beans/factory/xml/spring-util-4.1.xsd
http\://www.springframework.org/schema/util/spring-util-4.2.xsd=org/springframework/beans/factory/xml/spring-util-4.2.xsd
http\://www.springframework.org/schema/util/spring-util.xsd=org/springframework/beans/factory/xml/spring-util-4.2.xsd

二、spring中bean的解析和注册

首先，因为我们是在ClassPath下xml文件的方式配置的，所以Debug后会进入ClassPathXmlApplicationContext中，

    public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
        this(new String[]{configLocation}, true, (ApplicationContext)null);
    }

然后会调用同类中的多态的构造方法，如下。configLocations就是我们配置的xml文件的地址

    public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent) throws BeansException {
        super(parent);
        this.setConfigLocations(configLocations);
        if(refresh) {
            this.refresh();
        }

    }

在这里，通过 this .setConfigLocations(configLocations)方法，设置此次bean加载的配置文件位置。

然后进入到了refresh()方法。但从代码看，貌似进入的refresh()方法还是在ClassPathXmlApplicationContex里面，但是并不是。因为

public class ClassPathXmlApplicationContext extends AbstractXmlApplicationContext

因为继承关系，这个时候，进入的是 AbstractXmlApplicationContext中的refresh()方法。下面这个图，说明了接下来整个程序的调用图。其实核心代码并不多，但是在去往核心代码之前，会做一系列的准备和校验以及相应数据的转换。

进入到refresh()方法，可以看到这里包含了bean的主要处理逻辑。而我们要进入的是this.obtainFreshBeanFactory()。

    public void refresh() throws BeansException, IllegalStateException {
        Object var1 = this.startupShutdownMonitor;
        synchronized(this.startupShutdownMonitor) {
            this.prepareRefresh();
            ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
            this.prepareBeanFactory(beanFactory);

            try {
                this.postProcessBeanFactory(beanFactory);
                this.invokeBeanFactoryPostProcessors(beanFactory);
                this.registerBeanPostProcessors(beanFactory);
                this.initMessageSource();
                this.initApplicationEventMulticaster();
                this.onRefresh();
                this.registerListeners();
                this.finishBeanFactoryInitialization(beanFactory);
                this.finishRefresh();
            } catch (BeansException var9) {
                if(this.logger.isWarnEnabled()) {
                    this.logger.warn("Exception encountered during context initialization - cancelling refresh attempt: " + var9);
                }

                this.destroyBeans();
                this.cancelRefresh(var9);
                throw var9;
            } finally {
                this.resetCommonCaches();
            }

        }
    }

进入到obtainFreashBeanFactory方法，然后进入 this .refreshBeanFactory();在refreshBeanFactory中，Spring创建了一个beanFactory，

然后进入loadBeanDefinitions(beanFactory)。

    protected final void refreshBeanFactory() throws BeansException {
        if(this.hasBeanFactory()) {
            this.destroyBeans();
            this.closeBeanFactory();
        }

        try {
            DefaultListableBeanFactory beanFactory = this.createBeanFactory();
            beanFactory.setSerializationId(this.getId());
            this.customizeBeanFactory(beanFactory);
            this.loadBeanDefinitions(beanFactory);  //这里进入
            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);
        }
    }

在loadBeanDefinitions中，Spring为该beanFactory创建了一个beanDefinitionReader，看名字也知道这个reader是用来读取bean的定义信息。

然后进入this.loadBeanDefinitions(beanDefinitionReader)

    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
        this.initBeanDefinitionReader(beanDefinitionReader);
        this.loadBeanDefinitions(beanDefinitionReader);//进入这里
    }

进入之后，是如下的内容。在这里判断configResource是否为空，同时把第一步设置的configLocations，通过get方法读了出来，也就是配置文件spring.xml的地址和名字。

    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);//进入这里
        }

    }

连续进入loadBeanDefinition后，如图。这个时候在进入新的loadBeanDefinition之前，他会根据location的位置，获取Resource。

    public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
        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)) {
                Resource resource = resourceLoader.getResource(location);
                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 {
                    Resource[] resources = ((ResourcePatternResolver)resourceLoader).getResources(location);
                    loadCount = this.loadBeanDefinitions(resources);//进入这里
                    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);
                }
            }
        }
    }

接下来进入进入后，resource文件被转成了encodedResource，

var5 = this.doLoadBeanDefinitions(inputSource, encodedResource.getResource());

