Ktor API

Set up the REST API skeleton with Ktor.

Routing with the Api class

The second lesson adds the Api class. It takes a UserRepo; the route(path) method interprets a URL path. This mimics Ktor's routing layer at a small scale.

Example: class Api(val repo: UserRepo). Calling route("/users/1") extracts id=1, finds the user via repo.find(1) and returns its name. Unknown paths return "404". This pattern connects HTTP requests to business logic.

route and startsWith

The route method recognizes the path with path.startsWith("/users/"). With removePrefix it extracts the id part; with toInt() it converts it to a number.

Example: path.removePrefix("/users/").toInt(). The path "/users/1" gives id 1. This value is passed to repo.find(id). This pattern is a concise way to extract parameters from a URL; it resembles Ktor's @Location pattern.

Exercise Add the Api class

Keep the existing User and UserRepo definitions. Add class Api(val repo: UserRepo) with method fun route(path: String): String. The method, if path.startsWith("/users/"), extracts the id and returns repo.find(id)?.name ?: "not found"; otherwise returns "404". In main add User(1, "Alex"), create Api(repo) and print println(api.route("/users/1")).

Show example solution

data class User(val id: Int, val name: String)

class UserRepo {
    private val users = mutableListOf<User>()
    fun save(u: User) { users.add(u) }
    fun find(id: Int): User? = users.find { it.id == id }
}

class Api(val repo: UserRepo) {
    fun route(path: String): String {
        return when {
            path.startsWith("/users/") -> {
                val id = path.removePrefix("/users/").toInt()
                repo.find(id)?.name ?: "not found"
            }
            else -> "404"
        }
    }
}

fun main() {
    val repo = UserRepo()
    repo.save(User(1, "Alex"))
    val api = Api(repo)
    println(api.route("/users/1"))
}

Test yourself

Complete the Api class definition

____ Api(val repo: UserRepo)

Show answer

class Api(val repo: UserRepo)

Match the Api parts

class Api(val repo)

Api class
route(path)

Routing
startsWith("/users/")

Path match
removePrefix

Id extract
What does Api.route return for an unknown path?
- null
- Empty list
- "404"
- Throws an exception
Show answer

"404" — Unknown paths return "404".

Next lesson: Exposed DB

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