• 这一次搞懂SpringMVC原理


    @

    前言

    前面几篇文章,学习了Spring IOC、Bean实例化过程、AOP、事务的源码和设计思想,了解了Spring的整体运行流程,但如果是web开发,那么必不可少的还有Spring MVC,本篇主要分析在请求调用过程中SpringMVC的实现原理,通过本篇要搞懂它是怎么解决请求、参数、返回值映射等问题的。

    正文

    请求入口

    我们都知道前端调用后端接口时,都会通过Servlet进行转发,而Servlet的声明周期包含下面四个阶段:

    • 实例化(new)
    • 初始化(init)
    • 执行(service调用doGet/doPost)
    • 销毁(destroy)

    前两个阶段在Spring启动阶段就做好了(init根据配置可能是第一次请求时才会调用),销毁是服务关闭的时候进行,本文主要分析的就是请求执行阶段。我们知道SpringMVC的核心就是DispatcherServlet,该类是对Servlet的扩展,所以直接从该类的service方法开始,但在此类中没有service方法,那肯定是在其父类中,我们先来看看其继承体系:
    在这里插入图片描述
    逐个往上找,在FrameworkServlet方法中就有一个service方法:

    	protected void service(HttpServletRequest request, HttpServletResponse response)
    			throws ServletException, IOException {
    
    		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
    		if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
    			processRequest(request, response);
    		}
    		else {
    			super.service(request, response);
    		}
    	}
    
        protected void service(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
        {
            String method = req.getMethod();
    
            if (method.equals(METHOD_GET)) {
                long lastModified = getLastModified(req);
                if (lastModified == -1) {
                    doGet(req, resp);
                } else {
                    long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                    if (ifModifiedSince < lastModified) {
                        maybeSetLastModified(resp, lastModified);
                        doGet(req, resp);
                    } else {
                        resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                    }
                }
    
            } else if (method.equals(METHOD_HEAD)) {
                long lastModified = getLastModified(req);
                maybeSetLastModified(resp, lastModified);
                doHead(req, resp);
            } else if (method.equals(METHOD_POST)) {
                doPost(req, resp);
            } else if (method.equals(METHOD_PUT)) {
                doPut(req, resp);
            } else if (method.equals(METHOD_DELETE)) {
                doDelete(req, resp);
            } else if (method.equals(METHOD_OPTIONS)) {
                doOptions(req,resp);
            } else if (method.equals(METHOD_TRACE)) {
                doTrace(req,resp);
            } else {
                String errMsg = lStrings.getString("http.method_not_implemented");
                Object[] errArgs = new Object[1];
                errArgs[0] = method;
                errMsg = MessageFormat.format(errMsg, errArgs);
                
                resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
            }
        }
       
    

    但其主要还是调用父类HttpServlet中的方法,而该类又会根据不同的请求方式会调到子类中,最后的核心方法就是DispatcherServlet中的doDispatch方法:

    	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    		HttpServletRequest processedRequest = request;
    		HandlerExecutionChain mappedHandler = null;
    		boolean multipartRequestParsed = false;
    
    		//异步管理
    		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    
    		try {
    			ModelAndView mv = null;
    			Exception dispatchException = null;
    
    			try {
    				//文件上传
    				processedRequest = checkMultipart(request);
    				multipartRequestParsed = (processedRequest != request);
    
    				//这个方法很重要,重点看
    				// Determine handler for the current request.
    				mappedHandler = getHandler(processedRequest);
    				if (mappedHandler == null) {
    					noHandlerFound(processedRequest, response);
    					return;
    				}
    
    				//获取跟HandlerMethod匹配的HandlerAdapter对象
    				// 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;
    					}
    				}
    
    				//前置过滤器,如果为false则直接返回
    				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    					return;
    				}
    
    				//调用到Controller具体方法,核心方法调用,重点看看
    				// Actually invoke the handler.
    				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    
    				if (asyncManager.isConcurrentHandlingStarted()) {
    					return;
    				}
    
    				applyDefaultViewName(processedRequest, mv);
    
