Spring MVC 源码- ViewResolver 组件

ViewResolver 组件

ViewResolver 组件，视图解析器，根据视图名和国际化，获得最终的视图 View 对象

回顾

先来回顾一下在 DispatcherServlet 中处理请求的过程中哪里使用到 ViewResolver 组件，可以回到《一个请求响应的旅行过程》中的 DispatcherServlet 的 render 方法中看看，如下：

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {// Determine locale for request and apply it to the response.// <1> 解析 request 中获得 Locale 对象，并设置到 response 中Locale locale = (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());response.setLocale(locale);// 获得 View 对象View view;String viewName = mv.getViewName();// 情况一，使用 viewName 获得 View 对象if (viewName != null) {// We need to resolve the view name.// <2.1> 使用 viewName 获得 View 对象view = resolveViewName(viewName, mv.getModelInternal(), locale, request);if (view == null) { // 获取不到，抛出 ServletException 异常throw new ServletException("Could not resolve view with name '" + mv.getViewName() +"' in servlet with name '" + getServletName() + "'");}}// 情况二，直接使用 ModelAndView 对象的 View 对象else {// No need to lookup: the ModelAndView object contains the actual View object.// 直接使用 ModelAndView 对象的 View 对象view = mv.getView();if (view == null) { // 获取不到，抛出 ServletException 异常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.// 打印日志if (logger.isTraceEnabled()) {logger.trace("Rendering view [" + view + "] ");}try {// <3> 设置响应的状态码if (mv.getStatus() != null) {response.setStatus(mv.getStatus().value());}// <4> 渲染页面view.render(mv.getModelInternal(), request, response);}catch (Exception ex) {if (logger.isDebugEnabled()) {logger.debug("Error rendering view [" + view + "]", ex);}throw ex;}
}@Nullable
protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,Locale locale, HttpServletRequest request) throws Exception {if (this.viewResolvers != null) {// 遍历 ViewResolver 数组for (ViewResolver viewResolver : this.viewResolvers) {// 根据 viewName + locale 参数，解析出 View 对象View view = viewResolver.resolveViewName(viewName, locale);// 解析成功，直接返回 View 对象if (view != null) {return view;}}}return null;
}

如果 ModelAndView 对象不为null，且需要进行页面渲染，则调用 render 方法，如果设置的 View 对象是 String 类型，也就是 viewName，则需要调用 resolveViewName 方法，通过 ViewResolver 根据 viewName 和 locale 解析出对应的 View 对象

这是前后端未分离的情况下重要的一个组件

ViewResolver 接口

org.springframework.web.servlet.ViewResolver，视图解析器，根据视图名和国际化，获得最终的视图 View 对象，代码如下：

public interface ViewResolver {/*** 根据视图名和国际化，获得最终的 View 对象*/@NullableView resolveViewName(String viewName, Locale locale) throws Exception;
}

ViewResolver 接口体系的结构如下：

ViewResolver 的实现类比较多，其中 Spring MVC 默认使用 org.springframework.web.servlet.view.InternalResourceViewResolver 这个实现类

Spring Boot 中的默认实现类如下：

可以看到有三个实现类：

• org.springframework.web.servlet.view.ContentNegotiatingViewResolver

• org.springframework.web.servlet.view.ViewResolverComposite，默认没有实现类

• org.springframework.web.servlet.view.BeanNameViewResolver

• org.springframework.web.servlet.view.InternalResourceViewResolver

初始化过程

在 DispatcherServlet 的 initViewResolvers(ApplicationContext context) 方法，初始化 ViewResolver 组件，方法如下：

private void initViewResolvers(ApplicationContext context) {// 置空 viewResolvers 处理this.viewResolvers = null;// 情况一，自动扫描 ViewResolver 类型的 Bean 们if (this.detectAllViewResolvers) {// Find all ViewResolvers in the ApplicationContext, including ancestor contexts.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.AnnotationAwareOrderComparator.sort(this.viewResolvers);}}// 情况二，获得名字为 VIEW_RESOLVER_BEAN_NAME 的 Bean 们else {try {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./*** 情况三，如果未获得到，则获得默认配置的 ViewResolver 类* {@link org.springframework.web.servlet.view.InternalResourceViewResolver}*/if (this.viewResolvers == null) {this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);if (logger.isTraceEnabled()) {logger.trace("No ViewResolvers declared for servlet '" + getServletName() +"': using default strategies from DispatcherServlet.properties");}}
}

