Spring系列-Spring MVC处理请求流程

上一篇文章学习了Spring MVC的加载原理，那接下来，还有其它的一些知识点需要学习。当Spring MVC初始化完之后，是如何进行工作的呢？当Http请求到了我们Web服务后，是怎么样找到对应的Controller进行处理呢，又是怎么样返回给用户页面的呢？今天这篇文章就来学习一下这个流程。

之前分析到，HandlerMapping会持有一系列URL到Controller的映射，Spring MVC提供了一系列HandlerMapping的实现，下面是继承关系：

下面就以SimpleUrlHandlerMapping为例，分析一下HandlerMapping的设计。这一条继承关系是这样的：
SimpleUrlHandlerMapping继承了AbstractUrlHandlerMapping，AbstractUrlHandlerMapping继承了AbstractHandlerMapping，AbstractHandlerMapping实现了HandlerMapping接口。

在HandlerMapping接口中，定义了一个getHandler方法，这个方法就是获取http请求对应的HandlerExecutionChain。下面看下HandlerExecutionChain的定义：

public class HandlerExecutionChain {

	private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);

	private final Object handler;

	private HandlerInterceptor[] interceptors;

	private List interceptorList;

	private int interceptorIndex = -1;
	
	......
}

HandlerExecutionChain中定义了一个Object类型的handler，这个handler就是对应的Controller，还维护了一个拦截器链。

在SimpleUrlHandlerMapping中，定义了一个urlMap，存放映射关系。而且在继承关系中可以看到，实现了ApplicationContextAware接口，所以会有容器的回调，最终会调用initApplicationContext方法：

/**
 * Calls the {@link #registerHandlers} method in addition to the
 * superclass's initialization.
 */
@Override
public void initApplicationContext() throws BeansException {
	super.initApplicationContext();
	registerHandlers(this.urlMap);
}
/**
 * Register all handlers specified in the URL map for the corresponding paths.
 * @param urlMap Map with URL paths as keys and handler beans or bean names as values
 * @throws BeansException if a handler couldn't be registered
 * @throws IllegalStateException if there is a conflicting handler registered
 */
protected void registerHandlers(Map urlMap) throws BeansException {
	if (urlMap.isEmpty()) {
		logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
	}
	else {
		for (Map.Entry entry : urlMap.entrySet()) {
			String url = entry.getKey();
			Object handler = entry.getValue();
			// Prepend with slash if not already present.
			if (!url.startsWith("/")) {
				url = "/" + url;
			}
			// Remove whitespace from handler bean name.
			if (handler instanceof String) {
				handler = ((String) handler).trim();
			}
			registerHandler(url, handler);
		}
	}
}

一直顺着代码跟下去，就可以看到这个流程了，SimpleUrlHandlerMapping的注册就是把url和handler作为键值对，放到handlerMap中去。

接下来看下DispatcherServlet对请求的处理，这个处理逻辑是在doService方法中：

/**
 * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
 * for the actual dispatching.
 */
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
	if (logger.isDebugEnabled()) {
		String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
		logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
				" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
	}

	// Keep a snapshot of the request attributes in case of an include,
	// to be able to restore the original attributes after the include.
	Map attributesSnapshot = null;
	if (WebUtils.isIncludeRequest(request)) {
		attributesSnapshot = new HashMap();
		Enumeration attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) {
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}

	// Make framework objects available to handlers and view objects.
	request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
	request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
	request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
	request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

	FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
	if (inputFlashMap != null) {
		request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
	}
	request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
	request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

	try {
		doDispatch(request, response);
	}
	finally {
		if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			// Restore the original attribute snapshot, in case of an include.
			if (attributesSnapshot != null) {
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}
}

方法会对request设置一些Attribute，然后关键是调用了doDispatch方法：

/**
 * 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;
	boolean multipartRequestParsed = false;

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

	try {
		// 先创建一个ModelAndView，用来持有handler处理请求的结果。
		ModelAndView mv = null;
		Exception dispatchException = null;

		try {
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			// Determine handler for the current request.
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null || mappedHandler.getHandler() == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			// 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 (logger.isDebugEnabled()) {
					logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
				}
				if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}

			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}

			// 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;
		}
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) {
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Throwable err) {
		triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
			}
		}
	}
}

