一、注解配置SpringMVC

采用全注解开发，替代我们的web.xml和SpringMVC的核心配置文件
我们需要创建对应的配置类，继承AbstractAnnotationConfigDispatcherServletInitializer
- 使用的Servlet版本要求在3.0以上
- 项目启动后容器会找到配置了，基于我们的配置来初始化Servlet的上下文
接下来给出模板代码及注释，演示如何使用注解进行配置

准备一个新的模块，无需创建 web.xml,打包方式仍为war，导入之前的依赖，手动创建webapp、WEB-INF目录

编写我们的核心配置类 WebInit

package com.atguigu.mvc.config;

import org.springframework.web.filter.CharacterEncodingFilter;
import org.springframework.web.filter.HiddenHttpMethodFilter;
import org.springframework.web.servlet.support.AbstractAnnotationConfigDispatcherServletInitializer;

import javax.servlet.Filter;

/**
 * 代替我们的web.xml
 * @author Bonbons
 * @version 1.0
 */
public class WebInit extends AbstractAnnotationConfigDispatcherServletInitializer {
    /**
     * 指定Spring配置类
     * @return
     */
    @Override
    protected Class<?>[] getRootConfigClasses() {
        return new Class[]{SpringConfig.class};
    }

    /**
     * 指定SpringMVC配置类
     * @return
     */
    @Override
    protected Class<?>[] getServletConfigClasses() {
        return new Class[]{WebConfig.class};
    }

    /**
     * 指定前端控制器的映射规则， url-pattern
     * @return
     */
    @Override
    protected String[] getServletMappings() {
        return new String[]{"/"};
    }

    /**
     * 配置我们的过滤器
     * @return
     */
    @Override
    protected Filter[] getServletFilters() {
        // 编码过滤器
        CharacterEncodingFilter characterEncodingFilter = new CharacterEncodingFilter();
        characterEncodingFilter.setEncoding("UTF-8");
        characterEncodingFilter.setForceResponseEncoding(true);
        // 隐藏过滤器，将post转换为delete和put [没有啥初始化参数，就是注册一下就能用]
        HiddenHttpMethodFilter hiddenHttpMethodFilter = new HiddenHttpMethodFilter();
        // 返回我们创建的过滤器
        return new Filter[]{characterEncodingFilter, hiddenHttpMethodFilter};
    }
}

编写我们 SpringMVC 的核心配置类 WebConfig

package com.atguigu.mvc.config;

import com.atguigu.mvc.interceptor.TestInterceptor;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.context.ContextLoader;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.multipart.MultipartFile;
import org.springframework.web.multipart.MultipartResolver;
import org.springframework.web.multipart.commons.CommonsMultipartResolver;
import org.springframework.web.servlet.HandlerExceptionResolver;
import org.springframework.web.servlet.ViewResolver;
import org.springframework.web.servlet.config.annotation.*;
import org.springframework.web.servlet.handler.SimpleMappingExceptionResolver;
import org.thymeleaf.spring5.SpringTemplateEngine;
import org.thymeleaf.spring5.view.ThymeleafViewResolver;
import org.thymeleaf.templatemode.TemplateMode;
import org.thymeleaf.templateresolver.ITemplateResolver;
import org.thymeleaf.templateresolver.ServletContextTemplateResolver;

import java.util.List;
import java.util.Properties;

/**
 * @author Bonbons
 * @version 1.0
 */
@Configuration // 声明配置类
@ComponentScan ("com.atguigu.mvc.controller")// 扫描组件
@EnableWebMvc // 开启注解驱动
public class WebConfig implements WebMvcConfigurer {
    /**
     * 配置默认的Servlet，也就是处理我们的静态资源
     * @param configurer
     */
    @Override
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
        WebMvcConfigurer.super.configureDefaultServletHandling(configurer);
    }

    /**
     * 配置拦截器
     * @param registry
     */
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        // 创建我们自定义拦截器的对象
        TestInterceptor testInterceptor = new TestInterceptor();
        // 设置拦截规则
        registry.addInterceptor(testInterceptor).addPathPatterns("/**");
        WebMvcConfigurer.super.addInterceptors(registry);
    }

    /**
     * 配置视图控制器
     * @param registry
     */
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        // 前面的viewController是请求路径，后面的viewName是视图名
        registry.addViewController("/hello").setViewName("hello");
        WebMvcConfigurer.super.addViewControllers(registry);
    }

    /**
     * 配置我们的文件上传解析器
     * @return
     */
    public MultipartResolver multipartResolver(){
        // 上面这个类型实际是一个接口，所以我们需要创建对应实现类的对象
        CommonsMultipartResolver commonsMultipartResolver = new CommonsMultipartResolver();
        // 返回
        return commonsMultipartResolver;
    }

