精盡Spring MVC原始碼分析 - ViewResolver 元件

月圓吖發表於2020-12-23

該系列文件是本人在學習 Spring MVC 的原始碼過程中總結下來的,可能對讀者不太友好,請結合我的原始碼註釋 Spring MVC 原始碼分析 GitHub 地址 進行閱讀

Spring 版本:5.2.4.RELEASE

該系列其他文件請檢視:《精盡 Spring MVC 原始碼分析 - 文章導讀》

ViewResolver 元件

ViewResolver 元件,檢視解析器,根據檢視名和國際化,獲得最終的檢視 View 物件

回顧

先來回顧一下在 DispatcherServlet 中處理請求的過程中哪裡使用到 ViewResolver 元件,可以回到《一個請求的旅行過程》中的 DispatcherServletrender 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 中獲得 Locale 物件,並設定到 response 中
    Locale locale = (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
    response.setLocale(locale);

    // 獲得 View 物件
    View view;
    String viewName = mv.getViewName();
    // 情況一,使用 viewName 獲得 View 物件
    if (viewName != null) {
        // We need to resolve the view name.
        // <2.1> 使用 viewName 獲得 View 物件
        view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
        if (view == null) { // 獲取不到,丟擲 ServletException 異常
            throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
                    "' in servlet with name '" + getServletName() + "'");
        }
    }
    // 情況二,直接使用 ModelAndView 物件的 View 物件
    else {
        // No need to lookup: the ModelAndView object contains the actual View object.
        // 直接使用 ModelAndView 物件的 View 物件
        view = mv.getView();
        if (view == null) { // 獲取不到,丟擲 ServletException 異常
            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.
    // 列印日誌
    if (logger.isTraceEnabled()) {
        logger.trace("Rendering view [" + view + "] ");
    }
    try {
        // <3> 設定響應的狀態碼
        if (mv.getStatus() != null) {
            response.setStatus(mv.getStatus().value());
        }
        // <4> 渲染頁面
        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) {
        // 遍歷 ViewResolver 陣列
        for (ViewResolver viewResolver : this.viewResolvers) {
            // 根據 viewName + locale 引數,解析出 View 物件
            View view = viewResolver.resolveViewName(viewName, locale);
            // 解析成功,直接返回 View 物件
            if (view != null) {
                return view;
            }
        }
    }
    return null;
}

如果 ModelAndView 物件不為null,且需要進行頁面渲染,則呼叫 render 方法,如果設定的 View 物件是 String 型別,也就是 viewName,則需要呼叫 resolveViewName 方法,通過 ViewResolver 根據 viewNamelocale 解析出對應的 View 物件

這是前後端未分離的情況下重要的一個元件

ViewResolver 介面

org.springframework.web.servlet.ViewResolver,檢視解析器,根據檢視名和國際化,獲得最終的檢視 View 物件,程式碼如下:

public interface ViewResolver {
	/**
	 * 根據檢視名和國際化,獲得最終的 View 物件
	 */
	@Nullable
	View resolveViewName(String viewName, Locale locale) throws Exception;
}

ViewResolver 介面體系的結構如下:

精盡Spring MVC原始碼分析 - ViewResolver 元件

ViewResolver 的實現類比較多,其中 Spring MVC 預設使用 org.springframework.web.servlet.view.InternalResourceViewResolver 這個實現類

Spring Boot 中的預設實現類如下:

精盡Spring MVC原始碼分析 - ViewResolver 元件

可以看到有三個實現類:

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

  • org.springframework.web.servlet.view.ViewResolverComposite,預設沒有實現類

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

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

初始化過程

DispatcherServletinitViewResolvers(ApplicationContext context) 方法,初始化 ViewResolver 元件,方法如下:

private void initViewResolvers(ApplicationContext context) {
    // 置空 viewResolvers 處理
    this.viewResolvers = null;