    				//中置过滤器
    				mappedHandler.applyPostHandle(processedRequest, response, mv);
    			}
    			catch (Exception ex) {
    				dispatchException = ex;
    			}
    			catch (Throwable err) {
    				// As of 4.3, we're processing Errors thrown from handler methods as well,
    				// making them available for @ExceptionHandler methods and other scenarios.
    				dispatchException = new NestedServletException("Handler dispatch failed", err);
    			}
    
    			//视图渲染及后置过滤器执行
    			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    		}
    		catch (Exception ex) {
    			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    		}
    		catch (Throwable err) {
    			triggerAfterCompletion(processedRequest, response, mappedHandler,
    					new NestedServletException("Handler processing failed", err));
    		}
    		finally {
    			if (asyncManager.isConcurrentHandlingStarted()) {
    				// Instead of postHandle and afterCompletion
    				if (mappedHandler != null) {
    					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
    				}
    			}
    			else {
    				// Clean up any resources used by a multipart request.
    				if (multipartRequestParsed) {
    					cleanupMultipart(processedRequest);
    				}
    			}
    		}
    	}
    

    MVC的所有处理逻辑都在这个方法中,先总结一下这个方法的实现逻辑,首先根据请求的url拿到缓存中的HandlerMethod对象和执行链对象,HandlerMethod中封装了controller对象、方法对象和方法参数等信息,执行链则是包含了一个个HandlerInterceptor拦截器;然后再通过HandlerMethod拿到对应的HandlerAdapter,这个对象的作用就是去适配我们的controller;准备工作做完后,首先会执行前置过滤,如果被拦截则直接返回,否则就去调用controller中的方法执行我们的业务逻辑并返回一个ModelView对象;接着执行中置过滤器,以及处理全局异常捕获器捕获到异常;最后进行视图渲染返回并执行后置过滤器进行资源释放等工作。
    以上就是MVC的整体执行流程,下面就逐个来分析,首先进入getHandler方法:

    	protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    		//handlerMappering实例
    		if (this.handlerMappings != null) {
    			for (HandlerMapping mapping : this.handlerMappings) {
    				//获取HandlerMethod和过滤器链的包装类
    				HandlerExecutionChain handler = mapping.getHandler(request);
    				if (handler != null) {
    					return handler;
    				}
    			}
    		}
    		return null;
    	}
    

    是委托给HandlerMapping对象的,这是一个接口,主要的实现类是RequestMappingHandlerMapping,同样先来看看其继承体系:
    在这里插入图片描述
    这个类是管理请求和处理类之间的映射关系的,你是否疑惑它是在哪里实例化的呢?下面先来看看MVC组件的初始化。

    组件初始化

    这里我以自动化配置的注解方式说明,Spring提供了一个@EnableWebMvc,通过前面的学习我们知道在这个注解中必定导入了一个配置类,点进去可以看到是DelegatingWebMvcConfiguration,这个类就是负责MVC的组件和扩展实现的初始化,其本身我们先不看,先看其父类WebMvcConfigurationSupport,这个类我们应该不陌生,要做一些自定义扩展时就需要继承该类(如拦截器Interceptor),同样作用的类还有WebMvcConfigurerAdapter,这个类是对前者相对安全的扩展,为什么是相对安全呢?因为继承前者会导致自动配置失效,而使用后者则不必担心此问题,只需要在类上加上@EnableWebMvc注解。
    WebMvcConfigurationSupport中我们可以看到很多@Bean标注的方法,也就是mvc组件的实例化,这里主要看看requestMappingHandlerMapping,其余的可自行阅读理解,也就是一些Bean的注册:

    	public RequestMappingHandlerMapping requestMappingHandlerMapping() {
    		RequestMappingHandlerMapping mapping = createRequestMappingHandlerMapping();
    		mapping.setOrder(0);
    		mapping.setInterceptors(getInterceptors());
    		mapping.setContentNegotiationManager(mvcContentNegotiationManager());
    		mapping.setCorsConfigurations(getCorsConfigurations());
    
    		......省略
    
    		return mapping;
    	}
    

    这里主要看getInterceptors方法如何获取拦截器的:

    	protected final Object[] getInterceptors() {
    		if (this.interceptors == null) {
    			InterceptorRegistry registry = new InterceptorRegistry();
    			//钩子方法,需要自己定义
    			addInterceptors(registry);
    			registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService()));
    			registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider()));
    			this.interceptors = registry.getInterceptors();
    		}
    		return this.interceptors.toArray();
    	}
    

