DispatcherServlet執行流程及相關原始碼分析
DispatcherServlet執行流程及相關原始碼分析
在前一篇文章SpringMVC 啟動流程及相關原始碼分析中，詳細探討了Spring MVC在Web容器中部署後的啟動過程，以及相關原始碼分析，同時也討論了DispatcherServlet類的初始化建立過程，相關內容在此不再贅述，如有需求可查閱。
本文主要講解DispatcherServlet類獲取使用者請求到響應的全過程，並針對相關原始碼進行分析。對於基本的MVC架構本文不再進行講解，有需要的讀者可自行查閱。
首先，讓我們站在Spring MVC的四大元件:DispatcherServlet、HandlerMapping
HandlerAdapter以及ViewResolver的角度來看一下Spring MVC對使用者請求的處理過程，有如下時序圖:
DispatcherServlet執行序列圖
具體處理過程如下:
- 1、使用者請求傳送至
DispatcherServlet類進行處理。 - 2、
DispatcherServlet類遍歷所有配置的HandlerMapping類請求查詢Handler。 - 3、
HandlerMapping類根據request請求的URL等資訊查詢能夠進行處理的Handler，以及相關攔截器interceptor並構造HandlerExecutionChain。 - 4、
HandlerMapping類將構造的HandlerExecutionChain類的物件返回給前端控制器DispatcherServlet類。 - 5、前端控制器拿著上一步的
Handler遍歷所有配置的HandlerAdapter類請求執行Handler。 - 6、
HandlerAdapter類執行相關Handler並獲取ModelAndView類的物件。 - 7、
HandlerAdapter類將上一步Handler執行結果的ModelAndView 類的物件返回給前端控制器。 - 8、
DispatcherServlet類遍歷所有配置的ViewResolver類請求進行檢視解析。 - 9、
ViewResolver類進行檢視解析並獲取View物件。 - 10、
ViewResolver類向前端控制器返回上一步驟的View物件。 - 11、
DispatcherServlet類進行檢視View的渲染，填充Model。 - 12、
DispatcherServlet類向用戶返回響應。
通過時序圖和上面的講解不難發現，整個Spring MVC對於使用者請求的響應和處理都是以DispatcherServlet類為核心，其他三大元件均與前端控制器進行互動，三大元件之間沒有互動並且互相解耦，因此，三大元件可以替換不同的實現而互相沒有任何影響，提高了整個架構的穩定性並且降低了耦合度。接下來會按照上述的響應過程逐一進行講解。
DispatcherServlet類本質上依舊是一個Servlet並且其父類實現了Servlet介面，我們知道，Servlet執行Service()方法對使用者請求進行響應，根據前一篇文章的分析方法可以得到人如下的呼叫邏輯圖:
service方法呼叫邏輯
從上圖的原始碼呼叫邏輯可以看出，HttpServlet抽象類實現了Servlet介面的service(ServletRequest, ServletResponse)的方法，因此，使用者請求的第一執行方法為該方法，該方法緊接著直接呼叫了service(HttpServletRequest, HttpServletResponse)方法，其子類FrameworkServlet抽象類重寫了該方法，因為多型的特性最終是呼叫了FrameworkServlet抽象類的service(HttpServletRequest, HttpServletResponse)方法，FrameworkServlet抽象類同樣也重寫了doHead()、doPost()、doPut()、doDelete()、doOptions()、doTrace()方法，service(ServletRequest, ServletResponse)方法根據請求型別的不同分別呼叫上述方法，上述六個方法都呼叫了processRequest()方法，而該方法最終呼叫了DispatcherServlet類的doService()方法。通過層層分析，我們找到了最終要呼叫的處理使用者請求的方法，doService()之前的方法呼叫都比較簡單，這裡不再逐一來檢視原始碼，有興趣的讀者可以自行查閱。
檢視doService()的原始碼如下:
/**
* 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<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<String, Object>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}
// Make framework objects available to handlers and view objects.
/*
將當前Servlet的子IoC容器放入request請求中
由此，我們可以訪問到當前IoC子容器以及根IoC容器中的Bean
*/
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 {
//真正進行使用者請求的處理
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);
}
}
}
}
doService()方法主要進行一些引數的設定，並將部分引數放入request請求中，真正執行使用者請求並作出響應的方法則為doDispatch()方法，檢視doDispatch()方法的原始碼如下:
/**
* Process the actual dispatching to the handler.
* <p>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.
