Spring MVC源码分析（四）：请求处理器体系结构设计与实现

一、概述

• 在我的上一篇文章：Spring源码分析（三）：DispatcherServlet的设计与实现中提到，DispatcherServlet在接收到客户端请求时，会遍历DispatcherServlet自身维护的一个HandlerMapping集合，来查找该请求对应的请求处理器，然后由该请求处理器来执行请求处理。
• 在SpringMVC中，DispatcherServlet通过HandlerMapping接口来定义请求和请求处理器之间的映射关系，通过HandlerAdapter接口来提供一个模板实现，即以统一的方式，调用不同handler来执行请求处理。
• HandlerMapping和HandlerAdapter并不是跟名字所描述的这样，即每个HandlerMapping定义一个请求和请求处理器的映射对，这两个接口不是从请求处理方法的角度来定义的，而是从请求处理器的实现类型的角度来定义的。HandlerMapping包含Url和HandlerMethod两种实现，HandlerAdapter包含HttpMethod，Servlet，Controller，HttpRequestHandler四种实现。

二、HandlerMapping：请求与请求处理器映射

• DispatcherServlet通过HandlerMapping获取处理某个请求的请求处理器，请求处理器由请求执行器handler和匹配的拦截器链interceptors两部分组成，具体由HandlerExecutionChain定义请求处理器实现。所以在HandlerMapping接口的实现当中，需要维护两个重要的组件，分别是请求和请求处理器handler的映射map，总拦截器链interceptors，拦截器链在HandlerMapping接口的最顶层抽象实现类AbstractHandlerMapping定义。
• 其次在AbstractHandlerMapping中会定义Cors跨域访问的处理，通常也是在拦截器链添加一个跨域拦截器。然后通过请求的header的Origin的值和应用的跨域配置来执行请求拦截。

请求和请求处理器handler的映射map

a. 基于URL

• 主要在抽象类AbstractUrlHandlerMapping中定义，具体实现子类包含BeanNameUrlHandlerMapping和SimpleUrlHandlerMapping。其中BeanNameUrlHandlerMapping的映射map的key为请求URI，value是beanName，即处理匹配这个uri的请求的bean，在spring容器中的beanName，通常是以“/”开头，如beanName为“/foo”的bean处理URI为“/foo”的请求，同时可以指定beanName的别名alias，这样这些别名也作为匹配的请求URI。获取映射关系的源码如下：

  public class BeanNameUrlHandlerMapping extends AbstractDetectingUrlHandlerMapping {
  
  	/**
  	 * Checks name and aliases of the given bean for URLs, starting with "/".
  	 */
  	@Override
  	protected String[] determineUrlsForHandler(String beanName) {
  		List<String> urls = new ArrayList<>();
  		
  		// urls为以这个beanName为value的多个key，即map的key和value
  		// beanName以“/”开头
  		if (beanName.startsWith("/")) {
  			urls.add(beanName);
  		}
  		// 别名也作为value
  		String[] aliases = obtainApplicationContext().getAliases(beanName);
  		for (String alias : aliases) {
  			if (alias.startsWith("/")) {
  				urls.add(alias);
  			}
  		}
  		return StringUtils.toStringArray(urls);
  	}
  
  }
  

  SimpleUrlHandlerMapping与BeanNameUrlHandlerMapping类似，不过映射map的value的类型可以是beanName或者是bean实例。而这个value对应的请求执行器，在SimpleUrlHandlerMapping与BeanNameUrlHandlerMapping中，可以是Servlet，Controller（是Controller接口实现类，不是@Controller注解）HttpRequestHandler，这个是与HandlerAdapter的实现类对应的。

b. 基于HandlerMethod

• 我们在应用代码中通常使用@Controller和@RequestMapping来定义请求和请求处理方法直接的映射关系，这种方式的请求和请求处理方法映射map是基于HandlerMethod来实现的，即将每个处理方法在springMVC中都会抽象成一个HandlerMethod对象，而请求的匹配条件配置，则是通过RequestMappingInfo来定义的。而这个对应HandlerMapping体系结构的设计是RequestMappingHandlerMapping。

HandlerMethod：基于方法的请求执行器

• HandlerMethod主要用于封装@Controller注解的类的使用@RequestMapping（或@RequestMapping的变体，如@GetMapping）注解的方法信息，类设计如下：

