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Spring系列(六) Spring Web MVC 应用构建分析

DispatcherServlet

DispatcherServlet 是Spring MVC的前端控制器名称, 用户的请求到达这里进行集中处理, 在Spring MVC中, 它的作用是为不同请求匹配对应的处理器, 将结果传递给视图解析器最终呈现给客户端.

前端控制器模式(Front Controller Pattern)是用来提供一个集中的请求处理机制,所有的请求都将由一个单一的处理程序处理。该处理程序可以做认证/授权/记录日志,或者跟踪请求,然后把请求传给相应的处理程序。

Servlet WebApplicationContext 和 Root WebApplicationContext

Spring MVC 存在两个应用上下文, 分别为Servlet WebApplicationContext和Root WebApplicationContext. 他们分别初始化不同类型的bean.

下图来自Spring官方文档

在DispatcherServlet启动的时候, 它会创建Spring上下文Servlet WebApplicationContext, 其中包含Web相关的Controller,ViewResolver,HandlerMapping等.

另外一个上下文Root WebApplicationContext是由ContextLoaderListener创建的, 包含除了Web组件外的其他bean, 比如包含业务逻辑的Service, 还有数据库相关的组件等.

代码(JavaConfig方式的配置代码)

下面是用JavaConfig方式实现的配置代码, 我们先搭建好一个Spring MVC 项目,然后结合源码分析Spring如何注册DispatcherServlet实例的.

// 继承AbstractAnnotationConfigDispatcherServletInitializer并重写其中的三个方法public class MvcWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer { // 指定Root上下文的配置类 @Override protected Class<?>[] getRootConfigClasses() { return new Class[]{ RootConfig.class }; } // 指定Web上下文的配置类 @Override protected Class<?>[] getServletConfigClasses() { return new Class[]{ WebConfig.class }; } // url映射 @Override protected String[] getServletMappings() { return new String[]{"/"}; }}

通过重写AbstractAnnotationConfigDispatcherServletInitializer的三个方法完成配置, WebConfig用来配置Web组件, RootConfig用来配置非Web组件.

@EnableWebMvc // 启用MVC@ComponentScan(basePackages = {"com.xlx.mvc.web"}) // 启用组件扫描,只扫描web相关的组件@Configurationpublic class WebConfig implements WebMvcConfigurer { // 视图解析器,jsp @Bean public ViewResolver viewResolver(){ InternalResourceViewResolver resolver = new InternalResourceViewResolver(); resolver.setPrefix("/WEB-INF/views/"); resolver.setSuffix(".jsp"); resolver.setExposeContextBeansAsAttributes(true); return resolver; } // 重写以启用默认的处理器, 用来处理静态资源 @Override public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer){ configurer.enable(); }}@Configuration@ComponentScan(basePackages = {"com.xlx.mvc"}, excludeFilters = { @ComponentScan.Filter(type = FilterType.ANNOTATION,classes = EnableWebMvc.class)}) // 扫描包, 但排除EnableWebMvc注解的类public class RootConfig {}

源码分析

Servlet 3.0 旨在支持基于代码的方式配置Servlet容器, 当3.0兼容的servlet容器启动的时候会在ClassPath查找并调用实现了接口ServletContainerInitializer的类的onStartup()方法, Spring中提供了这个接口的一个实现类SpringServletContainerInitializer. 其启动方法的代码如下:

@Overridepublic void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException { List<WebApplicationInitializer> initializers = new LinkedList<>(); // 应用中WebApplicationInitializer的bean生成到一个列表中. if (webAppInitializerClasses != null) { for (Class<?> waiClass : webAppInitializerClasses) { if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) { try { initializers.add((WebApplicationInitializer) ReflectionUtils.accessibleConstructor(waiClass).newInstance()); } catch (Throwable ex) { throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex); } } } } if (initializers.isEmpty()) { servletContext.log("No Spring WebApplicationInitializer types detected on classpath"); return; } servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath"); AnnotationAwareOrderComparator.sort(initializers); // 遍历所有WebApplicationInitializer, 并调用其onStartup方法 for (WebApplicationInitializer initializer : initializers) { initializer.onStartup(servletContext); }}

在上面方法的最后, 可以看到其将控制权交给WebApplicationInitializer的实例并遍历调用了onStartup()方法, 而我们定义的类MvcWebAppInitializer 就是它的子类. 完整的继承关系为

WebApplicationInitializer <--AbstractContextLoaderInitializer <--AbstractDispatcherServletInitializer <--AbstractAnnotationConfigDispatcherServletInitializer <--MvcWebAppInitializer

在类 AbstractDispatcherServletInitializer 中实现了onStartup()方法, 最终调用registerDispatcherServlet()方法完成注册, 两个方法的代码如下:

