五、嵌入式Servlet容器启动原理

什么时候创建嵌入式Servlet容器工厂？什么时候获取嵌入式的Servlet容器并启动Tomcat？

获取嵌入式的Servlet容器工厂
1：SpringBoot应用启动run方法
2：refreshContext(context);SpringBoot刷新IOC容器，初始化IOC容器
此时需要进行一个判断：如果是web应用则创建
AnnotationConfigEmbeddedWebApplicationContext容器，如果不是web应用则创建AnnotationConfigApplicationContext容器

3：刷新初始化IOC容器主要步骤

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

4：onRefresh()，web的IOC容器重写了onRefresh方法
5：web的IOC容器会创建嵌入式的Servlet
（org.springframework.boot.context.embedded.EmbeddedWebApplicationContext#createEmbeddedServletContainer()）

6：获取嵌入式的Servlet容器工厂，从IOC容器中获取
EmbeddedServletContainerFactory组件，EmbeddedServletContainerFactory创建对象，后置处理器处理这个对象，就获取所有的定制器来定制Servlet容器的相关配置

7：现在已经获取到了容器工厂，如下就通过容器工厂获取嵌入式的Servlet容器（tomcat）

8：先启动嵌入式的Servlet容器，再将IOC容器中剩下没有创建出的对象取出来，IOC容器启动嵌入式的Servlet容器