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
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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
}
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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);
}
}
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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
}
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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);
}
}
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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;
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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);
}
}
}
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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);
}
}
}
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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();
}
}
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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