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本篇內容介紹了“Java源碼分析SpringMVC執行流程”的有關知識，在實際案例的操作過程中，不少人都會遇到這樣的困境，接下來就讓小編帶領大家學習一下如何處理這些情況吧！希望大家仔細閱讀，能夠學有所成！
先看一下SpringMVC執行流程再看源碼，有助理解：
?戶發送請求?前端控制器DispatcherServlet。
DispatcherServlet 收到請求調? 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?查找)，?成處理器及處理器攔截器(如果有則?成)?并返回給DispatcherServlet。
DispatcherServlet調?HandlerAdapter處理器適配器。
HandlerAdapter經過適配調?具體的處理器(Controller，也叫后端控制器)
Controller執?完成返回ModelAndView。
HandlerAdapter 將 Controller 執?結果 ModelAndView 返回給DispatcherServlet。
DispatcherServlet將ModelAndView傳給ViewReslover視圖解析器。
ViewReslover解析后返回具體View。
DispatcherServlet根據View進?渲染視圖（即將模型數據填充?視圖中）。
DispatcherServlet 響應?戶。
我們都知道當從用戶發起請求到后端是，首先走的就是DispatcherServlet，接著就會調用doService()方法執行業務邏輯，doService()方法也只是一個中轉站，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);
// 為當前請求獲取映射處理器
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// 獲取映射處理器適配器
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：
1.getHandler(HttpServletRequest request)方法：該方法是處理當前請求找到合適的HandlerMapping，并返回一個HandlerExecutionChain，HandlerExecutionChain，HandlerExecutionChain包含了具體的處理器(handler)和攔截器列表。
HandlerMapping 默認的實現有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;
}2.getHandlerAdapter(Object handler) 根據HandlerExecutionChain中的handler來獲取處理器適配器(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有兩個默認實現類，分別是 org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter和 org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter，前者用于沒有使用模板引擎的請求，后者用于使用了模板引擎的接口。
實際處理請求的是HandlerAdapter的handle方法，如果是沒有使用例如JSP等的模板引擎，handle方法就會返回null，如果使用了模板引擎就會返回一個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);
}3.processDispatchResult方法用于處理轉發結果，該結果要么是一個ModelAndView，要么拋異常。
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。
4.render方法用來通過名稱呈現視圖，它也是請求處理的最后一步。
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) {
// 通過視圖名稱獲取視圖
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的封裝，最后調用的還是View的render方法。
resolveViewName方法用于解析視圖名稱，它會通過視圖解析器ViewResolver的resolveViewName方法解析視圖并返回一個視圖View，然后再通過View的render方法渲染視圖，至于是怎么渲染視圖的這里就不介紹了，感興趣的可以自行查看源碼。
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;
}“Java源碼分析SpringMVC執行流程”的內容就介紹到這里了，感謝大家的閱讀。如果想了解更多行業相關的知識可以關注億速云網站，小編將為大家輸出更多高質量的實用文章！
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