概述

本章开始进入另一重要的组件，即视图组件，Spring MVC处理视图组件使用两个主要的接口是ViewResolver和View。根据名称可知，ViewResolver即视图解析器，其作用是把逻辑视图名称解析为真正的视图，然后通过View对象进行渲染。View接口的作用是用于处理视图进行渲染。

延用之前的介绍流程，本章分两部分进行阐述：启动初始化和请求处理。

本系列文章是基于Spring5.0.5RELEASE。

ViewResolver初始化

Spring MVC初始化视图解析器策略与初始化其他策略一样，其入口是DispatcherSerlvet的initStrategies(context)方法，代码如下：

/**
 *初始化策略对象
 */
protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    // 初始化视图解析器ViewResolver
    initViewResolvers(context);
    initFlashMapManager(context);
}

看过之前文章的可以了解，此方法是Spring MVC初始化策略组件的入口。针对视图解析器组件，调用initViewResolver(context)方法，加载视图处理策略，该方法源码如下：

private void initViewResolvers(ApplicationContext context) {
    // viewResolvers是视图解析器集合，接收到用户请求时，从该属性变量中获取到Spring MVC使用的视图解析器
    this.viewResolvers = null;
    // 是否从Spring上下文中加载ViewResolver，detectAllViewResolvers属性变量默认为true，可在web部署描述文件中修改
    if (this.detectAllViewResolvers) {
        // Find all ViewResolvers in the ApplicationContext, including ancestor contexts.
        // 按接口类型ViewResolver查找全部
        // key为bean的id(name)，value为bean的class对象
        Map<String, ViewResolver> matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.viewResolvers = new ArrayList<>(matchingBeans.values());
            // We keep ViewResolvers in sorted order.
            // 对ViewResolver进行排序，通过Ordered接口实现
            AnnotationAwareOrderComparator.sort(this.viewResolvers);
        }
    }
    else { 
        try {
            // 从Spring上下文中加载指定名字为"viewResolver"的bean，作为视图解析器
            ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
            this.viewResolvers = Collections.singletonList(vr);
        }
        catch (NoSuchBeanDefinitionException ex) {
            // Ignore, we'll add a default ViewResolver later.
        }
    }

    // Ensure we have at least one ViewResolver, by registering
    // a default ViewResolver if no other resolvers are found.
    // 为了确保至少有一个ViewResolver视图解析器，Spring MVC配置了默认的ViewResolver
    // 在DispatcherServlet.properties文件中定义，默认为InternalResourceViewResolver
    if (this.viewResolvers == null) {
        this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);
        if (logger.isDebugEnabled()) {
            logger.debug("No ViewResolvers found in servlet '" + getServletName() + "': using default");
        }
    }
}

至此，Spring MVC即完成加载初始化ViewResolver视图解析器，即Spring MVC以具备解析处理逻辑视图名称的能力。

ViewResolver使用

ViewResolver的使用是指用户发起请求到Spring，Spring MVC经过HandlerMapping查找处理的handler，然后通过HandlerAdapter进行适配后处理用户请求，返回ModelAndView，最后使用ViewResolver对ModelAndView进行解析，即把逻辑视图名解析为真正的视图对象，由视图对象进行渲染的过程。

用户的请求处理流程由DispatcherServlet的doDispatcher(request,response)方法进行控制，主要代码如下：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    
    ... ...
    // 找到请求对应的handler
    mappedHandler = getHandler(processedRequest);
    ... ...
    // 找到对应handler的适配器
    HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
    ... ...
    // 执行用户请求的拦截器前置处理方法
    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
        return;
    }
    // 调用实际的handler处理方法
    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    ... ...
    // 设置默认的逻辑视图名称
    // 如果handler返回null或者ViewName属性为null时，Spring进行取视图规则为：前缀+url+后缀
    // 比如访问url为:http://localhost:8086/test，前缀配置/WEB-INF/jsp，后缀配置为.jsp，那么最终查找的是：/WEB-INF/jsp/test.jsp
    applyDefaultViewName(processedRequest, mv);
    // 执行用户请求的拦截器后置处理方法
    mappedHandler.applyPostHandle(processedRequest, response, mv);
    ... ...
    // 
    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}

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

    boolean errorView = false;
    // 异常处理
    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?
    // 调用render方法
    if (mv != null && !mv.wasCleared()) {
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
    else {
        if (logger.isDebugEnabled()) {
            logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                    "': assuming HandlerAdapter completed request handling");
        }
    }

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

    if (mappedHandler != null) {
        mappedHandler.triggerAfterCompletion(request, response, null);
    }
}

/*
 *渲染ModelAndView
 */
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;
    // 获取逻辑视图名称
    String viewName = mv.getViewName();
    if (viewName != null) {
        // We need to resolve the view name.
        // 根据逻辑视图名解析视图，返回View对象
        // 此处是视图解析器使用入口，见下面的方法
        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() + "'");
        }
    }
    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());
        }
        // 调用View的render方法进行视图渲染
        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;
    }
}

@Nullable
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) {
            // 调用ViewResovler接口入口方法
            View view = viewResolver.resolveViewName(viewName, locale);
            if (view != null) {
                return view;
            }
        }
    }
    return null;
}

总结

本章概述了Spring MVC加载视图解析器策略，并且找到了ViewResolver的入口方法，下面的章节继续学习ViewResolver接口的实现类。

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