1. 如果“开启”探测功能，则扫描已注册的 ViewResolver 的 Bean 们，添加到 viewResolvers 中，默认开启

2. 如果“关闭”探测功能，则获得 Bean 名称为 "viewResolver" 对应的 Bean ，将其添加至 viewResolvers

3. 如果未获得到，则获得默认配置的 ViewResolver 类，调用 getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) 方法，就是从 DispatcherServlet.properties 文件中读取 ViewResolver 的默认实现类，如下：

org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver

在 Spring Boot 不是通过这样初始化的，感兴趣的可以去看看

ContentNegotiatingViewResolver

org.springframework.web.servlet.view.ContentNegotiatingViewResolver，实现 ViewResolver、Ordered、InitializingBean 接口，继承 WebApplicationObjectSupport 抽象类，基于内容类型来获取对应 View 的 ViewResolver 实现类。其中，内容类型指的是 Content-Type 和拓展后缀

构造方法

public class ContentNegotiatingViewResolver extends WebApplicationObjectSupportimplements ViewResolver, Ordered, InitializingBean {@Nullableprivate ContentNegotiationManager contentNegotiationManager;/*** ContentNegotiationManager 的工厂，用于创建 {@link #contentNegotiationManager} 对象*/private final ContentNegotiationManagerFactoryBean cnmFactoryBean = new ContentNegotiationManagerFactoryBean();/*** 在找不到 View 对象时，返回 {@link #NOT_ACCEPTABLE_VIEW}*/private boolean useNotAcceptableStatusCode = false;/*** 默认 View 数组*/@Nullableprivate List<View> defaultViews;/*** ViewResolver 数组*/@Nullableprivate List<ViewResolver> viewResolvers;/*** 顺序，优先级最高*/private int order = Ordered.HIGHEST_PRECEDENCE;
}

• viewResolvers：ViewResolver 数组。对于来说，ContentNegotiatingViewResolver 会使用这些 ViewResolver们，解析出所有的 View 们，然后基于内容类型，来获取对应的 View 们。此时的 View 结果，可能是一个，可能是多个，所以需要比较获取到最优的 View 对象。

• defaultViews：默认 View 数组。那么此处的默认是什么意思呢？在 viewResolvers 们解析出所有的 View 们的基础上，也会添加 defaultViews 到 View 结果中

• order：顺序，优先级最高。所以，这也是为什么它排在最前面

在上图中可以看到，在 Spring Boot 中 viewResolvers 属性有三个实现类，分别是 BeanNameViewResolver、ViewResolverComposite、InternalResourceViewResolver

initServletContext

实现 initServletContext(ServletContext servletContext) 方法，初始化 viewResolvers 属性，方法如下：

在父类 WebApplicationObjectSupport 的父类 ApplicationObjectSupport 中可以看到，因为实现了 ApplicationContextAware 接口，则在初始化该 Bean 的时候会调用 setApplicationContext(@Nullable ApplicationContext context) 方法，在这个方法中会调用 initApplicationContext(ApplicationContext context) 这个方法，这个方法又会调用initServletContext(ServletContext servletContext) 方法

@Override
protected void initServletContext(ServletContext servletContext) {// <1> 扫描所有 ViewResolver 的 Bean 们Collection<ViewResolver> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(obtainApplicationContext(), ViewResolver.class).values();// <1.1> 情况一，如果 viewResolvers 为空，则将 matchingBeans 作为 viewResolvers 。// BeanNameViewResolver、ThymeleafViewResolver、ViewResolverComposite、InternalResourceViewResolverif (this.viewResolvers == null) {this.viewResolvers = new ArrayList<>(matchingBeans.size());for (ViewResolver viewResolver : matchingBeans) {if (this != viewResolver) { // 排除自己this.viewResolvers.add(viewResolver);}}}// <1.2> 情况二，如果 viewResolvers 非空，则和 matchingBeans 进行比对，判断哪些未进行初始化，进行初始化else {for (int i = 0; i < this.viewResolvers.size(); i++) {ViewResolver vr = this.viewResolvers.get(i);// 已存在在 matchingBeans 中，说明已经初始化，则直接 continueif (matchingBeans.contains(vr)) {continue;}// 不存在在 matchingBeans 中，说明还未初始化，则进行初始化String name = vr.getClass().getName() + i;obtainApplicationContext().getAutowireCapableBeanFactory().initializeBean(vr, name);}}// <1.3> 排序 viewResolvers 数组AnnotationAwareOrderComparator.sort(this.viewResolvers);// <2> 设置 cnmFactoryBean 的 servletContext 属性this.cnmFactoryBean.setServletContext(servletContext);
}

