What happens during SpringBoot startup.

First is the SpringApplication.run method, which takes a primary source and then parses and loads the configuration (which is the App.class passed in).

Then a SpringApplication is created, which checks what type of project it is: this.webApplicationType = WebApplicationType.deduceFromClasspath();

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

The parameters are passed in, and then run() is called to create a ConfigurableApplicationContext based on the project type.

public ConfigurableApplicationContext run(String... args) {
    // Start the timer
		StopWatch stopWatch = new StopWatch();
		stopWatch.start();
		ConfigurableApplicationContext context = null;
		Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
		configureHeadlessProperty();
        // This is used to manage multiple listeners. When created, it retrieves the names of the listeners to be created from META-INF/spring.factories and creates instances.
		SpringApplicationRunListeners listeners = getRunListeners(args);
        // Start the listener
		listeners.starting();
		try {
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(
					args);
            // Create runtime arguments
			ConfigurableEnvironment environment = prepareEnvironment(listeners,
					applicationArguments);
			configureIgnoreBeanInfo(environment);
            // Output the Spring Boot banner
			Banner printedBanner = printBanner(environment);
            // Create the application context
			context = createApplicationContext();
            // Exception reporting
			exceptionReporters = getSpringFactoriesInstances(
					SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
            // Pre-configure the context
			prepareContext(context, environment, listeners, applicationArguments,
					printedBanner);
            // Refresh the context
			refreshContext(context);
			afterRefresh(context, applicationArguments);
            // Stop the timer
			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;
	}

The listeners are used to create these things:

# 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

This means that we can customize listeners to listen to the corresponding events by implementing ApplicationListener.

public class MyListener implements ApplicationListener {

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

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

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

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

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

Add the following to META-INF/spring.factories:

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

Or you can manually add it:

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