一、组件说明
- DispatcherServlet：前端控制器，用于请求到达前端控制器，由它调用其他组件处理用户的请求。
- HandlerMapping：处理器映射器，负责根据用户请求找到Handler(处理器)，springmvc提供了不同的映射器实现方式。
- Handler：处理器，对具体的用户请求进行处理。
- HandlerAdapter：处理器适配器，通过HandlerAdapter对处理器进行执行。
- View Resolver：视图解析器，负责将处理结果生成view视图。View Resolver首先根据逻辑视图名解析成物理视图名即具体的页面地址，再生成View视图对象，最后对View进行渲染将处理结果通过页面展示给用户。springmvc框架提供了很多的View视图类型，包括：jstlView、freemarkerView、pdfView等。
二、前端控制器
DispatcherServlet是springmvc的*调度器，DispatcherServlet创建时会默认从DispatcherServlet.properties文件加载springmvc所用的各种组件。
二、处理器映射器
处理器映射器负责根据request请求找到对应的Handler处理器以及Interceptor拦截器，将它们封装在HandlerExecutionChain对象中返回给前端控制器。
下面是两种常用的处理器映射器：
1、BeanNameURLHandlerMapping：根据请求的url与spring容器中定义的bean的name进行匹配，从而从spring容器中找到bean实例。
2、simpleURLHandlerMapping：它可以将url和处理器bean的id进行统一映射配置。
三、处理器适配器
HandlerAdapter会根据适配器接口对Handler进行包装适配，包装后即可对处理器进行执行(使用了适配器模式)。
下面是两种常用的处理器适配器：
1、SimpleControllerHandlerAdapter：简单控制器处理器适配器，所有实现了org.springframework.web.servlet.mvc.Controller 接口的bean通过此适配器进行适配执行。
2、HttpRequestHandlerAdapter:http请求处理器适配器，所有实现了org.springframework.web.HttpRequestHandler接口的bean通过此适配器进行适配执行。
注：处理器实现HttpRequestHandler接口，实现handleRequest方法，该方法没有返回、ModelAndView，可以通过response修改定义响应内容，比如返回json数据。
四、注解处理器映射器和适配器
可以使用组件扫描，而省去在spring容器中配置每个controller类。配置如下：
<context:component-scan base-package="com.demo.ssm.controller"></context:component-scan>
1、注解处理器映射器，对类中标记的@RequestMapping的方法进行映射，根据RequestMapping定义的url匹配RequestMapping标记的方法，匹配成功返回HandlerMethod对象给前端控制器。
<!-- 注解映射器 -->
<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"/>
2、注解处理器适配器，对标记的@RequestMapping的方法进行适配。
<!-- 注解适配器 -->
<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"/>
注：还可以使用下面的配置代替上面处理器映射器和适配器的配置：
<mvc:annotation-driven></mvc:annotation-driven>
五、总结
springmvc处理流程：
1、用户发送请求到DispatchServlet前端控制器。
/**
* 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;
} // Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); 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
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
2、DispatchServlet调用HandlerMapping处理器映射器根据url查找Handler。
/**
* Return the HandlerExecutionChain for this request.
* <p>Tries all handler mappings in order.
* @param request current HTTP request
* @return the HandlerExecutionChain, or {@code null} if no handler could be found
*/
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
}
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
return null;
}
3、DispatchServlet调用HandlerAdapter处理器适配器对HandlerMapping找到的Handler进行包装。
/**
* Return the HandlerAdapter for this handler object.
* @param handler the handler object to find an adapter for
* @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
*/
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
for (HandlerAdapter ha : this.handlerAdapters) {
if (logger.isTraceEnabled()) {
logger.trace("Testing handler adapter [" + ha + "]");
}
if (ha.supports(handler)) {
return ha;
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
4、执行Handler，并返回一个ModelAndView。
/**
* Determine an error ModelAndView via the registered HandlerExceptionResolvers.
* @param request current HTTP request
* @param response current HTTP response
* @param handler the executed handler, or {@code null} if none chosen at the time of the exception
* (for example, if multipart resolution failed)
* @param ex the exception that got thrown during handler execution
* @return a corresponding ModelAndView to forward to
* @throws Exception if no error ModelAndView found
*/
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
Object handler, Exception ex) throws Exception { // Check registered HandlerExceptionResolvers...
ModelAndView exMv = null;
for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
if (exMv != null) {
break;
}
}
if (exMv != null) {
if (exMv.isEmpty()) {
request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
return null;
}
// We might still need view name translation for a plain error model...
if (!exMv.hasView()) {
exMv.setViewName(getDefaultViewName(request));
}
if (logger.isDebugEnabled()) {
logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
}
WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
return exMv;
} throw ex;
}
5、DispatchServlet调用ViewResolver视图解析器进行视图解析，解析完成后返回一个View，再进行视图渲染并展示。
/**
* Render the given ModelAndView.
* <p>This is the last stage in handling a request. It may involve resolving the view by name.
* @param mv the ModelAndView to render
* @param request current HTTP servlet request
* @param response current HTTP servlet response
* @throws ServletException if view is missing or cannot be resolved
* @throws Exception if there's a problem rendering the view
*/
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine locale for request and apply it to the response.
Locale locale = this.localeResolver.resolveLocale(request);
response.setLocale(locale); View view;
if (mv.isReference()) {
// We need to resolve the view name.
view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
"' in servlet with name '" + getServletName() + "'");
}
}
else {
// No need to lookup: the ModelAndView object contains the actual View object.
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
"View object in servlet with name '" + getServletName() + "'");
}
} // Delegate to the View object for rendering.
if (logger.isDebugEnabled()) {
logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
}
try {
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" +
getServletName() + "'", ex);
}
throw ex;
}
}