Spring3.2 MVC 分析

jopen 10年前發布 | 29K 次閱讀 Spring3 Spring MVC Web框架

Spring3.2 MVC 分析：

SpringMVC現在應該用得很廣泛了，其配置清晰，靈活度，定制能力等都是很強的，相比Struts2也是勝過一籌，還是從源碼來分析一下，SpringMVC為我們做了什么。

先從配置文件開始，看web.xml，用過SpringMVC的同學應該都很熟悉：

<web-app>
    <listener>
    <listener-class>
        org.springframework.web.context.ContextLoaderListener<!-- Spring根容器在這里被建立起來 -->
    </listener-class>
    </listener>
    <context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath*:configs/beans.xml</param-value>
    </context-param>
    <!-- SpringMVC子容器會在DispatherServlet初始化過程中被建立起來 -->
    <servlet>
        <servlet-name>example</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>example</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

</web-app>

看看SpringMVC官方的請求處理流程圖，DispacherServlet就是那個FrontController, 根據用戶請求的url，匹配到我們自定義的控制器，然后進行業務處理，處理完成，將數據model返回給FrontController，然后它將數據＋視圖模版(如jsp, freemarker等)進行渲染，然后返回到用戶瀏覽器：

那到底要做些什么工作，才能達到上面這樣的流程呢，往下說。

看看DispatcherServlet怎么來的：

先從HttpServletBean類入手，人類都知道Servlet被Servlet容器創建時，會調用一次其init方法，我們就看看HttpServletBean的init方法，其主要工作就是將Servlet配置參數映射到我們的Servlet Bean里面，然后再初始化子類(initServletBean):

@Override
public final void init() throws ServletException {
    // Set bean properties from init parameters.
    PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
    BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
    ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
    bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
    initBeanWrapper(bw);
    bw.setPropertyValues(pvs, true);

    // Let subclasses do whatever initialization they like.
    initServletBean();
    ...
}

接著看initServletBean方法，該方法由FrameworkServlet, 從其名稱就知道，它時Spring Web框架的基礎Servlet, Spring這樣描述它的工作：

＊ Manages a WebApplicationContext instance per servlet. The servlet's configuration is determined by beans      in the servlet's namespace.
＊ Publishes events on request processing, whether or not a request is successfully handled.

它如何實現initServletBean, 它做的事很單一，就是初始化WebApplicationContext對象，initFrameworkServlet是個空方法，留給子類去實現：

protected final void initServletBean() throws ServletException {
    ...     
    this.webApplicationContext = initWebApplicationContext();
    initFrameworkServlet();
    ...     
}

看看FrameworkServlet怎么初始化WebApplicationContext的，其實類似于初始化IoC容器(如之前Spring Ioc文章講XmlClasspathApplicationContext):

protected WebApplicationContext initWebApplicationContext() {                                                                   // 通過ServletContext對象獲取到ContextLoaderListener建立的根容器，它到底怎么被建立起來的，后面再講
    WebApplicationContext rootContext =
        WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    WebApplicationContext wac = null;

    if (this.webApplicationContext != null) { //若通過構造參數傳遞了webApplicationContext
        // A context instance was injected at construction time -> use it
        wac = this.webApplicationContext;
        if (wac instanceof ConfigurableWebApplicationContext) {
            ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;                           if (!cwac.isActive()) {
                // The context has not yet been refreshed -> provide services such as
                // setting the parent context, setting the application context id, etc
                if (cwac.getParent() == null) {
                    // The context instance was injected without an explicit parent -> set
                    // the root application context (if any; may be null) as the parent
                    cwac.setParent(rootContext);
                }
                configureAndRefreshWebApplicationContext(cwac);
            }
        }
    }
    if (wac == null) {
        // 如果配置了contextAttribute屬性，通過其來獲取WebApplicationContext對象
        wac = findWebApplicationContext();
    }
    if (wac == null) { 
        // 這里創建FrameworkServlet的上下文實例，默認實現是XmlWebApplicationContext, 且以根上下文為父上下文
        wac = createWebApplicationContext(rootContext);
    }
    //觸發refresh事件, 會調用DispatcherServlet的onRefresh方法
    if (!this.refreshEventReceived) {
    }
    //默認會將FrameworkServlet對象的上下文對象存到ServletContext中
    if (this.publishContext) {
        // Publish the context as a servlet context attribute.
    String attrName = getServletContextAttributeName();
    getServletContext().setAttribute(attrName, wac);
    }
    return wac;
}

