60--SpringMVC返回视图

1. 引

前两节分析了HandlerAdapter的handler方法过程，接下来回到doDispatch方法里来看其后续处理工作都还有哪些：

1.1 applyDefaultViewName设置默认视图名

private void applyDefaultViewName(HttpServletRequest request, @Nullable ModelAndView mv) throws Exception {
    // ModelAndView不为空，但是没有View对象则尝试为其生成一个默认的视图名
    if (mv != null && !mv.hasView()) {
        String defaultViewName = getDefaultViewName(request);
        if (defaultViewName != null) {
            mv.setViewName(defaultViewName);
        }
    }
}


protected String getDefaultViewName(HttpServletRequest request) throws Exception {
    return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);
}

public String getViewName(HttpServletRequest request) {
    String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
    return (this.prefix + transformPath(lookupPath) + this.suffix);
}

protected String transformPath(String lookupPath) {
    String path = lookupPath;
    if (this.stripLeadingSlash && path.startsWith(SLASH)) {
        path = path.substring(1);
    }
    if (this.stripTrailingSlash && path.endsWith(SLASH)) {
        path = path.substring(0, path.length() - 1);
    }
    if (this.stripExtension) {
        path = StringUtils.stripFilenameExtension(path);
    }
    if (!SLASH.equals(this.separator)) {
        path = StringUtils.replace(path, SLASH, this.separator);
    }
    return path;
}

具体工作委托给了RequestToViewNameTranslator接口的实现类，该方法比较简单。

2.2 applyPostHandle 应用已注册拦截器的后置方法

/**
 * 应用已注册拦截器的后置方法。
 *
 * Apply postHandle methods of registered interceptors.
 */
void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception {
    HandlerInterceptor[] interceptors = getInterceptors();
    if (!ObjectUtils.isEmpty(interceptors)) {
        for (int i = interceptors.length - 1; i >= 0; i--) {
            HandlerInterceptor interceptor = interceptors[i];
            interceptor.postHandle(request, response, this.handler, mv);
        }
    }
}

以上这两步都比较简单，接下来看返回视图结果的处理。

2.processDispatchResult简析

private void processDispatchResult(
        HttpServletRequest request,
        HttpServletResponse response,
        @Nullable HandlerExecutionChain mappedHandler,
        @Nullable ModelAndView mv,
        @Nullable Exception exception) throws Exception {

    boolean errorView = false;

    // 处理异常信息
    if (exception != null) {
        if (exception instanceof ModelAndViewDefiningException) {
            logger.debug("ModelAndViewDefiningException encountered", exception);
            mv = ((ModelAndViewDefiningException) exception).getModelAndView();
        }
        else {
            Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
            mv = processHandlerException(request, response, handler, exception);
            errorView = (mv != null);
        }
    }

    // Did the handler return a view to render?
    // 尝试解析视图和模型；
    // wasCleared:判断当前模型和视图是否已经被标识为清空，且当前视图和模型是否同时为空
    if (mv != null && !mv.wasCleared()) {
        // 解析并呈现视图和模型
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
    else {
        if (logger.isTraceEnabled()) {
            logger.trace("No view rendering, null ModelAndView returned.");
        }
    }

    if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        // Concurrent handling started during a forward
        return;
    }

    // 处理注册的后置完成拦截器
    if (mappedHandler != null) {
        mappedHandler.triggerAfterCompletion(request, response, null);
    }
}

processDispatchResult处理程序选择和处理程序调用的结果，该结果要么是一个ModelAndView，要么是一个要解析为ModelAndView的异常。该方法的核心是render方法，用来解析并呈现视图和模型。这也是一次完整请求最后要处理的部分。

3. render方法分析

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
    // Determine locale for request and apply it to the response.
    // 确定请求的区域设置并将其应用于响应。
    Locale locale = (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
    response.setLocale(locale);

    View view;
    // 获取视图名
    String viewName = mv.getViewName();
    // 未能获取视图名，则解析视图名
    if (viewName != null) {
        // We need to resolve the view name.
        view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
        if (view == null) {
            throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                    "' in servlet with name '" + getServletName() + "'");
        }
    }
    // 获取到视图名，再次判断当前ModelAndView对象中是否包含真正的View对象，
    // 因为接下来需要调用View对象的render方法
    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 {
        // 设置返回状态码
        if (mv.getStatus() != null) {
            response.setStatus(mv.getStatus().value());
        }
        // 调用View对象的render方法完成视图解析
        view.render(mv.getModelInternal(), request, response);
    }
    catch (Exception ex) {
        if (logger.isDebugEnabled()) {
            logger.debug("Error rendering view [" + view + "]", ex);
        }
        throw ex;
    }
}

其核心处理委托给了View对象的render方法：

public void render(@Nullable Map<String, ?> model, HttpServletRequest request,
        HttpServletResponse response) throws Exception {

    if (logger.isDebugEnabled()) {
        logger.debug("View " + formatViewName() +
                ", model " + (model != null ? model : Collections.emptyMap()) +
                (this.staticAttributes.isEmpty() ? "" : ", static attributes " + this.staticAttributes));
    }
    // 合并模型
    Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
    // 如果当前请求为下载的话，预先处理请求头
    prepareResponse(request, response);
    // 为客户端返回视图
    renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}

这里我们以InternalResourceView为例看看一下具体的返回过程：

protected void renderMergedOutputModel(
			Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

    // Expose the model object as request attributes.
    // 曝光模型
    exposeModelAsRequestAttributes(model, request);

    // Expose helpers as request attributes, if any.
    // 空的模板方法 //todo
    exposeHelpers(request);

    // Determine the path for the request dispatcher.
    // 获取转发路径
    String dispatcherPath = prepareForRendering(request, response);

    // Obtain a RequestDispatcher for the target resource (typically a JSP).
    // 获取可应用于 forward/include 的RequestDispatcher
    RequestDispatcher rd = getRequestDispatcher(request, dispatcherPath);
    if (rd == null) {
        throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +
                "]: Check that the corresponding file exists within your web application archive!");
    }

    // 处理include
    // If already included or response already committed, perform include, else forward.
    if (useInclude(request, response)) {
        response.setContentType(getContentType());
        if (logger.isDebugEnabled()) {
            logger.debug("Including [" + getUrl() + "]");
        }
        rd.include(request, response);
    }

    // 处理转发
    else {
        // Note: The forwarded resource is supposed to determine the content type itself.
        if (logger.isDebugEnabled()) {
            logger.debug("Forwarding to [" + getUrl() + "]");
        }
        rd.forward(request, response);
    }
}

对于返回的普通的视图，如JSP等，最后还是调用的RequestDispatcher.forward方法进行转发而已。

4.总结

到这里除了一些后续处理，一次完成的SpringMVC请求工作就完成了。当然这里只分析了普通的ModelAndView对象，大家可以多结合自己的实例，多多分析。