springboot情操陶冶-web配置(二)

承接前文springboot情操陶冶-web配置(一)，在分析mvc的配置之前先了解下其默认的错误界面是如何显示的

404界面

springboot有个比较有趣的配置server.error.whitelabel.enabled，可用来管理404界面的显示方式，是简单的显示还是详细的显示。
指定为false的时候，则会简简单单的显示视图找不到的错误信息，如下

指定为true的时候(默认配置)，则会显示前文样例中的错误信息，如下

源码层分析

springboot安排了ErrorMvcAutoConfiguration自动配置类来处理错误页面的相关信息，笔者分几个步骤来进行分析

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No.1 脑壳上的注解看一发

@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass({ Servlet.class, DispatcherServlet.class })
// Load before the main WebMvcAutoConfiguration so that the error View is available
@AutoConfigureBefore(WebMvcAutoConfiguration.class)
@EnableConfigurationProperties({ ServerProperties.class, ResourceProperties.class })
public class ErrorMvcAutoConfiguration {
}

可以看出其是排在WebMvcAutoConfiguration配置类之前的，那么为什么需要排在前面呢？看注释是说这样才可以使error视图有效，那怎么实现的呢？笔者带着问题继续往下探索

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No.2 DefaultErrorViewResolverConfiguration内部类-错误视图解析器注册

	@Configuration
	static class DefaultErrorViewResolverConfiguration {

		private final ApplicationContext applicationContext;

		private final ResourceProperties resourceProperties;

		DefaultErrorViewResolverConfiguration(ApplicationContext applicationContext,
				ResourceProperties resourceProperties) {
			this.applicationContext = applicationContext;
			this.resourceProperties = resourceProperties;
		}

		// 注册了DefaultErrorViewResolver解析器
		@Bean
		@ConditionalOnBean(DispatcherServlet.class)
		@ConditionalOnMissingBean
		public DefaultErrorViewResolver conventionErrorViewResolver() {
			return new DefaultErrorViewResolver(this.applicationContext,
					this.resourceProperties);
		}

	}

DefaultErrorViewResolver这个默认的错误视图解析器很有意思，里面包含了一些默认的处理，也分几个小步骤来吧，这样会显得清晰

• 静态方法了解

	static {
		Map<Series, String> views = new EnumMap<>(Series.class);
		views.put(Series.CLIENT_ERROR, "4xx");
		views.put(Series.SERVER_ERROR, "5xx");
		SERIES_VIEWS = Collections.unmodifiableMap(views);
	}

应该是对HTTP状态码的映射处理，以4开头的是客户端错误，5开头的为服务端错误

• 构造函数了解

	public DefaultErrorViewResolver(ApplicationContext applicationContext,
			ResourceProperties resourceProperties) {
		Assert.notNull(applicationContext, "ApplicationContext must not be null");
		Assert.notNull(resourceProperties, "ResourceProperties must not be null");
		this.applicationContext = applicationContext;
		this.resourceProperties = resourceProperties;
		// 模板加载器
		this.templateAvailabilityProviders = new TemplateAvailabilityProviders(
				applicationContext);
	}

上述的模板加载器主要是读取所有spring.factories中的org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider对应的属性值，本质也就是模板的渲染器，比如我们常用的freemarker、velocity、jsp等等

• 视图对象获取了解

	@Override
	public ModelAndView resolveErrorView(HttpServletRequest request, HttpStatus status,
			Map<String, Object> model) {
		// 优先根据状态码来查找view静态资源,比如404则会查找error/404视图
		ModelAndView modelAndView = resolve(String.valueOf(status), model);
		if (modelAndView == null && SERIES_VIEWS.containsKey(status.series())) {
			// 上述不存在则再查找error/4xx或者error/5xx视图
			modelAndView = resolve(SERIES_VIEWS.get(status.series()), model);
		}
		return modelAndView;
	}

	private ModelAndView resolve(String viewName, Map<String, Object> model) {
		String errorViewName = "error/" + viewName;
		// 通过模板加载器查找是否含有符合要求的视图资源
		TemplateAvailabilityProvider provider = this.templateAvailabilityProviders
				.getProvider(errorViewName, this.applicationContext);
		if (provider != null) {
			return new ModelAndView(errorViewName, model);
		}
		return resolveResource(errorViewName, model);
	}
	
	// 默认查找staticLocation指定路径的资源，比如classpath:/static/error/404.html
	private ModelAndView resolveResource(String viewName, Map<String, Object> model) {
		for (String location : this.resourceProperties.getStaticLocations()) {
			try {
				Resource resource = this.applicationContext.getResource(location);
				resource = resource.createRelative(viewName + ".html");
				if (resource.exists()) {
					// view类型为HtmlResourceView，直接将html资源输出到response对象中
					return new ModelAndView(new HtmlResourceView(resource), model);
				}
			}
			catch (Exception ex) {
			}
		}
		return null;
	}