    第一次进来会调用addInterceptors添加拦截器,这是一个模板方法,在子类DelegatingWebMvcConfiguration中实现:

    	private final WebMvcConfigurerComposite configurers = new WebMvcConfigurerComposite();
    	
    	protected void addInterceptors(InterceptorRegistry registry) {
    		this.configurers.addInterceptors(registry);
    	}
    
    	public void addInterceptors(InterceptorRegistry registry) {
    		for (WebMvcConfigurer delegate : this.delegates) {
    			delegate.addInterceptors(registry);
    		}
    	}
    

    可以看到最终是调用WebMvcConfigureraddInterceptors方法,也就是我们对WebMvcConfigurerAdapter的自定义扩展。看到这里我们应该明白了MVC的组件是如何添加到IOC容器中的,但是DispatcherServlet又是怎么获取到它们的呢?回到之前的代码中,在DispatcherServlet这个类中有一个onRefresh方法,这个方法又调用了initStrategies方法完成了MVC九大组件的注册:

    	protected void onRefresh(ApplicationContext context) {
    		initStrategies(context);
    	}
    
    	protected void initStrategies(ApplicationContext context) {
    		initMultipartResolver(context);
    		initLocaleResolver(context);
    		initThemeResolver(context);
    		initHandlerMappings(context);
    		initHandlerAdapters(context);
    		initHandlerExceptionResolvers(context);
    		initRequestToViewNameTranslator(context);
    		initViewResolvers(context);
    		initFlashMapManager(context);
    	}
    
    	private void initHandlerMappings(ApplicationContext context) {
    		this.handlerMappings = null;
    
    		if (this.detectAllHandlerMappings) {
    			// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
    			Map<String, HandlerMapping> matchingBeans =
    					BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
    			if (!matchingBeans.isEmpty()) {
    				this.handlerMappings = new ArrayList<>(matchingBeans.values());
    				// We keep HandlerMappings in sorted order.
    				AnnotationAwareOrderComparator.sort(this.handlerMappings);
    			}
    		}
    		else {
    			try {
    				HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
    				this.handlerMappings = Collections.singletonList(hm);
    			}
    			catch (NoSuchBeanDefinitionException ex) {
    				// Ignore, we'll add a default HandlerMapping later.
    			}
    		}
    		
    		if (this.handlerMappings == null) {
    			this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
    		}
    	}
    

    initHandlerMappings为例,其它组件实现逻辑基本一样。首先从IOC容器中拿到handlerMappings的所有实现类(WebMvcConfigurationSupport中注入的对象就在这里被获取到),若没有,则从DispatcherServlet.properties配置文件中(这个配置在spring-webmvc工程下org/springframework/web/servlet/DispatcherServlet.properties)获取默认的配置:

    org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
    
    org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver
    
    org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,
    	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping
    
    org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,
    	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,
    	org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter
    
    org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,
    	org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,
    	org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver
    
    org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator
    
    org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
    
    org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
    

    但是onRefresh又是在什么时候调用的呢?有两个地方,一个是Servlet初始化时会调用到initWebApplicationContext进行容器的初始化,这个方法中就会触发onRefresh;另外还有一个,在FrameworkServlet中有一个onApplicationEvent方法,而这个方法又会被内部类ContextRefreshListener调用,这个类实现了ApplicationListener接口,表示会接收容器刷新事件。
    以上就就是MVC HandlerMapping组件的初始化逻辑,其它组件实现逻辑相同,下面不再分析。

    调用Controller

    回到getHandler方法,其调用的是AbstractHandlerMapping类的方法:

    	public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    		//根据请求的uri拿到对应的HandlerMethod对象
    		Object handler = getHandlerInternal(request);
    		if (handler == null) {
    			handler = getDefaultHandler();
    		}
    		if (handler == null) {
    			return null;
    		}
    		// Bean name or resolved handler?
    		if (handler instanceof String) {
    			String handlerName = (String) handler;
    			handler = obtainApplicationContext().getBean(handlerName);
    		}
    
