springboot内置Tomcat流程

1、org.springframework.boot.SpringApplication#initialize

setInitializers((Collection) getSpringFactoriesInstances(
				ApplicationContextInitializer.class));

加载了org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext

2、spring refresh 中onRefresh回调中启动了内置Tomcat

ServletWebServerApplicationContext继承AbstractApplicationContext，并且重新实现了onRefresh方法。

	//重写了refresh方法，启动
    @Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

    //启动Tomcat
	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();
	}

    //如果Tomcat启动了，就发布服务ServletWebServerInitializedEvent
	@Override
	protected void finishRefresh() {
		super.finishRefresh();
		WebServer webServer = startWebServer();
		if (webServer != null) {
			publishEvent(new ServletWebServerInitializedEvent(webServer, this));
		}
	}