How to Create a Database Seeder in Spring Boot

Spring Boot is an awesome Java web framework that is very comparable to Laravel web framework (in PHP). They both aim at making web application development fast and less rigorous for a developer.

I am a Java lover who courts Laravel due to professional requirements. Laravel framework has a feature that lets you create database seeders — i.e. default data to be inserted into the database during application installation. 

Let’s set out to achieve the same thing in Spring Boot. The technique we will use is simple and straightforward: first, we will create a listener that listens to the application’s ContextRefreshEvent.

The event is fired when the application has been totally bootstrapped and all bean objects have been instantiated. Then, we will use the Models and configured repositories to persist default data into the database.

Registering the Event Listener

In Spring Boot, we can register an event listener by just declaring that event as a parameter of a method and then annotating the method with @EventListener. During application startup, Spring Boot will find the event listener and register it automatically:

@EventListener
public void seed(ContextRefreshedEvent event) {
    seedUsersTable();
    seedCategoryTable();
    seedSectionsTable();
}

Creating the Database Seeder

So far, we have registered an event listener and have called different methods to seed the database accordingly. One crucial advantage of running the seeders when the ContextRefreshEvent is fired is that we get access to all auto-wired beans in the application — including models and repositories.

In the seedUsersTable() and other seeder methods, we first check if the data exists already by trying to select it using SQL. If the data does not exist, we use the model and repository to persist the data else we just log a message.

private void seedUsersTable() {
        String sql = "SELECT username, email FROM users U WHERE U.username = \"admin\" OR " +
                "U.email = \"test@test.com\" LIMIT 1";
        List<User> u = jdbcTemplate.query(sql, (resultSet, rowNum) -> null);
        if(u == null || u.size() <= 0) {
             User user = new User();
             user.setName("Spring Blog");
             user.setUsername("admin");
             user.setEmail("test@test.com");
             user.setPassword(new BCryptPasswordEncoder().encode("test123"));
             user.setRole(Roles.SUPER_ADMIN.toString());
             user.setBanned(false);
             user.setConfirmEmail(true);
             userRepository.save(user);
             logger.info("Users Seeded");
        } else {
            logger.info("Users Seeding Not Required");
        }
    }

Conclusion

We have seen how to seed the database on application startup. We can use seeders to create default admin accounts or other default data required by our application. 

The complete source code can be found here.