2 前端控制器DispatcherServlet

　　2.1 DispatcherServlet源码分析

　　1.继承关系分析：DispatcherServlet类-FrameworkServlet类-HttpServletBean类-HttpServlet类-GenericServlet类。

　　2.先找到DispatcherServlet前端控制器类中进行请求和响应转发的doDispatch方法。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

try {
try {
ModelAndView mv = null;
Object dispatchException = null;

try {
processedRequest = this.checkMultipart(request);
multipartRequestParsed = processedRequest != request;
// 确定当前请求的处理程序。获取Handler执行链。
mappedHandler = this.getHandler(processedRequest);
if (mappedHandler == null) {
this.noHandlerFound(processedRequest, response);
return;
}

HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
String method = request.getMethod();
boolean isGet = HttpMethod.GET.matches(method);
if (isGet || HttpMethod.HEAD.matches(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
return;
}
}

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}

this.applyDefaultViewName(processedRequest, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
} catch (Exception var20) {
dispatchException = var20;
} catch (Throwable var21) {
dispatchException = new NestedServletException("Handler dispatch failed", var21);
}

this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
} catch (Exception var22) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
} catch (Throwable var23) {
this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
}

} finally {
if (asyncManager.isConcurrentHandlingStarted()) {
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
} else if (multipartRequestParsed) {
this.cleanupMultipart(processedRequest);
}

}
}

　　3.处理器执行链HandlerExecutionChain处理request请求。继续跟踪HandlerMapping的getHandler方法。

@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
// handlerMappings是Servlet所支持的处理器映射器的集合，这里有N个处理器映射器
for (HandlerMapping mapping : this.handlerMappings) {
// 获取处理器执行链
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}

　　4.继续跟进HandlerExecutionChain getHandler(HttpServletRequest request)方法，此方法来自AbstractHandlerMapping类。

@Override
@Nullable
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Object handler = getHandlerInternal(request);
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = obtainApplicationContext().getBean(handlerName);
}

// Ensure presence of cached lookupPath for interceptors and others
if (!ServletRequestPathUtils.hasCachedPath(request)) {
initLookupPath(request);
}

// 将请求request对象和处理器handler对象封装成一个处理器执行链
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

if (logger.isTraceEnabled()) {
logger.trace("Mapped to " + handler);
}
else if (logger.isDebugEnabled() && !DispatcherType.ASYNC.equals(request.getDispatcherType())) {
logger.debug("Mapped to " + executionChain.getHandler());
}

if (hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) {
CorsConfiguration config = getCorsConfiguration(handler, request);
if (getCorsConfigurationSource() != null) {
CorsConfiguration globalConfig = getCorsConfigurationSource().getCorsConfiguration(request);
config = (globalConfig != null ? globalConfig.combine(config) : config);
}
if (config != null) {
config.validateAllowCredentials();
}
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}

return executionChain;
}

　　5.继续跟进getHandlerExecutionChain(Object handler, HttpServletRequest request)方法。

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
// 获取处理器执行链对象
HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
(HandlerExecutionChain) handler : new HandlerExecutionChain(handler));

for (HandlerInterceptor interceptor : this.adaptedInterceptors) {
if (interceptor instanceof MappedInterceptor) {
MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;
if (mappedInterceptor.matches(request)) {
// 这里可以看出具有多个拦截器，而且拦截器是根据请求路径做匹配的
chain.addInterceptor(mappedInterceptor.getInterceptor());
}
}
else {
chain.addInterceptor(interceptor);
}
}
// 处理器执行链chain对象中最终封装的信息为：interceptor拦截器和handler处理器
return chain;
}

　　2.2 contextConfigLocation参数

　　1.属性contextConfigLocation：表示用于加载Bean的配置文件。

　　2.如果没有显示的配置contextConfigLocation参数，SpringMVC默认会去“/WEB-INF/[servlet名字]-servlet.xml”读配置文件。默认配置文件设置可通过org.springframework.web.context.support.XmlWebApplicationContext类中的源码来确定。、

// 根上下文的默认位置是“/WEB-INF/applicationContext.xml”，对于命名空间为“test-servlet”的上下文，则是“/WEB-INF/test-servlet.xml”（类似于servlet名称为“test”的DispatcherServlet实例）。
@Override
protected String[] getDefaultConfigLocations() {
if (getNamespace() != null) {
return new String[] {DEFAULT_CONFIG_LOCATION_PREFIX + getNamespace() + DEFAULT_CONFIG_LOCATION_SUFFIX};
}
else {
return new String[] {DEFAULT_CONFIG_LOCATION};
}
}

　　说明：如果想查看XmlWebApplicationContext类的源码，可以通过Idea窗口的【Navigate】-【Search Everywhere】选项，在打开的窗口中搜索对应的类名称即可找到类的所在位置。

　　3.DispatcherServlet配置初始化参数contextConfigLocation方式见下。

<servlet>
<servlet-name>springmvc</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/springmvc-servlet.xml</param-value>
</init-param>
</servlet>

　　4.contextConfigLocation参数可以省略不写，但是必须保证Bean的配置文件按照“/WEB-INF/[servlet名字]-servlet.xml”规则进行定义;否则项目启动时将找不到Bean配置文件，会报FileNotFoundException的异常。