In this post, I am going to show you how to add Basic Authentication to your SparkJava webapp in Kotlin.

Spark Java :

Its a micro-framework for creating web applications in Kotlin and Java 8 with minimal effort.

Basic Authentication :

Its simply an Authorization header whose value is Basic base64encode(usename:password)
So if username and password is admin and admin. Base64 encoding of admin:admin is YWRtaW46YWRtaW4=
Value of header will be

Authorization : Basic YWRtaW46YWRtaW4=

SparkJava allows us to add before and after filters which are evaluated before and after each request.

I will be writing a BasicAuthFilter which takes in a username and password with which you want to protect your webapp. Simply add a before filter which takes in BasicAuthFilter instance.

before(BasicAuthFilter("admin", "admin"))
class BasicAuthFilter(private val username: String, private val password: String) : Filter {

    override fun handle(request: Request, response: Response) {
        if (!request.headers().contains("Authorization") || !authenticated(request)) {
            response.header("WWW-Authenticate", BASIC_AUTHENTICATION_TYPE)
            halt(401, "Not Authenticate")

    private fun authenticated(request: Request): Boolean {
        val encodedHeader = request.headers("Authorization").substringAfter("Basic").trim()
        val submittedCredentials = extractCredentials(encodedHeader)
        if (submittedCredentials != null && submittedCredentials.size == NUMBER_OF_AUTHENTICATION_FIELDS) {
            val submittedUsername = submittedCredentials[0]
            val submittedPassword = submittedCredentials[1]
            return username == submittedUsername && password == submittedPassword
        return false

    private fun extractCredentials(encodedHeader: String?): Array<String>? {
        return if (encodedHeader != null) {
            val decodedHeader = String(Base64.getDecoder().decode(encodedHeader))
        } else {

    companion object {
        private const val BASIC_AUTHENTICATION_TYPE = "Basic"
        private const val NUMBER_OF_AUTHENTICATION_FIELDS = 2


handle() method contains logic for extracting encoded header and decoding the header into username and password. Extracted username and password is checked with the main credentials that you supplied while creating the instance of BasicAuthFilter. If submitted username and password is incorrect request is halted using halt(401) else request is passed down for further processing.