概述

Spring Boot对所支持的Servlet Web服务器实现做了建模抽象:

Servlet容器类型	WebServer模型接口	WebServer工厂实现类
Tomcat	TomcatWebServer	TomcatServletWebServerFactory
Jetty	JettyWebServer	JettyServletWebServerFactory
Undertow	UndertowWebServer	UndertowServletWebServerFactory

基于此模型概念,在一个Servlet Web应用中，Spring Boot会使用上表中所说的WebServer工厂组件生成相应的WebServer实例。而这里的WebServer工厂组件又是从哪里来的呢 ？ 这就是自动配置类ServletWebServerFactoryAutoConfiguration的任务了。

自动配置类ServletWebServerFactoryAutoConfiguration首先通过注解声明自己的生效条件:

1. 类 ServletRequest 存在于 classpath 上时才生效,也就是要求javax.servlet-api包必须被引用;
2. 当前应用必须是Spring MVC应用才生效；

在以上条件被满足时，ServletWebServerFactoryAutoConfiguration引入了如下三个配置类 :

1. EmbeddedTomcat
2. EmbeddedJetty
3. EmbeddedUndertow

这三个配置类是ServletWebServerFactoryConfiguration的嵌套配置类，它们会分别检测classpath上存在的类，从而判断当前应用使用的是Tomcat/Jetty/Undertow中的哪一个Servlet Web服务器,从而决定定义相应的工厂组件bean : TomcatServletWebServerFactory/JettyServletWebServerFactory/UndertowServletWebServerFactory。

源代码

源代码版本 : spring-boot-autoconfigure-2.1.3.RELEASE

ServletWebServerFactoryAutoConfiguration

package org.springframework.boot.autoconfigure.web.servlet;

// 省略 import 行

/**
 * EnableAutoConfiguration Auto-configuration for servlet web servers.
 *
 */
@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
// 仅在类 ServletRequest 存在于 classpath 上时才生效
@ConditionalOnClass(ServletRequest.class)
// 仅在当前应用是 Servlet Web 应用时才生效
@ConditionalOnWebApplication(type = Type.SERVLET)
// 确保前缀为 server 的配置参数加载到 bean ServerProperties
@EnableConfigurationProperties(ServerProperties.class)
// 1. 导入 ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar 以注册
// BeanPostProcessor : WebServerFactoryCustomizerBeanPostProcessor 和 
// ErrorPageRegistrarBeanPostProcessor
// 2. 导入 EmbeddedTomcat/EmbeddedJetty/EmbeddedUndertow 这三个属于
// ServletWebServerFactoryConfiguration 的嵌套配置类，这三个配置类会分别检测
// classpath上存在的类，从而判断当前应用使用的是 Tomcat/Jetty/Undertow,
// 从而决定定义哪一个 Servlet Web服务器的工厂 bean :
// TomcatServletWebServerFactory/JettyServletWebServerFactory/UndertowServletWebServerFactory
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
		ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
		ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
		ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {

    // 定义 bean ServletWebServerFactoryCustomizer
	@Bean
	public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(
			ServerProperties serverProperties) {
		return new ServletWebServerFactoryCustomizer(serverProperties);
	}

    // 针对当前Servlet容器是Tomcat时定义该 bean，用于定制化 TomcatServletWebServerFactory 
	@Bean
    // 仅在类 org.apache.catalina.startup.Tomcat 存在于 classpath 上时才生效
	@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")
	public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(
			ServerProperties serverProperties) {
		return new TomcatServletWebServerFactoryCustomizer(serverProperties);
	}

	/**
	 * Registers a WebServerFactoryCustomizerBeanPostProcessor. Registered via
	 * ImportBeanDefinitionRegistrar for early registration.
     * 这是一个 ImportBeanDefinitionRegistrar， 它会向容器注入两个 BeanPostProcessor :
     * 1. WebServerFactoryCustomizerBeanPostProcessor
     * 该 BeanPostProcessor 会搜集容器中所有的 WebServerFactoryCustomizer，对当前应用所采用的 
     * WebServerFactory 被初始化前进行定制
     * 2. ErrorPageRegistrarBeanPostProcessor
     * 该 BeanPostProcessor 会搜集容器中所有的 ErrorPageRegistrar，添加到当前应用所采用的 
     * ErrorPageRegistry 中,实际上，这里的 ErrorPageRegistry 会是 ConfigurableWebServerFactory,
     * 具体实现上来讲，是一个 ConfigurableTomcatWebServerFactory,ConfigurableJettyWebServerFactory
     * 或者 ConfigurableUndertowWebServerFactory,分别对应 Tomcat, Jetty, Undertow 这三种
     * Servlet Web 容器的工厂类
	 */
	public static class BeanPostProcessorsRegistrar
			implements ImportBeanDefinitionRegistrar, BeanFactoryAware {

		private ConfigurableListableBeanFactory beanFactory;

		@Override
		public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
			if (beanFactory instanceof ConfigurableListableBeanFactory) {
				this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;
			}
		}

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

		private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,
				String name, Class<?> beanClass) {
			if (ObjectUtils.isEmpty(
					this.beanFactory.getBeanNamesForType(beanClass, true, false))) {
				RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);
				beanDefinition.setSynthetic(true);
				registry.registerBeanDefinition(name, beanDefinition);
			}
		}

	}

}

ServletWebServerFactoryConfiguration

ServletWebServerFactoryConfiguration是一个针对ServletWebServerFactory进行配置的配置类。它通过检测应用classpath存在的类，从而判断当前应用要使用哪个Servlet容器:Tomcat,Jetty还是Undertow。检测出来之后，定义相应的Servlet Web服务器工厂组件bean :

Servlet容器类型	WebServerFactory实现类
Tomcat	TomcatServletWebServerFactory
Jetty	JettyServletWebServerFactory
Undertow	UndertowServletWebServerFactory

注意，以上三个实现类都继承自抽象基类AbstractServletWebServerFactory,实现了接口WebServerFactory,ErrorPageRegistry。该特征会被WebServerFactoryCustomizerBeanPostProcessor和ErrorPageRegistrarBeanPostProcessor使用，用于对WebServerFactory进行定制化，以及设置相应的错误页面。

package org.springframework.boot.autoconfigure.web.servlet;

// 省略 import 行
/**
 * Configuration classes for servlet web servers
 * 
 * Those should be @Import in a regular auto-configuration class to guarantee
 * their order of execution.
 *
 */
@Configuration
class ServletWebServerFactoryConfiguration {

	@Configuration
	@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedTomcat {

		@Bean
		public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
			return new TomcatServletWebServerFactory();
		}

	}

	/**
	 * Nested configuration if Jetty is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Server.class, Loader.class,
			WebAppContext.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedJetty {

		@Bean
		public JettyServletWebServerFactory JettyServletWebServerFactory() {
			return new JettyServletWebServerFactory();
		}

	}

	/**
	 * Nested configuration if Undertow is being used.
	 */
	@Configuration
	@ConditionalOnClass({ Servlet.class, Undertow.class, SslClientAuthMode.class })
	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
	public static class EmbeddedUndertow {

		@Bean
		public UndertowServletWebServerFactory undertowServletWebServerFactory() {
			return new UndertowServletWebServerFactory();
		}

	}

}