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spring mvc允许注册多个viewResolver。spring mvc通过循环调用viewResolver解析View。View不为空则直接返回。相应代码在DispatcherServlet中

protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
			HttpServletRequest request) throws Exception {
 
		for (ViewResolver viewResolver : this.viewResolvers) {
			View view = viewResolver.resolveViewName(viewName, locale);
			if (view != null) {
				return view;
			}
		}
		return null;
	}

viewResolvers是List集合。由于List是有序的，所以viewResolvers的顺序很重要。使用spring boot自动配置viewResolver的话，ContentNegotiatingViewResolver将会是第一个元素。相应代码在WebMvcAutoConfigurationAdapter中

@Bean
		@ConditionalOnBean(ViewResolver.class)
		@ConditionalOnMissingBean(name = "viewResolver", value = ContentNegotiatingViewResolver.class)
		public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {
			ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();
			resolver.setContentNegotiationManager(
					beanFactory.getBean(ContentNegotiationManager.class));
			// ContentNegotiatingViewResolver uses all the other view resolvers to locate
			// a view so it should have a high precedence
			resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);
			return resolver;
		}

见注释，ContentNegotiatingViewResolver使用所有其他的view resolvers定位view，所以它有最高的优先级