    // 情況一,自動掃描 ViewResolver 型別的 Bean 們
    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);
        }
    }
    // 情況二,獲得名字為 VIEW_RESOLVER_BEAN_NAME 的 Bean 們
    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.
    /**
     * 情況三,如果未獲得到,則獲得預設配置的 ViewResolver 類
     * {@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“開啟”探測功能,則掃描已註冊的 ViewResolver 的 Bean 們,新增到 viewResolvers 中,預設開啟

  2. 如果“關閉”探測功能,則獲得 Bean 名稱為 "viewResolver" 對應的 Bean ,將其新增至 viewResolvers

  3. 如果未獲得到,則獲得預設配置的 ViewResolver 類,呼叫 getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) 方法,就是從 DispatcherServlet.properties 檔案中讀取 ViewResolver 的預設實現類,如下:

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

在 Spring Boot 不是通過這樣初始化的,感興趣的可以去看看

ContentNegotiatingViewResolver

org.springframework.web.servlet.view.ContentNegotiatingViewResolver,實現 ViewResolver、Ordered、InitializingBean 介面,繼承 WebApplicationObjectSupport 抽象類,基於內容型別來獲取對應 View 的 ViewResolver 實現類。其中,內容型別指的是 Content-Type 和擴充字尾

構造方法

public class ContentNegotiatingViewResolver extends WebApplicationObjectSupport
		implements ViewResolver, Ordered, InitializingBean {

	@Nullable
	private ContentNegotiationManager contentNegotiationManager;
	/**
	 * ContentNegotiationManager 的工廠,用於建立 {@link #contentNegotiationManager} 物件
	 */
	private final ContentNegotiationManagerFactoryBean cnmFactoryBean = new ContentNegotiationManagerFactoryBean();
	/**
	 * 在找不到 View 物件時,返回 {@link #NOT_ACCEPTABLE_VIEW}
	 */
	private boolean useNotAcceptableStatusCode = false;
	/**
	 * 預設 View 陣列
	 */
	@Nullable
	private List<View> defaultViews;
	/**
	 * ViewResolver 陣列
	 */
	@Nullable
	private List<ViewResolver> viewResolvers;
	/**
	 * 順序,優先順序最高
	 */
	private int order = Ordered.HIGHEST_PRECEDENCE;
}
  • viewResolvers:ViewResolver 陣列。對於來說,ContentNegotiatingViewResolver 會使用這些 ViewResolver們,解析出所有的 View 們,然後基於內容型別,來獲取對應的 View 們。此時的 View 結果,可能是一個,可能是多個,所以需要比較獲取到最優的 View 物件。
  • defaultViews:預設 View 陣列。那麼此處的預設是什麼意思呢?在 viewResolvers 們解析出所有的 View 們的基礎上,也會新增 defaultViews 到 View 結果中
  • order:順序,優先順序最高。所以,這也是為什麼它排在最前面

在上圖中可以看到,在 Spring Boot 中 viewResolvers 屬性有三個實現類,分別是 BeanNameViewResolverViewResolverCompositeInternalResourceViewResolver

initServletContext

實現 initServletContext(ServletContext servletContext) 方法,初始化 viewResolvers 屬性,方法如下:

在父類 WebApplicationObjectSupport 的父類 ApplicationObjectSupport 中可以看到,因為實現了 ApplicationContextAware 介面,則在初始化該 Bean 的時候會呼叫 setApplicationContext(@Nullable ApplicationContext context) 方法,在這個方法中會呼叫 initApplicationContext(ApplicationContext context) 這個方法,這個方法又會呼叫initServletContext(ServletContext servletContext) 方法

@Override
protected void initServletContext(ServletContext servletContext) {
    // <1> 掃描所有 ViewResolver 的 Bean 們
    Collection<ViewResolver> matchingBeans = BeanFactoryUtils.
        beansOfTypeIncludingAncestors(obtainApplicationContext(), ViewResolver.class).values();
    // <1.1> 情況一,如果 viewResolvers 為空,則將 matchingBeans 作為 viewResolvers 。
    // BeanNameViewResolver、ThymeleafViewResolver、ViewResolverComposite、InternalResourceViewResolver
    if (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> 情況二,如果 viewResolvers 非空,則和 matchingBeans 進行比對,判斷哪些未進行初始化,進行初始化
    else {
        for (int i = 0; i < this.viewResolvers.size(); i++) {
            ViewResolver vr = this.viewResolvers.get(i);
            // 已存在在 matchingBeans 中,說明已經初始化,則直接 continue
            if (matchingBeans.contains(vr)) {
                continue;
            }
            // 不存在在 matchingBeans 中,說明還未初始化,則進行初始化
            String name = vr.getClass().getName() + i;
            obtainApplicationContext().getAutowireCapableBeanFactory().initializeBean(vr, name);
        }

