首先是SpringApplication.run 方法，方法拿到一个 primary source 然后解析加载配置（就是那个传进来的App.class）。

然后创建了个SpringApplication,创建的时候会检测这个是什么类型的项目，this.webApplicationType = WebApplicationType.deduceFromClasspath();

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

把参数传进去，然后run(),根据项目类型创建ConfigurableApplicationContext，

public ConfigurableApplicationContext run(String... args) {
    // 启动计时器
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
        // 这个东西用来管理多个listener，在创建的时候，从 META-INF/spring.factories拿到要创建的listener名字，创建实例
		SpringApplicationRunListeners listeners = getRunListeners(args);
        // 启动listener
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
					args);
            // 创建运行参数
			ConfigurableEnvironment environment = prepareEnvironment(listeners,
					applicationArguments);
			configureIgnoreBeanInfo(environment);
            // 输出开头那段Spring boot 什么什么的
			Banner printedBanner = printBanner(environment);
            // 创建上下文
			context = createApplicationContext();
            // 异常报告
			exceptionReporters = getSpringFactoriesInstances(
					SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
            //前置上下文配置
			prepareContext(context, environment, listeners, applicationArguments,
					printedBanner);
            // 刷新上下文
			refreshContext(context);
			afterRefresh(context, applicationArguments);
            // 计时完毕
			stopWatch.stop();
			if (this.logStartupInfo) {
				new StartupInfoLogger(this.mainApplicationClass)
						.logStarted(getApplicationLog(), stopWatch);
			}
			listeners.started(context);
			callRunners(context, applicationArguments);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, listeners);
			throw new IllegalStateException(ex);
		}

		try {
			listeners.running(context);
		}
		catch (Throwable ex) {
			handleRunFailure(context, ex, exceptionReporters, null);
			throw new IllegalStateException(ex);
		}
		return context;
	}

listeners 是拿到这些名字来创建东西的：

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

也就是说，我们是可以自定义Listener然后，监听相应的Event

只要实现ApplicationListener

/**
 * @author dqn
 * created at 2019/2/10 19:20
 */
public class MyListener implements ApplicationListener {

    @Override
    public void onApplicationEvent(ApplicationEvent event) {
        // 监听ApplicationStartingEvent
        if (event instanceof ApplicationStartedEvent) {
            System.out.println("ApplicationStartedEvent listened");
        }

        // 监听ApplicationEnvironmentPreparedEvent
        else if (event instanceof ApplicationEnvironmentPreparedEvent) {
            System.out.println("ApplicationEnvironmentPreparedEvent listened");
        }

        // 监听ApplicationPreparedEvent
        else if (event instanceof ApplicationPreparedEvent) {
            System.out.println("ApplicationPreparedEvent listened");
        }

        // 监听ApplicationReadyEvent
        else if (event instanceof ApplicationReadyEvent) {
            System.out.println("ApplicationReadyEvent listened");
        }

        // 监听ApplicationFailedEvent
        else if (event instanceof ApplicationFailedEvent) {
            System.out.println("ApplicationFailedEvent listened");
        }
    }
}

在META-INF/spring.factories加上

org.springframework.context.ApplicationListener=\
me.dqn.listener.MyListener

即可。当然也可以手动添加：

public static void main(String[] args) {
    SpringApplication application = new SpringApplication(App.class);
    application.addListeners(new MyListener());
    application.run(args);
}