1. 扫描所有 ViewResolver 的 Bean 们 matchingBeans

1. 情况一，如果 viewResolvers 为空，则将 matchingBeans 作为 viewResolvers

2. 情况二，如果 viewResolvers 非空，则和 matchingBeans 进行比对，判断哪些未进行初始化，进行初始化

3. 排序 viewResolvers 数组

2. 设置 cnmFactoryBean 的 servletContext 属性为当前 Servlet 上下文

afterPropertiesSet

因为 ContentNegotiatingViewResolver 实现了 InitializingBean 接口，在 Sping 初始化该 Bean 的时候，会调用该方法，完成一些初始化工作，方法如下：

@Override
public void afterPropertiesSet() {// 如果 contentNegotiationManager 为空，则进行创建if (this.contentNegotiationManager == null) {this.contentNegotiationManager = this.cnmFactoryBean.build();}if (this.viewResolvers == null || this.viewResolvers.isEmpty()) {logger.warn("No ViewResolvers configured");}
}

resolveViewName

实现 resolveViewName(String viewName, Locale locale) 方法，代码如下：

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws Exception {RequestAttributes attrs = RequestContextHolder.getRequestAttributes();Assert.state(attrs instanceof ServletRequestAttributes, "No current ServletRequestAttributes");// <1> 获得 MediaType 数组List<MediaType> requestedMediaTypes = getMediaTypes(((ServletRequestAttributes) attrs).getRequest());if (requestedMediaTypes != null) {// <2> 获得匹配的 View 数组List<View> candidateViews = getCandidateViews(viewName, locale, requestedMediaTypes);// <3> 筛选最匹配的 View 对象View bestView = getBestView(candidateViews, requestedMediaTypes, attrs);// 如果筛选成功，则返回if (bestView != null) {return bestView;}}String mediaTypeInfo = logger.isDebugEnabled() && requestedMediaTypes != null ? " given " + requestedMediaTypes.toString() : "";// <4> 如果匹配不到 View 对象，则根据 useNotAcceptableStatusCode ，返回 NOT_ACCEPTABLE_VIEW 或 null if (this.useNotAcceptableStatusCode) {if (logger.isDebugEnabled()) {logger.debug("Using 406 NOT_ACCEPTABLE" + mediaTypeInfo);}return NOT_ACCEPTABLE_VIEW;}else {logger.debug("View remains unresolved" + mediaTypeInfo);return null;}
}

1. 调用 getMediaTypes(HttpServletRequest request) 方法，获得 MediaType 数组，详情见下文

2. 调用 getCandidateViews(String viewName, Locale locale, List<MediaType> requestedMediaTypes) 方法，获得匹配的 View 数组，详情见下文

3. 调用 getBestView(List<View> candidateViews, List<MediaType> requestedMediaTypes, RequestAttributes attrs) 方法，筛选出最匹配的 View 对象，如果筛选成功则直接返回，详情见下文

4. 如果匹配不到 View 对象，则根据 useNotAcceptableStatusCode，返回 NOT_ACCEPTABLE_VIEW 或 null，其中NOT_ACCEPTABLE_VIEW 变量，代码如下：

private static final View NOT_ACCEPTABLE_VIEW = new View() {@Override@Nullablepublic String getContentType() {return null;}@Overridepublic void render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) {response.setStatus(HttpServletResponse.SC_NOT_ACCEPTABLE);}
};

这个 View 对象设置状态码为 406

getMediaTypes

getCandidateViews(HttpServletRequest request)方法，获得 MediaType 数组，如下：

@Nullable
protected List<MediaType> getMediaTypes(HttpServletRequest request) {Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");try {// 创建 ServletWebRequest 对象ServletWebRequest webRequest = new ServletWebRequest(request);// 从请求中，获得可接受的 MediaType 数组。默认实现是，从请求头 ACCEPT 中获取List<MediaType> acceptableMediaTypes = this.contentNegotiationManager.resolveMediaTypes(webRequest);// 获得可产生的 MediaType 数组List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request);// 通过 acceptableTypes 来比对，将符合的 producibleType 添加到 compatibleMediaTypes 结果中Set<MediaType> compatibleMediaTypes = new LinkedHashSet<>();for (MediaType acceptable : acceptableMediaTypes) {for (MediaType producible : producibleMediaTypes) {if (acceptable.isCompatibleWith(producible)) {compatibleMediaTypes.add(getMostSpecificMediaType(acceptable, producible));}}}// 按照 MediaType 的 specificity、quality 排序List<MediaType> selectedMediaTypes = new ArrayList<>(compatibleMediaTypes);MediaType.sortBySpecificityAndQuality(selectedMediaTypes);return selectedMediaTypes;}catch (HttpMediaTypeNotAcceptableException ex) {if (logger.isDebugEnabled()) {logger.debug(ex.getMessage());}return null;}
}