我們可以看看createWebApplicationContext()怎么建立其XmlWebApplicationContext上下文對象的：

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
    Class<?> contextClass = getContextClass(); //沒有配置，則默認XmlWebApplicationContext.class
    ConfigurableWebApplicationContext wac =
    (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
    wac.setEnvironment(getEnvironment());
    wac.setParent(parent); //將根上下文作為父上下文
    wac.setConfigLocation(getContextConfigLocation());//是否再xml中配置了contextConfigLocation屬性
    configureAndRefreshWebApplicationContext(wac); //如同之前文章將的Spring IoC就是初始化Ioc容器
    return wac;
}

那XmlWebApplicationContext的初始化與普通容器如XmlClassPathApplicationContext有什么不同呢？其實現是很簡潔的：

public class XmlWebApplicationContext extends AbstractRefreshableWebApplicationContext {
    public static final String DEFAULT_CONFIG_LOCATION = "/WEB-INF/applicationContext.xml";
    public static final String DEFAULT_CONFIG_LOCATION_PREFIX = "/WEB-INF/";
    public static final String DEFAULT_CONFIG_LOCATION_SUFFIX = ".xml";

    @Override //這個方法就是再AbstractApplicationContext中的模版方法refresh中調用，和一般IoC容器初始化一樣
    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        initBeanDefinitionReader(beanDefinitionReader);
        loadBeanDefinitions(beanDefinitionReader);
    }

    protected void initBeanDefinitionReader(XmlBeanDefinitionReader beanDefinitionReader) {
    }

    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws IOException {
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            for (String configLocation : configLocations) {
                reader.loadBeanDefinitions(configLocation);
            }
        }
    }

    @Override
    protected String[] getDefaultConfigLocations() {
        if (getNamespace() != null) { 
            return new String[] {DEFAULT_CONFIG_LOCATION_PREFIX + getNamespace() + DEFAULT_CONFIG_LOCATION_SUFFIX};//就是web.xml中配置的DispatcherServlet的servlet-name,對應我們就會有/WEB-INF/example-servlet.xml
        }
        else {
            return new String[] {DEFAULT_CONFIG_LOCATION};
        }
    }

}

簡單就分析完FrameworkServlet的上下文對象的構建過程，繼續看DispatcherServlet怎么構建自己的，就從FrameworkServlet中initWebApplicationContext的onRefresh中，對應DispatcherServlet中的OnRefresh方法：

@Override
protected void onRefresh(ApplicationContext context) {
    initStrategies(context);
}
protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context); //初始化multipartResolver bean, 用于文件上傳處理
    initLocaleResolver(context);    //初始化localeResolver bean, 用于解析用戶端Locale信息，默認實現AcceptHeaderLocaleResolver
    initThemeResolver(context); //初始化themeResolver bean, 默認實現FixedThemeResolver
    initHandlerMappings(context); //初始化handlerMappings集，將用戶請求映射到我們定義的Controller中
    initHandlerAdapters(context); //初始化handlerAdapters
    initHandlerExceptionResolvers(context); //初始化異常處理解析器
    initRequestToViewNameTranslator(context); //初始化請求到視圖的翻譯器
    initViewResolvers(context); //初始化視圖解析器，如jsp, freemarker等
    initFlashMapManager(context); //初始化FlashMap管理器，實現再不同請求中共享參數，如重定向。
}

上面這些組件都有默認實現，spring將默認實現放在和DispatcherServlet.java同包下的DispatcherServlet.properties中：

# Default implementation classes for DispatcherServlet's strategy interfaces.
# Used as fallback when no matching beans are found in the DispatcherServlet context.
# Not meant to be customized by application developers.

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver

org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
    org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
    org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
    org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
    org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
    org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator

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

org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

重點看看handlerMappdings怎么建立起來的：

private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;
    //默認會從父上下文遞歸地去獲取HandlerMapping bean
    if (this.detectAllHandlerMappings) {
        // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
        Map<String, HandlerMapping> matchingBeans =
            BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                OrderComparator.sort(this.handlerMappings);
            }
    }else { //僅從當前上下文獲取HandlerMapping bean
        HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
        this.handlerMappings = Collections.singletonList(hm);       
    }
    if (this.handlerMappings == null) { //若沒有配置，根據上面的屬性文件提供默認實現
        this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
    }
}