这个方法是真正做事情的方法。凡是看到以do开头，那说明是真正做事情的。之前的都是各种转换和准备。需要说明的是，从resource到encodedResource再到inoputStream再到

inputSource，这时候xml文件并没有真正读取。如果你是debug，可以看到这几个里面存放的都是spring.xml的加载路径。

进入真正的方法。

    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 {
                InputStream inputStream = encodedResource.getResource().getInputStream();

                try {
                    InputSource inputSource = new InputSource(inputStream);
                    if(encodedResource.getEncoding() != null) {
                        inputSource.setEncoding(encodedResource.getEncoding());
                    }

                    var5 = this.doLoadBeanDefinitions(inputSource, encodedResource.getResource()); //这里do开头的方法，才是做事情的方法
                } finally {
                    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;
        }
    }

进入后，我们发现通过inputSource和resource生成了doc对象，在doLoadDocument方法里，进行了文档的解析。文档解析的过程也有一些需要注意的地方，但是这里我们先不管它，进入registerBeanDefinitions(doc,resource)。

   protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException {
        try {
            Document doc = this.doLoadDocument(inputSource, resource);
            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);
        }
    }

进入后，定义了一个BeanDefiniDocumReader，这个documentReader用来读取doc对象，并注册bean。

    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        BeanDefinitionDocumentReader documentReader = this.createBeanDefinitionDocumentReader();
        int countBefore = this.getRegistry().getBeanDefinitionCount();
        documentReader.registerBeanDefinitions(doc, this.createReaderContext(resource));//进入
        return this.getRegistry().getBeanDefinitionCount() - countBefore;
    }

进去以后，获取元素的根节点root，Debug的时候，可以看到这个root的name属性正是beans。这和我们一开始说的，整个xml文档是被<beans></beans>包起来的说法一致。

然后进入doRegisterBeanDefinitons(root)。

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

进入之后，如下图，这个时候我们继续进入注释表明的地方。

    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)) {
                    return;
                }
            }
        }

        this.preProcessXml(root);
        this.parseBeanDefinitions(root, this.delegate);//进入
        this.postProcessXml(root);
        this.delegate = parent;
    }

进入parseBeanDefinitions后，这里开始对Document对象里面的Element节点root，开始遍历解析。

Spring的默认标签会进入this.parseDefaultElement解析，我们自定义的标签会进入delegate.parseCustomElement解析。

自定义标签的解析，在另外一篇单独讲解。这里讲解spring自带标签的解析。

    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)) {
                        this.parseDefaultElement(ele, delegate);//进入
                    } else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        } else {
            delegate.parseCustomElement(root);
        }

    }

进入之后，可以看到spring默认的标签只有四个：import、alias、bean、beans。在我们的spring.xml里面，我们只定义了root节点beans和一个子节点bean，所以进入bean的解析方法。

    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if(delegate.nodeNameEquals(ele, "import")) {
            this.importBeanDefinitionResource(ele);
        } else if(delegate.nodeNameEquals(ele, "alias")) {
            this.processAliasRegistration(ele);
        } else if(delegate.nodeNameEquals(ele, "bean")) {
            this.processBeanDefinition(ele, delegate);  //进入
        } else if(delegate.nodeNameEquals(ele, "beans")) {
            this.doRegisterBeanDefinitions(ele);
        }

    }

进入之后如下，先获取了bean标签的 BeanDefinitionHolder，这个bdHolder是什么呢？我们知道bean标签里面，有name、id、class等属性，这个方法就是为了获取ele标签的属性。如果你想直接进入最终bean的注册地方，可以跳过下面橙色的地方。

我们先进入解析生成BeanDefinitionHolder的方法里，然后再回到核心的registerBeanDefinition。

    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if(bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);//进入

            try {
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, this.getReaderContext().getRegistry());//核心
            } catch (BeanDefinitionStoreException var5) {
                this.getReaderContext().error("Failed to register bean definition with name '" + bdHolder.getBeanName() + "'", ele, var5);
            }

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

    }

进入这个，

        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);

看标签的属性是怎么生成的，相应的注释直接写在里面了。

    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
        String id = ele.getAttribute("id");//获取id属性，userId
        String nameAttr = ele.getAttribute("name");//获取name属性，userName
        List<String> aliases = new ArrayList(); //别名，我们自己在bean中配置的name，属于别名
        if(StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, ",; ");
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;  //beanName是由id属性给的，而不是name属性
        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) {
            this.checkNameUniqueness(beanName, aliases, ele);
        }

