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Spring Boot启动流程及源码实现深度解析

一、启动流程概述

Spring Boot的启动流程围绕SpringApplication类展开，核心流程可分为以下几个阶段：

1. 初始化阶段：推断应用类型，加载ApplicationContextInitializer和ApplicationListener
2. 环境准备：加载配置文件和命令行参数
3. 上下文创建：实例化ApplicationContext
4. 上下文刷新：执行refresh()方法完成Bean加载
5. 后置处理：执行CommandLineRunner和ApplicationRunner

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二、源码解析

1. 入口类

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

2. SpringApplication初始化

// SpringApplication.java
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {return new SpringApplication(primarySource).run(args);
}public SpringApplication(Class<?>... primarySources) {this(null, primarySources);
}private SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {this.resourceLoader = resourceLoader;this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));this.webApplicationType = WebApplicationType.deduceFromClasspath(); // 推断应用类型setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class)); // 加载InitializerssetListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); // 加载Listenersthis.mainApplicationClass = deduceMainApplicationClass();
}

关键步骤解析：

• deduceFromClasspath()通过类路径判断应用类型（Servlet/Reactive/None）
• 从META-INF/spring.factories加载初始化器和监听器

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3. run()方法核心流程

public ConfigurableApplicationContext run(String... args) {StopWatch stopWatch = new StopWatch();stopWatch.start();ConfigurableApplicationContext context = null;SpringApplicationRunListeners listeners = getRunListeners(args);listeners.starting();try {ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);configureIgnoreBeanInfo(environment);Banner printedBanner = printBanner(environment);// 创建应用上下文context = createApplicationContext();context.setEnvironment(environment);// 准备上下文prepareContext(context, environment, listeners, applicationArguments, printedBanner);// 刷新上下文（核心）refreshContext(context);// 后置处理afterRefresh(context, applicationArguments);stopWatch.stop();// 发布启动完成事件listeners.started(context);callRunners(context, applicationArguments);} catch (Throwable ex) {handleRunFailure(context, ex, listeners);throw new IllegalStateException(ex);}listeners.running(context);return context;
}

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三、关键阶段详解

1. 环境准备（prepareEnvironment）

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments) {ConfigurableEnvironment environment = getOrCreateEnvironment();configureEnvironment(environment, applicationArguments.getSourceArgs());ConfigurationPropertySources.attach(environment);listeners.environmentPrepared(environment); // 发布环境准备事件bindToSpringApplication(environment);return environment;
}

• 加载application.properties/yml文件
• 处理命令行参数--开头的参数
• 触发ApplicationEnvironmentPreparedEvent事件

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2. 上下文创建（createApplicationContext）

根据应用类型创建不同的上下文：

protected ConfigurableApplicationContext createApplicationContext() {return this.applicationContextFactory.create(this.webApplicationType);
}// 默认实现
ApplicationContextFactory DEFAULT = (webApplicationType) -> {try {switch (webApplicationType) {case SERVLET:return new AnnotationConfigServletWebServerApplicationContext();case REACTIVE:return new AnnotationConfigReactiveWebServerApplicationContext();default:return new AnnotationConfigApplicationContext();}} catch (Exception ex) {throw new IllegalStateException(...);}
};

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3. 上下文刷新（refreshContext）

private void refreshContext(ConfigurableApplicationContext context) {refresh(context);if (this.registerShutdownHook) {try {context.registerShutdownHook();} catch (AccessControlException ex) {// Not allowed in some environments.}}
}protected void refresh(ApplicationContext applicationContext) {((AbstractApplicationContext) applicationContext).refresh();
}

最终调用AbstractApplicationContext.refresh()，这是Spring容器的核心方法：

public void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {prepareRefresh();ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();prepareBeanFactory(beanFactory);// ... [省略其他步骤]finishRefresh(); // 触发ContextRefreshedEvent}
}

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四、关键扩展点

1. ApplicationContextInitializer

public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {void initialize(C applicationContext);
}

• 执行时机：上下文准备阶段（prepareContext）
• 配置方式：通过spring.factories或SpringApplication.addInitializers()

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2. ApplicationRunner/CommandLineRunner

@Component
public class DemoRunner implements ApplicationRunner {@Overridepublic void run(ApplicationArguments args) {// 应用启动后执行}
}

• 执行顺序：通过@Order注解控制
• 执行时机：上下文刷新完成后

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五、总结

Spring Boot的启动流程通过智能的自动配置和扩展机制，显著简化了Spring应用的初始化过程。理解其核心流程和关键扩展点，可以帮助开发者：

1. 深入排查启动过程中的问题
2. 实现自定义的初始化逻辑
3. 优化应用启动性能
4. 扩展框架的核心功能

建议结合源码调试工具，通过断点跟踪SpringApplication.run()的执行过程，可以更直观地理解各阶段的实现细节。

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流程图文字描述：

main()
└─▶ SpringApplication.run()├─▶ 初始化应用类型和扩展组件├─▶ 准备环境（加载配置）├─▶ 创建ApplicationContext├─▶ 准备上下文（Bean定义加载）├─▶ 刷新上下文（Bean初始化）├─▶ 执行Runner接口└─▶ 完成启动

通过以上分析，读者可以系统地掌握Spring Boot的启动机制及其实现原理。实际开发中可结合具体需求，合理使用扩展点进行定制化开发。