    }
    // <1.3> 排序 viewResolvers 陣列
    AnnotationAwareOrderComparator.sort(this.viewResolvers);
    // <2> 設定 cnmFactoryBean 的 servletContext 屬性
    this.cnmFactoryBean.setServletContext(servletContext);
}
  1. 掃描所有 ViewResolver 的 Bean 們 matchingBeans

    1. 情況一,如果 viewResolvers 為空,則將 matchingBeans 作為 viewResolvers
    2. 情況二,如果 viewResolvers 非空,則和 matchingBeans 進行比對,判斷哪些未進行初始化,進行初始化
    3. 排序 viewResolvers 陣列
  2. 設定 cnmFactoryBeanservletContext 屬性為當前 Servlet 上下文

afterPropertiesSet

因為 ContentNegotiatingViewResolver 實現了 InitializingBean 介面,在 Sping 初始化該 Bean 的時候,會呼叫該方法,完成一些初始化工作,方法如下:

@Override
public void afterPropertiesSet() {
    // 如果 contentNegotiationManager 為空,則進行建立
    if (this.contentNegotiationManager == null) {
        this.contentNegotiationManager = this.cnmFactoryBean.build();
    }
    if (this.viewResolvers == null || this.viewResolvers.isEmpty()) {
        logger.warn("No ViewResolvers configured");
    }
}

resolveViewName

實現 resolveViewName(String viewName, Locale locale) 方法,程式碼如下:

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws Exception {
    RequestAttributes attrs = RequestContextHolder.getRequestAttributes();
    Assert.state(attrs instanceof ServletRequestAttributes, "No current ServletRequestAttributes");
    // <1> 獲得 MediaType 陣列
    List<MediaType> requestedMediaTypes = getMediaTypes(((ServletRequestAttributes) attrs).getRequest());
    if (requestedMediaTypes != null) {
        // <2> 獲得匹配的 View 陣列
        List<View> candidateViews = getCandidateViews(viewName, locale, requestedMediaTypes);
        // <3> 篩選最匹配的 View 物件
        View bestView = getBestView(candidateViews, requestedMediaTypes, attrs);
        // 如果篩選成功,則返回
        if (bestView != null) {
            return bestView;
        }
    }

    String mediaTypeInfo = logger.isDebugEnabled() && requestedMediaTypes != null ? " given " + requestedMediaTypes.toString() : "";

    // <4> 如果匹配不到 View 物件,則根據 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. 呼叫 getMediaTypes(HttpServletRequest request) 方法,獲得 MediaType 陣列,詳情見下文

  2. 呼叫 getCandidateViews(String viewName, Locale locale, List<MediaType> requestedMediaTypes) 方法,獲得匹配的 View 陣列,詳情見下文

  3. 呼叫 getBestView(List<View> candidateViews, List<MediaType> requestedMediaTypes, RequestAttributes attrs) 方法,篩選出最匹配的 View 物件,如果篩選成功則直接返回,詳情見下文

  4. 如果匹配不到 View 物件,則根據 useNotAcceptableStatusCode,返回 NOT_ACCEPTABLE_VIEWnull,其中NOT_ACCEPTABLE_VIEW 變數,程式碼如下:

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

    這個 View 物件設定狀態碼為 406

getMediaTypes

getCandidateViews(HttpServletRequest request)方法,獲得 MediaType 陣列,如下:

