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

private void initialize(Object[] sources) {
    if (sources != null && sources.length > 0) {
      this.sources.addAll(Arrays.asList(sources));
    }
    //判定是否为webEnvironment
    this.webEnvironment = deduceWebEnvironment();
    //实例化并加载所有可以加载的ApplicationContextInitializer
    setInitializers((Collection) getSpringFactoriesInstances(
        ApplicationContextInitializer.class));
    //实例化并加载所有可以加载的ApplicationListener
    setListeners((Collection)
    getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
  }

private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
      Class<?>[] parameterTypes, Object... args) {
      //拿到类加载器
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    // Use names and ensure unique to protect against duplicates
    //使用loadFactoryNames方法载入所有的ApplicationContextInitializer的类全限定名
    Set<String> names = new LinkedHashSet<String>(
        SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    //使用反射将所有的ApplicationContextInitializer实例化   
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
        classLoader, args, names);
    //排序    
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
  }

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    try {
      Enumeration<URL> urls = classLoader != null?classLoader.getResources("META-INF/spring.factories"):ClassLoader.getSystemResources("META-INF/spring.factories");
      ArrayList result = new ArrayList();
      while(urls.hasMoreElements()) {
        URL url = (URL)urls.nextElement();
        Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
        String factoryClassNames = properties.getProperty(factoryClassName);
        result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
      }
      return result;
    } catch (IOException var8) {
      throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
    }
  }

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

public ConfigurableApplicationContext run(String... args) {
    //开启启动计时器，项目启动完会打印执行时间出来
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    FailureAnalyzers analyzers = null;
    configureHeadlessProperty();
    //获取SpringApplicationRunListener并启动监听器
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(
          args);
      //环境变量的加载    
      ConfigurableEnvironment environment = prepareEnvironment(listeners,
          applicationArguments);
      //启动后console的打印出来的一堆配置信息    
      Banner printedBanner = printBanner(environment);
      //终极大boss->ApplicationContext实例化
      context = createApplicationContext();
      analyzers = new FailureAnalyzers(context);
      prepareContext(context, environment, listeners, applicationArguments,
          printedBanner);
      refreshContext(context);
      afterRefresh(context, applicationArguments);
      listeners.finished(context, null);
      stopWatch.stop();
      if (this.logStartupInfo) {
        new StartupInfoLogger(this.mainApplicationClass)
            .logStarted(getApplicationLog(), stopWatch);
      }
      return context;
    }
    catch (Throwable ex) {
      handleRunFailure(context, listeners, analyzers, ex);
      throw new IllegalStateException(ex);
    }
  }

private void prepareContext(ConfigurableApplicationContext context,
      ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
      ApplicationArguments applicationArguments, Banner printedBanner) {
    context.setEnvironment(environment);
    postProcessApplicationContext(context);
    applyInitializers(context);
    listeners.contextPrepared(context);
    if (this.logStartupInfo) {
      logStartupInfo(context.getParent() == null);
      logStartupProfileInfo(context);
    }
    // Add boot specific singleton beans
    context.getBeanFactory().registerSingleton("springApplicationArguments",
        applicationArguments);
    if (printedBanner != null) {
      context.getBeanFactory().registerSingleton("springBootBanner", printedBanner);
    }
    // Load the sources
    Set<Object> sources = getSources();
    Assert.notEmpty(sources, "Sources must not be empty");
    load(context, sources.toArray(new Object[sources.size()]));
    listeners.contextLoaded(context);
  }