getCandidateViews

getCandidateViews(String viewName, Locale locale, List<MediaType> requestedMediaTypes)方法，获得匹配的 View 数组，如下：

private List<View> getCandidateViews(String viewName, Locale locale, List<MediaType> requestedMediaTypes)throws Exception {// 创建 View 数组List<View> candidateViews = new ArrayList<>();// <1> 来源一，通过 viewResolvers 解析出 View 数组结果，添加到 candidateViews 中if (this.viewResolvers != null) {Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");// <1.1> 遍历 viewResolvers 数组for (ViewResolver viewResolver : this.viewResolvers) {// <1.2> 情况一，获得 View 对象，添加到 candidateViews 中View view = viewResolver.resolveViewName(viewName, locale);if (view != null) {candidateViews.add(view);}// <1.3> 情况二，带有拓展后缀的方式，获得 View 对象，添加到 candidateViews 中for (MediaType requestedMediaType : requestedMediaTypes) {// <1.3.2> 获得 MediaType 对应的拓展后缀的数组（默认情况下未配置）List<String> extensions = this.contentNegotiationManager.resolveFileExtensions(requestedMediaType);// <1.3.3> 遍历拓展后缀的数组for (String extension : extensions) {// <1.3.4> 带有拓展后缀的方式，获得 View 对象，添加到 candidateViews 中String viewNameWithExtension = viewName + '.' + extension;view = viewResolver.resolveViewName(viewNameWithExtension, locale);if (view != null) {candidateViews.add(view);}}}}}// <2> 来源二，添加 defaultViews 到 candidateViews 中if (!CollectionUtils.isEmpty(this.defaultViews)) {candidateViews.addAll(this.defaultViews);}return candidateViews;
}

1. 来源一，通过 viewResolvers 解析出 View 数组结果，添加到 List<View> candidateViews 中

1. 遍历 viewResolvers 数组

2. 情况一，通过当前 ViewResolver 实现类获得 View 对象，添加到 candidateViews 中

3. 情况二，遍历入参 List<MediaType> requestedMediaTypes，将带有拓展后缀的类型再通过当前 ViewResolver 实现类获得 View 对象，添加到 candidateViews 中
   2. 获得 MediaType 对应的拓展后缀的数组（默认情况下未配置）
   3. 遍历拓展后缀的数组
   4. 带有拓展后缀的方式，通过当前 ViewResolver 实现类获得 View 对象，添加到 candidateViews 中

2. 来源二，添加 defaultViews 到 candidateViews 中

getBestView

getBestView(List<View> candidateViews, List<MediaType> requestedMediaTypes, RequestAttributes attrs)方法，筛选出最匹配的 View 对象，如下：

@Nullable
private View getBestView(List<View> candidateViews, List<MediaType> requestedMediaTypes, RequestAttributes attrs) {// <1> 遍历 candidateView 数组，如果有重定向的 View 类型，则返回它for (View candidateView : candidateViews) {if (candidateView instanceof SmartView) {SmartView smartView = (SmartView) candidateView;if (smartView.isRedirectView()) {return candidateView;}}}// <2> 遍历 MediaType 数组（MediaTy数组已经根据pespecificity、quality进行了排序）for (MediaType mediaType : requestedMediaTypes) {// <2> 遍历 View 数组for (View candidateView : candidateViews) {if (StringUtils.hasText(candidateView.getContentType())) {// <2.1> 如果 MediaType 类型匹配，则返回该 View 对象MediaType candidateContentType = MediaType.parseMediaType(candidateView.getContentType());if (mediaType.isCompatibleWith(candidateContentType)) {if (logger.isDebugEnabled()) {logger.debug("Selected '" + mediaType + "' given " + requestedMediaTypes);}attrs.setAttribute(View.SELECTED_CONTENT_TYPE, mediaType, RequestAttributes.SCOPE_REQUEST);return candidateView;}}}}return null;
}

