Spring XmlBeanFactory的Bean注册流程

XmlBeanFactory beanFactory = new XmlBeanFactory(new ClassPathResource("com/zhiwei/ioc/applicationContext.xml"));

引发的思考？？？

Spring xml文件中的Bean定义是怎么反映到BeanFactory的，中间经过什么样的过程，这里从源码分析稍微说明一下。

①：XmlBeanFactory :简单介绍

private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {        super(parentBeanFactory);        this.reader.loadBeanDefinitions(resource);    }

XmlBeanFactory是在DefaultListableBeanFactory真正意义的IOC容器添加XML的新特性的扩展工厂
XmlBeanDefinitionReader：从其使用方法来看，XmlBeanDefinitionReader会和XmlBeanFactory进行绑定，充当一个工具类的作用，主要用于XML文件的解析，类似这种形式的用法，例如tomcat里面的事件监听支持类：

protected final PropertyChangeSupport support =            new PropertyChangeSupport(this);

②：分析XmlBeanDefinitionReader的loadBeanDefinitions方法
本质注册BeanDefinition的方法

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;    }

值得注意的是内部调用的createReaderContext工具方法

public XmlReaderContext createReaderContext(Resource resource) {        return new XmlReaderContext(resource, this.problemReporter, this.eventListener,                this.sourceExtractor, this, getNamespaceHandlerResolver());    }

createReaderContext通过this这个属性，将XmlReaderContext和BeanFactory绑定在一起，这样就可以专注于xml文件的解析，而不用总是通过XmlBeanDefinitionReader去获取BeanFactory的信息

③：关注documentReader.registerBeanDefinitions方法：
本质调用DefaultBeanDefinitionDocumentReader的doRegisterBeanDefinitions方法

protected void doRegisterBeanDefinitions(Element root) {        BeanDefinitionParserDelegate parent = this.delegate;        this.delegate = createDelegate(getReaderContext(), root, parent);        if (this.delegate.isDefaultNamespace(root)) {            String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);            if (StringUtils.hasText(profileSpec)) {                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(                        profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);                if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {                    if (logger.isInfoEnabled()) {                        logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +                                "] not matching: " + getReaderContext().getResource());                    }                    return;                }            }        }        preProcessXml(root);        parseBeanDefinitions(root, this.delegate);        postProcessXml(root);        this.delegate = parent;    }

preProcessXml：解析xml文件的前期处理工作
postProcessXml：解析xml文件的后期处理工作
parseBeanDefinitions：真正意义上的解析xml文件内容

④：关注parseBeanDefinitions方法

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

⑤：接着关注parseDefaultElement方法

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {            importBeanDefinitionResource(ele);        }        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {            processAliasRegistration(ele);        }        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {            processBeanDefinition(ele, delegate);        }        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {            // recurse            doRegisterBeanDefinitions(ele);        }    }

Spring xml文件解析真相大白，这里解析xml文件中各种标签元素

⑥：重点关注 processBeanDefinition方法

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);        if (bdHolder != null) {            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);            try {                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());            }            catch (BeanDefinitionStoreException ex) {                getReaderContext().error("Failed to register bean definition with name '" +                        bdHolder.getBeanName() + "'", ele, ex);            }            getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));        }    }

分析：借助于BeanDefinitionParserDelegate的帮助，解析xml文件元素获取BeanDefinitionHolder对象，然后通过BeanDefinitionReaderUtils.registerBeanDefinition()将BeanDefinition和BeanFactory进行绑定

⑦：关注：BeanDefinitionReaderUtils.registerBeanDefinition方法

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) {            for (String alias : aliases) {                registry.registerAlias(beanName, alias);            }        }    }

之前上面说到XmlReaderContext绑定了BeanFactory，因此这里是通过XmlReaderContext获取BeanFactory对象，然后进行Bean的注册的

⑧：关注registry.registerBeanDefinition方法
本质调用DefaultListableBeanFactory的registerBeanDefinition进行注册

@Override    public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)            throws BeanDefinitionStoreException {        Assert.hasText(beanName, "Bean name must not be empty");        Assert.notNull(beanDefinition, "BeanDefinition must not be null");        if (beanDefinition instanceof AbstractBeanDefinition) {            try {                ((AbstractBeanDefinition) beanDefinition).validate();            }            catch (BeanDefinitionValidationException ex) {                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,                        "Validation of bean definition failed", ex);            }        }        BeanDefinition oldBeanDefinition;        oldBeanDefinition = this.beanDefinitionMap.get(beanName);        if (oldBeanDefinition != null) {            if (!isAllowBeanDefinitionOverriding()) {                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,                        "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +                        "': There is already [" + oldBeanDefinition + "] bound.");            }            else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {                // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE                if (this.logger.isWarnEnabled()) {                    this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +                            "' with a framework-generated bean definition: replacing [" +                            oldBeanDefinition + "] with [" + beanDefinition + "]");                }            }            else if (!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 (hasBeanCreationStarted()) {                // Cannot modify startup-time collection elements anymore (for stable iteration)                synchronized (this.beanDefinitionMap) {                    this.beanDefinitionMap.put(beanName, beanDefinition);                    List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);                    updatedDefinitions.addAll(this.beanDefinitionNames);                    updatedDefinitions.add(beanName);                    this.beanDefinitionNames = updatedDefinitions;                    if (this.manualSingletonNames.contains(beanName)) {                        Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);                        updatedSingletons.remove(beanName);                        this.manualSingletonNames = updatedSingletons;                    }                }            }            else {                // Still in startup registration phase                this.beanDefinitionMap.put(beanName, beanDefinition);                this.beanDefinitionNames.add(beanName);                this.manualSingletonNames.remove(beanName);            }            this.frozenBeanDefinitionNames = null;        }        if (oldBeanDefinition != null || containsSingleton(beanName)) {            resetBeanDefinition(beanName);        }    }

分析：DefaultListableBeanFactory注册Bean的时候首先会判断Bean Factory是否存在同名的Bean,如果存在则进行BeanFactory是否允许重写、Bean的优先级等校验，然后在进行对应的处理，如果不能存在则直接创建，然后将数据填充到beanDefinitionMap这个Map的数据结构中，当需要使用getBean方法的时候，方便从beanDefinitionMap根据key(Bean名称)直接读取，如果是别名,IOC容器内部维护一个别名和Bean名称的Map，需要的时候会将别名转换为Bean名称，然后再按照正常流程处理。