* <p>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 {
//使用者的request請求
HttpServletRequest processedRequest = request;
//HandlerExecutionChain區域性變數
HandlerExecutionChain mappedHandler = null;
//判斷是否解析了檔案型別的資料，如果有最終需要清理
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
//ModelAndView區域性變數
ModelAndView mv = null;
//處理異常區域性變數
Exception dispatchException = null;
try {
//檢查是否包含檔案等型別的資料
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
//向HandlerMapping請求查詢HandlerExecutionChain
mappedHandler = getHandler(processedRequest);
//如果HandlerExecutionChain為null，則沒有能夠進行處理的Handler，丟擲異常
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
//根據查詢到的Handler請求查詢能夠進行處理的HandlerAdapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
//判斷自上次請求後是否有修改，沒有修改直接返回響應
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;
}
}
/*
按順序依次執行HandlerInterceptor的preHandle方法
如果任一HandlerInterceptor的preHandle方法沒有通過則不繼續進行處理
*/
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
//通過HandlerAdapter執行查詢到的handler
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
//逆序執行HandlerInterceptor的postHandle方法
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);
}
//渲染檢視填充Model，如果有異常渲染異常頁面
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
//如果有異常按倒序執行所有HandlerInterceptor的afterCompletion方法
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
//如果有異常按倒序執行所有HandlerInterceptor的afterCompletion方法
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
if (mappedHandler != null) {
//倒序執行所有HandlerInterceptor的afterCompletion方法
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
//如果請求包含檔案型別的資料則進行相關清理工作
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
根據上述原始碼並結合文章開始講解的DispatcherServlet類結合三大元件對使用者請求的處理過程不難理解相關處理流程。
doDispatch()方法通過呼叫getHandler()方法並傳入reuqest通過HandlerMapping查詢HandlerExecutionChain，檢視其原始碼如下:
/**
* Return the HandlerExecutionChain for this request.
* <p>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;
}
getHandler()方法遍歷了開發者配置的所有HandlerMapping類根據request請求來查詢HandlerExecutionChain，從這裡可以看出，Spring MVC是支援使用者配置多個HandlerMapping類的，在處理使用者請求時會逐一查詢，找到後立即返回，因此，如果多個HandlerMapping類都能夠處理同一request請求，只會返回第一個能夠處理的HandlerMapping類構造的HandlerExecutionChain，所以在配置HandlerMapping類時需要注意不要對同一請求多次進行處理，由於篇幅問題HandlerMapping類如何具體查詢Handler並構造HandlerExecutionChain的細節不在此進行講解，如有興趣可以查閱本系列文章的第三篇SpringMVC HandlerMapping原始碼分析。
如果沒有找到對應的HandlerExecutionChain物件，則會執行noHandlerFound()方法，繼續檢視其原始碼如下:
/**
* No handler found -> set appropriate HTTP response status.
* @param request current HTTP request
* @param response current HTTP response
* @throws Exception if preparing the response failed
*/
protected void noHandlerFound(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (pageNotFoundLogger.isWarnEnabled()) {
pageNotFoundLogger.warn("No mapping found for HTTP request with URI [" + getRequestUri(request) +
"] in DispatcherServlet with name '" + getServletName() + "'");
}
if (this.throwExceptionIfNoHandlerFound) {
throw new NoHandlerFoundException(request.getMethod(), getRequestUri(request),
new ServletServerHttpRequest(request).getHeaders());
}
else {
response.sendError(HttpServletResponse.SC_NOT_FOUND);
}
}
如果沒有找到對應的HandlerExecutionChain則會丟擲異常NoHandlerFoundException，在開發的過程中，如果我們將具體的URL寫錯了則會遇到這個404錯誤。
繼續檢視doDispatch()方法的原始碼，如果找到了HandlerExecutionChain接下來會呼叫getHandlerAdapter()方法來查詢能夠對Handler進行處理的HandlerAdapter，檢視其原始碼如下:
/**
* Return the HandlerAdapter for this handler object.
* @param handler the handler object to find an adapter for
* @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
*/
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
for (HandlerAdapter ha : this.handlerAdapters) {
if (logger.isTraceEnabled()) {
logger.trace("Testing handler adapter [" + ha + "]");
}
if (ha.supports(handler)) {
return ha;
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
與HandlerMapping類似，查詢能夠處理具體Handler的HandlerAdapter時同樣會遍歷所有配置了的HandlerAdapter，HandlerAdapter是一個介面包含一個support()方法，該方法根據Handler是否實現某個特定的介面來判斷該HandlerAdapter是否能夠處理這個具體的Handler，這裡使用介面卡模式，通過這樣的方式就可以支援不同型別的HandlerAdapter。如果沒有查詢到能夠處理Handler的HandlerAdapter則會丟擲異常，如果在開發的過程中Handler在實現介面時出現了問題就可能會遇到上述異常。
查詢到了對應的HandlerAdapter後就會呼叫HandlerExecutionChain的applyPreHandle()方法來執行配置的所有HandlerInteceptor的preHandle()方法，檢視其原始碼如下:
/**
* Apply preHandle methods of registered interceptors.
* @return {@code true} if the execution chain should proceed with the
* next interceptor or the handler itself. Else, DispatcherServlet assumes
* that this interceptor has already dealt with the response itself.
