前端控制器是整个mvc框架中最为核心的一块，它主要用来拦截符合要求的外部请求，并把请求分发到不同的控制器去处理，根据控制器处理后的结果，生成相应的响应发送到客户端。前端控制器既可以使用filter实现（struts2采用这种方式），也可以使用servlet来实现(spring mvc框架)。

dispatcherservlet 作为前置控制器是web服务器的入口，是spring mvc最重要的一个类，通过它的生命周期可以加深对web服务器的理解。

servlet的生命周期

首先我们回忆一下servlet的生命周期：

servlet生命周期分为三个阶段：【servlet生命周期与工作原理详解】

　　1.初始化阶段 调用init()方法。servlet被装载后，servlet容器创建一个servlet实例并且调用servlet的init()方法进行初始化。在servlet的整个生命周期内，init()方法只被调用一次。

　　2.响应客户请求阶段　　调用service()方法

　　3.终止阶段　　调用destroy()方法

servlet初始化阶段

　　在下列时刻servlet容器装载servlet：

　　1.servlet容器启动时自动装载某些servlet，实现它只需要在web.xml文件中的<servlet></servlet>之间添加如下代码：　　

<loadon-startup>1</loadon-startup>
　　2.在servlet容器启动后，客户首次向servlet发送请求

　　3.servlet类文件被更新后，重新装载servlet

dispatcherservlet的结构

复习了上述知识后我们来看看dispatcherservlet的结构：

dispatcherservlet继承自抽象类：frameworkservlet，间接继承了httpservlet （frameworkservlet继承自httpservletbean，而httpservletbean继承自httpservlet ）

servlet的初始化

? 1 2 3 4 5 6 7 8 9 10 11 protected void initstrategies(applicationcontext context) { initmultipartresolver(context); //文件上传解析，如果请求类型是multipart将通过multipartresolver进行文件上传解析； initlocaleresolver(context); //本地化解析 initthemeresolver(context);　　　//主题解析 inithandlermappings(context); //通过handlermapping，将请求映射到处理器 inithandleradapters(context); //通过handleradapter支持多种类型的处理器 inithandlerexceptionresolvers(context); //如果执行过程中遇到异常将交给handlerexceptionresolver来解析 initrequesttoviewnametranslator(context); //直接解析请求到视图名 initviewresolvers(context); //通过viewresolver解析逻辑视图名到具体视图实现 initflashmapmanager(context); //flash映射管理器 }

servlet如何处理请求：

servlet的service方法处理http请求。

frameworkservlet.java 定义了servlet的service和destroy方法，如下所示：

? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 /** * override the parent class implementation in order to intercept patch * requests. */ @override protected 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); } }

我们知道http请求类型有七种（外加一个option选项），定义如下：

? 1 2 3 4 public enum requestmethod { get, head, post, put, patch, delete, options, trace }

frameworkservlet的service()处理不同的请求，我们以常见的post来说明：

? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 /** * 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 抽象定义了处理流程，留待子类来实现该方法，完成具体的请求处理。

? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 /** * 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;

具体实现如下：

? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 /** * exposes the dispatcherservlet-specific request attributes and delegates to {@link #dodispatch} * for the actual dispatching. */ @override protected 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); } } }

重头戏，作为请求分发器的实现：

功能：1. 把请求分发到handler（按照配置顺序获取servlet的映射关系获取handler）；2. 根据servlet已安装的 handleradapters 去查询第一个能处理的handler；3. handler激发处理请求

? 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 /** * 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销毁

? 1 2 3 4 5 6 7 8 9 10 11 12 /** * close the webapplicationcontext of this servlet. * @see org.springframework.context.configurableapplicationcontext#close() */ @override public 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(); } }

小结：

本文因篇章限制，仅仅介绍了请求处理的流程，没有对代码进行深入的分析，接下来的文章将从细微处着手，分析spring的代码之美。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。

原文链接：http://www.cnblogs.com/davidwang456/p/4090058.html