前端控制器是整個mvc框架中最為核心的一塊，它主要用來攔截符合要求的外部請求，并把請求分發到不同的控制器去處理，根據控制器處理后的結果，生成相應的響應發送到客戶端。前端控制器既可以使用filter實現（struts2采用這種方式），也可以使用servlet來實現(spring mvc框架)。
dispatcherservlet 作為前置控制器是web服務器的入口，是spring mvc最重要的一個類，通過它的生命周期可以加深對web服務器的理解。
servlet的生命周期
首先我們回憶一下servlet的生命周期：
servlet生命周期分為三個階段：【servlet生命周期與工作原理詳解】
1.初始化階段 調用init()方法。servlet被裝載后，servlet容器創建一個servlet實例并且調用servlet的init()方法進行初始化。在servlet的整個生命周期內，init()方法只被調用一次。
2.響應客戶請求階段 調用service()方法
3.終止階段 調用destroy()方法
servlet初始化階段
在下列時刻servlet容器裝載servlet：
1.servlet容器啟動時自動裝載某些servlet，實現它只需要在web.xml文件中的<servlet></servlet>之間添加如下代碼：
<loadon-startup>1</loadon-startup>
2.在servlet容器啟動后，客戶首次向servlet發送請求
3.servlet類文件被更新后，重新裝載servlet
dispatcherservlet的結構
復習了上述知識后我們來看看dispatcherservlet的結構：
dispatcherservlet繼承自抽象類：frameworkservlet，間接繼承了httpservlet （frameworkservlet繼承自httpservletbean，而httpservletbean繼承自httpservlet ）
servlet的初始化
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protected void initstrategies(applicationcontext context) {initmultipartresolver(context); //文件上傳解析，如果請求類型是multipart將通過multipartresolver進行文件上傳解析；initlocaleresolver(context); //本地化解析initthemeresolver(context); //主題解析inithandlermappings(context); //通過handlermapping，將請求映射到處理器inithandleradapters(context); //通過handleradapter支持多種類型的處理器inithandlerexceptionresolvers(context); //如果執行過程中遇到異常將交給handlerexceptionresolver來解析initrequesttoviewnametranslator(context); //直接解析請求到視圖名initviewresolvers(context); //通過viewresolver解析邏輯視圖名到具體視圖實現initflashmapmanager(context); //flash映射管理器} |
servlet如何處理請求：
servlet的service方法處理http請求。
frameworkservlet.java 定義了servlet的service和destroy方法，如下所示：
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/*** override the parent class implementation in order to intercept patch* requests.*/@overrideprotected void service(httpservletrequest request, httpservletresponse response) throws servletexception, ioexception {string method = request.getmethod();if (method.equalsignorecase(requestmethod.patch.name())) { processrequest(request, response);}else { super.service(request, response);}} |
我們知道http請求類型有七種（外加一個option選項），定義如下：
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public enum requestmethod {get, head, post, put, patch, delete, options, trace} |
frameworkservlet的service()處理不同的請求，我們以常見的post來說明：
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/*** process this request, publishing an event regardless of the outcome.* <p>the actual event handling is performed by the abstract* {@link #doservice} template method.*/protected final void processrequest(httpservletrequest request, httpservletresponse response) throws servletexception, ioexception {long starttime = system.currenttimemillis();throwable failurecause = null;localecontext previouslocalecontext = localecontextholder.getlocalecontext();localecontext localecontext = buildlocalecontext(request);requestattributes previousattributes = requestcontextholder.getrequestattributes();servletrequestattributes requestattributes = buildrequestattributes(request, response, previousattributes);webasyncmanager asyncmanager = webasyncutils.getasyncmanager(request);asyncmanager.registercallableinterceptor(frameworkservlet.class.getname(), new requestbindinginterceptor());initcontextholders(request, localecontext, requestattributes);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 { resetcontextholders(request, previouslocalecontext, previousattributes); if (requestattributes != null) { requestattributes.requestcompleted(); } if (logger.isdebugenabled()) { if (failurecause != null) { this.logger.debug("could not complete request", failurecause); } else { if (asyncmanager.isconcurrenthandlingstarted()) { logger.debug("leaving response open for concurrent processing"); } else { this.logger.debug("successfully completed request"); } } } publishrequesthandledevent(request, starttime, failurecause);}} |
