Spring Boot的启动过程是一个精心设计的自动化流程，下面我将详细阐述从main方法开始到内嵌Tomcat启动的全过程。

1. 入口：main方法

一切始于一个简单的main方法：

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

2. SpringApplication初始化

SpringApplication.run()方法内部会创建一个SpringApplication实例：

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {return new SpringApplication(primarySource).run(args);
}

2.1 构造阶段

在SpringApplication构造函数中完成以下关键操作：

推断应用类型：判断是Servlet应用(Spring MVC)还是Reactive应用(Spring WebFlux)
加载ApplicationContextInitializer：通过META-INF/spring.factories加载
加载ApplicationListener：同样通过spring.factories机制加载
推断主配置类：通过堆栈分析找到包含main方法的类

3. 运行阶段：run()方法

run()方法是整个启动过程的核心：

public ConfigurableApplicationContext run(String... args) {// 1. 创建并启动计时器StopWatch stopWatch = new StopWatch();stopWatch.start();// 2. 初始化应用上下文和异常报告器ConfigurableApplicationContext context = null;Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();configureHeadlessProperty();// 3. 获取SpringApplicationRunListeners并启动SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting();try {// 4. 准备环境ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);// 5. 打印BannerBanner printedBanner = printBanner(environment);// 6. 创建应用上下文context = createApplicationContext();// 7. 准备应用上下文prepareContext(context, environment, listeners, applicationArguments, printedBanner);// 8. 刷新应用上下文(关键步骤)refreshContext(context);// 9. 刷新后处理afterRefresh(context, applicationArguments);// 10. 停止计时器并发布启动完成事件stopWatch.stop();if (this.logStartupInfo) {new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);}listeners.started(context);// 11. 执行RunnercallRunners(context, applicationArguments);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, listeners);throw new IllegalStateException(ex);}try {listeners.running(context);}catch (Throwable ex) {handleRunFailure(context, ex, exceptionReporters, null);throw new IllegalStateException(ex);}return context;
}

4. 创建应用上下文

createApplicationContext()方法根据应用类型创建不同的应用上下文：

Servlet环境：创建AnnotationConfigServletWebServerApplicationContext
Reactive环境：创建AnnotationConfigReactiveWebServerApplicationContext
普通环境：创建AnnotationConfigApplicationContext

对于Web应用，会创建AnnotationConfigServletWebServerApplicationContext，它继承自ServletWebServerApplicationContext。

5. 准备应用上下文

prepareContext()方法完成以下工作：

将环境绑定到上下文
后置处理上下文
应用所有初始化器
发布ContextPrepared事件
注册主配置类bean定义
发布ContextLoaded事件

6. 刷新应用上下文

refreshContext()最终调用AbstractApplicationContext.refresh()，这是Spring容器的核心刷新流程：

public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// 1. 准备刷新prepareRefresh();// 2. 获取新的BeanFactoryConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();// 3. 准备BeanFactoryprepareBeanFactory(beanFactory);try {// 4. 后置处理BeanFactorypostProcessBeanFactory(beanFactory);// 5. 调用BeanFactoryPostProcessorinvokeBeanFactoryPostProcessors(beanFactory);// 6. 注册BeanPostProcessorregisterBeanPostProcessors(beanFactory);// 7. 初始化MessageSourceinitMessageSource();// 8. 初始化事件广播器initApplicationEventMulticaster();// 9. 初始化特殊bean(由子类实现)onRefresh();// 10. 注册监听器registerListeners();// 11. 初始化所有非懒加载单例finishBeanFactoryInitialization(beanFactory);// 12. 完成刷新finishRefresh();}catch (BeansException ex) {// 处理异常...}}
}

7. 内嵌Tomcat启动的关键：onRefresh()

对于Servlet Web应用，ServletWebServerApplicationContext重写了onRefresh()方法：

protected void onRefresh() {super.onRefresh();try {createWebServer();}catch (Throwable ex) {throw new ApplicationContextException("Unable to start web server", ex);}
}

createWebServer()是内嵌服务器启动的关键：

private void createWebServer() {WebServer webServer = this.webServer;ServletContext servletContext = getServletContext();if (webServer == null && servletContext == null) {// 1. 获取WebServer工厂(Tomcat, Jetty或Undertow)ServletWebServerFactory factory = getWebServerFactory();// 2. 创建WebServerthis.webServer = factory.getWebServer(getSelfInitializer());}else if (servletContext != null) {try {getSelfInitializer().onStartup(servletContext);}catch (ServletException ex) {throw new ApplicationContextException("Cannot initialize servlet context", ex);}}initPropertySources();
}

8. Tomcat服务器创建过程

以Tomcat为例，TomcatServletWebServerFactory.getWebServer()方法：

public WebServer getWebServer(ServletContextInitializer... initializers) {// 1. 创建Tomcat实例Tomcat tomcat = new Tomcat();// 2. 配置基础目录File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");tomcat.setBaseDir(baseDir.getAbsolutePath());// 3. 配置连接器Connector connector = new Connector(this.protocol);connector.setThrowOnFailure(true);tomcat.getService().addConnector(connector);customizeConnector(connector);tomcat.setConnector(connector);// 4. 配置Hosttomcat.getHost().setAutoDeploy(false);configureEngine(tomcat.getEngine());// 5. 准备上下文prepareContext(tomcat.getHost(), initializers);// 6. 创建TomcatWebServer并启动return getTomcatWebServer(tomcat);
}

9. 启动Tomcat

在TomcatWebServer构造函数中完成Tomcat的启动：

public TomcatWebServer(Tomcat tomcat, boolean autoStart) {this.tomcat = tomcat;this.autoStart = autoStart;initialize();
}private void initialize() throws WebServerException {// 启动Tomcatthis.tomcat.start();// 启动一个守护线程来等待停止命令startDaemonAwaitThread();
}

10. 自动配置的关键

整个过程中，自动配置是通过@SpringBootApplication注解中的@EnableAutoConfiguration实现的：

在invokeBeanFactoryPostProcessors()阶段会处理自动配置
AutoConfigurationImportSelector会加载META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports文件中的配置类
对于Tomcat，会加载ServletWebServerFactoryAutoConfiguration
这个配置类通过@Import引入了EmbeddedTomcat等配置