SpringIoC初始化BeanDefinition解析——注册

    

   BeanDefinition的解析还差最后的注册过程，距离前两篇文章隔的有点久，现在补上。

    先来回顾一下上一篇文章里提到了DefaultBeanDefinitionDocumentReader中的方法：

<span style="font-weight: normal;"><span style="font-size:18px;">protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);if (bdHolder != null) {bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);try {// Register the final decorated instance.BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());}catch (BeanDefinitionStoreException ex) {getReaderContext().error("Failed to register bean definition with name '" +bdHolder.getBeanName() + "'", ele, ex);}// Send registration event.getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));}}</span></span>

   这个方法通过BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry())进行BeanDefinition的注册。BeanDefinition被包装在bdHolder中，而registry是从ReaderContext中取得的。这个registry就是spring注册bean定义的注册中心，所有的bean都要注册到这个注册中心去。这个registry到底是什么东西呢，这要要从XmlBeanDefinitionReader的创建说起，再回顾下第一篇文章中XmlWebApplicationContext触发BeanDefinition解析的方法

<span style="font-weight: normal;"><span style="font-size:18px;">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);}</span></span>

   创建XmlBeanDefinitionReader的时候传入的了一个beanFactory。这个beanFactory其实就是刚刚提到的注册中心registry。下面的代码印证了这个事实

<span style="font-weight: normal;"><span style="font-size:18px;">public XmlBeanDefinitionReader(BeanDefinitionRegistry registry) {super(registry);}</span></span>

<span style="font-weight: normal;"><span style="font-size:18px;">public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactoryimplements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable</span></span>

     DefaultListableBeanFactory 就是BeanDefinitionRegistry得子类，容器在创建XmlBeanDefinitionReader的时候把自己的beanFactory作为构造函数的参数传给了XmlBeanDefinitionReader，然后XmlBeanDefinitionReader在构造ReaderContext的时候又将自己传给了ReaderContext。

<span style="font-weight: normal;"><span style="font-size:18px;">protected XmlReaderContext createReaderContext(Resource resource) {if (this.namespaceHandlerResolver == null) {this.namespaceHandlerResolver = createDefaultNamespaceHandlerResolver();}return new XmlReaderContext(resource, this.problemReporter, this.eventListener,this.sourceExtractor, this, this.namespaceHandlerResolver);}</span></span>

    而ReaderContext中获取registry就是取的XmlBeanDefinitionReader中的registry。

<span style="font-size:18px;">public final BeanDefinitionRegistry getRegistry() {return this.reader.getRegistry();}</span>

    注册中心DefaultListableBeanFactory主要靠维护一个hashmap来保存所有的BeanDefinition，hashmap的key就是bean的名字。下面节选自DefaultListableBeanFactory的代码：

<span style="font-size:18px;">/** Map of bean definition objects, keyed by bean name */private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>();</span>

      说了这么多还没有解释注册的方法，下面就是BeanDefinitionReaderUtils的registerBeanDefinition方法：

<span style="font-size:18px;">public static void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)throws BeanDefinitionStoreException {// Register bean definition under primary name.String beanName = definitionHolder.getBeanName();registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());// Register aliases for bean name, if any.String[] aliases = definitionHolder.getAliases();if (aliases != null) {for (String aliase : aliases) {registry.registerAlias(beanName, aliase);}}}</span>

     这里做了两件事，第一件事利用registry把bean的名字和BeanDefinition注册进去，另外就是把所有bean的别名也进行了注册。接下来看看registry是怎么对bean的名字和bean的别名进行注册的：

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);}}synchronized (this.beanDefinitionMap) {Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);if (oldBeanDefinition != null) {if (!this.allowBeanDefinitionOverriding) {throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +"': There is already [" + oldBeanDefinition + "] bound.");}else {if (this.logger.isInfoEnabled()) {this.logger.info("Overriding bean definition for bean '" + beanName +"': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");}}}else {this.beanDefinitionNames.add(beanName);this.frozenBeanDefinitionNames = null;}this.beanDefinitionMap.put(beanName, beanDefinition);resetBeanDefinition(beanName);}}

public void registerAlias(String name, String alias) {Assert.hasText(name, "'name' must not be empty");Assert.hasText(alias, "'alias' must not be empty");if (alias.equals(name)) {this.aliasMap.remove(alias);}else {if (!allowAliasOverriding()) {String registeredName = this.aliasMap.get(alias);if (registeredName != null && !registeredName.equals(name)) {throw new IllegalStateException("Cannot register alias '" + alias + "' for name '" +name + "': It is already registered for name '" + registeredName + "'.");}}checkForAliasCircle(name, alias);this.aliasMap.put(alias, name);}}