Springmvc ViewResolver设计实现过程解析

总结：

ViewResolver 如果要改需要自己注入到容器中并进行修改， springmvc使用的是InterResourceViewResover
view不需要自己改，是springmvc根据return返回值选的

既然看到有ModelAndView直接跳转jsp的， 有请求转发的，有重定向的，这里整体是怎么设计的： (@ResponseBody的在此不作展开)

HiController:

@Controller
public class HiController {
  @RequestMapping("/hi")
  public ModelAndView getHi() {
    ModelAndView mav = new ModelAndView("me");
    return mav;
  }

  @RequestMapping("/yes")
  public String forwardYes() {
    return "forward:patch";
  }

  @RequestMapping("/no")
  public String RedirectNo() {
    return "redirect:patch";
  }

  @ResponseBody
  @RequestMapping("/patch")
  public String redirectNo() {
    return "from forward or redirect request";   // 这种情况没有view，在这里不讨论
  }
}

主要代码：

DispatcherServlet.doDispatch(）里的：

DispatcherServlet.render方法：

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 != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
    response.setLocale(locale);

    View view;
    String viewName = mv.getViewName();
    if (viewName != null) {
      // We need to resolve the view name.
      view = resolveViewName(viewName, mv.getModelInternal(), locale, request);   // 1
      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.isTraceEnabled()) {
      logger.trace("Rendering view [" + view + "] ");
    }
    try {
      if (mv.getStatus() != null) {
        response.setStatus(mv.getStatus().value());
      }
      view.render(mv.getModelInternal(), request, response);  // 2
    }
    catch (Exception ex) {
      if (logger.isDebugEnabled()) {
        logger.debug("Error rendering view [" + view + "]", ex);
      }
      throw ex;
    }
  }

1. view = resolveViewName()会根据不同的路径生成不同的view, return mav 会返回JstlView, return "forward:/patch" 会返回InternalResourceView， return "direct:/patch" 会返回IndirectView

2. 不同的view去走不同的view.render()， 根据不同的view重写abstract void renderMergedOutputModel方法

再来看是如何生成不同的view:[/code][code]view = resolveViewName() 进去，走到
DiapatcherServlet先有ViewResolver这个，用来生成不同的view

protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
      Locale locale, HttpServletRequest request) throws Exception {

    if (this.viewResolvers != null) {
      for (ViewResolver viewResolver : this.viewResolvers) {
        View view = viewResolver.resolveViewName(viewName, locale);
        if (view != null) {
          return view;
        }
      }
    }
    return null;
  }

ViewResolver 接口只有一个方法

public interface ViewResolver {
  @Nullable
  View resolveViewName(String viewName, Locale locale) throws Exception;

}

要配置具体的视图解析器，springMVC中使用的是InterResourceViewResover，InterResourceViewResover 和他的父类UrlBasedViewResolver中都没有重写resolveViewName方法，再上一层的父类AbstractCahingViewResolver实现了resolveViewName方法

AbstractCahingViewResolver：

@Override
  @Nullable
  public View resolveViewName(String viewName, Locale locale) throws Exception {
    if (!isCache()) {
      return createView(viewName, locale);
    }
    else {
      Object cacheKey = getCacheKey(viewName, locale);
      View view = this.viewAccessCache.get(cacheKey);
      if (view == null) {
        synchronized (this.viewCreationCache) {
          view = this.viewCreationCache.get(cacheKey);
          if (view == null) {
            // Ask the subclass to create the View object.
            view = createView(viewName, locale);
            if (view == null && this.cacheUnresolved) {
              view = UNRESOLVED_VIEW;
            }
            if (view != null && this.cacheFilter.filter(view, viewName, locale)) {
              this.viewAccessCache.put(cacheKey, view);
              this.viewCreationCache.put(cacheKey, view);
            }
          }
        }
      }
      else {
        if (logger.isTraceEnabled()) {
          logger.trace(formatKey(cacheKey) + "served from cache");
        }
      }
      return (view != UNRESOLVED_VIEW ? view : null);
    }
  }

InterResourceViewResover中没有createView方法，所以是调用它父类UrlBasedViewResolver的createView方法：

@Override
  protected View createView(String viewName, Locale locale) throws Exception {
    // If this resolver is not supposed to handle the given view,
    // return null to pass on to the next resolver in the chain.
    if (!canHandle(viewName, locale)) {
      return null;
    }

    // Check for special "redirect:" prefix.
    if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
      String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
      RedirectView view = new RedirectView(redirectUrl,
          isRedirectContextRelative(), isRedirectHttp10Compatible());
      String[] hosts = getRedirectHosts();
      if (hosts != null) {
        view.setHosts(hosts);
      }
      return applyLifecycleMethods(REDIRECT_URL_PREFIX, view);  // return "direct:/patch"在这里构造view
    }

    // Check for special "forward:" prefix.
    if (viewName.startsWith(FORWARD_URL_PREFIX)) {
      String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
      InternalResourceView view = new InternalResourceView(forwardUrl);
      return applyLifecycleMethods(FORWARD_URL_PREFIX, view);   // return "forward:/patch" 在这里构造view
    } 

    // Else fall back to superclass implementation: calling loadView.
    return super.createView(viewName, locale);          // return mav 在这里构造view
  }

关于ViewResolver的代码执行顺序， 前面分析那么多，这里再打断点快速验证一下：

进DispatcherServlet的doDispatch看到就是这个解析器：

断点放在这里，

然后下一步：

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