一行 run，背后走了四段路

new SpringApplication(ShopApplication.class)          // 1 判断应用类型，读 spring.factories 里的扩展
        .run(args);

public ConfigurableApplicationContext run(String... args) {
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting(bootstrapContext, mainApplicationClass);              // 2 ApplicationStartingEvent
    try {
        ConfigurableEnvironment env = prepareEnvironment(listeners, …);   // 3 读配置
        Banner banner = printBanner(env);                                   // 4
        context = createApplicationContext();                               // 5 一个空容器
        prepareContext(bootstrapContext, context, env, listeners, …);       // 6 跑 Initializer，放进主类
        refreshContext(context);                                            // 7 ~ 11 都在这一行里
        afterRefresh(context, arguments);                                   // 空方法，留给子类
        // 12 打印 "Started ShopApplication in … seconds"
        listeners.started(context, timeTakenToStartup);                     // 12 ApplicationStartedEvent
        callRunners(context, arguments);                                    // 13 ApplicationRunner / CommandLineRunner
    } catch (Throwable ex) {
        throw handleRunFailure(context, ex, listeners);   // 失败：发 ApplicationFailedEvent，FailureAnalyzer 写报告
    }
    listeners.ready(context, timeTakenToReady);                             // 14 ApplicationReadyEvent
    return context;
}