        AbstractBeanDefinition beanDefinition = this.parseBeanDefinitionElement(ele, beanName, containingBean);//进入
        if(beanDefinition != null) {
            if(!StringUtils.hasText(beanName)) {
                try {
                    if(containingBean != null) {
                        beanName = BeanDefinitionReaderUtils.generateBeanName(beanDefinition, this.readerContext.getRegistry(), true);
                    } else {
                        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;
        }
    }

接下来进入this.parseBeanDefinitionElement中，在这个方法里，传入了beanName先被保存起来。然后获取class属性。

在this.createBeanDefinition中，通过反射获取class属性的类，然后保存在bd中。下方的注释说明了主要的作用，希望能进去看一下。

    public AbstractBeanDefinition parseBeanDefinitionElement(Element ele, String beanName, BeanDefinition containingBean) {
        this.parseState.push(new BeanEntry(beanName));
        String className = null;
        if(ele.hasAttribute("class")) {
            className = ele.getAttribute("class").trim();
        }

        try {
            String parent = null;
            if(ele.hasAttribute("parent")) {
                parent = ele.getAttribute("parent");
            }

            AbstractBeanDefinition bd = this.createBeanDefinition(className, parent);//反射获取class属性指定的类，保存该类和类名
            this.parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);//获取ele标签的所有属性，即bean的所有属性。
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, "description"));
            this.parseMetaElements(ele, bd);
            this.parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            this.parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
            this.parseConstructorArgElements(ele, bd);
            this.parsePropertyElements(ele, bd);
            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;
    }

上面一部分一下bdHolder的作用。

你需要知道的是bdHolder里面保存着该标签的所有属性。

然后回到正题，进入标签的注册阶段，这里就是bean标签的注册阶段。

BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, this.getReaderContext().getRegistry());

进入这个方法，

    public static void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) throws BeanDefinitionStoreException {
        String beanName = definitionHolder.getBeanName();//获取bean的名字，userId
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());//注册的核心
        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);
            }
        }

    }
进入注册的核心registerBeanDefinition（...），终于到最终的注册方法上了。可能你之前也听过，容器其实是一个map，这里你将会看到它。

   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 var9) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName, "Validation of bean definition failed", var9);
            }
        }

        BeanDefinition oldBeanDefinition = (BeanDefinition)this.beanDefinitionMap.get(beanName);//从容器beanDefinitionMap中获取beanName对应的value
        if(oldBeanDefinition != null) {//如果不是null，说明该beanName已经在容器中存在了，抛异常
            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 {//创建bean
            if(this.hasBeanCreationStarted()) {
                Map var4 = this.beanDefinitionMap;   //存储bean的容器
                synchronized(this.beanDefinitionMap) {
                    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);//将bean注册到容器中
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }

            this.frozenBeanDefinitionNames = null;
        }

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

    }

上面就是核心的注册功能，在完成注册后，程序回到下面的地方.注册完bean后，开始进行别名alias的处理。这里还是有需要说道的地方，可别因为看到了核心的注册，后面别名这里

就不看了。进入下面注释说明的registerAlias()

    public static void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) throws BeanDefinitionStoreException {
        String beanName = definitionHolder.getBeanName();
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());//注册bean
        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);//进入
            }
        }

    }

进入后，代码如下。name就是beanName，在我的配置文件里就是userId，alias就是在配置文件里定义的name属性的值，也就是userName.我特意将id和name属性用不同的

名称，就是为了这里。

    public void registerAlias(String name, String alias) {
        Assert.hasText(name, "'name' must not be empty");//name也就是 id属性的value:userId
        Assert.hasText(alias, "'alias' must not be empty");alias也就是配置文件中name属性的value:userName
        if(alias.equals(name)) {//id和name属性的值一样，就从存放别名的map中删除。
            this.aliasMap.remove(alias);
        } else {
            String registeredName = (String)this.aliasMap.get(alias);
            if(registeredName != null) {
                if(registeredName.equals(name)) {
                    return;
                }

                if(!this.allowAliasOverriding()) {
                    throw new IllegalStateException("Cannot register alias '" + alias + "' for name '" + name + "': It is already registered for name '" + registeredName + "'.");
                }
            }

            this.checkForAliasCircle(name, alias);
            this.aliasMap.put(alias, name);//将alisas(userName) 和 name(userId)存入别名map
        }

    }

到这里，整个流程基本就讲解完了。有疑问或者错误，欢迎留言。