@Nullable
protected List<MediaType> getMediaTypes(HttpServletRequest request) {
    Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");
    try {
        // 建立 ServletWebRequest 物件
        ServletWebRequest webRequest = new ServletWebRequest(request);
        // 從請求中,獲得可接受的 MediaType 陣列。預設實現是,從請求頭 ACCEPT 中獲取
        List<MediaType> acceptableMediaTypes = this.contentNegotiationManager.resolveMediaTypes(webRequest);
        // 獲得可產生的 MediaType 陣列
        List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request);
        // 通過 acceptableTypes 來比對,將符合的 producibleType 新增到 mediaTypesToUse 結果陣列中
        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,獲得匹配的 View 陣列,如下:

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

    // 建立 View 陣列
    List<View> candidateViews = new ArrayList<>();
    // <1> 來源一,通過 viewResolvers 解析出 View 陣列結果,新增到 candidateViews 中
    if (this.viewResolvers != null) {
        Assert.state(this.contentNegotiationManager != null, "No ContentNegotiationManager set");
        // <1.1> 遍歷 viewResolvers 陣列
        for (ViewResolver viewResolver : this.viewResolvers) {
            // <1.2> 情況一,獲得 View 物件,新增到 candidateViews 中
            View view = viewResolver.resolveViewName(viewName, locale);
            if (view != null) {
                candidateViews.add(view);
            }
            // <1.3> 情況二,帶有擴充字尾的方式,獲得 View 物件,新增到 candidateViews 中
            for (MediaType requestedMediaType : requestedMediaTypes) {
                // <1.3.2> 獲得 MediaType 對應的擴充字尾的陣列(預設情況下未配置)
                List<String> extensions = this.contentNegotiationManager.resolveFileExtensions(requestedMediaType);
                // <1.3.3> 遍歷擴充字尾的陣列
                for (String extension : extensions) {
                    // <1.3.4> 帶有擴充字尾的方式,獲得 View 物件,新增到 candidateViews 中
                    String viewNameWithExtension = viewName + '.' + extension;
                    view = viewResolver.resolveViewName(viewNameWithExtension, locale);
                    if (view != null) {
                        candidateViews.add(view);
                    }
                }
            }
        }
    }
    // <2> 來源二,新增 defaultViews 到 candidateViews 中
    if (!CollectionUtils.isEmpty(this.defaultViews)) {
        candidateViews.addAll(this.defaultViews);
    }
    return candidateViews;
}
  1. 來源一,通過 viewResolvers 解析出 View 陣列結果,新增到 List<View> candidateViews

    1. 遍歷 viewResolvers 陣列
    2. 情況一,通過當前 ViewResolver 實現類獲得 View 物件,新增到 candidateViews
    3. 情況二,遍歷入參 List<MediaType> requestedMediaTypes,將帶有擴充字尾的型別再通過當前 ViewResolver 實現類獲得 View 物件,新增到 candidateViews
      2. 獲得 MediaType 對應的擴充字尾的陣列(預設情況下未配置)
      3. 遍歷擴充字尾的陣列
      4. 帶有擴充字尾的方式,通過當前 ViewResolver 實現類獲得 View 物件,新增到 candidateViews
  2. 來源二,新增 defaultViewscandidateViews

getBestView

getBestView(List<View> candidateViews, List<MediaType> requestedMediaTypes, RequestAttributes attrs)方法,篩選出最匹配的 View 物件,如下:

@Nullable
private View getBestView(List<View> candidateViews, List<MediaType> requestedMediaTypes, RequestAttributes attrs) {
    // <1> 遍歷 candidateView 陣列,如果有重定向的 View 型別,則返回它
    for (View candidateView : candidateViews) {
        if (candidateView instanceof SmartView) {
            SmartView smartView = (SmartView) candidateView;
            if (smartView.isRedirectView()) {
                return candidateView;
            }
        }
    }
    // <2> 遍歷 MediaType 陣列(MediaTy陣列已經根據pespecificity、quality進行了排序)
    for (MediaType mediaType : requestedMediaTypes) {
        // <2> 遍歷 View 陣列
        for (View candidateView : candidateViews) {
            if (StringUtils.hasText(candidateView.getContentType())) {
                // <2.1> 如果 MediaType 型別匹配,則返回該 View 物件
                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. 遍歷 candidateView 陣列,如果有重定向的 View 型別,則返回它。也就是說,重定向的 View ,優先順序更高。
  2. 遍歷 MediaType 陣列(MediaTy陣列已經根據pespecificityquality進行了排序)和 candidateView 陣列
    1. 如果 MediaType 型別匹配該 View 物件,則返回該 View 物件。也就是說,優先順序的匹配規則,由 ViewResolver 在 viewResolvers 的位置,越靠前,優先順序越高。

