Spring MVC源码分析（四）：请求处理器体系结构设计与实现

以你之姓@ 2022-03-25 06:58 150阅读 0赞

## 一、概述 ##

*  在我的上一篇文章：[Spring源码分析（三）：DispatcherServlet的设计与实现][Spring_DispatcherServlet]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. * * 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. * 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：![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTAwMTM1NzM_size_16_color_FFFFFF_t_70] 
 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. * 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} * 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. * 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. * 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. * 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. * * 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;
 	}
 }

[Spring_DispatcherServlet]: https://blog.csdn.net/u010013573/article/details/86550201
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