  /**
   * Encapsulates information about a handler method consisting of a
   * {@linkplain #getMethod() method} and a {@linkplain #getBean() bean}.
   * Provides convenient access to method parameters, the method return value,
   * method annotations, etc.
   *
   * <p>The class may be created with a bean instance or with a bean name
   * (e.g. lazy-init bean, prototype bean). Use {@link #createWithResolvedBean()}
   * to obtain a {@code HandlerMethod} instance with a bean instance resolved
   * through the associated {@link BeanFactory}.
   * @since 3.1
   */
  public class HandlerMethod {
  	private final Object bean;
  	@Nullable
  	private final BeanFactory beanFactory;
  	private final Class<?> beanType;
  	private final Method method;
  	private final Method bridgedMethod;
  	private final MethodParameter[] parameters;
  	@Nullable
  	private HttpStatus responseStatus;
  	@Nullable
  	private String responseStatusReason;
  	@Nullable
  	private HandlerMethod resolvedFromHandlerMethod;
  	@Nullable
  	private volatile List<Annotation[][]> interfaceParameterAnnotations;
  	
  	...
  
  }
  

  核心属性为bean，即@Controller注解类对象；method请求方法，主要用于反射调用；parameters方法参数，类型为MethodParameter，MethodParameter封装了参数的位置parameterIndex，参数类型parameterType，参数注解parameterAnnotations等信息。

RequestMappingInfo：请求的匹配条件

• 主要是对@RequestMapping注解的相关属性进行封装，然后作为请求和请求处理器映射map的key。通常@RequstMapping可以设置value，params，method，consumes，produces等参数来精确匹配请求。
• value为匹配的路径；params是匹配的请求参数值；method为处理的http请求方法，consumers为请求的mediaType，produces为响应的mediaType。value，params，consumes，produces都可以设置多个，如下为一个用例：

  @RestController  
  @RequestMapping("/home")  
  public class IndexController {  
      @RequestMapping(value = {
       "/head",
       "/index"
      }, 
      method = RequestMethod.GET,
      headers = {  
          "content-type=text/plain",  
          "content-type=text/html"  
      }, consumers = {
      	"application/JSON",  
      	"application/XML"  
      }, produces = {
      	"application/JSON"
      }) String post() {  
          return "Mapping applied along with headers";  
      }  
  }  
  

• RequestMappingInfo的类设计如下：

  public final class RequestMappingInfo implements RequestCondition<RequestMappingInfo> {
  	@Nullable
  	private final String name;
  	private final PatternsRequestCondition patternsCondition;
  	private final RequestMethodsRequestCondition methodsCondition;
  	private final ParamsRequestCondition paramsCondition;
  	private final HeadersRequestCondition headersCondition;
  	private final ConsumesRequestCondition consumesCondition;
  	private final ProducesRequestCondition producesCondition;
  	private final RequestConditionHolder customConditionHolder;
  	
  	...
  
  		/**
  	 * Checks if all conditions in this request mapping info match the provided request and returns
  	 * a potentially new request mapping info with conditions tailored to the current request.
  	 * <p>For example the returned instance may contain the subset of URL patterns that match to
  	 * the current request, sorted with best matching patterns on top.
  	 * @return a new instance in case all conditions match; or {@code null} otherwise
  	 */
  	@Override
  	@Nullable
  	public RequestMappingInfo getMatchingCondition(HttpServletRequest request) {
  		RequestMethodsRequestCondition methods = this.methodsCondition.getMatchingCondition(request);
  		ParamsRequestCondition params = this.paramsCondition.getMatchingCondition(request);
  		HeadersRequestCondition headers = this.headersCondition.getMatchingCondition(request);
  		ConsumesRequestCondition consumes = this.consumesCondition.getMatchingCondition(request);
  		ProducesRequestCondition produces = this.producesCondition.getMatchingCondition(request);
  
  		if (methods == null || params == null || headers == null || consumes == null || produces == null) {
  			return null;
  		}
  
  		PatternsRequestCondition patterns = this.patternsCondition.getMatchingCondition(request);
  		if (patterns == null) {
  			return null;
  		}
  
