JAVA面試題之-SpringMVC的執行流程
1. 前言:
在java的面試過程中，如果是讨論架構方面的話，這個問題被問到的幾率就很大；
身邊朋友在面試的時候也會時不時的碰到，固總結在此。
2.概念:
那什麼是springMVC呢？
它其實是一種我們做javaWeb開發的一種架構；包括MVC三個層次的架構；
M:modle：業務模型（也就是sevice+do/mapper層）
V:View,視圖層（如jsp等前端顯示層）
C：Controller，控制器（sevlet/javabean）；
3.SpringMVC的執行流程:
4. 源碼分析:
DispatcherServlet其實是一個Servlet,我們都知道在處理請求的時候會交給service方法進行處理；在DispatcherServlet類中繼承了FrameworkServlet，
在FrameworkServlet中的service方法中:
protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
if (httpMethod != HttpMethod.PATCH && httpMethod != null) {
//其他請求類型的方法，經由父類，也就是HttpServlet處理；
super.service(request, response);
} else {
//如果請求類型的方法位PATCH，那麼就用processRequest進行處理；
this.processRequest(request, response);
}
}
request.getMethod();是表示得到前端用戶端傳向伺服器傳送資料的方法；如GET,POST,HREADER,TRACE等；
而HttpMtheod中的resolve代碼如下:還是前端的那些傳送資料的方法，隻是進行了封裝；
public enum HttpMethod {
GET,
HEAD,
POST,
PUT,
PATCH,
DELETE,
OPTIONS,
TRACE;
private static final Map<String, HttpMethod> mappings = new HashMap(16);
private HttpMethod() {
}
@Nullable
public static HttpMethod resolve(@Nullable String method) {
return method != null ? (HttpMethod)mappings.get(method) : null;
}
如果為PATCH,就會調用processRequest方法；而此方法中父類的doservice，因為子類實作了此方法，根據多态實際上使用的是DIsptcherServet中的doService方法；
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = this.buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new FrameworkServlet.RequestBindingInterceptor(null));
this.initContextHolders(request, localeContext, requestAttributes);
try {
this.doService(request, response);
} catch (IOException | ServletException var16) {
failureCause = var16;
throw var16;
} catch (Throwable var17) {
failureCause = var17;
throw new NestedServletException("Request processing failed", var17);
} finally {
this.resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
this.logResult(request, response, (Throwable)failureCause, asyncManager);
this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause);
}
}
再看DispatcherServlet中的doService方法:可以看到，會執行本類中的doDispatch方法；
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
this.logRequest(request);
//給request中的屬性做一份快照；
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap();
//getAttribute ,setAttribute ,
Enumeration attrNames = request.getAttributeNames();
label95:
while(true) {
String attrName;
do {
if (!attrNames.hasMoreElements()) {
break label95;
}
attrName = (String)attrNames.nextElement();
} while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
// 如果我們沒有配置類似本地化或者主題的處理器之類的，springMVC 會使用預設的值；預設的配置檔案是，DispathcerServlet.properties;
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());
if (this.flashMapManager != null) {
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 {
//開始執行
this.doDispatch(request, response);
} finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
this.restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
DispatcherServlet中的doDispatch方法；
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
//springMVC中的異步請求的管理
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
try {
ModelAndView mv = null;
Object dispatchException = null;
try {
//先檢查是不是multipart類型的，比如上傳等，
//如果是multipart類型的，就轉化為MultipartHttpServletRequest類型；
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
//獲得目前請求的Handler
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null) {
this.noHandlerFound(processedRequest, response);
return;
}
// 獲得目前請求的HandlerAdapter
HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
//對于header中的last-modified的處理
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;
}
}
//攔截器的preHandle方法進行處理；
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
//真正調用handle的地方
