为什么SpringBoot项目不需要web.xml原创

/*
* Central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers
* or HTTP-based remote service exporters. Dispatches to registered handlers for processing
* a web request, providing convenient mapping and exception handling facilities.
 
* 用于HTTP请求处理程序/控制器的中央调度器，例如用于Web UI控制器或基于HTTP的远程服务导出器。
* 调度到注册的处理程序以处理Web请求，从而提供方便的映射和异常处理工具。
*/

@SpringBootApplication
public class DemoSpringmvcApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoSpringmvcApplication.class, args);
    }
}

public class SpringApplication {
  ...
  public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
    return new SpringApplication(primarySources).run(args);
  } 
  ...
  public ConfigurableApplicationContext run(String... args) {
    ...
		try {
			...
			context = createApplicationContext();
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			refreshContext(context);
			afterRefresh(context, applicationArguments);
			...
			}
			...
		}
		...
	}
  ...
  private void refreshContext(ConfigurableApplicationContext context) {
		refresh(context);
		...
	}
  ...
  protected void refresh(ApplicationContext applicationContext) {
		Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
		((AbstractApplicationContext) applicationContext).refresh();
	}
}

protected ConfigurableApplicationContext createApplicationContext() {
   Class<?> contextClass = this.applicationContextClass;
   if (contextClass == null) {
      try {
         switch (this.webApplicationType) {
         case SERVLET:
            contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
            break;
         case REACTIVE:
            contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
            break;
         default:
            contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
         }
      }
      ...
   }
   return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}

public AnnotationConfigServletWebServerApplicationContext() {
   this.reader = new AnnotatedBeanDefinitionReader(this);
   this.scanner = new ClassPathBeanDefinitionScanner(this);
}

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
   ...
   AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}

public abstract class AnnotationConfigUtils {
  public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
        BeanDefinitionRegistry registry, @Nullable Object source) {
     ...
     Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);
     if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
        RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
        def.setSource(source);
        beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
     }
     ...
	}
}

public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext {
		@Override
    public void refresh() throws BeansException, IllegalStateException {
       synchronized (this.startupShutdownMonitor) {
          // 为刷新做准备，例如
          // 初始化上下文环境中的所有占位符属性源;验证是否所有标记为必需的属性都是可解析的
          prepareRefresh();
          // 获得子类中刷新的Bean工厂
          ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
          // 准备好在此上下文中使用的bean工厂
          prepareBeanFactory(beanFactory);
          try {
             // 允许在上下文子类中对bean工厂进行后处理
             postProcessBeanFactory(beanFactory);
            // 调用上下文中注册为bean的工厂处理器
            invokeBeanFactoryPostProcessors(beanFactory);
             ...
          }
          ...
       }
    }		
    protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
      PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
      ...
    }
}

public Iterable<Group.Entry> getImports() {
  for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
    this.group.process(deferredImport.getConfigurationClass().getMetadata(),
                       deferredImport.getImportSelector());
  }
  return this.group.selectImports();
}

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
      AnnotationMetadata annotationMetadata) {
   ...
   List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
   configurations = removeDuplicates(configurations);
   ...
}

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
         getBeanClassLoader());
  ...
}

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
		MultiValueMap<String, String> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		try {
			Enumeration<URL> urls = (classLoader != null ?
					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
			result = new LinkedMultiValueMap<>();
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
					String factoryTypeName = ((String) entry.getKey()).trim();
					for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryTypeName, factoryImplementationName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}

/* 
* Interface used to configure a Servlet 3.0+ {@link ServletContext context}
* programmatically. Unlike {@link WebApplicationInitializer}, classes that implement this
* interface (and do not implement {@link WebApplicationInitializer}) will <b>not</b> be
* detected by {@link SpringServletContainerInitializer} and hence will not be
* automatically bootstrapped by the Servlet container.
* <p>
* This interface is designed to act in a similar way to
* {@link ServletContainerInitializer}, but have a lifecycle that's managed by Spring and
* not the Servlet container.

* 用于以编程方式配置Servlet 3.0+{@link ServletContext Context}的接口。
* 与{@link WebApplicationInitializer}不同，
* 实现此接口(且不实现{@link WebApplicationInitializer})的类不会被{@link SpringServletContainerInitializer}检测到，
* 因此不会由Servlet容器自动引导。此接口的设计方式与{@link ServletContainerInitializer}类似，但其生命周期由Spring管理，而不是Servlet容器

public class ServletWebServerApplicationContext extends GenericWebApplicationContext
		implements ConfigurableWebServerApplicationContext {
		...
		@Override
    protected void onRefresh() {
       super.onRefresh();
       try {
          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) {
        ServletWebServerFactory factory = getWebServerFactory();
        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();
    }
		...
}

public interface ServletContainerInitializer {  
    void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException;
}

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
	public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
      throws ServletException {
     List<WebApplicationInitializer> initializers = new LinkedList<>();
     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);
              }
           }
        }
     }
     ...
     for (WebApplicationInitializer initializer : initializers) {
        initializer.onStartup(servletContext);
     }
  }
}

protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
   SpringApplicationBuilder builder = createSpringApplicationBuilder();
	 ...
   builder.initializers(new ServletContextApplicationContextInitializer(servletContext));
   ...
   return run(application);
}

private void prepareContext(ConfigurableApplicationContext context, ConfigurableEnvironment environment,
      SpringApplicationRunListeners listeners, ApplicationArguments applicationArguments, Banner printedBanner) {
   context.setEnvironment(environment);
   postProcessApplicationContext(context);
   applyInitializers(context);
   ...
}