工作三年，小胖问我 SpringBoot 是怎么启动的？真的离谱！

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>xxx</version>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>xxx</version>
</dependency>

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

o.s.b.w.embedded.tomcat.TomcatWebServer :Tomcat started on port(s): 8080 (http) with context path

<dependency>
 <groupId>org.springframework.boot</groupId>
 <artifactId>spring-boot-starter-web</artifactId>
 <!-- 移处 Tomcat -->
 <exclusions>
  <exclusion>
   <groupId>org.springframework.boot</groupId>
   <artifactId>spring-boot-starter-tomcat</artifactId>
  </exclusion>
 </exclusions>
</dependency>
<!-- 换成 jetty 容器 -->
<dependency>
 <groupId>org.springframework.boot</groupId>
 <artifactId>spring-boot-starter-jetty</artifactId>
</dependency>

o.e.jetty.server.AbstractConnector: Started ServerConnector@53f9009d{HTTP/1.1, (http/1.1)}{0.0.0.0:8080}
o.s.b.web.embedded.jetty.JettyWebServer

SpringApplication.run (Application.class, args)

public ConfigurableApplicationContext run(String...args) {

    // 1.创建并启动计时监控类
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    // 2.声明应用上下文对象和异常报告集合
    ConfigurableApplicationContext context = null;
    Collection < SpringBootExceptionReporter > exceptionReporters = new ArrayList();
    // 3.设置系统属性 headless 的值
    this.configureHeadlessProperty();
    // 4.创建所有 Spring 运行监听器并发布应用启动事件
    SpringApplicationRunListeners listeners = this.getRunListeners(args);
    listeners.starting();
    Collection exceptionReporters;
    
    try {
        // 5.处理 args 参数
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        // 6.准备环境
        ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
        this.configureIgnoreBeanInfo(environment);
        // 7.创建 Banner 的打印类
        Banner printedBanner = this.printBanner(environment);
        // 8.创建应用上下文
        context = this.createApplicationContext();
        // 9.实例化异常报告器
        exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[] {
            ConfigurableApplicationContext.class
        }, context);
        // 10.准备应用上下文
        this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        // 11.刷新应用上下文
        this.refreshContext(context);
        // 12.应用上下文刷新之后的事件的处理
        this.afterRefresh(context, applicationArguments);
        // 13.停止计时监控类
        stopWatch.stop();
        // 14.输出日志记录执行主类名、时间信息
        if (this.logStartupInfo) {
            (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
        }
        // 15.发布应用上下文启动完成事件
        listeners.started(context);
        // 16.执行所有 Runner 运行器
        this.callRunners(context
    } catch (Throwable var10
        this.handleRunFailure(context, var10, exceptionReporters, listeners
        throw new IllegalStateException(var10);
    }

    try {
        // 17.发布应用上下文就绪事件
        listeners.running(context);
        // 18.返回应用上下文对象
        return context;
    } catch (Throwable var9) {
        this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners) null);
        throw new IllegalStateException(var9);
    }

}

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