  		RequestConditionHolder custom = this.customConditionHolder.getMatchingCondition(request);
  		if (custom == null) {
  			return null;
  		}
  
  		return new RequestMappingInfo(this.name, patterns,
  				methods, params, headers, consumes, produces, custom.getCondition());
  	}
  	
  	...
  }
  

RequestMappingHandlerMapping：基于HandlerMethod和RequestMappingInfo的HandlerMapping实现

• RequestMappingHandlerMapping是HandlerMapping的一个实现，其请求和请求处理器的映射map是以RequestMappingInfo为key，HandlerMethod为value的。这样设计可以通过从客户端请求request对象中获取URI，http method，http header等信息，从而找到匹配的RequestMappingInfo。以该RequestMappingInfo作为key，在映射map中找到请求处理器或者说是请求处理方法。
• RequestMappingHandlerMapping的类结构设计如下：
  核心关注：RequestMappingHandlerMapping -> RequestMappingInfoHandlerMapping -> AbstractHandlerMethodMapping -> AbstractHandlerMapping，InitializingBean这个继承体系设计。同时从请求和请求处理器映射map的创建和初始化；给定请求，从请求和请求处理器映射map查找请求处理器两个角度来分析：

  1. map的创建和初始化
     由类的继承体系可知，实现了InitializingBean接口，故spring容器在创建这个bean时，填充好所有属性之后，会调用InitializingBean的afterPropertiesSet方法，以下为从RequestMappingHandlerMapping -> RequestMappingInfoHandlerMapping的afterPropertiesSet实现：
     RequestMappingHandlerMapping的afterPropertiesSet方法定义：

     	@Override
     	public void afterPropertiesSet() {
     		this.config = new RequestMappingInfo.BuilderConfiguration();
     		this.config.setUrlPathHelper(getUrlPathHelper());
     		this.config.setPathMatcher(getPathMatcher());
     		this.config.setSuffixPatternMatch(this.useSuffixPatternMatch);
     		this.config.setTrailingSlashMatch(this.useTrailingSlashMatch);
     		this.config.setRegisteredSuffixPatternMatch(this.useRegisteredSuffixPatternMatch);
     		this.config.setContentNegotiationManager(getContentNegotiationManager());
     		
     		// 调用RequestMappingInfoHandlerMapping的afterPropertiesSet方法
     		
     		super.afterPropertiesSet();
     	}
     

     RequestMappingInfoHandlerMapping的afterPropertiesSet方法：定义initHandlerMethods完成请求和请求处理器映射map的初始化，故map的初始化是定义在RequestMappingInfoHandlerMapping中的，其整体实现源码如下:

     	// Handler method detection
     	/**
     	 * Detects handler methods at initialization.
     	 * @see #initHandlerMethods
     	 */
     	@Override
     	public void afterPropertiesSet() {
     		initHandlerMethods();
     	}
     
     	/**
     	 * Scan beans in the ApplicationContext, detect and register handler methods.
     	 * @see #getCandidateBeanNames()
     	 * @see #processCandidateBean
     	 * @see #handlerMethodsInitialized
     	 */
     	protected void initHandlerMethods() {
     		
     		// 遍历所有bean
     		
     		for (String beanName : getCandidateBeanNames()) {
     			if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
     				
     				// 查找是handler的bean
     				
     				processCandidateBean(beanName);
     			}
     		}
     		handlerMethodsInitialized(getHandlerMethods());
     	}
     

     获取所有Object类型的bean：getCandidateBeanNames

     	// 获取所有Object类型的bean
     	
     	/**
     	 * Determine the names of candidate beans in the application context.
     	 * @since 5.1
     	 * @see #setDetectHandlerMethodsInAncestorContexts
     	 * @see BeanFactoryUtils#beanNamesForTypeIncludingAncestors
     	 */
     	protected String[] getCandidateBeanNames() {
     		return (this.detectHandlerMethodsInAncestorContexts ?
     				BeanFactoryUtils.beanNamesForTypeIncludingAncestors(obtainApplicationContext(), Object.class) :
     				obtainApplicationContext().getBeanNamesForType(Object.class));
     	}
     

     processCandidateBean：通过isHandler方法，判断某个bean是否是handler，如果是则通过detectHandlerMethods方法，获取该handler的所有方法，然后保存注册到请求和请求处理器映射map。

