SpringFramework之ViewResolver优化

Springboot版本是2.x

Springboot中，对mvc进行自动化配置时在WebMvcAutoConfiguration中会自动注入InternalResourceViewResolver(渲染jsp的)、BeanNameViewResolver、ContentNegotiatingViewResolver。

但是我们一般是直接返回json或者使用thymeleaf，所以InternalResourceViewResolver和BeanNameViewResolver是用不到的，但是springmvc的render中这几个都会被调用到，所以如果我们将InternalResourceViewResolver和BeanNameViewResolver去掉，不加入到mvc的ViewResolver集合中那么就可以优化渲染的过程。

来看ContentNegotiatingViewResolver#resolveViewName，如下List-1

List-1

public View resolveViewName(String viewName, Locale locale) throws Exception {
    RequestAttributes attrs = RequestContextHolder.getRequestAttributes();
    Assert.state(attrs instanceof ServletRequestAttributes, "No current ServletRequestAttributes");
    List<MediaType> requestedMediaTypes = getMediaTypes(((ServletRequestAttributes) attrs).getRequest());
    if (requestedMediaTypes != null) {
        List<View> candidateViews = getCandidateViews(viewName, locale, requestedMediaTypes);
        View bestView = getBestView(candidateViews, requestedMediaTypes, attrs);
        if (bestView != null) {
            return bestView;
        }
    }
    ...
}

...

private List<View> getCandidateViews(String viewName, Locale locale, List<MediaType> requestedMediaTypes)
        throws Exception {

    List<View> candidateViews = new ArrayList<>();
    if (this.viewResolvers != null) {
        Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");
        for (ViewResolver viewResolver : this.viewResolvers) {
            View view = viewResolver.resolveViewName(viewName, locale);
            if (view != null) {
                candidateViews.add(view);
            }
            for (MediaType requestedMediaType : requestedMediaTypes) {
                List<String> extensions = this.contentNegotiationManager.resolveFileExtensions(requestedMediaType);
                for (String extension : extensions) {
                    String viewNameWithExtension = viewName + '.' + extension;
                    view = viewResolver.resolveViewName(viewNameWithExtension, locale);
                    if (view != null) {
                        candidateViews.add(view);
                    }
                }
            }
        }
    }
    if (!CollectionUtils.isEmpty(this.defaultViews)) {
        candidateViews.addAll(this.defaultViews);
    }
    return candidateViews;
}

List-1中resolveViewName方法中调用getCandidateViews获取候选View集合，getCandidateViews方法中循坏遍历viewResolvers，逐个拿到对应的View然后加入到结果集合中，我们可以debug下这个viewResolvers的值有InternalResourceViewResolver、BeanNameViewResolver、ThymeleafViewResolver、ContentNegotiatingViewResolver，所以默认情况下执行了很多无用的代码。

我们怎么将viewResolvers的集合中将InternalResourceViewResolver、BeanNameViewResolver去掉呢？

ContentNegotiatingViewResolver是WebMvcAutoConfiguration中自动注入进去的，是单例bean，那么我们可以在spring完成这个bean的创建、初始化等后来修改里面属性viewResolvers的值。

我们需要知道spring上下文什么时候启动完成，回到AbstractApplicationContext#refresh()，上下文启动完成后调用finishRefresh()，进而调用publishEvent(new ContextRefreshedEvent(this))，即发布ContextRefreshedEvent事件，所以我们可以通过监听这个事件然后在spring上下文启动完成后对ContentNegotiatingViewResolver这个bean进行修改。

创建ApplicationListener来监听ContextRefreshedEvent事件，如下List-2

List-2

@Component
public class ExclusiveViewResolverApplicationListener implements ApplicationListener<ContextRefreshedEvent> {
    private static final String EXCLUSIVE_VIEW_RESOLVER="web-support.exclusive-view-resolver";
    private static final Class<ViewResolver> VIEW_RESOLVER_CLASS = ViewResolver.class;
    private final String[] exclusiveViewResolverBeanNameList;

    public ExclusiveViewResolverApplicationListener(@Value("${" + EXCLUSIVE_VIEW_RESOLVER+ "}") String exclusiveViewResolverBeanName) {
        this.exclusiveViewResolverBeanNameList = exclusiveViewResolverBeanName.split(",");
    }

    @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        ApplicationContext applicationContext = event.getApplicationContext();
        this.configureExclusiveViewResolver(applicationContext);
    }

    private void configureExclusiveViewResolver(ApplicationContext applicationContext) {
        Map<String, ViewResolver> viewResolversMap = BeanFactoryUtils
                .beansOfTypeIncludingAncestors(applicationContext, VIEW_RESOLVER_CLASS);
        int size = viewResolversMap.size();
        if (size >= 2) {
            for (String exclusiveViewResolverName: exclusiveViewResolverBeanNameList) {
                ViewResolver exclusiveViewResolver = viewResolversMap.remove(exclusiveViewResolverName);
                if (exclusiveViewResolver == null) {
                    throw new NoSuchBeanDefinitionException(VIEW_RESOLVER_CLASS,exclusiveViewResolverName);
                }
            }
            List<ViewResolver> viewResolvers = new ArrayList<>(viewResolversMap.values());
            //要把ContentNegotiatingViewResolver排除,不然会递归调用导致栈溢出
            viewResolvers.removeIf(viewResolver -> viewResolver instanceof ContentNegotiatingViewResolver);
            ContentNegotiatingViewResolver contentNegotiatingViewResolver = applicationContext.getBean(ContentNegotiatingViewResolver.class);
            contentNegotiatingViewResolver.setViewResolvers(viewResolvers);
        }
    }
}

然后在application.properties中配置，如List-3，这里的defaultViewResolver和beanNameViewResolver是固定的，不能修改，因为这是InternalResourceViewResolver和BeanNameViewResolver的bean name——怎么知道的呢，从WebMvcAutoConfiguration中知道，这个bean name是方法名称。

List-3

web-support.exclusive-view-resolver=defaultViewResolver,beanNameViewResolver

List-2中

收到spring上下文启动完成的事件后，从ApplicationContext中获取所有ViewResolver实例，移除我们配置的defaultViewResolver和beanNameViewResolver
从applicationContext中获取ContentNegotiatingViewResolver实例，之后将过滤后的viewResolver设置给其属性

这样，回到List-1中，去获取候选View的时候，遍历的viewResolver就只有ThymeleafViewResolver了。