BeanNameViewResolver

org.springframework.web.servlet.view.BeanNameViewResolver,實現 ViewResolver、Ordered 介面,繼承 WebApplicationObjectSupport 抽象類,基於 Bean 的名字獲得 View 物件的 ViewResolver 實現類

構造方法

public class BeanNameViewResolver extends WebApplicationObjectSupport implements ViewResolver, Ordered {
    /**
	 * 順序,優先順序最低
	 */
	private int order = Ordered.LOWEST_PRECEDENCE;  // default: same as non-Ordered
}

resolveViewName

實現 resolveViewName(String viewName, Locale locale) 方法,根據名稱獲取 View 型別對應的 Bean(View 物件),如下:

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws BeansException {
    ApplicationContext context = obtainApplicationContext();
    // 如果對應的 Bean 物件不存在,則返回 null
    if (!context.containsBean(viewName)) {
        // Allow for ViewResolver chaining...
        return null;
    }
    // 如果 Bean 對應的 Bean 型別不是 View ,則返回 null
    if (!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;
    }
    // 獲得 Bean 名字對應的 View 物件
    return context.getBean(viewName, View.class);
}

ViewResolverComposite

org.springframework.web.servlet.view.ViewResolverComposite,實現 ViewResolver、Ordered、InitializingBean、ApplicationContextAware、ServletContextAware 介面,複合的 ViewResolver 實現類

構造方法

public class ViewResolverComposite implements ViewResolver, Ordered, InitializingBean,
		ApplicationContextAware, ServletContextAware {
	/**
	 * ViewResolver 陣列
	 */
	private final List<ViewResolver> viewResolvers = new ArrayList<>();

	/**
	 * 順序,優先順序最低
	 */
	private int order = Ordered.LOWEST_PRECEDENCE;
}

afterPropertiesSet

因為 ViewResolverComposite 實現了 InitializingBean 介面,在 Sping 初始化該 Bean 的時候,會呼叫該方法,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

實現 resolveViewName(String viewName, Locale locale) 方法,程式碼如下:

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws Exception {
    // 遍歷 viewResolvers 陣列,逐個進行解析,但凡成功,則返回該 View 物件
    for (ViewResolver viewResolver : this.viewResolvers) {
        // 執行解析
        View view = viewResolver.resolveViewName(viewName, locale);
        // 解析成功,則返回該 View 物件
        if (view != null) {
            return view;
        }
    }
    return null;
}

AbstractCachingViewResolver

org.springframework.web.servlet.view.AbstractCachingViewResolver,實現 ViewResolver 介面,繼承 WebApplicationObjectSupport 抽象類,提供通用的快取的 ViewResolver 抽象類。對於相同的檢視名,返回的是相同的 View 物件,所以通過快取,可以進一步提供效能。

構造方法

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
		@Nullable
		public String getContentType() {
			return null;
		}
		@Override
		public 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; // 快取上限。如果 cacheLimit = 0 ,表示禁用快取

	/** Whether we should refrain from resolving views again if unresolved once. */
	private boolean cacheUnresolved = true; // 是否快取空 View 物件

	/** 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 的快取的對映

	/** Map from view key to View instance, synchronized for View creation. */
	// View 的快取的對映。相比 {@link #viewAccessCache} 來說,增加了 synchronized 鎖
	@SuppressWarnings("serial")
	private final Map<Object, View> viewCreationCache =
			new LinkedHashMap<Object, View>(DEFAULT_CACHE_LIMIT, 0.75f, true) {
				@Override
				protected boolean removeEldestEntry(Map.Entry<Object, View> eldest) {
					if (size() > getCacheLimit()) {
						viewAccessCache.remove(eldest.getKey());
						return true;
					}
					else {
						return false;
					}
				}
			};
}