看看HandlerMapping接口的定義，十分簡潔，就一個getHandler接口：

public interface HandlerMapping {
   ...
   //根據當前request對象，獲取HandlerExecutionChain對象，其包括一個handler對象(我們定義的Controller)和一些定義的攔截器
   HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;
}

其繼承樹：

HandlerExecutionChain確實保存了handler和interceptors:

public class HandlerExecutionChain {

    private final Object handler;

    private HandlerInterceptor[] interceptors;

    private List<HandlerInterceptor> interceptorList;

    private int interceptorIndex = -1;
    ...
}

看一個HandlerMapping的簡單實現SimpleUrlHandlerMapping, 我們需要配置urlMap屬性，通過registerHandlers方法將urlMap的屬性注冊到父類的handlerMap中，其保存了url請求與handler bean的對應關系：

public class SimpleUrlHandlerMapping extends AbstractUrlHandlerMapping {
    private final Map<String, Object> urlMap = new HashMap<String, Object>();

    protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {
        if (urlMap.isEmpty()) {
            logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
        } else {
            for (Map.Entry<String, Object> entry : urlMap.entrySet()) {
            String url = entry.getKey();
            Object handler = entry.getValue();
            // Prepend with slash if not already present.
            if (!url.startsWith("/")) {
                url = "/" + url;
            }
            // Remove whitespace from handler bean name.
            if (handler instanceof String) {
                handler = ((String) handler).trim();
            }
            registerHandler(url, handler); //由父類實現
            }                                                                                                    ｝
}

繼續看AbstractUrlHandlerMapping怎么實現registerHandler()方法：

protected void registerHandler(String urlPath, Object handler) throws BeansException, IllegalStateException {
    Object resolvedHandler = handler;
    // Eagerly resolve handler if referencing singleton via name.
    if (!this.lazyInitHandlers && handler instanceof String) {//若為非懶加載且是單例bean，則從容器中獲取
        String handlerName = (String) handler;
        if (getApplicationContext().isSingleton(handlerName)) {
            resolvedHandler = getApplicationContext().getBean(handlerName);
        }
    }

    Object mappedHandler = this.handlerMap.get(urlPath);
    if (mappedHandler != null) { //一個url只能有一個對應的handler
        if (mappedHandler != resolvedHandler) {
            throw new IllegalStateException(
                "Cannot map " + getHandlerDescription(handler) + " to URL path [" + urlPath +
                "]: There is already " + getHandlerDescription(mappedHandler) + " mapped.");
        }
    } else {
        if (urlPath.equals("/")) {
            setRootHandler(resolvedHandler);
        } else if (urlPath.equals("/*")) {
            setDefaultHandler(resolvedHandler);
        } else {
            this.handlerMap.put(urlPath, resolvedHandler);
        }
    }
}

這上面就是HandlerMapping bean怎么被注冊起來的，下面是HandlerMapping Bean怎么完成請求的映射處理的。

看看AbstractHandlerMapping的getHandler(), 根據當前請求來獲取對應的HandlerExecuteChain:

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
    Object handler = getHandlerInternal(request); // 獲取handler對象
    if (handler == null) {
        handler = getDefaultHandler(); //默認Handler對象，就是上面注冊的"/"的handler對象
    }
    if (handler == null) {
        return null;
    }
    // Bean name or resolved handler?
    if (handler instanceof String) { //如果是bean name就從容器中獲取
        String handlerName = (String) handler;
        handler = getApplicationContext().getBean(handlerName);
    }
    return getHandlerExecutionChain(handler, request); //獲取handlerExecutionChain
}

接著看getHandlerInternal怎么獲得handler對象(AbstractUrlHanlderMapping):

protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
    String lookupPath = getUrlPathHelper().getLookupPathForRequest(request); //提取請求url
    Object handler = lookupHandler(lookupPath, request); //根據url查找handler
    if (handler == null) {
        // We need to care for the default handler directly, since we need to
        // expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
        Object rawHandler = null;
        if ("/".equals(lookupPath)) {
            rawHandler = getRootHandler();
        }
        if (rawHandler == null) {
            rawHandler = getDefaultHandler();
        }
        if (rawHandler != null) {
            // Bean name or resolved handler?
            if (rawHandler instanceof String) {
                String handlerName = (String) rawHandler;
                rawHandler = getApplicationContext().getBean(handlerName);
            }
            validateHandler(rawHandler, request);
            handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
        }
    }
    return handler;
}

