淺談springMVC中的設計模式(1)——責任鏈模式
最近終於閒了下來,準備自己記錄些東西。網上關於springMVC的資料很多,但關於設計模式的還有限,我就想把springMVC原始碼中的設計模式抽出來做成一個系列,簡單的談一下其中的實現原理,作為一種學習分享,以後有更深的感悟也會更新。
先從一張圖對整個springMVC的運作流程有一個大致的瞭解,圖片侵刪。
HandlerExecutionChain
其中我們可以看到,在springMVC中,DispatcherServlet這個核心類中使用到了HandlerExecutionChain這個類,他就是責任鏈模式實行的具體類。在DispatcherServlet的doDispatch這個方法中,我們可以看到它貫穿了整個請求dispatch的流程:
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);
// 獲取該請求的handler,每個handler實為HandlerExecutionChain,它為一個處理鏈,負責處理整個請求
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
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;
}
}
// 責任鏈執行預處理方法,實則是將請求交給註冊的請求攔截器執行
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// 實際的執行邏輯的部分,也就是你加了@RequestMapping註解的方法
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 責任鏈執行後處理方法,實則是將請求交給註冊的請求攔截器執行
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
// 處理返回的結果,觸發責任鏈上註冊的攔截器的AfterCompletion方法,其中也用到了HandlerExecutionChain註冊的handler來處理錯誤結果
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
// 觸發責任鏈上註冊的攔截器的AfterCompletion方法
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
從上面的程式碼中我們可以看到,HandlerExecutionChain主要負責請求的攔截器的執行和請求的處理,但是他本身不處理請求,只是將請求分配給在鏈上註冊的處理器執行,這是一種責任鏈的實現方式,減少了責任鏈本身與處理邏輯之間的耦合的同時,規範了整個處理請求的流程,下面我們看一下上面程式碼中涉及到的方法在HandlerExecutionChain類中對應的程式碼。
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;
}
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);
}
}
}
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);
}
}
}
}
程式碼很容易理解,這裡不詳細說明。需要注意的是,HandlerExecutionChain維護了HandlerInterceptor(攔截器)的集合,可以向其中註冊相應的HandlerInterceptor。
總結
個人的理解,責任鏈模式可以很好的將原本耦合的順序過程處理的程式碼和邏輯,解耦成執行的順序邏輯,和一個個相對應的處理器(責任人),對應的責任鏈只需要關心責任處理的順序,而不需要關心具體的處理邏輯,將這些邏輯交給註冊的責任人去處理。從springMVC的原始碼中,我們可以看到這一設計模式的應用,將原本複雜的請求處理邏輯表現的清楚明白。