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

时间:2021-04-21 15:07     来源/作者:一天不进步,就是退步

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)。

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

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

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

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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:

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/**
* 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:

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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:

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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。

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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;

具体实现如下:

 

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/**
* 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

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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销毁

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/**
* 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

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