OrderRepository.kt

import org.springframework.jdbc.core.JdbcTemplate
import org.springframework.jdbc.core.RowMapper
import org.springframework.stereotype.Repository
import java.util.*

@Repository
class OrderRepository(private val jdbcTemplate: JdbcTemplate) {
    private val rowMapper: RowMapper<Order> = RowMapper { rs, i -> Order(rs.getLong("id")) }

    fun findOrderById(id: Long): Optional<Order> {
        val result = jdbcTemplate.query<Order>("select * from orders where id=:id", rowMapper, id)
        if (result.size > 1) throw RuntimeException("Too many results from findOrderById.")
        return if (result.size == 1) Optional.of(result.get(0)) else Optional.empty()
    }
}