@Overridepublic void onStartup(ServletContext servletContext) throws ServletException { super.onStartup(servletContext); registerDispatcherServlet(servletContext);}protected void registerDispatcherServlet(ServletContext servletContext) { // 获取Sevlet名称, 这个方法返回了默认值"dispatcher" String servletName = getServletName(); Assert.hasLength(servletName, "getServletName() must not return null or empty"); // 此处调用的方法是抽象方法, 由子类AbstractAnnotationConfigDispatcherServletInitializer实现, 其最终调用了自定义类的getServletConfigClasses()方法获取配置信息(源码附在本段后面). 用来生成Servlet上下文. WebApplicationContext servletAppContext = createServletApplicationContext(); Assert.notNull(servletAppContext, "createServletApplicationContext() must not return null"); // 生成dispatcherServlet实例 FrameworkServlet dispatcherServlet = createDispatcherServlet(servletAppContext); Assert.notNull(dispatcherServlet, "createDispatcherServlet(WebApplicationContext) must not return null"); dispatcherServlet.setContextInitializers(getServletApplicationContextInitializers()); // 注册DispatcherServlet ServletRegistration.Dynamic registration = servletContext.addServlet(servletName, dispatcherServlet); if (registration == null) { throw new IllegalStateException("Failed to register servlet with name "" + servletName + "". " + "Check if there is another servlet registered under the same name."); } registration.setLoadOnStartup(1); registration.addMapping(getServletMappings()); registration.setAsyncSupported(isAsyncSupported()); Filter[] filters = getServletFilters(); if (!ObjectUtils.isEmpty(filters)) { for (Filter filter : filters) { registerServletFilter(servletContext, filter); } } customizeRegistration(registration);}

下面附读取Servlet配置类的代码: 类AbstractAnnotationConfigDispatcherServletInitializer实现了createServletApplicationContext(), 可以看到代码中调用了方法getServletConfigClasses(), 这是个抽象方法, 声明为protected abstract Class<?>[] getServletConfigClasses();. 最终的实现正是在我们自定义的子类MvcWebAppInitializer中.

@Overrideprotected WebApplicationContext createServletApplicationContext() { AnnotationConfigWebApplicationContext context = new AnnotationConfigWebApplicationContext(); // 读取配置类 Class<?>[] configClasses = getServletConfigClasses(); if (!ObjectUtils.isEmpty(configClasses)) { context.register(configClasses); } return context;}

上面完成了DispatcherServlet的注册和启动, 接下来可以定义Controller了.

请求映射

在此之前需要了解下关于URL映射的Servlet规范, 注意这是Servlet的规范, 当然也适用于DispatcherServlet, 代码中我们为DispatcherServlet映射为"/", 规范中"/"为使用"default"Servlet, 也就意味着所有的请求默认通过DispatcherServlet处理.

为了处理静态资源, 在WebConfig中覆盖了方法configureDefaultServletHandling()已启用静态资源处理器DefaultServletHttpRequestHandler, 它的优先级是最低, 这意味着在匹配不到其他handler的时候,servlet会将请求交给这个handler处理.

规则按顺序执行,匹配到就直接返回.

    精确匹配, url完全与模式匹配最长路径匹配, 查找模式中路径最长的匹配项, 例如/user/list/1匹配模式/user/list/, 而不是/user/扩展名匹配默认Servlet

代码

@Controller@RequestMapping(value = "/home")public class HomeController { @RequestMapping(value = "/default",method = RequestMethod.GET) public String home(){ return "home"; }}

源码分析

我们的Controller以注解(@RequestMapping,@GetMapping等)方式定义, RequestMappingHandlerMapping用来生成请求url与处理方法的映射关系(mapping),这个mapping最终是由DispatcherServlet调用找到匹配到url对应的controller方法并调用.

通过查看Spring的bean依赖关系图(找到类WebConfig, Ctrl+Alt+U并选spring beans dependency)可以找到RequestMappingHandlerMapping生成的线索.

简化的关系图如下:

可以看到WebmvcConfigurationSupport中有个@Bean注解的方法生成RequestMappingHandlerMapping的实例, 而WebmvcConfigurationSupport继承了DelegatingWebMvcConfiguration, 后者是由@EnableWebMvc注解导入.