1. 遍历 candidateView 数组，如果有重定向的 View 类型，则返回它。也就是说，重定向的 View ，优先级更高。

2. 遍历 MediaType 数组（MediaTy数组已经根据pespecificity、quality进行了排序）和 candidateView 数组

1. 如果 MediaType 类型匹配该 View 对象，则返回该 View 对象。也就是说，优先级的匹配规则，由 ViewResolver 在 viewResolvers 的位置，越靠前，优先级越高。

BeanNameViewResolver

org.springframework.web.servlet.view.BeanNameViewResolver，实现 ViewResolver、Ordered 接口，继承 WebApplicationObjectSupport 抽象类，基于 Bean 的名字获得 View 对象的 ViewResolver 实现类

构造方法

public class BeanNameViewResolver extends WebApplicationObjectSupport implements ViewResolver, Ordered {/*** 顺序，优先级最低*/private int order = Ordered.LOWEST_PRECEDENCE;  // default: same as non-Ordered
}

resolveViewName

实现 resolveViewName(String viewName, Locale locale) 方法，根据名称获取 View 类型对应的 Bean（View 对象），如下：

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws BeansException {ApplicationContext context = obtainApplicationContext();// 如果对应的 Bean 对象不存在，则返回 nullif (!context.containsBean(viewName)) {// Allow for ViewResolver chaining...return null;}// 如果 Bean 对应的 Bean 类型不是 View ，则返回 nullif (!context.isTypeMatch(viewName, View.class)) {if (logger.isDebugEnabled()) {logger.debug("Found bean named '" + viewName + "' but it does not implement View");}// Since we're looking into the general ApplicationContext here,// let's accept this as a non-match and allow for chaining as well...return null;}// 获得 Bean 名字对应的 View 对象return context.getBean(viewName, View.class);
}

ViewResolverComposite

org.springframework.web.servlet.view.ViewResolverComposite，实现 ViewResolver、Ordered、InitializingBean、ApplicationContextAware、ServletContextAware 接口，复合的 ViewResolver 实现类

构造方法

public class ViewResolverComposite implements ViewResolver, Ordered, InitializingBean,ApplicationContextAware, ServletContextAware {/*** ViewResolver 数组*/private final List<ViewResolver> viewResolvers = new ArrayList<>();/*** 顺序，优先级最低*/private int order = Ordered.LOWEST_PRECEDENCE;
}

afterPropertiesSet

因为 ViewResolverComposite 实现了 InitializingBean 接口，在 Sping 初始化该 Bean 的时候，会调用该方法，完成一些初始化工作，方法如下：

@Override
public void afterPropertiesSet() throws Exception {for (ViewResolver viewResolver : this.viewResolvers) {if (viewResolver instanceof InitializingBean) {((InitializingBean) viewResolver).afterPropertiesSet();}}
}

resolveViewName

实现 resolveViewName(String viewName, Locale locale) 方法，代码如下：

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws Exception {// 遍历 viewResolvers 数组，逐个进行解析，但凡成功，则返回该 View 对象for (ViewResolver viewResolver : this.viewResolvers) {// 执行解析View view = viewResolver.resolveViewName(viewName, locale);// 解析成功，则返回该 View 对象if (view != null) {return view;}}return null;
}

AbstractCachingViewResolver

org.springframework.web.servlet.view.AbstractCachingViewResolver，实现 ViewResolver 接口，继承 WebApplicationObjectSupport 抽象类，提供通用的缓存的 ViewResolver 抽象类。对于相同的视图名，返回的是相同的 View 对象，所以通过缓存，可以进一步提供性能。

构造方法

public abstract class AbstractCachingViewResolver extends WebApplicationObjectSupport implements ViewResolver {/** Default maximum number of entries for the view cache: 1024. */public static final int DEFAULT_CACHE_LIMIT = 1024;/** Dummy marker object for unresolved views in the cache Maps. */private static final View UNRESOLVED_VIEW = new View() {@Override@Nullablepublic String getContentType() {return null;}@Overridepublic void render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) {}};/** The maximum number of entries in the cache. */private volatile int cacheLimit = DEFAULT_CACHE_LIMIT; // 缓存上限。如果 cacheLimit = 0 ，表示禁用缓存/** Whether we should refrain from resolving views again if unresolved once. */private boolean cacheUnresolved = true; // 是否缓存空 View 对象/** Fast access cache for Views, returning already cached instances without a global lock. */private final Map<Object, View> viewAccessCache = new ConcurrentHashMap<>(DEFAULT_CACHE_LIMIT); // View 的缓存的映射/** Map from view key to View instance, synchronized for View creation. */// View 的缓存的映射。相比 {@link #viewAccessCache} 来说，增加了 synchronized 锁@SuppressWarnings("serial")private final Map<Object, View> viewCreationCache =new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) {@Overrideprotected boolean removeEldestEntry(Map.Entry<Object, View> eldest) {if (size() > getCacheLimit()) {viewAccessCache.remove(eldest.getKey());return true;}else {return false;}}};
}

