SpringMVC執行流程在面試中經常會被問到，本篇文章通過源碼的方式簡單的了解一下SpringMVC執行流程。

先看流程

先看一下SpringMVC執行流程再看源碼，有助了解：

⽤戶發送請求⾄前端控制器DispatcherServlet。
DispatcherServlet 收到請求調⽤ HandlerMapping 處理器映射器。
處理器映射器找到具體的處理器(可以根據xml配置、注解進⾏查找)，⽣成處理器及處理器攔截器(如果有則⽣成)⼀并傳回給DispatcherServlet。
DispatcherServlet調⽤HandlerAdapter處理器擴充卡。
HandlerAdapter經過适配調⽤具體的處理器(Controller，也叫後端控制器)
Controller執⾏完成傳回ModelAndView。
HandlerAdapter 将 Controller 執⾏結果 ModelAndView 傳回給DispatcherServlet。
DispatcherServlet将ModelAndView傳給ViewReslover視圖解析器。
ViewReslover解析後傳回具體View。
DispatcherServlet根據View進⾏渲染視圖（即将模型資料填充⾄視圖中）。
DispatcherServlet 響應⽤戶。

SpringMVC執行流程

再看源碼

我們都知道當從使用者發起請求到後端是，首先走的就是DispatcherServlet，接着就會調用doService()方法執行業務邏輯，doService()方法也隻是一個中轉站，實際執行邏輯的是doDispatch()方法，且看源碼：

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    logRequest(request);
    // 省略部分源碼
    try {
        // 執行實際邏輯
        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);
            }
        }
        ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
    }
}

doDispatch 方法：

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

            // 為目前請求擷取映射處理器
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // 擷取映射處理器擴充卡
            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;
            }

            // 實際調用的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);
        }
        //處理轉發結果
        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);
            }
        }
    }
}

下面來看一下其中幾個重要的方法：

getHandler(HttpServletRequest request)方法：該方法是處理目前請求找到合适的HandlerMapping，并傳回一個HandlerExecutionChain，它包含了具體的處理器(handler)和攔截器清單。HandlerMapping 預設的實作有org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping 和 org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping。

@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;
}

getHandlerAdapter(Object handler) 根據HandlerExecutionChain中的handler來擷取處理器擴充卡(HandlerAdapter)，

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
    if (this.handlerAdapters != null) {
        for (HandlerAdapter adapter : this.handlerAdapters) {
            if (adapter.supports(handler)) {
                return adapter;
            }
        }
    }
    throw new ServletException("No adapter for handler [" + handler +
            "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}

HandlerAdapter有兩個預設實作類，分别是 org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter和 org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter，前者用于沒有使用模闆引擎的請求，後者用于使用了模闆引擎的接口。

實際處理請求的是HandlerAdapter的handle方法，如果是沒有使用例如JSP等的模闆引擎，handle方法就會傳回null，如果使用了模闆引擎就會傳回一個ModelAndView對象。

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

handle方法最終調用的是Controller接口的 handleRequest(HttpServletRequest request, HttpServletResponse response) 方法來處理請求。

以SimpleControllerHandlerAdapter#handle方法源碼為例：

@Override
@Nullable
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
        throws Exception {
    return ((Controller) handler).handleRequest(request, response);
}

processDispatchResult方法用于處理轉發結果，該結果要麼是一個ModelAndView對象，要麼抛異常。

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
   @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
   @Nullable 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);
            //處理異常
            mv = processHandlerException(request, response, handler, exception);
            errorView = (mv != null);
        }
    }

    // Did the handler return a view to render?
    if (mv != null && !mv.wasCleared()) {
        //加載視圖
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    } else {
        if (logger.isTraceEnabled()) {
            logger.trace("No view rendering, null ModelAndView returned.");
        }
    }

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

    if (mappedHandler != null) {
        // Exception (if any) is already handled..
        mappedHandler.triggerAfterCompletion(request, response, null);
    }
}

processDispatchResult方法中在正常情況下會調用render方法。

render方法用來通過名稱呈現視圖，它也是請求處理的最後一步。

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 != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
    response.setLocale(locale);

    View view;
    String viewName = mv.getViewName();
    if (viewName != null) {
        // 通過視圖名稱擷取視圖
        view = resolveViewName(viewName, 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.isTraceEnabled()) {
        logger.trace("Rendering view [" + view + "] ");
    }
    try {
        if (mv.getStatus() != null) {
            response.setStatus(mv.getStatus().value());
        }
        //渲染視圖
        view.render(mv.getModelInternal(), request, response);
    }
    catch (Exception ex) {
        if (logger.isDebugEnabled()) {
            logger.debug("Error rendering view [" + view + "]", ex);
        }
        throw ex;
    }
}

DispatcherServlet的render方法是對視圖View的render方法封裝。

resolveViewName方法用于解析視圖名稱，它會通過視圖解析器ViewResolver的resolveViewName方法解析視圖并傳回一個視圖View，然後再通過View的render方法渲染視圖，至于是怎麼渲染視圖的這裡就不介紹了，感興趣的可以自行檢視源碼。

protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
   Locale locale, HttpServletRequest request) throws Exception {
    if (this.viewResolvers != null) {
        for (ViewResolver viewResolver : this.viewResolvers) {
            View view = viewResolver.resolveViewName(viewName, locale);
            if (view != null) {
                return view;
            }
        }
    }
    return null;
}

總結

以上就是正常調用下DispatcherServlet主要流程的源碼，面試中問流程的比較多，了解源碼會加分，最好是知道一點，這裡也是簡單的介紹一下，想了解更多的源碼可以自行檢視。

從源碼角度解析SpringMVC執行流程

先看流程

再看源碼

總結

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