    @Override
    public void configureHandlerExceptionResolvers(List<HandlerExceptionResolver> resolvers) {

        SimpleMappingExceptionResolver exceptionResolver = new SimpleMappingExceptionResolver();
        // Properties 集合存储异常和跳转页面的映射
        Properties prop = new Properties();
        // 添加
        prop.setProperty("java.lang.ArithmeticException", "error");
        // 将其添加到我们的SimpleMappingExceptionResolver自定义异常对象中
        exceptionResolver.setExceptionMappings(prop);
        // 将异常信息添加到共享域中
        exceptionResolver.setExceptionAttribute("exception");
        // 将配置添加到我们的解析器中
        resolvers.add(exceptionResolver);
        WebMvcConfigurer.super.configureHandlerExceptionResolvers(resolvers);
    }

    //配置生成模板解析器
    @Bean
    public ITemplateResolver templateResolver() {
        WebApplicationContext webApplicationContext = ContextLoader.getCurrentWebApplicationContext();
        // ServletContextTemplateResolver需要一个ServletContext作为构造参数，可通过WebApplicationContext 的方法获得
        ServletContextTemplateResolver templateResolver = new ServletContextTemplateResolver(webApplicationContext.getServletContext());
        templateResolver.setPrefix("/WEB-INF/templates/");
        templateResolver.setSuffix(".html");
        templateResolver.setCharacterEncoding("UTF-8");
        templateResolver.setTemplateMode(TemplateMode.HTML);
        return templateResolver;
    }

    //生成模板引擎并为模板引擎注入模板解析器
    @Bean
    public SpringTemplateEngine templateEngine(ITemplateResolver templateResolver) {
        SpringTemplateEngine templateEngine = new SpringTemplateEngine();
        templateEngine.setTemplateResolver(templateResolver);
        return templateEngine;
    }

    //生成视图解析器并未解析器注入模板引擎
    @Bean
    public ViewResolver viewResolver(SpringTemplateEngine templateEngine) {
        ThymeleafViewResolver viewResolver = new ThymeleafViewResolver();
        viewResolver.setCharacterEncoding("UTF-8");
        viewResolver.setTemplateEngine(templateEngine);
        return viewResolver;
    }
}

编写我们Spring的核心配置类 SpringConfig 【只是定义了这个类，但是没配置】

package com.atguigu.mvc.config;

import org.springframework.context.annotation.Configuration;

/**
 * @author Bonbons
 * @version 1.0
 */
@Configuration
public class SpringConfig {
}

编写我们拦截器的配置类 TestInterceptor

package com.atguigu.mvc.interceptor;

import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * @author Bonbons
 * @version 1.0
 */
public class TestInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        System.out.println("TestInterceptor-->preHandle");
        return HandlerInterceptor.super.preHandle(request, response, handler);
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        HandlerInterceptor.super.postHandle(request, response, handler, modelAndView);
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        HandlerInterceptor.super.afterCompletion(request, response, handler, ex);
    }
}

编写我们的控制器 TestController

package com.atguigu.mvc.controller;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;

/**
 * @author Bonbons
 * @version 1.0
 */
@Controller
public class TestController {
    @RequestMapping("/")
    public String index(){
        return "index";
    }
}

编写我们的首页 index

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
hello
</body>
</html>

编写我们的 hello 页面，用于视图控制器【还应该有个异常跳转页面 error】

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
hello
</body>
</html>

这部分内容主要看一下应该如何配置，我觉得实际开发的时候，不会采用全部用注解进行配置

在这里插入图片描述

二、SpringMVC的执行流程

$ SpringMVC常用组件

DispatcherServlet：前端控制器，不需要工程师开发，由框架提供
- 作用：统一处理请求和响应，整个流程控制的中心，由它调用其它组件处理用户的请求
HandlerMapping：处理器映射器，不需要工程师开发，由框架提供
- 作用：根据请求的url、method等信息查找Handler，即控制器方法
Handler：处理器，需要工程师开发
- 作用：在DispatcherServlet的控制下Handler对具体的用户请求进行处理
HandlerAdapter：处理器适配器，不需要工程师开发，由框架提供
- 作用：通过HandlerAdapter对处理器（控制器方法）进行执行
ViewResolver：视图解析器，不需要工程师开发，由框架提供
- 作用：进行视图解析，得到相应的视图，ThymeleafView、InternalResourceView、RedirectView【Thymeleaf、转发、重定向】
View：视图
- 作用：将模型数据通过页面展示给用户