通过 viewAccessCache 属性，提供更快的访问 View 缓存

通过 viewCreationCache 属性，提供缓存的上限的功能

KEY 是通过 getCacheKey(String viewName, Locale locale) 方法，获得缓存 KEY，方法如下：

protected Object getCacheKey(String viewName, Locale locale) {return viewName + '_' + locale;
}

resolveViewName

实现 resolveViewName(String viewName, Locale locale) 方法，代码如下：

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws Exception {// 如果禁用缓存，则创建 viewName 对应的 View 对象if (!isCache()) {return createView(viewName, locale);}else {// 获得缓存 KEYObject cacheKey = getCacheKey(viewName, locale);// 从 viewAccessCache 缓存中，获得 View 对象View view = this.viewAccessCache.get(cacheKey);// 如果获得不到缓存，则从 viewCreationCache 中，获得 View 对象if (view == null) {synchronized (this.viewCreationCache) {// 从 viewCreationCache 中，获得 View 对象view = this.viewCreationCache.get(cacheKey);if (view == null) {// Ask the subclass to create the View object.// 创建 viewName 对应的 View 对象view = createView(viewName, locale);// 如果创建失败，但是 cacheUnresolved 为 true ，则设置为 UNRESOLVED_VIEWif (view == null && this.cacheUnresolved) {view = UNRESOLVED_VIEW;}// 如果 view 非空，则添加到 viewAccessCache 缓存中if (view != null) {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);}
}@Nullable
protected View createView(String viewName, Locale locale) throws Exception {return loadView(viewName, locale);
}
@Nullable
protected abstract View loadView(String viewName, Locale locale) throws Exception;

逻辑比较简单，主要是缓存的处理，需要通过子类去创建对应的 View 对象

UrlBasedViewResolver

org.springframework.web.servlet.view.UrlBasedViewResolver，实现 Ordered 接口，继承 AbstractCachingViewResolver 抽象类，基于 Url 的 ViewResolver 实现类

构造方法

public class UrlBasedViewResolver extends AbstractCachingViewResolver implements Ordered {public static final String REDIRECT_URL_PREFIX = "redirect:";public static final String FORWARD_URL_PREFIX = "forward:";/*** View 的类型，不同的实现类，会对应一个 View 的类型*/@Nullableprivate Class<?> viewClass;/*** 前缀*/private String prefix = "";/*** 后缀*/private String suffix = "";/*** ContentType 类型*/@Nullableprivate String contentType;private boolean redirectContextRelative = true;private boolean redirectHttp10Compatible = true;@Nullableprivate String[] redirectHosts;/*** RequestAttributes 暴露给 View 使用时的属性*/@Nullableprivate String requestContextAttribute;/** Map of static attributes, keyed by attribute name (String). */private final Map<String, Object> staticAttributes = new HashMap<>();/*** 是否暴露路径变量给 View 使用*/@Nullableprivate Boolean exposePathVariables;@Nullableprivate Boolean exposeContextBeansAsAttributes;@Nullableprivate String[] exposedContextBeanNames;/*** 是否只处理指定的视图名们*/@Nullableprivate String[] viewNames;/*** 顺序，优先级最低*/private int order = Ordered.LOWEST_PRECEDENCE;
}

initApplicationContext

实现 initApplicationContext() 方法，进一步初始化，代码如下：

在父类 WebApplicationObjectSupport 的父类 ApplicationObjectSupport 中可以看到，因为实现了 ApplicationContextAware 接口，则在初始化该 Bean 的时候会调用 setApplicationContext(@Nullable ApplicationContext context) 方法，在这个方法中会调用 initApplicationContext(ApplicationContext context) 这个方法，这个方法又会调用initApplicationContext() 方法

@Override
protected void initApplicationContext() {super.initApplicationContext();if (getViewClass() == null) {throw new IllegalArgumentException("Property 'viewClass' is required");}
}

在子类中会看到 viewClass 属性一般会在构造方法中设置

getCacheKey

重写 getCacheKey(String viewName, Locale locale) 方法，忽略 locale 参数，仅仅使用 viewName 作为缓存 KEY，如下：

@Override
protected Object getCacheKey(String viewName, Locale locale) {// 重写了父类的方法，去除locale直接返回viewNamereturn viewName;
}

