spring的applicationContext.xml文件与NamespaceHandler解析

http\://www.springframework.org/schema/mvc=org.springframework.web.servlet.config.MvcNamespaceHandler

package org.springframework.web.servlet.config;
/**
 * {@link NamespaceHandler} for Spring MVC configuration namespace.
 *
 * @author Keith Donald
 * @author Jeremy Grelle
 * @author Sebastien Deleuze
 * @since 3.0
 */
public class MvcNamespaceHandler extends NamespaceHandlerSupport {
	@Override
	public void init() {
		registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
		registerBeanDefinitionParser("default-servlet-handler", new DefaultServletHandlerBeanDefinitionParser());
		registerBeanDefinitionParser("interceptors", new InterceptorsBeanDefinitionParser());
		registerBeanDefinitionParser("resources", new ResourcesBeanDefinitionParser());
		registerBeanDefinitionParser("view-controller", new ViewControllerBeanDefinitionParser());
		registerBeanDefinitionParser("redirect-view-controller", new ViewControllerBeanDefinitionParser());
		registerBeanDefinitionParser("status-controller", new ViewControllerBeanDefinitionParser());
		registerBeanDefinitionParser("view-resolvers", new ViewResolversBeanDefinitionParser());
		registerBeanDefinitionParser("tiles-configurer", new TilesConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("freemarker-configurer", new FreeMarkerConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("groovy-configurer", new GroovyMarkupConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("script-template-configurer", new ScriptTemplateConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("cors", new CorsBeanDefinitionParser());
	}
}

public BeanDefinition parseCustomElement(Element ele, @Nullable BeanDefinition containingBd) {
	String namespaceUri = getNamespaceURI(ele);
	if (namespaceUri == null) {
		return null;
	}
	// 获取一个NamespaceHandler
	NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
	if (handler == null) {
		error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
		return null;
	}
	// 通过这个NamespaceHandler
	// 在它里面维护的parser集合中找到
	// 与该标签对应的parser，由该parser来执行解析
	return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

/**
 * Load the specified NamespaceHandler mappings lazily.
 */
private Map<String, Object> getHandlerMappings() {
	Map<String, Object> handlerMappings = this.handlerMappings;
	if (handlerMappings == null) {
		synchronized (this) {
			handlerMappings = this.handlerMappings;
			if (handlerMappings == null) {
				if (logger.isTraceEnabled()) {
					logger.trace("Loading NamespaceHandler mappings from [" + this.handlerMappingsLocation + "]");
				}
				try {
					Properties mappings =
							PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
					if (logger.isTraceEnabled()) {
						logger.trace("Loaded NamespaceHandler mappings: " + mappings);
					}
					handlerMappings = new ConcurrentHashMap<>(mappings.size());
					CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
					this.handlerMappings = handlerMappings;
				}
				catch (IOException ex) {
					throw new IllegalStateException(
							"Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
				}
			}
		}
	}
	return handlerMappings;
}

  /**
   * Locate the {@link NamespaceHandler} for the supplied namespace URI
   * from the configured mappings.
   * @param namespaceUri the relevant namespace URI
   * @return the located {@link NamespaceHandler}, or {@code null} if none found
   */
  @Override
  @Nullable
  public NamespaceHandler resolve(String namespaceUri) {
      // 懒加载NamespaceHandler的handlerMappings
  	Map<String, Object> handlerMappings = getHandlerMappings();
  	Object handlerOrClassName = handlerMappings.get(namespaceUri);
  	if (handlerOrClassName == null) {
  		return null;
  	}
  	// 不是第一次调用，则已经是NamespaceHandler类型了，可以直接返回
  	else if (handlerOrClassName instanceof NamespaceHandler) {
  		return (NamespaceHandler) handlerOrClassName;
  	}
  	// 第一次调用，由上面分析可知：
  	// 刚开始从META-INF/spring.handlers
  	// 读出时，handlerMappings的value是字符串
  	else {
  		String className = (String) handlerOrClassName;
  		try {
  		// 加载NamespaceHandler
  			Class<?> handlerClass = ClassUtils.forName(className, this.classLoader);
  			if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
  				throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
  						"] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
  			}
  			NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
  			// 调用init方法完成parsers的初始化
  			namespaceHandler.init();
  			handlerMappings.put(namespaceUri, namespaceHandler);
  			return namespaceHandler;
  		}
  		catch (ClassNotFoundException ex) {
  			throw new FatalBeanException("Could not find NamespaceHandler class [" + className +
  					"] for namespace [" + namespaceUri + "]", ex);
  		}
  		catch (LinkageError err) {
  			throw new FatalBeanException("Unresolvable class definition for NamespaceHandler class [" +
  					className + "] for namespace [" + namespaceUri + "]", err);
  		}
  	}
  }
  protected final void registerBeanDefinitionParser(String elementName, BeanDefinitionParser parser) {
    this.parsers.put(elementName, parser);
  }

/**
 * {@link org.springframework.beans.factory.xml.NamespaceHandler}
 * for the '{@code context}' namespace.
 *
 * @author Mark Fisher
 * @author Juergen Hoeller
 * @since 2.5
 */
public class ContextNamespaceHandler extends NamespaceHandlerSupport {
	@Override
	public void init() {
		registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
		registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
		registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
		// <context:component-scan />
    	registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
		registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
		registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
		registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
		registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
	}
}