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//處理預設視圖名，即添加字首字尾名
this.applyDefaultViewName(processedRequest, mv);
//攔截器postHandle進行處理
mappedHandler.applyPostHandle(processedRequest, response, mv);
} catch (Exception var20) {
dispatchException = var20;
} catch (Throwable var21) {
dispatchException = new NestedServletException("Handler dispatch failed", var21);
}
//處理最後的結果，渲染之類的都在這裡
this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
}
} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
// Clean up any resources used by a multipart request
} else if (multipartRequestParsed) {
this.cleanupMultipart(processedRequest);
}
}
}
//springmvc中關于dispatcherServlet中的步驟都在這；
查找請求對應的handle對象
對應的上面 mappedHandler = this.getHandler(processedRequest)代碼；
具體getHandler（）方法如下:
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
//周遊所有的handlerMapping
Iterator var2 = this.handlerMappings.iterator();
while(var2.hasNext()) {
HandlerMapping mapping = (HandlerMapping)var2.next();
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
這個方法又調用了getHandler（request）；在AbstractHandlerMapping類中，
@Nullable
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
// 根據request，擷取handler；
Object handler = this.getHandlerInternal(request);
//如果沒有找到就使用預設的handler
if (handler == null) {
handler = this.getDefaultHandler();
}
if (handler == null) {
return null;
} else {
//如果handler屬于String類，表示是一個bean的名稱
//需要對照對應的bean
if (handler instanceof String) {
String handlerName = (String)handler;
handler = this.obtainApplicationContext().getBean(handlerName);
}
//封裝handler執行鍊
HandlerExecutionChain executionChain = this.getHandlerExecutionChain(handler, request);
//列印相關log資訊
if (this.logger.isTraceEnabled()) {
this.logger.trace("Mapped to " + handler);
} else if (this.logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
this.logger.debug("Mapped to " + executionChain.getHandler());
}
//判斷是否是跨域問題；
if (this.hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) {
//與跨域配置資訊映射的容器
CorsConfiguration config = this.corsConfigurationSource != null ? this.corsConfigurationSource.getCorsConfiguration(request) : null;
CorsConfiguration handlerConfig = this.getCorsConfiguration(handler, request);
config = config != null ? config.combine(handlerConfig) : handlerConfig;
//得到跨域的handeler執行鍊
executionChain = this.getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
}
再看， 根據request，擷取handler，
Object handler = this.getHandlerInternal(request);
往下追,在AbstractHandlerMethodMapping類中，源碼為:
protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
//擷取request中的url，用來比對handler
String lookupPath = this.getUrlPathHelper().getLookupPathForRequest(request);
request.setAttribute(LOOKUP_PATH, lookupPath);
this.mappingRegistry.acquireReadLock();
HandlerMethod var4;
try {
//根據路徑尋找Handler；
HandlerMethod handlerMethod = this.lookupHandlerMethod(lookupPath, request);
// 根據HandlerMethod中的bean來執行個體化Handler并添加進HandlerMethod；
var4 = handlerMethod != null ? handlerMethod.createWithResolvedBean() : null;
} finally {
this.mappingRegistry.releaseReadLock();
}
return var4;
}
根據路徑來尋找HandlerMethod；
HandlerMethod handlerMethod = this.lookupHandlerMethod(lookupPath, request);
源碼:
@Nullable
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List<AbstractHandlerMethodMapping<T>.Match> matches = new ArrayList();//直接比對
List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
//如果有就直接添加到比對清單中
if (directPathMatches != null) {
this.addMatchingMappings(directPathMatches, matches, request);
}
// 還沒有比對的，就周遊所有的處理方法查找
if (matches.isEmpty()) {
// No choice but to go through all
this.addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
}
// 找到了1比對的