也就是说，不支持 Locale 特性

canHandle

canHandle(String viewName, Locale locale) 方法，判断传入的视图名是否可以被处理，如下：

protected boolean canHandle(String viewName, Locale locale) {String[] viewNames = getViewNames();return (viewNames == null || PatternMatchUtils.simpleMatch(viewNames, viewName));
}@Nullable
protected String[] getViewNames() {return this.viewNames;
}

一般情况下，viewNames 指定的视图名们为空，所以会满足 viewNames == null 代码块。也就说，所有视图名都可以被处理

applyLifecycleMethods

applyLifecycleMethods(String viewName, AbstractUrlBasedView view) 方法，代码如下：

protected View applyLifecycleMethods(String viewName, AbstractUrlBasedView view) {// 情况一，如果 viewName 有对应的 View Bean 对象，则使用它ApplicationContext context = getApplicationContext();if (context != null) {Object initialized = context.getAutowireCapableBeanFactory().initializeBean(view, viewName);if (initialized instanceof View) {return (View) initialized;}}// 情况二，直接返回 viewreturn view;
}

createView

重写 createView(String viewName, Locale locale) 方法，增加了对 REDIRECT、FORWARD 的情况的处理，如下：

@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.// 是否能处理该视图名称if (!canHandle(viewName, locale)) {return null;}// Check for special "redirect:" prefix.if (viewName.startsWith(REDIRECT_URL_PREFIX)) { // 如果是 REDIRECT 开头，创建 RedirectView 视图String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());String[] hosts = getRedirectHosts();if (hosts != null) {// 设置 RedirectView 对象的 hosts 属性view.setHosts(hosts);}// 应用return applyLifecycleMethods(REDIRECT_URL_PREFIX, view);}// Check for special "forward:" prefix.if (viewName.startsWith(FORWARD_URL_PREFIX)) { // 如果是 FORWARD 开头，创建 InternalResourceView 视图String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());InternalResourceView view = new InternalResourceView(forwardUrl);// 应用return applyLifecycleMethods(FORWARD_URL_PREFIX, view);}// Else fall back to superclass implementation: calling loadView.// 创建视图名对应的 View 对象return super.createView(viewName, locale);
}

loadView

实现 loadView(String viewName, Locale locale) 方法，加载 viewName 对应的 View 对象，方法如下：

@Override
protected View loadView(String viewName, Locale locale) throws Exception {// <x> 创建 viewName 对应的 View 对象AbstractUrlBasedView view = buildView(viewName);// 应用View result = applyLifecycleMethods(viewName, view);return (view.checkResource(locale) ? result : null);
}

其中，<x> 处，调用 buildView(String viewName) 方法，创建 viewName 对应的 View 对象，方法如下：

protected AbstractUrlBasedView buildView(String viewName) throws Exception {Class<?> viewClass = getViewClass();Assert.state(viewClass != null, "No view class");// 创建 AbstractUrlBasedView 对象AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(viewClass);// 设置各种属性view.setUrl(getPrefix() + viewName + getSuffix());String contentType = getContentType();if (contentType != null) {view.setContentType(contentType);}view.setRequestContextAttribute(getRequestContextAttribute());view.setAttributesMap(getAttributesMap());Boolean exposePathVariables = getExposePathVariables();if (exposePathVariables != null) {view.setExposePathVariables(exposePathVariables);}Boolean exposeContextBeansAsAttributes = getExposeContextBeansAsAttributes();if (exposeContextBeansAsAttributes != null) {view.setExposeContextBeansAsAttributes(exposeContextBeansAsAttributes);}String[] exposedContextBeanNames = getExposedContextBeanNames();if (exposedContextBeanNames != null) {view.setExposedContextBeanNames(exposedContextBeanNames);}return view;
}

requiredViewClass

requiredViewClass() 方法，定义了产生的视图，代码如下：

protected Class<?> requiredViewClass() {return AbstractUrlBasedView.class;
}

InternalResourceViewResolver

org.springframework.web.servlet.view.InternalResourceViewResolver，继承 UrlBasedViewResolver 类，解析出 JSP 的 ViewResolver 实现类

