How to add sample data to database with springboot?

I've a Spring Boot application with the Spring Data JPA and two entities mapped to a database. My application is set to recreate the database on every startup. Now i want to create some instances of these POJOs, persist them in the database so I've some data to test my application with in the ongoing development process.

What is the best way to do that?

What i tried so far:
My class to add the sample data

public class DBSampleAdditor {
    @PersistenceContext
    private EntityManager em;

    @Transactional
    public void addSampleData() {
        // creating POJO instances and persist them with em.perists()
    }
}

I tried to call these functions in the main of my ApplicationClass

@SpringBootApplication()
public class ApiApplication {
	public static void main(String[] args) {
		SpringApplication.run(ApiApplication.class, args);
        DBSampleAdditor dbsa = new DBSampleAdditor();
        dbsa.addSampleData();
	}
}

That didnt work at all, as the EntityManager never got an Instance from the persistence unit.

I also tried to create a CommandLineRunner:

@Component
public class PTCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("Adding sample data...");
        DBSampleAdditor dbsa = new DBSampleAdditor();
        dbsa.addSampleData();
    }
}

But that one seemed to never been called in the startup process.

You can use method with @PostConstruct with @Component to insert data at startup.

@PostConstruct will be invoked after the bean has been created, dependencies have been injected, all managed properties are set, and before the bean is actually set into scope.

Just inject your database repository in the class marked with @Component and call your injected database repository inside the method marked with @PostConstruct

Example:

@Component
public class DbInit {
 
    @Autowired
    private DBSampleAdditor dbsa;
 
    @PostConstruct
    private void postConstruct() {
        dbsa.addSampleData();
    }
}

In order to use spring managed components like the EntityManager the objects need to spring Beans, which basically means they are created inside the spring context and can utilise the IoC container.

In the example above, running the DbSampleAdditor class from the main method is outside the spring context, so the beans like PersistenceContext wont get injected.

One fairly simple way to do this inside the spring context is to add an ApplicationListener for the ApplicationReadyEvent

@Component
class Initialise implements ApplicationListener<ApplicationReadyEvent> {

    @Autowired
    private final DbSampleAdditor db;

    @Override
    public void onApplicationEvent(ApplicationReadyEvent applicationReadyEvent) {
        db.addSampleData();
   }
}

When the application ready event fires all the necessary spring bean wiring is set up so when it runs the addSampleData method, things like the EM are good to go.

Other approaches for setting up a DB for spring boot are documented here

Instead of creating an object DBSampleAdditor, try to autowire it so that it is available on application startup. @Autowired DBSampleAdditor DBSampleAdditor Then you try to add sample data within the run method