IOC之loadBeanDefinitions主流程

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
  //前面一些列操作，加载资源把Resource转化为InputStream，再包装成inputSource
   Assert.notNull(encodedResource, "EncodedResource must not be null");
   if (logger.isInfoEnabled()) {
      logger.info("Loading XML bean definitions from " + encodedResource.getResource());
   }
   Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
   if (currentResources == null) {
      currentResources = new HashSet<>(4);
      this.resourcesCurrentlyBeingLoaded.set(currentResources);
   }
   if (!currentResources.add(encodedResource)) {
      throw new BeanDefinitionStoreException(
            "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
   }
   try {
      InputStream inputStream = encodedResource.getResource().getInputStream();
      try {
         InputSource inputSource = new InputSource(inputStream);
         if (encodedResource.getEncoding() != null) {
            inputSource.setEncoding(encodedResource.getEncoding());
         }
        //重点就是在doLoadBeanDefinitions
         return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
      }
      finally {
         inputStream.close();
      }
   }
   catch (IOException ex) {
      throw new BeanDefinitionStoreException(
            "IOException parsing XML document from " + encodedResource.getResource(), ex);
   }
   finally {
      currentResources.remove(encodedResource);
      if (currentResources.isEmpty()) {
         this.resourcesCurrentlyBeingLoaded.remove();
      }
   }
}

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
      throws BeanDefinitionStoreException {
   try {
     //重点就两行代码 
     //1.把inputsource转化为Document对象
     //2.开始重点 registerBeanDefinitions
      Document doc = doLoadDocument(inputSource, resource);
      return registerBeanDefinitions(doc, resource);
   }
   catch (BeanDefinitionStoreException ex) {
      throw ex;
   }
   catch (SAXParseException ex) {
      throw new XmlBeanDefinitionStoreException(resource.getDescription(),
            "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
   }
   catch (SAXException ex) {
      throw new XmlBeanDefinitionStoreException(resource.getDescription(),
            "XML document from " + resource + " is invalid", ex);
   }
   catch (ParserConfigurationException ex) {
      throw new BeanDefinitionStoreException(resource.getDescription(),
            "Parser configuration exception parsing XML from " + resource, ex);
   }
   catch (IOException ex) {
      throw new BeanDefinitionStoreException(resource.getDescription(),
            "IOException parsing XML document from " + resource, ex);
   }
   catch (Throwable ex) {
      throw new BeanDefinitionStoreException(resource.getDescription(),
            "Unexpected exception parsing XML document from " + resource, ex);
   }
}

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
  //实例化了一个DefaultBeanDefinitionDocumentReader
   BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
   int countBefore = getRegistry().getBeanDefinitionCount();
   //终于到注册BeanDefintions的环节，委托给了DefaultBeanDefinitionDocumentReader
   documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
   return getRegistry().getBeanDefinitionCount() - countBefore;
}

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

protected void doRegisterBeanDefinitions(Element root) {
//这里就是通过项目不同的profile的设置 来获取xml上相应的profile bean
   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;
         }
      }
   }
	//前置处理xml
   preProcessXml(root);
 	//重点注册又出现在这里
   parseBeanDefinitions(root, this.delegate);
  //后置处理xml
   postProcessXml(root);

   this.delegate = parent;
}

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
  //判断root节点是否为根节点，若是则遍历其子节点做处理,子节点存在<bean> <import> <alias>这些标签
  //若不存在命名空间则调用parseCustomElement逻辑，处理自定义节点，自定义节点得需要有相应的处理器才行，
  //否则啥也解析不了
		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)) {
            
           // 处理正常的Element
						parseDefaultElement(ele, delegate);
					}
					else {
						delegate.parseCustomElement(ele);
					}
				}
			}
		}
		else {
			delegate.parseCustomElement(root);
		}
	}

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)) {
     //分析注册bean
      processBeanDefinition(ele, delegate);
   }
   else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
      // recurse
      doRegisterBeanDefinitions(ele);
   }
}

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
  //BeanDefinitionHolder 就是BeanDefinition的强化版，持有 BeanDefinition，带有BeanName和alias
  //所以看名字还是很容易理解变量的作用的
   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));
   }
}

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));
   }
}