springMVC如何解析视图

SpringMVC如何从请求中分发解析视图

首先了解SpringMVC的9大组件

// 文件解析
/** MultipartResolver used by this servlet. */
@Nullable
private MultipartResolver multipartResolver;
// 国际化解析
/** LocaleResolver used by this servlet. */
@Nullable
private LocaleResolver localeResolver;
// 主题解析器
/** ThemeResolver used by this servlet. */
@Nullable
private ThemeResolver themeResolver;
// 映射表
/** List of HandlerMappings used by this servlet. */
@Nullable
private List<HandlerMapping> handlerMappings;
// 适配器表
/** List of HandlerAdapters used by this servlet. */
@Nullable
private List<HandlerAdapter> handlerAdapters;
// 异常处理
/** List of HandlerExceptionResolvers used by this servlet. */
@Nullable
private List<HandlerExceptionResolver> handlerExceptionResolvers;
//请求转视图名
/** RequestToViewNameTranslator used by this servlet. */
@Nullable
private RequestToViewNameTranslator viewNameTranslator;
// 重定向数据
/** FlashMapManager used by this servlet. */
@Nullable
private FlashMapManager flashMapManager;
// 视图解析器
/** List of ViewResolvers used by this servlet. */
@Nullable
private List<ViewResolver> viewResolvers;

1. 收到htttp请求

2. 过滤请求 包含一系列的过滤 一般都是dofilter方法

3. 通过DispatchSerlet.class来处理请求（核心类）

4. 根据请求利用handleMappings找到应该处理的Controller以及目标方法

// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);

如何找到的呢？

@Nullable
	protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		if (this.handlerMappings != null) {
			for (HandlerMapping mapping : this.handlerMappings) {
				HandlerExecutionChain handler = mapping.getHandler(request);
				if (handler != null) {
					return handler;
				}
			}
		}
		return null;
	}

很简单的遍历所有的HandlerMapping

在初始化HandlerMapping的过程中，至少有一个handlerMapping

// handlerMappings的初始化
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.
			}
		}

		// Ensure we have at least one HandlerMapping, by registering
		// a default HandlerMapping if no other mappings are found.
		if (this.handlerMappings == null) {
			this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
			if (logger.isTraceEnabled()) {
				logger.trace("No HandlerMappings declared for servlet '" + getServletName() +
						"': using default strategies from DispatcherServlet.properties");
			}
		}
	}

5. 根据handler选择合适的handlerAdapter

				// Determine handler adapter for the current request.
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

6. 执行目标方法并生成ModelAndView

适配器

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());   

测试对于POJO，String 类型实际上只有返回值那么 此时的mav = null;

7.根据ModelAndView返回结果

有视图render视图，没有就跳过装载视图

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

流程图如下

DispatcherServlet核心方法dodispatch:

	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
                		// 是否为文件上传
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);
				// 找到处理这个请求的Controller
				// Determine handler for the current request.
				mappedHandler = getHandler(processedRequest);
                		// 没有找到处理的Controller准备返回
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

                  		// 处理适配器
				// 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;
					}
				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}
				// 生成ModelAndView （最最最核心的部分）
				// Actually invoke the handler.
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}

				applyDefaultViewName(processedRequest, mv);
				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);
			}
            	// 渲染ModelAndView 到页面
			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);
				}
			}
		}
	}

结果

显然SpringMVC 使用了适配器模式

根据不同的方法执行选择不同的类