if (!matches.isEmpty()) {
//排序之後，擷取第一個
AbstractHandlerMethodMapping<T>.Match bestMatch = (AbstractHandlerMethodMapping.Match)matches.get(0);
//如果有多個比對的，會找到第二個最适合的進行比較一下
if (matches.size() > 1) {
Comparator<AbstractHandlerMethodMapping<T>.Match> comparator = new AbstractHandlerMethodMapping.MatchComparator(this.getMappingComparator(request));
matches.sort(comparator);
bestMatch = (AbstractHandlerMethodMapping.Match)matches.get(0);
if (this.logger.isTraceEnabled()) {
this.logger.trace(matches.size() + " matching mappings: " + matches);
}
if (CorsUtils.isPreFlightRequest(request)) {
return PREFLIGHT_AMBIGUOUS_MATCH;
}
AbstractHandlerMethodMapping<T>.Match secondBestMatch = (AbstractHandlerMethodMapping.Match)matches.get(1);
if (comparator.compare(bestMatch, secondBestMatch) == 0) {
Method m1 = bestMatch.handlerMethod.getMethod();
Method m2 = secondBestMatch.handlerMethod.getMethod();
String uri = request.getRequestURI();
throw new IllegalStateException("Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
}
}
request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.handlerMethod);
//設定request參數
this.handleMatch(bestMatch.mapping, lookupPath, request);
//傳回比對處理url的方法
return bestMatch.handlerMethod;
} else {// 最後還沒有找到，傳回null
return this.handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
}
}
在AbstractHandlerMapping類中的getHandler方法中，對handler進行根據request的handler直接擷取。
（具體在AbstracHandlerMethodMapping類中）
然候如果沒有就
擷取預設Handler
如果上面沒有擷取到Handler，就會擷取預設的Handler。
如果還擷取不到就傳回null。
處理String類型的Handler
如果上面處理完的Handler是String類型的，
就會根據這個handlerName擷取bean。
封裝Handler執行鍊
上面擷取完Handler，就開始封裝執行鍊了，
就是将我們配置的攔截器加入到執行鍊中去，getHandlerExecutionChain：
protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
//如果目前Handler不是執行鍊類型，就使用一個新的執行鍊執行個體封裝起來
HandlerExecutionChain chain =
(handler instanceof HandlerExecutionChain) ?
(HandlerExecutionChain) handler : new HandlerExecutionChain(handler);
//根據請求查詢路徑，獲得目前的url
String lookupPath = this.urlPathHelper.getLookupPathForRequest(request, LOOKUP_PATH);
//先擷取适配類型的攔截器添加進去攔截器鍊的疊代器
Iterator var5 = this.adaptedInterceptors.iterator();
while(var5.hasNext()) {
HandlerInterceptor interceptor = (HandlerInterceptor)var5.next();
if (interceptor instanceof MappedInterceptor) {
MappedInterceptor mappedInterceptor = (MappedInterceptor)interceptor;
if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
//周遊攔截器，找到跟目前url對應的，添加進執行鍊中去
chain.addInterceptor(mappedInterceptor.getInterceptor());
}
} else {
//擷取适配類型的攔截器添加進去攔截器鍊
chain.addInterceptor(interceptor);
}
}
return chain;
}
擷取對應請求的Handler擴充卡： HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());（DispatcherServlet類中的doDipatch（）方法中）
getHandlerAdapter：
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
if (this.handlerAdapters != null) {
Iterator var2 = this.handlerAdapters.iterator();
//周遊所有的HandlerAdapter，找到和目前Handler比對的就傳回
//我們這裡會比對到RequestMappingHandlerAdapter
while(var2.hasNext()) {
HandlerAdapter adapter = (HandlerAdapter)var2.next();
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");
}
緩存的處理
也就是對last-modified的處理
執行攔截器的preHandle方法
就是周遊所有的我們定義的interceptor，執行preHandle方法
使用Handler擴充卡執行目前的Handler：也就是:
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
ha.handle執行目前handlerInternal，我們這裡使用的是RequestMappingHandlerAdapter，首先會進入AbstractHandlerMethodAdapter的handle方法,然後該方法調用了本類的抽象的handlerInternal方法，RequestMappingHandlerAdapter內建了它：
protected ModelAndView handleInternal(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
this.checkRequest(request);
ModelAndView mav;
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
// Execute invokeHandlerMethod in synchronized block if required.