通過 viewAccessCache 屬性,提供更快的訪問 View 快取

通過 viewCreationCache 屬性,提供快取的上限的功能

KEY 是通過 getCacheKey(String viewName, Locale locale) 方法,獲得快取 KEY,方法如下:

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

resolveViewName

實現 resolveViewName(String viewName, Locale locale) 方法,程式碼如下:

@Override
@Nullable
public View resolveViewName(String viewName, Locale locale) throws Exception {
    // 如果禁用快取,則建立 viewName 對應的 View 物件
    if (!isCache()) {
        return createView(viewName, locale);
    }
    else {
        // 獲得快取 KEY
        Object cacheKey = getCacheKey(viewName, locale);
        // 從 viewAccessCache 快取中,獲得 View 物件
        View view = this.viewAccessCache.get(cacheKey);
        // 如果獲得不到快取,則從 viewCreationCache 中,獲得 View 物件
        if (view == null) {
            synchronized (this.viewCreationCache) {
                // 從 viewCreationCache 中,獲得 View 物件
                view = this.viewCreationCache.get(cacheKey);
                if (view == null) {
                    // Ask the subclass to create the View object.
                    // 建立 viewName 對應的 View 物件
                    view = createView(viewName, locale);
                    // 如果建立失敗,但是 cacheUnresolved 為 true ,則設定為 UNRESOLVED_VIEW
                    if (view == null && this.cacheUnresolved) {
                        view = UNRESOLVED_VIEW;
                    }
                    // 如果 view 非空,則新增到 viewAccessCache 快取中
                    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;

邏輯比較簡單,主要是快取的處理,需要通過子類去建立對應的 View 物件

UrlBasedViewResolver

org.springframework.web.servlet.view.UrlBasedViewResolver,實現 Ordered 介面,繼承 AbstractCachingViewResolver 抽象類,基於 Url 的 ViewResolver 實現類

構造方法

public class UrlBasedViewResolver extends AbstractCachingViewResolver implements Ordered {

	public static final String REDIRECT_URL_PREFIX = "redirect:";

	public static final String FORWARD_URL_PREFIX = "forward:";

	/**
	 * View 的型別,不同的實現類,會對應一個 View 的型別
	 */
	@Nullable
	private Class<?> viewClass;
	/**
	 * 字首
	 */
	private String prefix = "";
	/**
	 * 字尾
	 */
	private String suffix = "";
	/**
	 * ContentType 型別
	 */
	@Nullable
	private String contentType;

	private boolean redirectContextRelative = true;

	private boolean redirectHttp10Compatible = true;

	@Nullable
	private String[] redirectHosts;
	/**
	 * RequestAttributes 暴露給 View 使用時的屬性
	 */
	@Nullable
	private String requestContextAttribute;

	/** Map of static attributes, keyed by attribute name (String). */
	private final Map<String, Object> staticAttributes = new HashMap<>();
	/**
	 * 是否暴露路徑變數給 View 使用
	 */
	@Nullable
	private Boolean exposePathVariables;

	@Nullable
	private Boolean exposeContextBeansAsAttributes;

	@Nullable
	private String[] exposedContextBeanNames;
	/**
	 * 是否只處理指定的檢視名們
	 */
	@Nullable
	private String[] viewNames;
	/**
	 * 順序,優先順序最低
	 */
	private int order = Ordered.LOWEST_PRECEDENCE;
}

initApplicationContext

實現 initApplicationContext() 方法,進一步初始化,程式碼如下:

在父類 WebApplicationObjectSupport 的父類 ApplicationObjectSupport 中可以看到,因為實現了 ApplicationContextAware 介面,則在初始化該 Bean 的時候會呼叫 setApplicationContext(@Nullable ApplicationContext context) 方法,在這個方法中會呼叫 initApplicationContext(ApplicationContext context) 這個方法,這個方法又會呼叫initApplicationContext() 方法