通过上述的代码注释，基本可以得知错误视图的查找规则，所以用户可以简单的在static目录下配置对应状态码的页面比如error/404.html或者error/500.html；当然也可以配置统一的页面error/4xx.html或者error/5xx.html

那如果我们啥也不指定，那上述的错误提示信息是如何展示的呢？

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No.3 WhitelabelErrorViewConfiguration-白板错误视图配置

	// server.error.whitelabel.enabled开关，默认是打开的
	@Configuration
	@ConditionalOnProperty(prefix = "server.error.whitelabel", name = "enabled", matchIfMissing = true)
	@Conditional(ErrorTemplateMissingCondition.class)
	protected static class WhitelabelErrorViewConfiguration {
		
		// 熟悉的打印信息
		private final SpelView defaultErrorView = new SpelView(
				"<html><body><h1>Whitelabel Error Page</h1>"
						+ "<p>This application has no explicit mapping for /error, so you are seeing this as a fallback.</p>"
						+ "<div id='created'>${timestamp}</div>"
						+ "<div>There was an unexpected error (type=${error}, status=${status}).</div>"
						+ "<div>${message}</div></body></html>");

		// 创建了名为error的视图对象
		@Bean(name = "error")
		@ConditionalOnMissingBean(name = "error")
		public View defaultErrorView() {
			return this.defaultErrorView;
		}

		// 与上面的View对象搭配使用
		@Bean
		@ConditionalOnMissingBean
		public BeanNameViewResolver beanNameViewResolver() {
			BeanNameViewResolver resolver = new BeanNameViewResolver();
			resolver.setOrder(Ordered.LOWEST_PRECEDENCE - 10);
			return resolver;
		}
	}

上述就是我们开头可见的错误信息的处理处，详细的用户可自行查阅代码

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No.4 构造函数了解

	public ErrorMvcAutoConfiguration(ServerProperties serverProperties,
			ObjectProvider<List<ErrorViewResolver>> errorViewResolversProvider) {
		this.serverProperties = serverProperties;
		this.errorViewResolvers = errorViewResolversProvider.getIfAvailable();
	}

上述的errorViewResolverProvider便会加载第二步骤的DefaultViewResolver，当然用户也可以自定义去实现ErrorViewResolver接口。这些错误的视图解析器将会在下一步骤的controller层被调用

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No.5 error控制器注册

	@Bean
	@ConditionalOnMissingBean(value = ErrorAttributes.class, search = SearchStrategy.CURRENT)
	public DefaultErrorAttributes errorAttributes() {
		return new DefaultErrorAttributes(
				this.serverProperties.getError().isIncludeException());
	}

	// 创建BasicErrorController控制器用于响应server.error.path指定的路径，默认为/error
	@Bean
	@ConditionalOnMissingBean(value = ErrorController.class, search = SearchStrategy.CURRENT)
	public BasicErrorController basicErrorController(ErrorAttributes errorAttributes) {
		return new BasicErrorController(errorAttributes, this.serverProperties.getError(),
				this.errorViewResolvers);
	}

此处的BasicErrorController对象则会默认响应/error的请求，其内部写了一个返回html页面的响应方法

	@RequestMapping(produces = "text/html")
	public ModelAndView errorHtml(HttpServletRequest request,
			HttpServletResponse response) {
		HttpStatus status = getStatus(request);
		Map<String, Object> model = Collections.unmodifiableMap(getErrorAttributes(
				request, isIncludeStackTrace(request, MediaType.TEXT_HTML)));
		// 状态码设置
		response.setStatus(status.value());
		// 调用errorViewResolvers集合去获取对应的错误视图
		ModelAndView modelAndView = resolveErrorView(request, response, status, model);
		// 如果没指定相应的视图，则会采用默认的名为error的视图
		return (modelAndView != null ? modelAndView : new ModelAndView("error", model));
	}

对上述代码的注释作下简单的解释，帮助读者们理顺下思路

1. 首先其会调用所有实现了ErrorViewResolver接口的视图解析器去找寻相应的错误视图，并支持通过Order接口进行排序。所以此处默认情况下会调用DefaultErrorViewResolver来获取view，具体的如果获取可见上文的讲解
2. 如果上述找到了，那么也就么事了，但是如果还没找到，则会默认指定名为error的视图。
3. 那么如何去解析默认名为error的视图呢？答案在DispatcherServlet在最终确定渲染视图的时候，会统一调用所有实现了ViewResolver接口的视图解析器去获取视图对象，那么第三步骤中的BeanNameViewResolver对象便会找寻到对应的SpelView视图，由其来进行相应的渲染

在此处笔者回答下开头的问题，为什么ErrorMvcAutoConfiguration需要放在DispatcherServletAutoConfiguration之前，其实最主要的是后者并没有去注册BeanViewResolver，此处上了一份保险，好让能正确的找到SpelView对象

error请求问题

经过上文的分析，我们知道了BasicErrorController用来处理访问方式为GET [/error]的请求并处理得到相应的错误视图，那么最重要的问题来了，到底怎么在出现资源找不到的时候去路由至此路径上呢？笔者继续带着这个问题去探索

