Separation of different application responsibilities

SpringBootKotlinApplication.kt

import jakarta.validation.Valid
import org.apache.logging.log4j.LogManager
import org.springframework.data.annotation.Id
import org.springframework.data.relational.core.mapping.Column
import org.springframework.data.relational.core.mapping.Table
import org.springframework.data.repository.CrudRepository
import org.springframework.http.ResponseEntity
import org.springframework.stereotype.Repository
import org.springframework.stereotype.Service
import org.springframework.transaction.annotation.Transactional
import org.springframework.web.bind.annotation.PostMapping
import org.springframework.web.bind.annotation.RequestBody
import org.springframework.web.bind.annotation.RestController
import java.net.URI
import java.time.LocalDateTime


package api

// generated in api model package, classes have no logic!
class Address(val postalCode: String, val houseNumber: String)
class User(val name: String, val age: Int, val address: Address, val id: String? = null)

// generated api interface, needs to be implemented by the application
interface UserApi {
    @PostMapping("/users")
    fun createUser(@Valid @RequestBody user: api.User): ResponseEntity<Void>
}

// controller implementation of the generated api interface
@RestController
class UserController(private val userService: UserService): UserApi {
    override fun createUser(user: api.User): ResponseEntity<Void> {
        val newUser = userService.createUser(domain.User(name = user.name, age = user.age, address = domain.Address(postalCode = user.address.postalCode, houseNumber = user.address.houseNumber)))
        return ResponseEntity.created(URI.create("/users/${newUser.id}")).build()
    }
}


package domain

// domain objects
class Address(val postalCode: String, val houseNumber: String)
class User(val name: String, val age: Int, val address: Address? = null, val id: Long? = null) {
    // domain objects can (should?) include domain logic, for example:
    fun isAdult() = age > 17
    fun isRetired() = age > 67
}

// This service implements all the user related application logic
@Service
class UserService(
    private val userRepository: UserRepository,
    private val eventPublisher: EventPublisher
) {

    private val log = LogManager.getLogger()
    
    /**
     * This function implements everything that is needed for creating a user.
     */
    @Transactional
    fun createUser(user: domain.User): domain.User {
        // We might want to verify some (business) rules before creating the user, for example:
        val existingUser = userRepository.findByName(user.name)
        if(existingUser != null) {
            throw UserAlreadyExistsException("User with name: ${user.name} already exists!")
        }
        // Store user in datastore?
        val newUser = userRepository.save(repository.User(name = user.name, age = user.age))
        log.info("New user created! $newUser")

        // Maybe publish some event to a message queue here...
        eventPublisher.publishUserCreatedEvent(event.UserCreatedEvent( /* etc */ ))

        // And when everything is successfully done, return the newly created user
        return domain.User(name = newUser.name, age = newUser.age, id = newUser.id)
    }
}

class UserAlreadyExistsException(msg: String) : RuntimeException(msg)


package repository

// All datastore related logic, like mapping to a table, etc.

@Table("users")
class User(
    @Id val id: Long? = null,
    @Column("name")
    val name: String,
    @Column("age")
    val age: Int,
    @Column("creation_date_time")
    val creationDateTime: LocalDateTime = LocalDateTime.now()
    /* etc */
)

@Repository
interface UserRepository : CrudRepository<repository.User, Long> {
    fun findByName(name: String): repository.User?
}


package event

// event publishing related logic, again a different concern.
class UserCreatedEvent(/* properties like name? id? creation date, etc. */)

@Service
class EventPublisher {
    fun publishUserCreatedEvent(event: UserCreatedEvent) {
        /* etc */
    }
}