这个doDispatch算是处理请求的核心方法了，方法一开始先创建一个ModelAndView，后面用来持有handler处理的结果。
getHandler方法，会根据当前的request请求，来决定使用哪个handler，可以看下getHandler详细的执行流程：

/**
 * 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
 */
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
	for (HandlerMapping hm : this.handlerMappings) {
		if (logger.isTraceEnabled()) {
			logger.trace(
					"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
		}
		HandlerExecutionChain handler = hm.getHandler(request);
		if (handler != null) {
			return handler;
		}
	}
	return null;
}

/**
 * Look up a handler for the given request, falling back to the default
 * handler if no specific one is found.
 * @param request current HTTP request
 * @return the corresponding handler instance, or the default handler
 * @see #getHandlerInternal
 */
@Override
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
	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 = getApplicationContext().getBean(handlerName);
	}

	HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
	if (CorsUtils.isCorsRequest(request)) {
		CorsConfiguration globalConfig = this.corsConfigSource.getCorsConfiguration(request);
		CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
		CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
		executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
	}
	return executionChain;
}


@Override
protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
	String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
	Object handler = lookupHandler(lookupPath, request);
	if (handler == null) {
		// We need to care for the default handler directly, since we need to
		// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
		Object rawHandler = null;
		if ("/".equals(lookupPath)) {
			rawHandler = getRootHandler();
		}
		if (rawHandler == null) {
			rawHandler = getDefaultHandler();
		}
		if (rawHandler != null) {
			// Bean name or resolved handler?
			if (rawHandler instanceof String) {
				String handlerName = (String) rawHandler;
				rawHandler = getApplicationContext().getBean(handlerName);
			}
			validateHandler(rawHandler, request);
			handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
		}
	}
	if (handler != null && logger.isDebugEnabled()) {
		logger.debug("Mapping [" + lookupPath + "] to " + handler);
	}
	else if (handler == null && logger.isTraceEnabled()) {
		logger.trace("No handler mapping found for [" + lookupPath + "]");
	}
	return handler;
}

getHandler方法中会遍历handlerMappings列表，然后调用每个HandlerMapping的getHandler方法，获取HandlerExecutionChain，如果当前的HandlerMapping获取成功，则直接返回。HandlerMapping的getHandler入参是个HttpServletRequest，会先调用getHandlerInternal方法获取handler，如果获取到的为空，则返回默认的handler。

还是回到doDispatch方法中继续看，通过getHandler获得handler之后，还会通过getHandlerAdapter方法，获得HandlerAdapter，后面会调用HandlerAdapter的handle方法，这个方法会真正触发对Controller中方法的调用，比如，可以看下SimpleControllerHandlerAdapter中这个方法的实现：

@Override
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
		throws Exception {

	return ((Controller) handler).handleRequest(request, response);
}

@Override
public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response)
		throws Exception {

	// Delegate to WebContentGenerator for checking and preparing.
	checkRequest(request);
	prepareResponse(response);

	// Execute handleRequestInternal in synchronized block if required.
	if (this.synchronizeOnSession) {
		HttpSession session = request.getSession(false);
		if (session != null) {
			Object mutex = WebUtils.getSessionMutex(session);
			synchronized (mutex) {
				return handleRequestInternal(request, response);
			}
		}
	}

	return handleRequestInternal(request, response);
}

还是回到doDispatch的逻辑中，仔细观察下，其实在调用HandlerAdapter的handle方法之前和之后，还有两个方法的调用，applyPreHandle和applyPostHandle，这两个方法就是执行拦截器中的方法。

之后，会调用processDispatchResult方法，进行结果的处理：

/**
 * Handle the result of handler selection and handler invocation, which is
 * either a ModelAndView or an Exception to be resolved to a ModelAndView.
 */
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
		HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {

	boolean errorView = false;

	if (exception != null) {
		if (exception instanceof ModelAndViewDefiningException) {
			logger.debug("ModelAndViewDefiningException encountered", exception);
			mv = ((ModelAndViewDefiningException) exception).getModelAndView();
		}
		else {
			Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
			mv = processHandlerException(request, response, handler, exception);
			errorView = (mv != null);
		}
	}

	// Did the handler return a view to render?
	if (mv != null && !mv.wasCleared()) {
		render(mv, request, response);
		if (errorView) {
			WebUtils.clearErrorRequestAttributes(request);
		}
	}
	else {
		if (logger.isDebugEnabled()) {
			logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
					"': assuming HandlerAdapter completed request handling");
		}
	}

	if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
		// Concurrent handling started during a forward
		return;
	}

	if (mappedHandler != null) {
		mappedHandler.triggerAfterCompletion(request, response, null);
	}
}