     	/**
     	 * Determine the type of the specified candidate bean and call
     	 * {@link #detectHandlerMethods} if identified as a handler type.
     	 * <p>This implementation avoids bean creation through checking
     	 * {@link org.springframework.beans.factory.BeanFactory#getType}
     	 * and calling {@link #detectHandlerMethods} with the bean name.
     	 * @param beanName the name of the candidate bean
     	 * @since 5.1
     	 * @see #isHandler
     	 * @see #detectHandlerMethods
     	 */
     	protected void processCandidateBean(String beanName) {
     		Class<?> beanType = null;
     		try {
     			beanType = obtainApplicationContext().getType(beanName);
     		}
     		catch (Throwable ex) {
     			// An unresolvable bean type, probably from a lazy bean - let's ignore it.
     			if (logger.isTraceEnabled()) {
     				logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
     			}
     		}
     		
     		// 使用isHandler方法，判断bean是否是
     		// 请求执行器handler
     		// 这个是一个重要方法，在RequestMappingInfoHandlerMapping中实现
     		
     		if (beanType != null && isHandler(beanType)) {
     			detectHandlerMethods(beanName);
     		}
     	}
     		
     	/**
     	 * Look for handler methods in the specified handler bean.
     	 * @param handler either a bean name or an actual handler instance
     	 * @see #getMappingForMethod
     	 */
     	protected void detectHandlerMethods(Object handler) {
     		Class<?> handlerType = (handler instanceof String ?
     				obtainApplicationContext().getType((String) handler) : handler.getClass());
     
     		if (handlerType != null) {
     			Class<?> userType = ClassUtils.getUserClass(handlerType);
     			Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
     					(MethodIntrospector.MetadataLookup<T>) method -> {
     						try {
     							return getMappingForMethod(method, userType);
     						}
     						catch (Throwable ex) {
     							throw new IllegalStateException("Invalid mapping on handler class [" +
     									userType.getName() + "]: " + method, ex);
     						}
     					});
     			if (logger.isTraceEnabled()) {
     				logger.trace(formatMappings(userType, methods));
     			}
     			methods.forEach((method, mapping) -> {
     				Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
     				registerHandlerMethod(handler, invocableMethod, mapping);
     			});
     		}
     	}	
     

     isHandler方法：判断bean是否是handler，这个是在主类RequestMappingHandlerMapping中定义的：判断bean是否使用@Controller或者@RequestMapping注解

     	/**
     	 * {@inheritDoc}
     	 * <p>Expects a handler to have either a type-level @{@link Controller}
     	 * annotation or a type-level @{@link RequestMapping} annotation.
     	 */
     	@Override
     	protected boolean isHandler(Class<?> beanType) {
     		return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
     				AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
     	}
     

  2. 从请求和请求处理器映射map查找请求处理器：主要是在DispatcherServlet中调用，对应RequestMappingHandlerMapping的底层实现如下：主要是在AbstractHandlerMethodMapping的getHandlerInternal方法定义，AbstractHandlerMethodMapping直接继承于AbstractHandlerMapping。getHandlerInternal的源码实现逻辑如下：

     public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMapping implements InitializingBean {
     	/**
     	 * Look up a handler method for the given request.
     	 */
     	@Override
     	protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
     		String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
     		this.mappingRegistry.acquireReadLock();
     		try {
     			HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
     			return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
     		}
     		finally {
     			this.mappingRegistry.releaseReadLock();
     		}
     	}
     
     	/**
     	 * Look up the best-matching handler method for the current request.
     	 * If multiple matches are found, the best match is selected.
     	 * @param lookupPath mapping lookup path within the current servlet mapping
     	 * @param request the current request
     	 * @return the best-matching handler method, or {@code null} if no match
     	 * @see #handleMatch(Object, String, HttpServletRequest)
     	 * @see #handleNoMatch(Set, String, HttpServletRequest)
     	 */
     	@Nullable
     	protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
     		List<Match> matches = new ArrayList<>();
     		List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
     		if (directPathMatches != null) {
     			addMatchingMappings(directPathMatches, matches, request);
     		}
     		if (matches.isEmpty()) {
     			// No choice but to go through all mappings...
     			addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
     		}
     