$ DispatcherServlet初始化过程

DispatcherServlet 本质上是一个 Servlet，所以天然的遵循 Servlet 的生命周期。所以宏观上是 Servlet 生命周期来进行调度。

在这里插入图片描述

a>初始化WebApplicationContext

所在类：org.springframework.web.servlet.FrameworkServlet

protected WebApplicationContext initWebApplicationContext() {
    WebApplicationContext rootContext =
        WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    WebApplicationContext wac = null;

    if (this.webApplicationContext != null) {
        // 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) {
        // No context instance was injected at construction time -> see if one
        // has been registered in the servlet context. If one exists, it is assumed
        // that the parent context (if any) has already been set and that the
        // user has performed any initialization such as setting the context id
        wac = findWebApplicationContext();
    }
    if (wac == null) {
        // No context instance is defined for this servlet -> create a local one
        // 创建WebApplicationContext
        wac = createWebApplicationContext(rootContext);
    }

    if (!this.refreshEventReceived) {
        // Either the context is not a ConfigurableApplicationContext with refresh
        // support or the context injected at construction time had already been
        // refreshed -> trigger initial onRefresh manually here.
        synchronized (this.onRefreshMonitor) {
            // 刷新WebApplicationContext
            onRefresh(wac);
        }
    }

    if (this.publishContext) {
        // Publish the context as a servlet context attribute.
        // 将IOC容器在应用域共享
        String attrName = getServletContextAttributeName();
        getServletContext().setAttribute(attrName, wac);
    }

    return wac;
}

b>创建WebApplicationContext

所在类：org.springframework.web.servlet.FrameworkServlet

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
    Class<?> contextClass = getContextClass();
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException(
            "Fatal initialization error in servlet with name '" + getServletName() +
            "': custom WebApplicationContext class [" + contextClass.getName() +
            "] is not of type ConfigurableWebApplicationContext");
    }
    // 通过反射创建 IOC 容器对象
    ConfigurableWebApplicationContext wac =
        (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

    wac.setEnvironment(getEnvironment());
    // 设置父容器
    wac.setParent(parent);
    String configLocation = getContextConfigLocation();
    if (configLocation != null) {
        wac.setConfigLocation(configLocation);
    }
    configureAndRefreshWebApplicationContext(wac);

    return wac;
}

c>DispatcherServlet初始化策略

FrameworkServlet创建WebApplicationContext后，刷新容器，调用onRefresh(wac)，此方法在DispatcherServlet中进行了重写，调用了initStrategies(context)方法，初始化策略，即初始化DispatcherServlet的各个组件

所在类：org.springframework.web.servlet.DispatcherServlet

protected void initStrategies(ApplicationContext context) {
   initMultipartResolver(context);
   initLocaleResolver(context);
   initThemeResolver(context);
   initHandlerMappings(context);
   initHandlerAdapters(context);
   initHandlerExceptionResolvers(context);
   initRequestToViewNameTranslator(context);
   initViewResolvers(context);
   initFlashMapManager(context);
}

$ DispatcherServlet调用组件处理请求

a>processRequest()

FrameworkServlet重写HttpServlet中的service()和doXxx()，这些方法中调用了processRequest(request, response)

所在类：org.springframework.web.servlet.FrameworkServlet

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {

    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;

    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    LocaleContext localeContext = buildLocaleContext(request);

    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

    initContextHolders(request, localeContext, requestAttributes);

    try {
		// 执行服务，doService()是一个抽象方法，在DispatcherServlet中进行了重写
        doService(request, response);
    }
    catch (ServletException | IOException ex) {
        failureCause = ex;
        throw ex;
    }
    catch (Throwable ex) {
        failureCause = ex;
        throw new NestedServletException("Request processing failed", ex);
    }

    finally {
        resetContextHolders(request, previousLocaleContext, previousAttributes);
        if (requestAttributes != null) {
            requestAttributes.requestCompleted();
        }
        logResult(request, response, failureCause, asyncManager);
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

b>doService()

所在类：org.springframework.web.servlet.DispatcherServlet

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    logRequest(request);

    // Keep a snapshot of the request attributes in case of an include,
    // to be able to restore the original attributes after the include.
    Map<String, Object> attributesSnapshot = null;
    if (WebUtils.isIncludeRequest(request)) {
        attributesSnapshot = new HashMap<>();
        Enumeration<?> attrNames = request.getAttributeNames();
        while (attrNames.hasMoreElements()) {
            String attrName = (String) attrNames.nextElement();
            if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
    }

    // Make framework objects available to handlers and view objects.
    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());

    if (this.flashMapManager != null) {
        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);
    }

