How can I abort Spring-Boot startup?

Get the Application Context, e.g.:

@Autowired
private ConfigurableApplicationContext ctx;

Then call the close method if you cannot find the directory:

ctx.close();

That gracefully shutdowns the Application Context and thus the Spring Boot Application itself.

Update:

A more detailed example based on the code provided in the Question.

Main Class

@SpringBootApplication
public class GracefulShutdownApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext ctx = SpringApplication.run(GracefulShutdownApplication.class, args);
        try{
            ctx.getBean("watchService");
        }catch(NoSuchBeanDefinitionException e){
            System.out.println("No folder to watch...Shutting Down");
            ctx.close();
        }
    }

}

WatchService Configuration

@Configuration
public class WatchServiceConfig {

    @Value("${dirPath}")
    private String dirPath;

    @Conditional(FolderCondition.class)
    @Bean
    public WatchService watchService() throws IOException {
        WatchService watchService = null;
        watchService = FileSystems.getDefault().newWatchService();
        Paths.get(dirPath).register(watchService, ENTRY_CREATE);
        System.out.println("Started watching directory");
        return watchService;
    }

Folder Condition

public class FolderCondition implements Condition{

    @Override
    public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
        String folderPath = conditionContext.getEnvironment().getProperty("dirPath");
        File folder = new File(folderPath);
        return folder.exists();
    }
}

Make the WatchService Bean @Conditional based on whether the directory is present or not. Then in your Main Class, check whether the WatchService Bean exists, and if not shutdown the Application Context by calling close().

The accepted answer is correct, but unnecessarily complex. There's no need for a Condition, and then checking for the existence of the bean, and then closing the ApplicationContext. Simply checking for the presence of the directory during WatchService creation, and throwing an exception will abort the application startup due to failure to create the bean.