    		//获取HandlerMethod和过滤器链的包装类
    		HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
    
    		if (logger.isTraceEnabled()) {
    			logger.trace("Mapped to " + handler);
    		}
    		else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
    			logger.debug("Mapped to " + executionChain.getHandler());
    		}
    
    		//是否是跨域请求,就是查看request请求头中是否有Origin属性
    		if (CorsUtils.isCorsRequest(request)) {
    			//自定义的钩子方法获取跨域配置
    			CorsConfiguration globalConfig = this.corsConfigurationSource.getCorsConfiguration(request);
    			//注解获取跨域配置
    			CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
    			CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
    			//这里设置了跨域的过滤器CorsInterceptor
    			executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
    		}
    
    		return executionChain;
    	}
    

    先看AbstractHandlerMethodMapping.getHandlerInternal

    	protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
    		//从request对象中获取uri,/common/query2
    		String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
    		this.mappingRegistry.acquireReadLock();
    		try {
    			//根据uri从映射关系中找到对应的HandlerMethod对象
    			HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
    			//把Controller类实例化
    			return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
    		}
    		finally {
    			this.mappingRegistry.releaseReadLock();
    		}
    	}
    
    	protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
    		List<Match> matches = new ArrayList<>();
    		// 根据url拿到对应的RequestMappingInfo
    		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);
    				//如果两个RequestMappinginfo什么都相同,报错
    				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);
    		}
    	}
    
    	private void addMatchingMappings(Collection<T> mappings, List<Match> matches, HttpServletRequest request) {
    		for (T mapping : mappings) {
    			// 拿到匹配的RequestMappingInfo对象,有可能url相同,@RequestMapping的属性(请求方式、参数等)匹配不上
    			T match = getMatchingMapping(mapping, request);
    			if (match != null) {
    				//RequestMappingInfo对象和HandlerMethod对象封装到Match对象中,其实就是注解属性和Method对象的映射
    				matches.add(new Match(match, this.mappingRegistry.getMappings().get(mapping)));
    			}
    		}
    	}
    
    

    这里逻辑很简单,就是通过请求url从urlLookup中拿到对应的RequestMappingInfo(每一个 @RequestMapping对应一个RequestMappingInfo对象)对象,再根据RequestMappingInfo对象从mappingLookup拿到对应的HandlerMethod并返回。
    但这里你可能会比较好奇urlLookupmappingLookup从哪里来的,仔细观察你会发现当前这个类实现了一个接口InitializingBean,实现了这个接口的类会在该类的Bean实例化完成后调用afterPropertiesSet方法,上面的映射关系就是在这个方法中做的。实际上这个方法不止完成了上面两个映射关系,还有下面两个:

    • corsLookup:handlerMethod -> corsConfig
    • registry:RequestMappingInfo -> MappingRegistration(包含url、handlerMethod、RequestMappingInfo、name等信息)

    这里就不展开分析了,奉上一张时序图,读者可根据下面的时序图自行分析:
    在这里插入图片描述
    拿到HandlerMethod对象后,又会通过getHandlerExecutionChain方法去获取到所有的HandlerInterceptor拦截器对象,并连同HandlerMethod对象一起封装为HandlerExecutionChain。之后是获取跨域配置,这里不详细分析。
    拿到HandlerExecutionChain对象后返回到doDispatch方法,又调用了getHandlerAdapter
    方法拿到HandlerAdapter

    	protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
    		//根据handlerMethod对象,找到合适的HandlerAdapter对象,这里用到了策略模式
    		if (this.handlerAdapters != null) {
    			for (HandlerAdapter adapter : this.handlerAdapters) {
    				if (adapter.supports(handler)) {
    					return adapter;
    				}
    			}
    		}
    	}
    