    RequestPath requestPath = null;
    if (this.parseRequestPath && !ServletRequestPathUtils.hasParsedRequestPath(request)) {
        requestPath = ServletRequestPathUtils.parseAndCache(request);
    }

    try {
        // 处理请求和响应
        doDispatch(request, response);
    }
    finally {
        if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
        if (requestPath != null) {
            ServletRequestPathUtils.clearParsedRequestPath(request);
        }
    }
}

c>doDispatch()

所在类：org.springframework.web.servlet.DispatcherServlet

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

    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

    try {
        ModelAndView mv = null;
        Exception dispatchException = null;

        try {
            processedRequest = checkMultipart(request);
            multipartRequestParsed = (processedRequest != request);

            // Determine handler for the current request.
            /*
            	mappedHandler：调用链
                包含handler、interceptorList、interceptorIndex
            	handler：浏览器发送的请求所匹配的控制器方法
            	interceptorList：处理控制器方法的所有拦截器集合
            	interceptorIndex：拦截器索引，控制拦截器afterCompletion()的执行
            */
            mappedHandler = getHandler(processedRequest);
            if (mappedHandler == null) {
                noHandlerFound(processedRequest, response);
                return;
            }

            // Determine handler adapter for the current request.
           	// 通过控制器方法创建相应的处理器适配器，调用所对应的控制器方法
            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;
                }
            }
			
            // 调用拦截器的preHandle()
            if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                return;
            }

            // Actually invoke the handler.
            // 由处理器适配器调用具体的控制器方法，最终获得ModelAndView对象
            mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

            if (asyncManager.isConcurrentHandlingStarted()) {
                return;
            }

            applyDefaultViewName(processedRequest, mv);
            // 调用拦截器的postHandle()
            mappedHandler.applyPostHandle(processedRequest, response, mv);
        }
        catch (Exception ex) {
            dispatchException = ex;
        }
        catch (Throwable err) {
            // As of 4.3, we're processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        // 后续处理：处理模型数据和渲染视图
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
    catch (Exception ex) {
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    }
    catch (Throwable err) {
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                               new NestedServletException("Handler processing failed", err));
    }
    finally {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        }
        else {
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }
}

d>processDispatchResult()

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
                                   @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
                                   @Nullable Exception exception) throws Exception {

    boolean errorView = false;

    if (exception != null) {
        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()) {
        // 处理模型数据和渲染视图
        render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    }
    else {
        if (logger.isTraceEnabled()) {
            logger.trace("No view rendering, null ModelAndView returned.");
        }
    }

    if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        // Concurrent handling started during a forward
        return;
    }

    if (mappedHandler != null) {
        // Exception (if any) is already handled..
        // 调用拦截器的afterCompletion()
        mappedHandler.triggerAfterCompletion(request, response, null);
    }
}

$ SpringMVC的执行流程

用户向服务器发送请求，请求被SpringMVC 前端控制器 DispatcherServlet捕获。
DispatcherServlet对请求URL进行解析，得到请求资源标识符（URI），判断请求URI对应的映射：

a) 不存在

i. 再判断是否配置了mvc:default-servlet-handler

ii. 如果没配置，则控制台报映射查找不到，客户端展示404错误

在这里插入图片描述

iii. 如果有配置，则访问目标资源（一般为静态资源，如：JS,CSS,HTML），找不到客户端也会展示404错误
在这里插入图片描述

b) 存在则执行下面的流程

根据该URI，调用HandlerMapping获得该Handler配置的所有相关的对象（包括Handler对象以及Handler对象对应的拦截器），最后以HandlerExecutionChain执行链对象的形式返回。
DispatcherServlet 根据获得的Handler，选择一个合适的HandlerAdapter。
如果成功获得HandlerAdapter，此时将开始执行拦截器的preHandler(…)方法【正向】
提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)方法，处理请求。在填充Handler的入参过程中，根据你的配置，Spring将帮你做一些额外的工作：

a) HttpMessageConveter：将请求消息（如Json、xml等数据）转换成一个对象，将对象转换为指定的响应信息

b) 数据转换：对请求消息进行数据转换。如String转换成Integer、Double等

c) 数据格式化：对请求消息进行数据格式化。如将字符串转换成格式化数字或格式化日期等

d) 数据验证：验证数据的有效性（长度、格式等），验证结果存储到BindingResult或Error中

Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象。
此时将开始执行拦截器的postHandle(…)方法【逆向】。
根据返回的ModelAndView（此时会判断是否存在异常：如果存在异常，则执行HandlerExceptionResolver进行异常处理）选择一个适合的ViewResolver进行视图解析，根据Model和View，来渲染视图。
渲染视图完毕执行拦截器的afterCompletion(…)方法【逆向】。
将渲染结果返回给客户端。