Kotlin JDBC layer in Javalin REST API

Main.kt

import com.zaxxer.hikari.HikariDataSource
import io.javalin.Javalin
import org.slf4j.LoggerFactory
import java.sql.PreparedStatement
import java.sql.ResultSet
import java.sql.SQLException

val ds = HikariDataSource().apply {
    jdbcUrl = "jdbc:pgsql://localhost/test"
    username = "api"
    password = "dummy123"
}

fun ResultSet.toList(): List<Map<String, Any>> {
    val metadata = this.metaData
    val rows = mutableListOf<Map<String, Any>>()
    while (this.next()) {
        val row = mutableMapOf<String, Any>()
        for (i in 1..metadata.columnCount) {
            row[metadata.getColumnLabel(i)] = this.getObject(i)
        }
        rows.add(row)
    }
    return rows
}

class SQL(private val query: String) {
    private var generated: Array<out String>? = null

    interface UpdateResult {
        val affected: Int
        val generatedKeys: List<Map<String, Any>>
    }

    fun asList(vararg params: Any): List<Map<String, Any>> {
        return prepare(params) { stmt ->
            val rows: List<Map<String, Any>>
            stmt.executeQuery().use { rs ->
                rows = rs.toList()
            }
            rows
        }
    }

    fun generatedKeys(vararg keys: String): SQL {
        generated = keys
        return this
    }

    fun update(vararg params: Any): UpdateResult {
        return prepare(params, generated) { stmt ->
            val affected = stmt.executeUpdate()
            object : UpdateResult {
                override val affected = affected
                override val generatedKeys = stmt.generatedKeys.toList()
            }
        }
    }

    private fun <T> prepare(
        params: Array<out Any>,
        generatedKeys: Array<out String>? = null,
        callback: (stmt: PreparedStatement) -> T
    ): T {
        ds.connection.use { con ->
            when (generatedKeys) {
                null -> con.prepareStatement(query)
                else -> con.prepareStatement(query, generatedKeys)
            }.use { stmt ->
                var idx = 1
                for (param in params) {
                    stmt.setObject(idx++, param)
                }
                return callback(stmt)
            }
        }
    }
}

fun main() {
    val app = Javalin.create().start(8100)

    val logger = LoggerFactory.getLogger("test-api")

    app.exception(SQLException::class.java) { e, ctx ->
        logger.error("An SQLException occurred.", e)
        ctx.result("Sorry, it's on us.")
        ctx.status(500)
    }

    app.get("/") { ctx ->
        ctx.json(
            SQL("SELECT name FROM people").asList()
        )
    }

    app.get("/:length") { ctx ->
        ctx.json(
            SQL("SELECT name FROM people WHERE LENGTH(name) = ?")
                .asList(ctx.pathParam<Int>("length").get())
        )
    }

    app.post("/add") { ctx ->
        val result = SQL("INSERT INTO people (name) VALUES (?)")
            .generatedKeys("id")
            .update(ctx.formParam<String>("name").get())
        ctx.result(
            "Affected ${result.affected} row with id=${result.generatedKeys[0]["id"]} and name=${ctx.formParam("name")}."
        )
    }
    
    app.delete("/delete") { ctx ->
        ctx.json(
            SQL("DELETE FROM people WHERE id = ?")
                .update(ctx.formParam<Int>("id").get())
                .affected
        )
    }
}