    这里的handlerAdapters变量值从哪里来?相信不用我再分析,主要看这里的设计思想,典型的策略模式
    之后调用完前置过滤器后,才是真正调用我们controller方法的逻辑,通过HandlerAdapter.handle去调用,最终会调用到ServletInvocableHandlerMethod.invokeAndHandle

    	public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
    
    		//具体调用逻辑,重点看
    		Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
    		setResponseStatus(webRequest);
    
    		if (returnValue == null) {
    			if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
    				mavContainer.setRequestHandled(true);
    				return;
    			}
    		}
    		else if (StringUtils.hasText(getResponseStatusReason())) {
    			mavContainer.setRequestHandled(true);
    			return;
    		}
    
    		mavContainer.setRequestHandled(false);
    		Assert.state(this.returnValueHandlers != null, "No return value handlers");
    		try {
    			//返回值处理
    			this.returnValueHandlers.handleReturnValue(
    					returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
    		}
    		catch (Exception ex) {
    			if (logger.isTraceEnabled()) {
    				logger.trace(formatErrorForReturnValue(returnValue), ex);
    			}
    			throw ex;
    		}
    	}
    

    这个方法里面主要看invokeForRequesthandleReturnValue的调用,前者是完成参数绑定并调用controller,后者则是对返回值进行处理并封装到ModelAndViewContainer中。先来看invokeForRequest

    	public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
    
    		//获取参数数组
    		Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
    		if (logger.isTraceEnabled()) {
    			logger.trace("Arguments: " + Arrays.toString(args));
    		}
    		return doInvoke(args);
    	}
    

    doInvoke就是完成反射调用,主要还是看参数绑定的实现逻辑,在getMethodArgumentValues方法中:

    	protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
    			Object... providedArgs) throws Exception {
    
    		if (ObjectUtils.isEmpty(getMethodParameters())) {
    			return EMPTY_ARGS;
    		}
    		//入参的包装类,里面包装了参数类型,参数名称,参数注解等等信息
    		MethodParameter[] parameters = getMethodParameters();
    		Object[] args = new Object[parameters.length];
    		for (int i = 0; i < parameters.length; i++) {
    			MethodParameter parameter = parameters[i];
    			//设置参数名称解析器
    			parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
    			args[i] = findProvidedArgument(parameter, providedArgs);
    			if (args[i] != null) {
    				continue;
    			}
    			//典型的策略模式,根据parameter能否找到对应参数的处理类,能找到就返回true
    			if (!this.resolvers.supportsParameter(parameter)) {
    				throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
    			}
    			try {
    				//具体参数值解析过程,重点看看
    				args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
    			}
    			catch (Exception ex) {
    				// Leave stack trace for later, exception may actually be resolved and handled..
    				if (logger.isDebugEnabled()) {
    					String error = ex.getMessage();
    					if (error != null && !error.contains(parameter.getExecutable().toGenericString())) {
    						logger.debug(formatArgumentError(parameter, error));
    					}
    				}
    				throw ex;
    			}
    		}
    		return args;
    	}
    

    参数、返回值解析

    因为参数类型非常多,同时还会伴随各种注解,如:@RequestBody、@RequestParam、@PathVariable等,所以参数解析的工作是非常繁杂的,同时还要考虑到扩展性,所以SpringMVC依然采用了策略模式来完成对各种参数类型的解析绑定,其顶层接口就是HandlerMethodArgumentResolver,而默认SpringMVC提供的解析方式就高达20多种:
    在这里插入图片描述
    上面是类图,读者可根据自己熟悉的参数类型找到对应的类进行分析,最核心的还是要掌握这里的设计思想。
    接着方法调用完成后就是对返回值的处理,同样的,返回值类型也是非常多,也可以使用各种注解标注,所以也是使用策略模式实现,其顶层接口是HandlerMethodReturnValueHandler,实现类如下:
    在这里插入图片描述
    调用完成之后就是执行后续操作了:执行中置过滤器、处理全局异常、视图渲染以及执行后置过滤器,这些与主流程没有太大关系,本篇不展开分析了,最后是MVC的执行时序图:
    在这里插入图片描述

    总结

    本篇是Spring核心原理系列的最后一篇,前前后后花了一个月时间,终于从宏观上大致上理解了Spring的实现原理和运行机制,明白了之前项目中一些坑是如何产生的,最主要的是学到设计模式的运用以及如何利用Spring的一些常用的扩展点进行自定义扩展。但对于Spring这个庞大的体系来说,还有很多是要去理解学习的,尤其是设计思想,只有长期琢磨才能深刻的理解掌握。在我之前的文章中包括本篇还有很多没分析到的细节,在后面我会不定期分享出来。

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  • 原文地址:https://www.cnblogs.com/yewy/p/13149882.html
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