synchronized(mutex) {
mav = this.invokeHandlerMethod(request, response, handlerMethod);
}
} else {
mav = this.invokeHandlerMethod(request, response, handlerMethod);
}
} else {
mav = this.invokeHandlerMethod(request, response, handlerMethod);
}
if (!response.containsHeader("Cache-Control")) {
if (this.getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
//Always prevent caching in case of session attribute management.
this.applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
} else {
this.prepareResponse(response);
}
}
//傳回封裝的ModelAndView的mav
return mav;
}
再往下追，就是把handler封裝的handlerMethod，完成請求參數的封裝，和jason格式的轉換；
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ServletWebRequest webRequest = new ServletWebRequest(request, response);
Object result;
try {
WebDataBinderFactory binderFactory = this.getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = this.getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod invocableMethod = this.createInvocableHandlerMethod(handlerMethod);
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
modelFactory.initModel(webRequest, mavContainer, invocableMethod);
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.setTaskExecutor(this.taskExecutor);
asyncManager.setAsyncWebRequest(asyncWebRequest);
asyncManager.registerCallableInterceptors(this.callableInterceptors);
asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);
if (asyncManager.hasConcurrentResult()) {
result = asyncManager.getConcurrentResult();
mavContainer = (ModelAndViewContainer)asyncManager.getConcurrentResultContext()[0];
asyncManager.clearConcurrentResult();
LogFormatUtils.traceDebug(this.logger, (traceOn) -> {
String formatted = LogFormatUtils.formatValue(result, !traceOn);
return "Resume with async result [" + formatted + "]";
});
invocableMethod = invocableMethod.wrapConcurrentResult(result);
}
invocableMethod.invokeAndHandle(webRequest, mavContainer, new Object[0]);
if (!asyncManager.isConcurrentHandlingStarted()) {
ModelAndView var15 = this.getModelAndView(mavContainer, modelFactory, webRequest);
return var15;
}
result = null;
} finally {
webRequest.requestCompleted();
}
return (ModelAndView)result;
}
組裝預設視圖名稱
字首和字尾名都加上
執行攔截器的postHandle方法
周遊intercepter的postHandle方法。
處理最後的結果，渲染之類的
DispatcherServlet類中的processDispatchResult方法：
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) {
this.logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException)exception).getModelAndView();
} else {
Object handler = mappedHandler != null ? mappedHandler.getHandler() : null;
mv = this.processHandlerException(request, response, handler, exception);
errorView = mv != null;
}
}
// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
//渲染
this.render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
} else if (this.logger.isTraceEnabled()) {
this.logger.trace("No view rendering, null ModelAndView returned.");
}
if
// Concurrent handling started during a forward
(!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
}
}
}
重點看下render方法，進行渲染：
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
//設定本地化
Locale locale = this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale();
response.setLocale(locale);
String viewName = mv.getViewName();
View view;
if (viewName != null) {
//解析視圖名，得到視圖
view = this.resolveViewName(viewName, mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException("Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + this.getServletName() + "'");
}
} else {
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a View object in servlet with name '" + this.getServletName() + "'");
}
}
if (this.logger.isTraceEnabled()) {
this.logger.trace("Rendering view [" + view + "] ");
}
try {
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
//委托給視圖進行渲染
view.render(mv.getModelInternal(), request, response);
} catch (Exception var8) {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Error rendering view [" + view + "]", var8);
}
throw var8;
}
}
view.render就是進行視圖的渲染，然後跳轉頁面等處理。
到這裡大概的流程就走完了。其中涉及到的東西還有很多，暫先不做詳細處理。
![JAVA并發多線程面試真題 2020最新最全丨來自阿裡、百度、美團、京東多家大廠寫在前面Java并發知識思維導圖2020阿裡、百度、京東、順豐、美團等多家大廠多線程與高并發面試真題第1章多線程基礎第二章Atomic類第3章Lock與Condition第4章同步工具類第5章并發容器第6章線程池與Future第7章ForkJoi nPool第8章CompletableFuture108道多線程&并發面試題最後[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)