这个方法中，关键的是render方法，这个方法是对ModelAndView对象进行处理的。

/**
 * Render the given ModelAndView.
 * 
This is the last stage in handling a request. It may involve resolving the view by name.
 * @param mv the ModelAndView to render
 * @param request current HTTP servlet request
 * @param response current HTTP servlet response
 * @throws ServletException if view is missing or cannot be resolved
 * @throws Exception if there's a problem rendering the view
 */
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
	// Determine locale for request and apply it to the response.
	Locale locale = this.localeResolver.resolveLocale(request);
	response.setLocale(locale);

	View view;
	if (mv.isReference()) {
		// We need to resolve the view name.
		view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
		if (view == null) {
			throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
					"' in servlet with name '" + getServletName() + "'");
		}
	}
	else {
		// No need to lookup: the ModelAndView object contains the actual View object.
		view = mv.getView();
		if (view == null) {
			throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
					"View object in servlet with name '" + getServletName() + "'");
		}
	}

	// Delegate to the View object for rendering.
	if (logger.isDebugEnabled()) {
		logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
	}
	try {
		view.render(mv.getModelInternal(), request, response);
	}
	catch (Exception ex) {
		if (logger.isDebugEnabled()) {
			logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
					getServletName() + "'", ex);
		}
		throw ex;
	}
}

在这个方法中，会获取到View视图对象，调用视图对象的render方法，进行视图的呈现。在获取视图对象的时候，会判断如果当前的ModelAndView是个引用的话，就调用resolveViewName方法，对视图名进行解析。

protected View resolveViewName(String viewName, Map model, Locale locale,
		HttpServletRequest request) throws Exception {

	for (ViewResolver viewResolver : this.viewResolvers) {
		View view = viewResolver.resolveViewName(viewName, locale);
		if (view != null) {
			return view;
		}
	}
	return null;
}

接下来再看下视图对象的render方法：

@Override
public void render(Map model, HttpServletRequest request, HttpServletResponse response) throws Exception {
	if (logger.isTraceEnabled()) {
		logger.trace("Rendering view with name '" + this.beanName + "' with model " + model +
			" and static attributes " + this.staticAttributes);
	}

	Map mergedModel = createMergedOutputModel(model, request, response);
	prepareResponse(request, response);
	renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}

方法入参包含一个model参数，这个就是我们在业务逻辑中，设置到ModelAndView中去的，render方法会把所有的信息设置到一个Map中，然后通过response，把视图呈现给http客户端。

/**
 * Creates a combined output Map (never {@code null}) that includes dynamic values and static attributes.
 * Dynamic values take precedence over static attributes.
 */
protected Map createMergedOutputModel(Map model, HttpServletRequest request,
		HttpServletResponse response) {

	@SuppressWarnings("unchecked")
	Map pathVars = (this.exposePathVariables ?
			(Map) request.getAttribute(View.PATH_VARIABLES) : null);

	// Consolidate static and dynamic model attributes.
	int size = this.staticAttributes.size();
	size += (model != null ? model.size() : 0);
	size += (pathVars != null ? pathVars.size() : 0);

	Map mergedModel = new LinkedHashMap(size);
	mergedModel.putAll(this.staticAttributes);
	if (pathVars != null) {
		mergedModel.putAll(pathVars);
	}
	if (model != null) {
		mergedModel.putAll(model);
	}

	// Expose RequestContext?
	if (this.requestContextAttribute != null) {
		mergedModel.put(this.requestContextAttribute, createRequestContext(request, response, mergedModel));
	}

	return mergedModel;
}

到这里，流程就基本上完成了，其实renderMergedOutputModel还有一些内容，这里就不再分析了。
总结一下，http请求过来时，先到DispatcherServlet的doService方法，然后调用doDispatch方法，doDispatch里面会根据request的请求url，返回对应的handler，然后根据handler获取HandlerAdapter，调用HandlerAdapter的handle方法，处理请求。之后对返回的ModelAndView处理，获取view对象，调用view的render对视图处理，返回到页面。

参考资料：
1.《Spring技术内幕》 计文柯 著