由lookupHandler(url，request)查找handler對象：

protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
    // Direct match?
    Object handler = this.handlerMap.get(urlPath); //精確匹配獲取handler
    if (handler != null) {
        // Bean name or resolved handler?
        if (handler instanceof String) { 
            String handlerName = (String) handler;
            handler = getApplicationContext().getBean(handlerName);
        }
        validateHandler(handler, request); //驗證handler，由子類實現
        return buildPathExposingHandler(handler, urlPath, urlPath, null);
    }
    //正則匹配獲取handler
    List<String> matchingPatterns = new ArrayList<String>();
    for (String registeredPattern : this.handlerMap.keySet()) {
        if (getPathMatcher().match(registeredPattern, urlPath)) {
            matchingPatterns.add(registeredPattern);
        }
    }
    String bestPatternMatch = null;
    Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
    if (!matchingPatterns.isEmpty()) {
        Collections.sort(matchingPatterns, patternComparator);
        bestPatternMatch = matchingPatterns.get(0);
    }
    if (bestPatternMatch != null) {
        handler = this.handlerMap.get(bestPatternMatch);
        // Bean name or resolved handler?
        if (handler instanceof String) {
            String handlerName = (String) handler;
            handler = getApplicationContext().getBean(handlerName);
        }
        validateHandler(handler, request);
        String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestPatternMatch, urlPath);

        // There might be multiple 'best patterns', let's make sure we have the correct URI template variables
        // for all of them
        Map<String, String> uriTemplateVariables = new LinkedHashMap<String, String>();
        for (String matchingPattern : matchingPatterns) {
            if (patternComparator.compare(bestPatternMatch, matchingPattern) == 0) {
                Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
                Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
                uriTemplateVariables.putAll(decodedVars);
            }
        }
        return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);
    }
    // No handler found...
    return null;
}

上面就明白了根據請求獲取handler對象的過程，下面講講請求是怎么被DispatcherServlet分發處理的.

DispatcherServlet分發處理主要方法為doServlce方法開始：

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    // 如果request請求是include請求，則對請求屬性作一個快照
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
       attributesSnapshot = new HashMap<String, Object>();
       Enumeration<?> attrNames = request.getAttributeNames();
       while (attrNames.hasMoreElements()) {
          String attrName = (String) attrNames.nextElement();
          if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
              attributesSnapshot.put(attrName, request.getAttribute(attrName));
          }
       }
    }

    //設置一些涉及MVC框架的屬性，以方便handler或view等引用.
    request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
    //更新FlashMap屬性，該屬性是基于Session實現，主要是在不同request請求間傳遞參數，典型的就是Redirect這種情況
    FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
    if (inputFlashMap != null) {
        request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
    }
    request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
    request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

    try {
        doDispatch(request, response); //真正實現將請求進行轉發
    } finally {
        if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            return;
        }
        // Restore the original attribute snapshot, in case of an include.
        if (attributesSnapshot != null) {
            restoreAttributesAfterInclude(request, attributesSnapshot);
        }
    }
}

看doDispatch():

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    ModelAndView mv = null;
    Exception dispatchException = null;
    processedRequest = checkMultipart(request);//檢查request是否Multipart, 即是否上傳文件
    multipartRequestParsed = processedRequest != request;

    //獲取當前請求對應的HandlerExecutionChain對象
    mappedHandler = getHandler(processedRequest, false);
    if (mappedHandler == null || mappedHandler.getHandler() == null) {
         noHandlerFound(processedRequest, response);
         return;
    }
    // Determine handler adapter for the current request.
    HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

    // Process last-modified header, if supported by the handler.
    String method = request.getMethod();
    boolean isGet = "GET".equals(method);
    if (isGet || "HEAD".equals(method)) {
       long lastModified = ha.getLastModified(request, mappedHandler.getHandler());          
       if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
          return;
       }
    }
    if (!mappedHandler.applyPreHandle(processedRequest, response)) {//前置處理,調用chain的攔截器預處理方法preHandle
          return;
    }
    ...
    //調用handler處理方法，最終會調用Controller的handleRequest方法，并返回ModelAndView對象
    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    ...
    applyDefaultViewName(request, mv);//解析視圖名稱，就是我們配置的prefix + uri + suffix
    mappedHandler.applyPostHandle(processedRequest, response, mv);//后置處理,調用chain的攔截器postHandle方法
    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); //進行視圖渲染
    ...
}

