spring mvc DispatcherServlet之前端控制器架构详解

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

 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：

 /**
 * 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：

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：

 /**
 * 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。

/**
 * 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：

 /**
 * 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

 /**
 * 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

 /**
 * 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。