frameworkservlet 抽象定義了處理流程，留待子類來實現該方法，完成具體的請求處理。
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/** * subclasses must implement this method to do the work of request handling, * receiving a centralized callback for get, post, put and delete. * <p>the contract is essentially the same as that for the commonly overridden * {@code doget} or {@code dopost} methods of httpservlet. * <p>this class intercepts calls to ensure that exception handling and * event publication takes place. * @param request current http request * @param response current http response * @throws exception in case of any kind of processing failure * @see javax.servlet.http.httpservlet#doget * @see javax.servlet.http.httpservlet#dopost */ protected abstract void doservice(httpservletrequest request, httpservletresponse response) throws exception; |
具體實現如下：
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/*** exposes the dispatcherservlet-specific request attributes and delegates to {@link #dodispatch}* for the actual dispatching.*/@overrideprotected 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("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);}finally { if (webasyncutils.getasyncmanager(request).isconcurrenthandlingstarted()) { return; } // restore the original attribute snapshot, in case of an include. if (attributessnapshot != null) { restoreattributesafterinclude(request, attributessnapshot); }}} |
重頭戲，作為請求分發器的實現：
功能：1. 把請求分發到handler（按照配置順序獲取servlet的映射關系獲取handler）；2. 根據servlet已安裝的 handleradapters 去查詢第一個能處理的handler；3. handler激發處理請求
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/*** 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 {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); // determine handler for the current request. mappedhandler = gethandler(processedrequest); 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()) { logger.debug("last-modified value for [" + getrequesturi(request) + "] is: " + lastmodified); } if (new servletwebrequest(request, response).checknotmodified(lastmodified) && isget) { return; } } if (!mappedhandler.applyprehandle(processedrequest, response)) { return; } try { // actually invoke the handler. mv = ha.handle(processedrequest, response, mappedhandler.gethandler()); } finally { if (asyncmanager.isconcurrenthandlingstarted()) { return; } } applydefaultviewname(request, mv); mappedhandler.applyposthandle(processedrequest, response, mv); } catch (exception ex) { dispatchexception = ex; } processdispatchresult(processedrequest, response, mappedhandler, mv, dispatchexception);}catch (exception ex) { triggeraftercompletion(processedrequest, response, mappedhandler, ex);}catch (error err) { triggeraftercompletionwitherror(processedrequest, response, mappedhandler, err);}finally { if (asyncmanager.isconcurrenthandlingstarted()) { // instead of posthandle and aftercompletion mappedhandler.applyafterconcurrenthandlingstarted(processedrequest, response); return; } // clean up any resources used by a multipart request. if (multipartrequestparsed) { cleanupmultipart(processedrequest); }}} |
servlet銷毀
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/*** close the webapplicationcontext of this servlet.* @see org.springframework.context.configurableapplicationcontext#close()*/@overridepublic void destroy() {getservletcontext().log("destroying spring frameworkservlet '" + getservletname() + "'");// only call close() on webapplicationcontext if locally managed...if (this.webapplicationcontext instanceof configurableapplicationcontext && !this.webapplicationcontextinjected) { ((configurableapplicationcontext) this.webapplicationcontext).close();}} |
小結：
本文因篇章限制，僅僅介紹了請求處理的流程，沒有對代碼進行深入的分析，接下來的文章將從細微處著手，分析spring的代碼之美。
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持服務器之家。
原文鏈接：http://www.cnblogs.com/davidwang456/p/4090058.html