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No.1 ErrorPageCustomizer-错误页面配置

	@Bean
	public ErrorPageCustomizer errorPageCustomizer() {
		return new ErrorPageCustomizer(this.serverProperties);
	}

	private static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {

		private final ServerProperties properties;

		protected ErrorPageCustomizer(ServerProperties properties) {
			this.properties = properties;
		}

		@Override
		public void registerErrorPages(ErrorPageRegistry errorPageRegistry) {
			// 默认路径为/error
			ErrorPage errorPage = new ErrorPage(
					this.properties.getServlet().getServletPrefix()
							+ this.properties.getError().getPath());
			// 注册
			errorPageRegistry.addErrorPages(errorPage);
		}

		@Override
		public int getOrder() {
			return 0;
		}

	}

上述的errorPage貌似展示了一点信息，可能是会去访问/error的源头，那么ErrorPageCustomizer#registerErrorPages()是如何被调用的呢？继续往下

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No.2 ServletWebAutoConfiguration引入的时候还注册了一个BeanPostProcessor

		@Override
		public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
				BeanDefinitionRegistry registry) {
			if (this.beanFactory == null) {
				return;
			}
			registerSyntheticBeanIfMissing(registry,
					"webServerFactoryCustomizerBeanPostProcessor",
					WebServerFactoryCustomizerBeanPostProcessor.class);
			// 就是这个
			registerSyntheticBeanIfMissing(registry,
					"errorPageRegistrarBeanPostProcessor",
					ErrorPageRegistrarBeanPostProcessor.class);
		}

我们直接去关注其主要的方法

	// 注册了相应的错误界面
	private void postProcessBeforeInitialization(ErrorPageRegistry registry) {
		for (ErrorPageRegistrar registrar : getRegistrars()) {
			registrar.registerErrorPages(registry);
		}
	}

	private Collection<ErrorPageRegistrar> getRegistrars() {
		if (this.registrars == null) {
			// Look up does not include the parent context
			this.registrars = new ArrayList<>(this.beanFactory
					.getBeansOfType(ErrorPageRegistrar.class, false, false).values());
			this.registrars.sort(AnnotationAwareOrderComparator.INSTANCE);
			this.registrars = Collections.unmodifiableList(this.registrars);
		}
		return this.registrars;
	}

至于为什么在该类中去注册这个processor去执行注册错误页面，看来这个路径的转发应该与web容器有关。其实追踪源头其实将错误界面注册到了相应的web容器中(Tomcat)，具体的读者可自行去分析。

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No.4 web容器加载(插曲，顺带提一下)
我们都知道springboot对环境为Servlet所采用的ApplicationContext为AnnotationConfigServletWebServerApplicationContext，其父类在刷新上下文过程中的onRefresh()方法便去启动了web容器

	@Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			// 创建web服务器
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			// 默认为TomcatServletWebServerFactory
			ServletWebServerFactory factory = getWebServerFactory();
			// 初始化servlet/filter等
			this.webServer = factory.getWebServer(getSelfInitializer());
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context",
						ex);
			}
		}
		initPropertySources();
	}

上述的代码主要会在ServletContext上注册Filters和Servlets集合并且注册ErrorPages，限于代码过长，读者可自行分析。而具体的去启动web容器则是在finishRefresh()方法中

	@Override
	protected void finishRefresh() {
		super.finishRefresh();
		// 启动
		WebServer webServer = startWebServer();
		if (webServer != null) {
			publishEvent(new ServletWebServerInitializedEvent(webServer, this));
		}
	}

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No.5 StandardHostValve-错误界面应用

    private void status(Request request, Response response) {

        int statusCode = response.getStatus();

        ....
		// 优先查找404对应的ErrorPage
        ErrorPage errorPage = context.findErrorPage(statusCode);
        if (errorPage == null) {
            // 0-默认的ErrorPage，此处便是上文注册的
            errorPage = context.findErrorPage(0);
        }
        if (errorPage != null && response.isErrorReportRequired()) {
            ....
            // 在custom方法中会调用RequestDispatcher对象进行后端路由重置到/error请求
            if (custom(request, response, errorPage)) {
                response.setErrorReported();
                try {
                    response.finishResponse();
                } catch (ClientAbortException e) {
                    // Ignore
                } catch (IOException e) {
                    container.getLogger().warn("Exception Processing " + errorPage, e);
                }
            }
        }
    }

此源码来源于tomcat，这让笔者想起了针对状态码的page配置

    <!--404 error page specified based on Tomcat-->
    <error-page>
        <error-code>404</error-code>
        <location>/404.html</location>
    </error-page>

小结

本文的内容较多，需要耐心阅读，读者只需要了解View视图的解析加载便可通读全文，如果想要自定义状态码视图则直接在classpath:/static/error目录下新建相应的状态码HTML文件即可，具体可参照本文的讲述。