接著看processDispatchResult()方法：

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
            HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
    boolean errorView = false;
    if (exception != null) { //如果有異常
        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?
    if (mv != null && !mv.wasCleared()) {
        render(mv, request, response); //渲染視圖的最后一個步驟
     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);
    }
}

看看render怎么工作的：

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.resolveLocale(request);
    response.setLocale(locale);

    View view;
    if (mv.isReference()) { // 若ModalAndView中保存了view的引用
        //根據我們配置的ViewReslover bean，解析視圖名稱
        view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request);
        if (view == null) {
            throw new ServletException(
                "Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" +
                                getServletName() + "'");
        }
    } else {  //ModalAndView中保存了實際的view對象
        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() + "'");
        }
    }
    view.render(mv.getModelInternal(), request, response); //渲染視圖
}

上面講述了DispatcherServlet的請求分發過程及視圖渲染大致流程。接下來得說說視圖渲染得話題，默認DispatcherServlet的默認視圖實是 InternalResourceViewResolver, 也就是jsp視圖，看看jsp視圖是怎么被呈現的。

先看AbstractView的render方法：

public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
    Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);//合并動態和靜態屬性
    prepareResponse(request, response);
    renderMergedOutputModel(mergedModel, request, response); //用模型渲染視圖
}

看看InternalResourceView的renderMergeOutputModel怎么實現的：

protected void renderMergedOutputModel(
            Map<String, Object> model, HttpServletRequest request, HttpServletResponse response){

    // Determine which request handle to expose to the RequestDispatcher.
    HttpServletRequest requestToExpose = getRequestToExpose(request);

    //將model放入到request屬性中
    exposeModelAsRequestAttributes(model, requestToExpose);

    // Expose helpers as request attributes, if any.
    exposeHelpers(requestToExpose);

    //獲取request對象dispath的path
    String dispatcherPath = prepareForRendering(requestToExpose, response);

    //獲取RequestDispatcher對象
    RequestDispatcher rd = getRequestDispatcher(requestToExpose, 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(requestToExpose, response)) {
        response.setContentType(getContentType());
        rd.include(requestToExpose, response);
    }else {
        // Note: The forwarded resource is supposed to determine the content type itself.
        exposeForwardRequestAttributes(requestToExpose);
        rd.forward(requestToExpose, response);
    }
}

這上面就講述了MVC中視圖的工作。

關于Spring根容器的建立, 之前說了，Spring會建立根上下文，再建立MVC上下文，我們簡略說下根上下文的建立，也就是web.xml的ContextLoaderListener配置：

<listener>
    <listener-class>
        org.springframework.web.context.ContextLoaderListener
    </listener-class>
</listener>

看看ContextLoaderListener, 它繼承自ContextLoader, 主要初始化工作還是由ContextLoader來完成

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
   ...
}

看看ContextLoader的initWebApplicationContext方法：

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
    if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {
    ...
    }
    long startTime = System.currentTimeMillis();

    // Store context in local instance variable, to guarantee that
    // it is available on ServletContext shutdown.
    if (this.context == null) {
        this.context = createWebApplicationContext(servletContext);//創建根上下文
    }
    if (this.context instanceof ConfigurableWebApplicationContext) {
        ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
        if (!cwac.isActive()) {
            // The context has not yet been refreshed -> provide services such as
            // setting the parent context, setting the application context id, etc
            if (cwac.getParent() == null) {
            // The context instance was injected without an explicit parent ->
            // determine parent for root web application context, if any.
            ApplicationContext parent = loadParentContext(servletContext);
                cwac.setParent(parent);
            }
            configureAndRefreshWebApplicationContext(cwac, servletContext);//構建上下文，類似IoC容器構建
        }
    }                                                                                                           //將創建的根上下文保存到ServletContext, 所以再MVC上下文中可以獲取到
    servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);
    ClassLoader ccl = Thread.currentThread().getContextClassLoader();
    if (ccl == ContextLoader.class.getClassLoader()) {
        currentContext = this.context;
    } else if (ccl != null) {
        currentContextPerThread.put(ccl, this.context);
    }
    return this.context;
}

protected WebApplicationContext createWebApplicationContext(ServletContext sc) {
    Class<?> contextClass = determineContextClass(sc);//默認實現為XmlWebApplicationContext
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Custom context class [" + contextClass.getName() +
            "] is not of type [" + ConfigurableWebApplicationContext.class.getName() + "]");
    }
    return (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
}

這就簡略說了Spring根上下文的建立過程。

來自：http://my.oschina.net/indestiny/blog/197061

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