JavaWeb学习之——Spring篇之Spring MVC组件简介

说明

1.  Handler——http请求具体的处理类，简单理解为Controller中的@RequestMapping注解方法
2.  HandlerMapping——把reuqest请求映射到Handler
3.  HandleAdapter——怎么把Servlet请求转换Handler的须要参数形式，并使用Handler来处理request请求

1. HandlerMapping介绍

HandlerMapping就是根据Request找到对应的处理Handler和Interceptors。HandlerMapping 的方法只有一个，根据request获取HandlerExecutionChain

HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;

HandlerExecutionChain里面包含了若干Interceptor和一个具体的Handler。

在DispatcherServlet中，有个方法

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;
}

就是经过request，在HandlerMapping列表中查找，找到第一个就返回HandlerExecutionChain

2. HandlerAdapter

从下面注释能够看出，HandlerAdapter的做用就是用request来调用handler的处理类（由于每一个controller的方法的参数都是不同的）

public interface HandlerAdapter {

   /**
    * 判断当前adapter是否支持该handler示例
    */
   boolean supports(Object handler);

   /**
    * 使用参数中的handler处理参数中的request
    */
   ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

   long getLastModified(HttpServletRequest request, Object handler);
}

在DispatcherServlet中的getHandlerAdpater方法中，也是遍历全部的handlerAdpater，找出第一个能support的去处理。

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");
}

3. HandlerExceptionResolver

当在上面的处理过程当中出现Exception 的时候，就须要一个专门处理异常的类（就算出问题了，总也得给点反馈吧）。而HandlerExceptionResolver就是来作这种事情的。咱们作统一错误页或者记录错误日志的时候就能够实现该接口

public interface HandlerExceptionResolver {

   /**
    * 出现异常的时候会调用改方法
    */
   ModelAndView resolveException(
         HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex);
}

4. ViewResolver

这个组件的做用就是渲染页面，在Dispatcher中的应用以下：

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
   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 {
      if (mv.getStatus() != null) {
         response.setStatus(mv.getStatus().value());
      }
      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;
   }
}

首先是先找出渲染视图locale(地区，例如zn），而后根据locale调用ViewResolver的解析方法生成View。最后用view渲染。

ViewResolver的做用就是根据视图名（viewName）找出对应的视图（能够理解为一个地址），而视图的做用就是填充模板中的一些参数（JSTL或是Freemaker）。接口以下：

public interface ViewResolver {

   View resolveViewName(String viewName, Locale locale) throws Exception;

}

5. RequestToViewNameTranslator

前面介绍ViewResolver的做用是根据viewName来查找出对应的视图，可是有些handler处理过程当中并无设置视图，而RequestToViewNameTranslator就是根据request找出视图。

6. MultipartResolver

用于处理上传请求。将普通的requet封装为MultipartHttpServletRequest。后者直接能够获取文件。

7. FlashMapManager

这个主要是再出redirect的时候传递参数。

public interface FlashMapManager {

   /**
    * 恢复和更新参数
    */
   FlashMap retrieveAndUpdate(HttpServletRequest request, HttpServletResponse response);

   /**
    * 将参数保存到FlashMap中
    */
   void saveOutputFlashMap(FlashMap flashMap, HttpServletRequest request, HttpServletResponse response);

}