Spring Boot啟動流程及源碼實現深度解析

一、啟動流程概述

Spring Boot的啟動流程圍繞SpringApplication類展開，核心流程可分為以下幾個階段：

1. 初始化階段：推斷應用類型，加載ApplicationContextInitializer和ApplicationListener
2. 環境準備：加載配置文件和命令行參數
3. 上下文創建：實例化ApplicationContext
4. 上下文刷新：執行refresh()方法完成Bean加載
5. 后置處理：執行CommandLineRunner和ApplicationRunner

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二、源碼解析

1. 入口類

@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(); // 推斷應用類型setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class)); // 加載InitializerssetListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class)); // 加載Listenersthis.mainApplicationClass = deduceMainApplicationClass();
}

關鍵步驟解析：

• deduceFromClasspath()通過類路徑判斷應用類型（Servlet/Reactive/None）
• 從META-INF/spring.factories加載初始化器和監聽器

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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);// 創建應用上下文context = createApplicationContext();context.setEnvironment(environment);// 準備上下文prepareContext(context, environment, listeners, applicationArguments, printedBanner);// 刷新上下文（核心）refreshContext(context);// 后置處理afterRefresh(context, applicationArguments);stopWatch.stop();// 發布啟動完成事件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、關鍵階段詳解

1. 環境準備（prepareEnvironment）

private ConfigurableEnvironment prepareEnvironment(SpringApplicationRunListeners listeners,ApplicationArguments applicationArguments) {ConfigurableEnvironment environment = getOrCreateEnvironment();configureEnvironment(environment, applicationArguments.getSourceArgs());ConfigurationPropertySources.attach(environment);listeners.environmentPrepared(environment); // 發布環境準備事件bindToSpringApplication(environment);return environment;
}

• 加載application.properties/yml文件
• 處理命令行參數--開頭的參數
• 觸發ApplicationEnvironmentPreparedEvent事件

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2. 上下文創建（createApplicationContext）

根據應用類型創建不同的上下文：

protected ConfigurableApplicationContext createApplicationContext() {return this.applicationContextFactory.create(this.webApplicationType);
}// 默認實現
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();
}

最終調用AbstractApplicationContext.refresh()，這是Spring容器的核心方法：

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

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四、關鍵擴展點

1. ApplicationContextInitializer

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

• 執行時機：上下文準備階段（prepareContext）
• 配置方式：通過spring.factories或SpringApplication.addInitializers()

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2. ApplicationRunner/CommandLineRunner

@Component
public class DemoRunner implements ApplicationRunner {@Overridepublic void run(ApplicationArguments args) {// 應用啟動后執行}
}

• 執行順序：通過@Order注解控制
• 執行時機：上下文刷新完成后

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五、總結

Spring Boot的啟動流程通過智能的自動配置和擴展機制，顯著簡化了Spring應用的初始化過程。理解其核心流程和關鍵擴展點，可以幫助開發者：

1. 深入排查啟動過程中的問題
2. 實現自定義的初始化邏輯
3. 優化應用啟動性能
4. 擴展框架的核心功能

建議結合源碼調試工具，通過斷點跟蹤SpringApplication.run()的執行過程，可以更直觀地理解各階段的實現細節。

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流程圖文字描述：

main()
└─? SpringApplication.run()├─? 初始化應用類型和擴展組件├─? 準備環境（加載配置）├─? 創建ApplicationContext├─? 準備上下文（Bean定義加載）├─? 刷新上下文（Bean初始化）├─? 執行Runner接口└─? 完成啟動

通過以上分析，讀者可以系統地掌握Spring Boot的啟動機制及其實現原理。實際開發中可結合具體需求，合理使用擴展點進行定制化開發。