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Spring 是如何解析 bean 标签的？( 二 )

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property 标签的解析算是相对复杂的，其他标签（meta、constructor-arg 等）的解析过程大体是类似的，不再一一分析 。
经过 BeanDefinitionParserDelegate#parseBeanDefinitionElement 方法的解析和封装后，就得到了保存我们自定义 bean 信息的 BeanDefinition，即 GenericBeanDefinition 。Spring 又把 BeanDefinition 和别名信息封装成了 BeanDefinitionHolder：
public class BeanDefinitionParserDelegate {public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {String id = ele.getAttribute(ID_ATTRIBUTE);String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);// ...// 解析后得到的 BeanDefinitionAbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);if (beanDefinition != null) {// ...String[] aliasesArray = StringUtils.toStringArray(aliases);return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);}return null;}}此外，在向 IoC 容器注册之前，还有一个 decorateBeanDefinitionIfRequired 方法，它主要是用来处理默认名称空间（即 www.springframework.org/schema/bean… ）之外的 bean 定义，比如 <tx>、<context> 等，这里仍然先沿着主线走，暂不深入分析 。
接下来就是将 BeanDefinition 注册到 IoC 容器：
public class DefaultBeanDefinitionDocumentReader implements BeanDefinitionDocumentReader {// ...protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {// 解析后的 BeanDefinition 封装成的 BeanDefinitionHolderBeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);if (bdHolder != null) {// 处理非默认名称空间的 beanbdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);try {// Register the final decorated instance.// 注册 BeanDefinitionBeanDefinitionReaderUtils.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));}}}public abstract class BeanDefinitionReaderUtils {// ...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 alias : aliases) {registry.registerAlias(beanName, alias);}}}}IoC 容器是哪个？如何注册呢？
前文提到过，Spring 默认的 IoC 容器是 DefaultListableBeanFactory，来看下它的继承结构：

Spring 是如何解析 bean 标签的？

文章插图
 
可以看到 DefaultListableBeanFactory 实现了 BeanDefinitionRegistry 接口 。
所谓的“注册”到 IoC 容器，其实就是把 BeanDefinition 保存到了 DefaultListableBeanFactory 持有的一个 Map 中，如下：
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactoryimplements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {// ...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);}}// 获取已存在的 BeanDefinitionBeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);if (existingDefinition != null) {if (!isAllowBeanDefinitionOverriding()) {throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);}// 这几个异常信息是不是有点眼熟？else if (existingDefinition.getRole() < beanDefinition.getRole()) {// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTUREif (logger.isInfoEnabled()) {logger.info("Overriding user-defined bean definition for bean '" + beanName +"' with a framework-generated bean definition: replacing [" +existingDefinition + "] with [" + beanDefinition + "]");}}else if (!beanDefinition.equals(existingDefinition)) {if (logger.isDebugEnabled()) {logger.debug("Overriding bean definition for bean '" + beanName +"' with a different definition: replacing [" + existingDefinition +"] with [" + beanDefinition + "]");}}else {if (logger.isTraceEnabled()) {logger.trace("Overriding bean definition for bean '" + beanName +"' with an equivalent definition: replacing [" + existingDefinition +"] 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<>(this.beanDefinitionNames.size() + 1);updatedDefinitions.addAll(this.beanDefinitionNames);updatedDefinitions.add(beanName);this.beanDefinitionNames = updatedDefinitions;removeManualSingletonName(beanName);}}else {// 注册到 Map 中// Still in startup registration phasethis.beanDefinitionMap.put(beanName, beanDefinition);this.beanDefinitionNames.add(beanName);removeManualSingletonName(beanName);}this.frozenBeanDefinitionNames = null;}if (existingDefinition != null || containsSingleton(beanName)) {resetBeanDefinition(beanName);}else if (isConfigurationFrozen()) {clearByTypeCache();}}}


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