DispatcherServlet類結構圖
DispatcherServlet原始碼分析
1. 加載組態檔
/**
* This implementation calls {@link #initStrategies}.
*/
@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
/**
* 初始化定位決議器、主題決議器、處理器映射器、處理器配接器、例外決議器、視圖決議器等等
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
initStrategies()方法我們可以看出DispatcherServlet實體化時會初始化web層相關的bean，如HandlerMapping，HandlerAdapter等，并且如果我們沒有進行配置，DispatcherServlet會提供默認的配置，以上的Servlet的體系結構以及DispatcherServlet的實體化程序我們可以看出主要完成以下幾個事情：
（1）通過配置Servlet實作SpringMVC核心控制器DispatcherServlet的初始化；
（2）通過ServletContext共享Spring根背景關系，使得每一個Servlet實體獲取根背景關系中的bean，用于實體化SpringMVC web層的相關bean，
（3）初始化DispatcherServlet作為核心控制器，接收處理請求需要的相關資源，如HandlerMapping，HandlerAdapter等，
（4）通過Servlet體系結構中的繼承關系以及抽象方法，可以根據具體的需求對各個層級的Servlet抽象方法進行重寫以滿足不同的功能需要，父類中只定義流程和方法參考，具體實作由子Servlet完成，實作定義與實作的分離，便于擴展，
2. processRequest()方法
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
// Expose current LocaleResolver and request as LocaleContext.
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContextHolder.setLocaleContext(buildLocaleContext(request), this.threadContextInheritable);
// Expose current RequestAttributes to current thread.
RequestAttributes previousRequestAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = null;
if (previousRequestAttributes == null || previousRequestAttributes.getClass().equals(ServletRequestAttributes.class)) {
requestAttributes = new ServletRequestAttributes(request);
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace("Bound request context to thread: " + request);
}
try {
doService(request, response);
}
catch (ServletException ex) {
failureCause = ex;
throw ex;
}
catch (IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
// Clear request attributes and reset thread-bound context.
LocaleContextHolder.setLocaleContext(previousLocaleContext, this.threadContextInheritable);
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(previousRequestAttributes, this.threadContextInheritable);
requestAttributes.requestCompleted();
}
if (logger.isTraceEnabled()) {
logger.trace("Cleared thread-bound request context: " + request);
}
if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
this.logger.debug("Successfully completed request");
}
}
if (this.publishEvents) {
// Whether or not we succeeded, publish an event.
long processingTime = System.currentTimeMillis() - startTime;
this.webApplicationContext.publishEvent(
new ServletRequestHandledEvent(this,
request.getRequestURI(), request.getRemoteAddr(),
request.getMethod(), getServletConfig().getServletName(),
WebUtils.getSessionId(request), getUsernameForRequest(request),
processingTime, failureCause));
}
}
}
DispatcherServlet也是通過自己的service()方法來接收和轉發Http請求到具體的doGet()或doPost()這些方法的，以一次典型的GET請求為例，經過HttpServlet基類中service()方法的委派，請求會被轉發到doGet()方法中，doGet()方法，在DispatcherServlet的父類FrameworkServlet類中被覆寫，
processRequest()方法理解的要點是以doService()方法為區隔，前一部分是將當前請求的Locale物件和屬性，分別設定到LocaleContextHolder和RequestContextHolder這兩個抽象類中的ThreadLocal物件中，也就是分別將這兩個東西和請求執行緒做了系結，在doService()處理結束后，再恢復回請求前的LocaleContextHolder和RequestContextHolder，也即解除執行緒系結，每次請求處理結束后，容器背景關系都發布了一個ServletRequestHandledEvent事件，你可以注冊監聽器來監聽該事件，
可以看到，processRequest()方法只是做了一些執行緒安全的隔離，真正的請求處理，發生在doService()方法中，
3. doService()方法
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
String requestUri = urlPathHelper.getRequestUri(request);
logger.debug("DispatcherServlet with name '" + getServletName() + "' processing " + request.getMethod() +
" request for [" + requestUri + "]");
}
// 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)) {
logger.debug("Taking snapshot of request attributes before include");
attributesSnapshot = new HashMap<string, object="">();
Enumeration attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}
// 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 {
doDispatch(request, response); //這邊最終也是呼叫了doDispatch方法，該方法主要用來處理SPring框架的具體業務分發邏輯，
}
finally {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
doService()方法中requet.setAttribute()方法的呼叫，將前面在初始化流程中實體化的物件設定到http請求的屬性中，供下一步處理使用，其中有容器的背景關系物件、本地化決議器等SpringMVC特有的編程元素，不同于Struts2中的ValueStack，SpringMVC的資料并沒有從HttpServletRequest物件中抽離出來再存進另外一個編程元素，這也跟SpringMVC的設計思想有關，因為從一開始，SpringMVC的設計者就認為，不應該將請求處理程序和Web容器完全隔離，所以，真正發生請求轉發的方法doDispatch()中，它的引數是HttpServletRequest和HttpServletResponse物件，
4. doDispatch()方法
//Spring框架最終的分發都是通過該方法的
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
int interceptorIndex = -1;
try {
ModelAndView mv;
boolean errorView = false;
try {
processedRequest = checkMultipart(request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest, false);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
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 (logger.isDebugEnabled()) {
String requestUri = urlPathHelper.getRequestUri(request);
logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
// 這里是處理前置攔截器
HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
if (interceptors != null) {
for (int i = 0; i < interceptors.length; i++) { HandlerInterceptor interceptor = interceptors[i]; if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) { triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null); return; } interceptorIndex = i; } } //處理最終的Action邏輯 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); // Do we need view name translation? if (mv != null && !mv.hasView()) { mv.setViewName(getDefaultViewName(request)); } //處理后置攔截器 if (interceptors != null) { for (int i = interceptors.length - 1; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);
}
}
}
catch (ModelAndViewDefiningException ex) {
logger.debug("ModelAndViewDefiningException encountered", ex);
mv = ex.getModelAndView();
}
catch (Exception ex) {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(processedRequest, response, handler, ex);
errorView = (mv != null);
}
// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
render(mv, processedRequest, 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");
}
}
// Trigger after-completion for successful outcome.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
}
catch (Exception ex) {
// Trigger after-completion for thrown exception.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
throw ex;
}
catch (Error err) {
ServletException ex = new NestedServletException("Handler processing failed", err);
// Trigger after-completion for thrown exception.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
throw ex;
}
finally {
// Clean up any resources used by a multipart request.
if (processedRequest != request) {
cleanupMultipart(processedRequest);
}
}
}
doDispatch()是整個請求轉發流程中最核心的方法，DispatcherServlet所接收的Http請求，經過層層轉發，最終都是匯總到這個方法中來進行最后的請求分發和處理，它通過高度抽象的介面，描述出了一個MVC（Model-View-Controller）設計模式的實作方案，Model、View、Controller三種層次的編程元素，在SpringMVC中都有大量的實作類，各種處理細節也是千差萬別，但是，它們最后都是由，也都能由doDispatch()方法來統一描述，這就是介面和抽象的威力，萬變不離其宗，
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