     		if (!matches.isEmpty()) {
     			Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
     			matches.sort(comparator);
     			Match bestMatch = matches.get(0);
     			if (matches.size() > 1) {
     				if (logger.isTraceEnabled()) {
     					logger.trace(matches.size() + " matching mappings: " + matches);
     				}
     				if (CorsUtils.isPreFlightRequest(request)) {
     					return PREFLIGHT_AMBIGUOUS_MATCH;
     				}
     				Match secondBestMatch = matches.get(1);
     				if (comparator.compare(bestMatch, secondBestMatch) == 0) {
     					Method m1 = bestMatch.handlerMethod.getMethod();
     					Method m2 = secondBestMatch.handlerMethod.getMethod();
     					String uri = request.getRequestURI();
     					throw new IllegalStateException(
     							"Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
     				}
     			}
     			request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.handlerMethod);
     			handleMatch(bestMatch.mapping, lookupPath, request);
     			return bestMatch.handlerMethod;
     		}
     		else {
     			return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
     		}
     	}
     

三、DispatcherServlet的handlerMappings集合

1. 初始化

• DispatcherServlet可以包含多个HandlerMapping接口的实现类对象，其中RequestMappingHandlerMapping就是其中一个，用于获取使用@Controller和@RequestMapping注解的类和方法，RequestMappingHandlerMapping获取这些Controller类和其方法，从而生成请求和请求处理器映射map，供DispatcherServlet在接收到请求时查找并调用。

• handlerMappings集合中的HandlerMapping对象的创建和初始化时机是在创建DispatcherServlet自身的bean容器WebApplicationContext的时候，具体为在解析WebApplicationContext的配置xml文件，如WEB-INF/myDispatcherServlet-servlet.xml时，通过spring-webmvc子项目的META-INF/spring.handlers获取命名空间处理器MvcNamespaceHandler，MvcNamespaceHandler注册并使用AnnotationDrivenBeanDefinitionParser来处理xml文件的annotation-driven标签：

  
  /**
   * {@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());
  		
  		...
  		
  	}
  
  }
  

• RequestMappingHandlerMapping类型的HandlerMapping的创建：AnnotationDrivenBeanDefinitionParser在解析annotation-driven标签时，调用parse方法：此时创建RequestMappingHandlerMapping类型的bean：

  	@Override
  	@Nullable
  	public BeanDefinition parse(Element element, ParserContext context) {
  		
  		...
  	
  		// 创建RequestMappingHandlerMapping的BeanDefinition，
  		// 之后创建bean实例
  	
  		RootBeanDefinition handlerMappingDef = new RootBeanDefinition(RequestMappingHandlerMapping.class);
  		handlerMappingDef.setSource(source);
  		handlerMappingDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
  		handlerMappingDef.getPropertyValues().add("order", 0);
  		handlerMappingDef.getPropertyValues().add("contentNegotiationManager", contentNegotiationManager);
  
  		if (element.hasAttribute("enable-matrix-variables")) {
  			Boolean enableMatrixVariables = Boolean.valueOf(element.getAttribute("enable-matrix-variables"));
  			handlerMappingDef.getPropertyValues().add("removeSemicolonContent", !enableMatrixVariables);
  		}
  
  		configurePathMatchingProperties(handlerMappingDef, element, context);
  		readerContext.getRegistry().registerBeanDefinition(HANDLER_MAPPING_BEAN_NAME , handlerMappingDef);
  
  		RuntimeBeanReference corsRef = MvcNamespaceUtils.registerCorsConfigurations(null, context, source);
  		handlerMappingDef.getPropertyValues().add("corsConfigurations", corsRef);
  
  		...
  		
