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

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

    public SpringApplication(ResourceLoader resourceLoader, Class... primarySources) {
        this.sources = new LinkedHashSet();
        this.bannerMode = Mode.CONSOLE;
        this.logStartupInfo = true;
        this.addCommandLineProperties = true;
        this.addConversionService = true;
        this.headless = true;
        this.registerShutdownHook = true;
        this.additionalProfiles = new HashSet();
        this.isCustomEnvironment = false;
        this.resourceLoader = resourceLoader;
        Assert.notNull(primarySources, "PrimarySources must not be null");
    // 把項(xiàng)目啟動(dòng)類.class設(shè)置為屬性存儲起來
        this.primarySources = new LinkedHashSet(Arrays.asList(primarySources));
    // 判斷當(dāng)前webApplicationType應(yīng)用的類型
        this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // 設(shè)置初始化器(Initializer),最后會調(diào)用這些初始化器
        this.setInitializers(this.getSpringFactoriesInstances(ApplicationContextInitializer.class));
    // 設(shè)置監(jiān)聽器(Listener)
        this.setListeners(this.getSpringFactoriesInstances(ApplicationListener.class));
    // 用于推斷并設(shè)置項(xiàng)目main()方法啟動(dòng)的主程序啟動(dòng)類
        this.mainApplicationClass = this.deduceMainApplicationClass();

  public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
        this.configureHeadlessProperty();
    // 第一步：獲取并啟動(dòng)監(jiān)聽器
        SpringApplicationRunListeners listeners = this.getRunListeners(args);
        listeners.starting();
        Collection exceptionReporters;
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
    // 第二步：根據(jù)SpringApplicationRunListeners以及參數(shù)來準(zhǔn)備環(huán)境
            ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
            this.configureIgnoreBeanInfo(environment);
    // 準(zhǔn)備Banner打印器 - 就是啟動(dòng)Spring Boot的時(shí)候打印在console上的ASCII藝術(shù)字體
            Banner printedBanner = this.printBanner(environment);
    // 第三步：創(chuàng)建Spring容器
            context = this.createApplicationContext();
            exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, new Object[]{context});
    // 第四步：Spring容器前置處理
            this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
    // 第五步：刷新容器
            this.refreshContext(context);
    // 第六步：Spring容器后置處理
            this.afterRefresh(context, applicationArguments);
            stopWatch.stop();
            if (this.logStartupInfo) {
                (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
            }
    // 第七步：發(fā)出結(jié)束執(zhí)行的事件
            listeners.started(context);
    // 返回容器
            this.callRunners(context, applicationArguments);
        } catch (Throwable var10) {
            this.handleRunFailure(context, var10, exceptionReporters, listeners);
            throw new IllegalStateException(var10);
        }
        try {
            listeners.running(context);
            return context;
        } catch (Throwable var9) {
            this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners) null);
            throw new IllegalStateException(var9);
        }
    }