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

在子類中會看到 viewClass 屬性一般會在構造中法中設定

getCacheKey

重寫 getCacheKey(String viewName, Locale locale) 方法,忽略 locale 引數,僅僅使用 viewName 作為快取 KEY,如下:

@Override
protected Object getCacheKey(String viewName, Locale locale) {
    // 重寫了父類的方法,去除locale直接返回viewName
    return viewName;
}

也就是說,不支援 Locale 特性

canHandle

canHandle(String viewName, Locale locale) 方法,判斷傳入的檢視名是否可以被處理,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;
}

一般情況下,viewNames 指定的檢視名們為空,所以會滿足 viewNames == null 程式碼塊。也就說,所有檢視名都可以被處理

applyLifecycleMethods

applyLifecycleMethods(String viewName, AbstractUrlBasedView view) 方法,程式碼如下:

protected View applyLifecycleMethods(String viewName, AbstractUrlBasedView view) {
    // 情況一,如果 viewName 有對應的 View Bean 物件,則使用它
    ApplicationContext context = getApplicationContext();
    if (context != null) {
        Object initialized = context.getAutowireCapableBeanFactory().initializeBean(view, viewName);
        if (initialized instanceof View) {
            return (View) initialized;
        }
    }
    // 情況二,直接返回 view
    return view;
}

createView

重寫 createView(String viewName, Locale locale) 方法,增加了對 REDIRECT、FORWARD 的情況的處理,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.
    // 是否能處理該檢視名稱
    if (!canHandle(viewName, locale)) {
        return null;
    }

    // Check for special "redirect:" prefix.
    if (viewName.startsWith(REDIRECT_URL_PREFIX)) { // 如果是 REDIRECT 開頭,建立 RedirectView 檢視
        String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
        RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
        String[] hosts = getRedirectHosts();
        if (hosts != null) {
            // 設定 RedirectView 物件的 hosts 屬性
            view.setHosts(hosts);
        }
        // 應用
        return applyLifecycleMethods(REDIRECT_URL_PREFIX, view);
    }

    // Check for special "forward:" prefix.
    if (viewName.startsWith(FORWARD_URL_PREFIX)) { // 如果是 FORWARD 開頭,建立 InternalResourceView 檢視
        String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
        InternalResourceView view = new InternalResourceView(forwardUrl);
        // 應用
        return applyLifecycleMethods(FORWARD_URL_PREFIX, view);
    }

    // Else fall back to superclass implementation: calling loadView.
    // 建立檢視名對應的 View 物件
    return super.createView(viewName, locale);
}

loadView

實現 loadView(String viewName, Locale locale) 方法,載入 viewName 對應的 View 物件,方法如下:

@Override
protected View loadView(String viewName, Locale locale) throws Exception {
    // <x> 建立 viewName 對應的 View 物件
    AbstractUrlBasedView view = buildView(viewName);
    // 應用
    View result = applyLifecycleMethods(viewName, view);
    return (view.checkResource(locale) ? result : null);
}

其中,<x> 處,呼叫 buildView(String viewName) 方法,建立 viewName 對應的 View 物件,方法如下:

protected AbstractUrlBasedView buildView(String viewName) throws Exception {
    Class<?> viewClass = getViewClass();
    Assert.state(viewClass != null, "No view class");

    // 建立 AbstractUrlBasedView 物件
    AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(viewClass);

    // 設定各種屬性
    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,定義了產生的檢視,程式碼如下:

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

InternalResourceViewResolver

org.springframework.web.servlet.view.InternalResourceViewResolver,繼承 UrlBasedViewResolver 類,解析出 JSP 的 ViewResolver 實現類

構造方法

public class InternalResourceViewResolver extends UrlBasedViewResolver {

	/**
	 * 判斷 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());

	@Nullable
	private Boolean alwaysInclude;

	public InternalResourceViewResolver() {
		// 獲得 viewClass
		Class<?> viewClass = requiredViewClass();
		if (InternalResourceView.class == viewClass && jstlPresent) {
			viewClass = JstlView.class;
		}
		// 設定 viewClass
		setViewClass(viewClass);
	}
}