构造方法

public class InternalResourceViewResolver extends UrlBasedViewResolver {/*** 判断 javax.servlet.jsp.jstl.core.Config 是否存在*/private static final boolean jstlPresent = ClassUtils.isPresent("javax.servlet.jsp.jstl.core.Config", InternalResourceViewResolver.class.getClassLoader());@Nullableprivate Boolean alwaysInclude;public InternalResourceViewResolver() {// 获得 viewClassClass<?> viewClass = requiredViewClass();if (InternalResourceView.class == viewClass && jstlPresent) {viewClass = JstlView.class;}// 设置 viewClasssetViewClass(viewClass);}
}

从构造方法中，可以看出，视图名会是 InternalResourceView 或 JstlView 类。实际上，JstlView 是 InternalResourceView 的子类。

buildView

重写 buildView(String viewName) 方法，代码如下：

@Override
protected AbstractUrlBasedView buildView(String viewName) throws Exception {// 调用父方法InternalResourceView view = (InternalResourceView) super.buildView(viewName);if (this.alwaysInclude != null) {view.setAlwaysInclude(this.alwaysInclude);}// 设置 View 对象的相关属性view.setPreventDispatchLoop(true);return view;
}

设置两个属性

View

org.springframework.web.servlet.View，Spring MVC 中的视图对象，用于视图渲染，代码如下：

public interface View {String RESPONSE_STATUS_ATTRIBUTE = View.class.getName() + ".responseStatus";String PATH_VARIABLES = View.class.getName() + ".pathVariables";String SELECTED_CONTENT_TYPE = View.class.getName() + ".selectedContentType";@Nullabledefault String getContentType() {return null;}/*** 渲染视图*/void render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)throws Exception;
}

View 接口体系的结构如下：

可以看到 View 的实现类非常多，本文不会详细分析，简单讲解两个方法

在 DispatcherServlet 中会直接调用 View 的 render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) 来进行渲染页面

// AbstractView.java
@Override
public void render(@Nullable Map<String, ?> model, HttpServletRequest request,HttpServletResponse response) throws Exception {// 合并返回结果，将 Model 中的静态数据和请求中的动态数据进行合并Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);// 进行一些准备工作（修复 IE 中存在的 BUG）兼容性处理prepareResponse(request, response);// 进行渲染renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}

1. 将 Model 对象与请求中的数据进行合并，生成一个 Map 对象，保存进入页面的一些数据

2. 进行一些准备工作（修复 IE 中存在的 BUG）

3. 调用 renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) 方法，页面渲染，如下：

// InternalResourceView.java
@Override
protected void renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {// Expose the model object as request attributes.exposeModelAsRequestAttributes(model, request);// Expose helpers as request attributes, if any.// 往请求中设置一些属性，Locale、TimeZone、LocalizationContextexposeHelpers(request);// Determine the path for the request dispatcher.// 获取需要转发的路径String dispatcherPath = prepareForRendering(request, response);// Obtain a RequestDispatcher for the target resource (typically a JSP).// 获取请求转发器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!");}// 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);} else {// Note: The forwarded resource is supposed to determine the content type itself.if (logger.isDebugEnabled()) {logger.debug("Forwarding to [" + getUrl() + "]");}// 最后进行转发rd.forward(request, response);}
}

是不是很熟悉？

通过 Servlet 的 javax.servlet.RequestDispatcher 请求派发着，转到对应的 URL

总结

本文分析了 ViewResolver 组件，视图解析器，根据视图名和国际化，获得最终的视图 View 对象。Spring MVC 执行完处理器后生成一个 ModelAndView 对象，如果该对象不为 null 并且有对应的 viewName，那么就需要通过 ViewResolver 根据 viewName 解析出对应的 View 对象。

在 Spring MVC 和 Spring Boot 中最主要的还是 InternalResourceViewResolver 实现类，例如这么配置：

<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver"><!-- 自动给后面 action 的方法 return 的字符串加上前缀和后缀，变成一个可用的地址 --><property name="prefix" value="/WEB-INF/jsp/" /><property name="suffix" value=".jsp" />
</bean>

当返回的视图名称为 login 时，View 对象的 url 就是 /WEB-INF/jsp/login.jsp，调用 View 的 render 方法进行页面渲染时，请求会转发到这个 url

当然，还有其他的 ViewResolver 实现类，例如 BeanNameViewResolver，目前大多数都是前后端分离的项目，这个组件也许你很少用到

至此，
《Spring MVC 源码分析》
系列最后一篇文档已经讲述完了，对于 Spring MVC 中大部分的内容都有分析到，你会发现 Spring MVC 原来是这么回事， 其中涉及到 Spring 思想相关内容在努力阅读中，敬请期待~
希望这系列文档能够帮助你对 Spring MVC 有进一步的理解，路漫漫其修远兮~