  }
  

2. 请求处理

• DispatcherServlet遍历自身的handlerMappings集合，从每个handlerMapping的map中找到处理这个请求的请求处理器：找到一个则返回，其中handlerMappings集合的实例类型，可以是AbstractUrlHandlerMapping的实现，如BeanNameUrlHandlerMapping和SimpleUrlHandlerMapping；或者是AbstractHandlerMethodMapping的实现，如RequestMappingHandlerMapping。

  /**
   * Return the HandlerExecutionChain for this request.
   * <p>Tries all handler mappings in order.
   * @param request current HTTP request
   * @return the HandlerExecutionChain, or {@code null} if no handler could be found
   */
  @Nullable
  protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
  	if (this.handlerMappings != null) {
  		for (HandlerMapping mapping : this.handlerMappings) {
  		
  			// 找到一个则返回
  			
  			HandlerExecutionChain handler = mapping.getHandler(request);
  			if (handler != null) {
  				return handler;
  			}
  		}
  	}
  	return null;
  }
  

• 调用HandlerMapping的getHandler方法获取一个请求处理器HandlerExecutionChain。

3. 请求执行

• HandlerExecutionChain：由一个请求执行器handler和多个请求拦截器interceptors组成，handler在处理请求之前，需要先调用interceptors的preHandler方法来对请求进行拦截处理，只有通过了全部的拦截器，才能交给请求执行器handler完成真正的请求处理。源码设计如下：

  /**
   * Handler execution chain, consisting of handler object and any handler interceptors.
   * Returned by HandlerMapping's {@link HandlerMapping#getHandler} method.
   */
  public class HandlerExecutionChain {
  	private final Object handler;
  
  	@Nullable
  	private HandlerInterceptor[] interceptors;
  
  	@Nullable
  	private List<HandlerInterceptor> interceptorList;
  
  	private int interceptorIndex = -1;
  
  	...
  	
  }
  

4. 完整请求处理过程

• DispatcherServlet中使用doDispatch方法完成整个请求处理，核心处理逻辑如下

	/**
	 * Process the actual dispatching to the handler.
	 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
	 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
	 * to find the first that supports the handler class.
	 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
	 * themselves to decide which methods are acceptable.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @throws Exception in case of any kind of processing failure
	 */
	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;

		...
			// multipart请求加工
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			// 查找请求处理器handler
			
			// Determine handler for the current request.
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			// 获取请求执行器的执行适配器
			
			// Determine handler adapter for the current request.
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			// Process last-modified header, if supported by the handler.
			String method = request.getMethod();
			boolean isGet = "GET".equals(method);
			if (isGet || "HEAD".equals(method)) {
				long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
				if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}

			// 拦截器链前置处理applyPreHandle
			
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}
			
			// 通过请求执行器适配器来调用请求执行器（如handlerMethod）处理请求
			
			// Actually invoke the handler.
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}
			
			// 拦截器链后置处理applyPostHandle
			
			applyDefaultViewName(processedRequest, mv);
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		
		...
		
		// 响应客户端
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}

四、HandlerAdapter：DispatcherServlet的请求执行适配器

• DispatcherServlet通过HandlerAdapter来间接调用实际的请求执行器handler。由上面分析可知请求执行器包含：Servlet，Controller，HttpRequestHandler，HandlerMethod。
• 所以与上面这些请求执行器配套的HandlerAdapter分别为：SimpleServletHandlerAdapter，SimpleControllerHandlerAdapter，HttpRequestHandlerAdapter和RequestMappingHandlerAdapter。
• 如下为SimpleControllerHandlerAdapter的源码：其他类似

  package org.springframework.web.servlet.mvc;
  /**
   * Adapter to use the plain {@link Controller} workflow interface with
   * the generic {@link org.springframework.web.servlet.DispatcherServlet}.
   * Supports handlers that implement the {@link LastModified} interface.
   *
   * <p>This is an SPI class, not used directly by application code.
   *
   * @author Rod Johnson
   * @author Juergen Hoeller
   * @see org.springframework.web.servlet.DispatcherServlet
   * @see Controller
   * @see LastModified
   * @see HttpRequestHandlerAdapter
   */
  public class SimpleControllerHandlerAdapter implements HandlerAdapter {
  	@Override
  	public boolean supports(Object handler) {
  		return (handler instanceof Controller);
  	}
  	@Override
  	@Nullable
  	public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
  			throws Exception {
  
  		return ((Controller) handler).handleRequest(request, response);
  	}
  	@Override
  	public long getLastModified(HttpServletRequest request, Object handler) {
  		if (handler instanceof LastModified) {
  			return ((LastModified) handler).getLastModified(request);
  		}
  		return -1L;
  	}
  }