從構造方法中,可以看出,檢視名會是 InternalResourceView 或 JstlView 類。? 實際上,JstlView 是 InternalResourceView 的子類。

buildView

重寫 buildView(String viewName) 方法,程式碼如下:

@Override
protected AbstractUrlBasedView buildView(String viewName) throws Exception {
    // 呼叫父方法
    InternalResourceView view = (InternalResourceView) super.buildView(viewName);
    if (this.alwaysInclude != null) {
        view.setAlwaysInclude(this.alwaysInclude);
    }
    // 設定 View 物件的相關屬性
    view.setPreventDispatchLoop(true);
    return view;
}

設定兩個屬性

View

org.springframework.web.servlet.View,Spring MVC 中的檢視物件,用於檢視渲染,程式碼如下:

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";

	@Nullable
	default String getContentType() {
		return null;
	}

	/**
	 * 渲染檢視
	 */
	void render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response)
			throws Exception;
}

View 介面體系的結構如下:

精盡Spring MVC原始碼分析 - ViewResolver 元件

可以看到 View 的實現類非常多,本文不會詳細分析,簡單講解兩個方法

在 DispatcherServlet 中會直接呼叫 View 的 render(@Nullable Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) 來進行渲染頁面

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

   // 合併返回結果,將 Model 中的靜態資料和請求中的動態資料進行合併
   Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
   // 進行一些準備工作(修復 IE 中存在的 BUG)
   prepareResponse(request, response);
   // 進行渲染
   renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}
  1. 將 Model 物件與請求中的資料進行合併,生成一個 Map 物件,儲存進入頁面的一些資料

  2. 進行一些準備工作(修復 IE 中存在的 BUG)

  3. 呼叫 renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) 方法,頁面渲染,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.
       // 往請求中設定一些屬性,Locale、TimeZone、LocalizationContext
       exposeHelpers(request);
    
       // Determine the path for the request dispatcher.
       // 獲取需要轉發的路徑
       String dispatcherPath = prepareForRendering(request, response);
    
       // Obtain a RequestDispatcher for the target resource (typically a JSP).
       // 獲取請求轉發器
       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() + "]");
          }
          // 最後進行轉發
          rd.forward(request, response);
       }
    }
    

是不是很熟悉??

通過 Servlet 的 javax.servlet.RequestDispatcher 請求派發著,轉到對應的 URL

總結

本文分析了 ViewResolver 元件,檢視解析器,根據檢視名和國際化,獲得最終的檢視 View 物件。Spring MVC 執行完處理器後生成一個 ModelAndView 物件,如果該物件不為 null 並且有對應的 viewName,那麼就需要通過 ViewResolver 根據 viewName 解析出對應的 View 物件。

在 Spring MVC 和 Spring Boot 中最主要的還是 InternalResourceViewResolver 實現類,例如這麼配置:

<bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
    <!-- 自動給後面 action 的方法 return 的字串加上字首和字尾,變成一個可用的地址 -->
    <property name="prefix" value="/WEB-INF/jsp/" />
    <property name="suffix" value=".jsp" />
</bean>

當返回的檢視名稱為 login 時,View 物件的 url 就是 /WEB-INF/jsp/login.jsp,呼叫 View 的 render 方法進行頁面渲染時,請求會轉發到這個 url

當然,還有其他的 ViewResolver 實現類,例如 BeanNameViewResolver,目前大多數都是前後端分離的專案,這個元件也許你很少用到

至此,《精盡 Spring MVC 原始碼分析》系列最後一篇文件已經講述完了,筆者寫的過程中也是懵懵懂懂的狀態,寫完之後才更加的理解,對於 Spring MVC 中大部分的內容都有分析到,你會發現 Spring MVC 原來是這麼回事,開心~ ??? 其中涉及到 Spring 相關內容在努力閱讀中,敬請期待~

希望這系列文件能夠幫助你對 Spring MVC 的理解,路漫漫其修遠兮~

參考文章:芋道原始碼《精盡 Spring MVC 原始碼分析》

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