分析SpringBoot的启动原理

@Configuration
@AutoConfigureOrder(-2147483648)
@ConditionalOnClass({ServletRequest.class})
@ConditionalOnWebApplication(
  type = Type.SERVLET
)
@EnableConfigurationProperties({ServerProperties.class})
@Import({ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class, EmbeddedTomcat.class, EmbeddedJetty.class, EmbeddedUndertow.class})
public class ServletWebServerFactoryAutoConfiguration {
  public ServletWebServerFactoryAutoConfiguration() {
  }

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

  @Bean
  @ConditionalOnClass(
    name = {"org.apache.catalina.startup.Tomcat"}
  )
  public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(ServerProperties serverProperties) {
    return new TomcatServletWebServerFactoryCustomizer(serverProperties);
  }

  public static class BeanPostProcessorsRegistrar implements ImportBeanDefinitionRegistrar, BeanFactoryAware {
    private ConfigurableListableBeanFactory beanFactory;
    public BeanPostProcessorsRegistrar() {
    }

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

    }

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

    }
  }
}

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-web</artifactId>
  <version>${spring.boot.version}</version>
  <exclusions>
    <exclusion>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-logging</artifactId>
    </exclusion>
    <exclusion>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-tomcat</artifactId>
    </exclusion>
    <exclusion>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
    </exclusion>
  </exclusions>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-jetty</artifactId>
  <version>${spring.boot.version}</version>
  <exclusions>
    <exclusion>
      <groupId>org.eclipse.jetty.aggregate</groupId>
      <artifactId>jetty-all</artifactId>
    </exclusion>
  </exclusions>
</dependency>