springmvc请求处理流程

上次明白了springmvc的初始化流程，现在来看看springmvc如何具体处理一个请求。

当一个请求到达服务器后，会交给servlet，如果是交给了DispatchServlet，那么就会调到

##doService方法

这个方法主要是往request里设置一些属性。方便之后handler和view获取。

// 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 {
    //属性设置完后将request和response交给doDispatch
    doDispatch(request, response);
}
finally {

doDispatch方法

这里包含了完整的对这个请求的处理。

找到这个处理这个请求的handler

HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;

WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

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

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

        //通过getHandler决定用哪个handler，并返回一个HandlerExecutionChain对象。
        //这个对象不仅包含handler，还提供了prehandler,posthandler,completionHandler拦截器的方法。
        mappedHandler = getHandler(processedRequest);
        if (mappedHandler == null || mappedHandler.getHandler() == null) {
            noHandlerFound(processedRequest, response);
            return;
        }

///可以看到getHandler，实际上就是轮询哪个HandlerMapping能够处理这个request
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;
}

///那this.handlerMapping是怎么初始化的呢？就是在所有的ApplicationContext，中找到实现了HandlerMapping的beans.如果我们没特别设置，会有一些默认的handlerMapping。
private void initHandlerMappings(ApplicationContext context){
    this.handlerMappings = null;

    if (this.detectAllHandlerMappings) {
        // 在所有的ApplicationContext，中找到实现了HandlerMapping的beans.
        Map<String, HandlerMapping> matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerMappings = new ArrayList<HandlerMapping>(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.isDebugEnabled()) {
            logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
        }
    }
}

ok,通过上面的代码，我们知道了handlerMapping是如何产生的，以及如何根据request找到对应的HandlerExecutionChain。

该执行handler了

在执行前，spring将handler封装为一个HandlerAdapter。为什么要这么做呢？看看这个接口的定义

/**
Interface that must be implemented for each handler type to handle a request.
 * This interface is used to allow the {@link DispatcherServlet} to be indefinitely
 * extensible. The {@code DispatcherServlet} accesses all installed handlers through
 * this interface, meaning that it does not contain code specific to any handler type.
 */
大概是说为了扩展性，DispatchServlet不应该和任何具体的handler直接打交道，都通过HanlderAdapter来调用。

所以我们在doDispatch中看到了：

// Determine handler adapter for the current request.
//挑选一个HandlerAdapter，所有handler的调用都通过adapter。
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

不过还有拦截器，如果设置了拦截器，那么我们应该先执行preHandler，在执行handler，再postHandler，最后在view渲染完后执行completionhandler。

//如果prehandler返回false，直接结束掉
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    return;
}

// 通过adapter调用handler。
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

applyDefaultViewName(processedRequest, mv);
//执行posthandler
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
//处理渲染，之后执行completionhandler
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

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()) {
           //在render中会去根据viewname在viewresolver中找到对应的view进行render。
		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) {
           //render完成了，执行completionhander。
		mappedHandler.triggerAfterCompletion(request, response, null);
	}
}

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()) {
		// 根据viewname找到对应的view
		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 {
           //渲染view
		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;
	}
}

ok支持整个request的流程就处理完了。