/** * * 返回排序为0的RequestMappingHandlerMapping实例bean, 用来处理注解方式的Controller请求. */@Beanpublic RequestMappingHandlerMapping requestMappingHandlerMapping() { RequestMappingHandlerMapping mapping = createRequestMappingHandlerMapping(); // 顺序为0, 顺便提一句, 静态资源的处理器Handler的顺序为Integer.Max mapping.setOrder(0); mapping.setInterceptors(getInterceptors()); mapping.setContentNegotiationManager(mvcContentNegotiationManager()); mapping.setCorsConfigurations(getCorsConfigurations()); PathMatchConfigurer configurer = getPathMatchConfigurer(); Boolean useSuffixPatternMatch = configurer.isUseSuffixPatternMatch(); if (useSuffixPatternMatch != null) { mapping.setUseSuffixPatternMatch(useSuffixPatternMatch); } Boolean useRegisteredSuffixPatternMatch = configurer.isUseRegisteredSuffixPatternMatch(); if (useRegisteredSuffixPatternMatch != null) { mapping.setUseRegisteredSuffixPatternMatch(useRegisteredSuffixPatternMatch); } Boolean useTrailingSlashMatch = configurer.isUseTrailingSlashMatch(); if (useTrailingSlashMatch != null) { mapping.setUseTrailingSlashMatch(useTrailingSlashMatch); } UrlPathHelper pathHelper = configurer.getUrlPathHelper(); if (pathHelper != null) { mapping.setUrlPathHelper(pathHelper); } PathMatcher pathMatcher = configurer.getPathMatcher(); if (pathMatcher != null) { mapping.setPathMatcher(pathMatcher); } Map<String, Predicate<Class<?>>> pathPrefixes = configurer.getPathPrefixes(); if (pathPrefixes != null) { mapping.setPathPrefixes(pathPrefixes); } return mapping;}

好了, 现在有了DispatcherServlet, 并且有了可以处理映射关系的RequestMappingHandlerMapping, 接下来再看下当请求到达时, DispatcherServlet 如何为Url找到对应的Handler方法.

DispatcherServlet中定义了处理请求的doService()方法, 最终这个方法委托doDispatch()处理请求, 特别注意中文注释的几个语句, 除此之外, 这个方法还提供了生命周期的一些处理工作.

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); // 获取当前请求对应的handler mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest, response); return; } // 获取当前请求对应handler的适配器 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)) { return; } // 最终调用Handler的方法 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(processedRequest, mv); 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); } 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); } } }}

上面代码中, 重点关注getHandler方法.

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { if (this.handlerMappings != null) { for (HandlerMapping mapping : this.handlerMappings) { HandlerExecutionChain handler = mapping.getHandler(request); if (handler != null) { return handler; } } } return null;}

可以看到请求所需的handler是取自实例变量this.handlerMappings,接下来顺藤摸瓜, 看这个变量是何时初始化的.通过引用, 我们查找到了下面方法.

private void initHandlerMappings(ApplicationContext context) { this.handlerMappings = null; if (this.detectAllHandlerMappings) { // 找到上下文中的所有HandlerMapping, 包括祖先上下文 Map<String, HandlerMapping> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerMappings = new ArrayList<>(matchingBeans.values()); // HandlerMapping排序 AnnotationAwareOrderComparator.sort(this.handlerMappings); } } else { try { HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class); this.handlerMappings = Collections.singletonList(hm); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we"ll add a default HandlerMapping later. // 这个注释... } } // 保证至少要有一个HandlerMapping. if (this.handlerMappings == null) { this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class); if (logger.isTraceEnabled()) { logger.trace("No HandlerMappings declared for servlet "" + getServletName() + "": using default strategies from DispatcherServlet.properties"); } }}

整理下调用关系: DispatcherServlet initHandlerMappings <-- initStrategies <-- onRefresh <--FrameworkServlet initWebApplicationContext <-- initServletBean <--HttpServletBean init <--GenericServlet init(ServletConfig config)最后的GenericServlet是servlet Api的.

Spring Boot 中的DispatcherServlet

Spring Boot微服务中的DispatcherServlet装配, 因为其一般使用内置的Servlet容器, 是通过DispatcherServletAutoConfiguration来完成的. 下面是生成DispatcherServlet bean的代码, 这个bean在内部静态类DispatcherServletConfiguration中.

@Bean(name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)public DispatcherServlet dispatcherServlet() { DispatcherServlet dispatcherServlet = new DispatcherServlet(); dispatcherServlet.setDispatchOptionsRequest( this.webMvcProperties.isDispatchOptionsRequest()); dispatcherServlet.setDispatchTraceRequest( this.webMvcProperties.isDispatchTraceRequest()); dispatcherServlet.setThrowExceptionIfNoHandlerFound( this.webMvcProperties.isThrowExceptionIfNoHandlerFound()); return dispatcherServlet;}

上面我们通过注解方式构建了一个MVC应用程序, 并且通过源码分析其构建原理, 其中Spring使用的前端控制器实现类是DispatcherServlet, 其在Servlet容器启动的时候实例化, 并初始化容器中的Handler处理器. 当请求到达DispatcherServlet时会调用其doDispatcher()方法选择最合适的处理器. 最后我们扫了一眼Spring Boot的自动装配DispatcherServlet方式.

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