Kotlin extension methods to configure resttemplate to support various oauth2 grant types

BearerAuthorizationInterceptor.kt

import org.springframework.boot.web.client.RestTemplateBuilder
import org.springframework.http.HttpEntity
import org.springframework.http.HttpHeaders
import org.springframework.http.HttpRequest
import org.springframework.http.MediaType
import org.springframework.http.client.ClientHttpRequestExecution
import org.springframework.http.client.ClientHttpRequestInterceptor
import org.springframework.http.client.ClientHttpResponse
import org.springframework.util.Assert
import org.springframework.util.Base64Utils
import org.springframework.util.LinkedMultiValueMap
import org.springframework.util.MultiValueMap
import java.nio.charset.StandardCharsets

class BearerAuthorizationInterceptor(private val token: String) : ClientHttpRequestInterceptor {

    init {
        Assert.hasLength(token, "Token must not be empty")
    }

    override fun intercept(request: HttpRequest, body: ByteArray, execution: ClientHttpRequestExecution): ClientHttpResponse {
        request.headers.add("Authorization", "Bearer " + token)
        return execution.execute(request, body)
    }

}

fun RestTemplateBuilder.oauth2ClientAuthorization(clientId: String, clientSecret: String): RestTemplateBuilder {

    val restTemplate = this.build()

    val headers = createClientCredentialsHeader(clientId, clientSecret)

    val map = LinkedMultiValueMap<String, String>()
    map.add("grant_type", "client_credentials")
    val request = HttpEntity<MultiValueMap<String, String>>(map, headers)

    val token = restTemplate.postForObject("/oauth/token", request, Map::class.java)["access_token"].toString()

    return this.interceptors(BearerAuthorizationInterceptor(token))
}

private fun createClientCredentialsHeader(clientId: String, clientSecret: String): HttpHeaders {
    val headers = HttpHeaders()
    headers.contentType = MediaType.APPLICATION_FORM_URLENCODED
    val encodedAuth = Base64Utils.encodeToString("$clientId:$clientSecret".toByteArray(StandardCharsets.UTF_8))
    headers["Authorization"] = "Basic $encodedAuth"
    return headers
}

fun RestTemplateBuilder.oauth2PasswordAuthorization(clientId: String, clientSecret: String, username: String, password: String): RestTemplateBuilder {

    val restTemplate = this.build()

    val headers = createClientCredentialsHeader(clientId, clientSecret)

    val map = LinkedMultiValueMap<String, String>()
    map.add("grant_type", "password")
    map.add("username", username)
    map.add("password", password)
    val request = HttpEntity<MultiValueMap<String, String>>(map, headers)

    val token = restTemplate.postForObject("/oauth/token", request, Map::class.java)["access_token"].toString()

    return this.interceptors(BearerAuthorizationInterceptor(token))
}