*/
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = 0; i < interceptors.length; i++) {
HandlerInterceptor interceptor = interceptors[i];
if (!interceptor.preHandle(request, response, this.handler)) {
triggerAfterCompletion(request, response, null);
return false;
}
this.interceptorIndex = i;
}
}
return true;
}
HandlerExecutionChain的applyPreHandle()方法會按照順序依次呼叫HandlerInterceptor的preHandle()方法，但當任一HandlerInterceptor的preHandle()方法返回了false就不再繼續執行其他HandlerInterceptor的preHandle()方法，而是直接跳轉執行triggerAfterCompletion()方法，檢視該方法原始碼如下:
/**
* Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
* Will just invoke afterCompletion for all interceptors whose preHandle invocation
* has successfully completed and returned true.
*/
void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = this.interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
try {
interceptor.afterCompletion(request, response, this.handler, ex);
}
catch (Throwable ex2) {
logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
}
}
}
}
這裡遍歷的下標為interceptorIndex，該變數在前一個方法applyPreHandle()方法中賦值，如果preHandle()方法返回true該變數加一，因此該方法會逆序執行所有preHandle()方法返回了true的HandlerInterceptor的afterCompletion()方法。到這裡讀者已經掌握了HandlerInterceptor的preHandle()方法以及afterCompletion()方法的執行順序，這些內容並不需要我們死記，需要知道其執行順序檢視原始碼是最好的方法。
繼續閱讀doDispatch()方法的原始碼，如果所有攔截器的preHandle()方法都返回了true沒有進行攔截，接下來前端控制器會請求執行上文獲取的Handler，這個Handler就是開發的時候編寫的Controller，根據實現介面的不同執行相關方法，並獲取到ModelAndView類的物件。
接下來會執行HandlerInterceptor的postHandle()方法，具體原始碼如下:
/**
* Apply postHandle methods of registered interceptors.
*/
void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = interceptors.length - 1; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
interceptor.postHandle(request, response, this.handler, mv);
}
}
}
可以發現，postHandle()方法是按照逆序執行。
執行完postHandle()方法後，doDispatch()方法呼叫了processDispatchResult()方法，其原始碼如下:
/**
* 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;
//判斷HandlerMapping、HandlerAdapter處理時的異常是否為空
if (exception != null) {
//上述兩個元件處理時的異常不為空
//如果為ModelAndViewDefiningException異常，則獲取一個異常檢視
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
//如果不為ModelAndViewDefiningException異常，進行異常檢視的獲取
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?
//判斷mv是否為空，不管是正常的ModelAndView還是異常的ModelAndView，只要存在mv就進行檢視渲染
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
//否則記錄無檢視
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;
}
//執行相關HandlerInterceptor的afterCompletion()方法
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}
該方法傳入了一個異常類的物件dispatchException，閱讀doDispatch()方法的原始碼可以看出，Spring MVC對整個doDispatch()方法用了巢狀的try-catch語句，內層的try-catch用於捕獲HandlerMapping進行對映查詢HandlerExecutionChain以及HandlerAdapter執行具體Handler時的處理異常，並將異常傳入到上述processDispatchResult()方法中。
processDispatchResult()方法主要用於針對產生的異常來構造異常檢視，接著不管檢視是正常檢視還是異常檢視均呼叫render()方法來渲染，檢視render()方法的具體原始碼如下:
/**
* Render the given ModelAndView.
* <p>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.
// 解析檢視名稱獲取對應檢視View
view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
//如果檢視View為空丟擲異常
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 {
//設定Http響應狀態字
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
//呼叫檢視View的render方法通過Model來渲染檢視
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;
}
}
render()方法通過呼叫resolveViewName()方法根據檢視名稱解析對應的檢視View，該方法原始碼如下:
/**
* Resolve the given view name into a View object (to be rendered).
* <p>The default implementations asks all ViewResolvers of this dispatcher.
* Can be overridden for custom resolution strategies, potentially based on
* specific model attributes or request parameters.
* @param viewName the name of the view to resolve
* @param model the model to be passed to the view
* @param locale the current locale
* @param request current HTTP servlet request
* @return the View object, or {@code null} if none found
* @throws Exception if the view cannot be resolved
* (typically in case of problems creating an actual View object)
* @see ViewResolver#resolveViewName
*/
protected View resolveViewName(String viewName, Map<String, Object> 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;
}
resolveViewName()方法通過遍歷配置的所有ViewResolver類根據檢視名稱來解析對應的檢視View，如果找到則返回對應檢視View，沒有找到則返回null。
回到前一個render()方法，如果上述方法返回的檢視為null則丟擲異常，這個異常相信大多數人也見過，當開發時寫錯了返回的View檢視名稱時就會丟擲該異常。接下來呼叫具體檢視的render()方法來進行Model資料的渲染填充，最終構造成完整的檢視。
到這裡，doDispatch()的外層try-catch異常的作用我們就知道了，為了捕獲渲染檢視時的異常，通過兩層巢狀的try-catch，Spring MVC就能夠捕獲到三大元件在處理使用者請求時的異常，通過這樣的方法能夠很方便的實現統一的異常處理。
總結
通過前文的原始碼分析，我們能夠清楚的認識到Spring MVC對使用者請求的處理過程，進一步加深對Spring MVC的理解。