This code demonstrates a method of signing OkHttp requests with an OAuth access token. An Authenticator is provided to renew an expired token using a refresh token.

LICENSE

Copyright (C) 2017 Tristan Waddington <tristan.waddington@gmail.com>

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

TokenAuth.kt

/* 
 * Copyright (C) 2017 Tristan Waddington <tristan.waddington@gmail.com>
 *
 * This software may be modified and distributed under the terms
 * of the MIT license.  See the LICENSE file for details.
 */

data class TokenAuth(
        val accessToken: String,
        val refreshToken: String,
        val expiresIn: Long,
        val expiresAt: Instant = Instant.now().plusSeconds(expiresIn)
)

TokenAuthenticator.kt

/* 
 * Copyright (C) 2017 Tristan Waddington <tristan.waddington@gmail.com>
 *
 * This software may be modified and distributed under the terms
 * of the MIT license.  See the LICENSE file for details.
 */

import okhttp3.Authenticator
import okhttp3.Request
import okhttp3.Response
import okhttp3.Route
import retrofit2.Call
import retrofit2.http.Body
import retrofit2.http.POST
import java.io.IOException

interface AuthProviderApi {
    @POST("/oauth/renew")
    fun renew(@Body req: RenewRequest): Call<RenewResponse>   
}

data class RenewRequest(
        val refresh_token: String,
        val grant_type: String = "refresh_token"
)

data class RenewResponse(
        val access_token: String,
        val expires_in: Long,
        val expires_at: Instant = Instant.now().plusSeconds(expires_in)
)

class TokenAuthenticator(
        private val store: TokenStore,
        private val service: AuthProviderApi
) : Authenticator {
    override fun authenticate(route: Route, response: Response): Request? {
        if (response.code() != 401) return null
        val tokens = store.getTokens() ?: return null
        
        synchronized(this) {
            val request = response.request()
            
            // Validate that the tokens were not mutated by another thread
            if (tokens != store.getTokens()) {
                val accessToken = store.getTokens()?.accessToken ?: ""
                
                // Tokens have been mutated, retry request with updated auth
                if (!accessToken.isEmpty()) return request.withAuthorization(accessToken)
            }
            
            // Request fresh access token
            try {
                val renewRequest = RenewRequest(tokens.refreshToken)
                val renewResponse = service.renew(renewRequest).execute()
                val body = renewResponse.body()
                
                if (renewResponse.isSuccessful && body != null) {
                    val newToken = body.access_token
                    val usedToken = response.request().getAuthorizationToken()
                    
                    if (newToken != usedToken) {
                        store.setTokens(tokens.copy(
                                accessToken = newToken,
                                expiresAt = body.expires_at,
                                expiresIn = body.expiresIn))
                        return request.withAuthorization(newToken)
                    }
                }
            } catch (e: IOException) {
                // todo: Log the exception   
            }
        }
        return null
    }
}

internal fun Request.withAuthorization(accessToken: String) =
        this.newBuilder()
                .header("Authorization", "Bearer $accessToken")
                .build()

internal fun Request.getAuthorizationToken(): String? =
        this.header("Authorization")
                ?.replace("Bearer", "")
                ?.trim()

TokenAuthInterceptor.kt

/* 
 * Copyright (C) 2017 Tristan Waddington <tristan.waddington@gmail.com>
 *
 * This software may be modified and distributed under the terms
 * of the MIT license.  See the LICENSE file for details.
 */

import okhttp3.Interceptor
import okhttp3.Interceptor.Chain
import okhttp3.Response

class TokenAuthInterceptor(private val store: TokenStore) : Interceptor {
    override fun intercept(chain: Chain): Response {
        val tokens = store.getTokens() ?: return chain.proceed(chain.request())   
        
        val req = chain.request().newBuilder()
                .header("Authorization", "Bearer ${tokens.accessToken}")
                .build()
        return chain.proceed(req)
    }
}

TokenStore.kt

/* 
 * Copyright (C) 2017 Tristan Waddington <tristan.waddington@gmail.com>
 *
 * This software may be modified and distributed under the terms
 * of the MIT license.  See the LICENSE file for details.
 */

interface TokenStore {
    fun hasTokens(): Boolean
    fun getTokens(): TokenAuth?
    fun setTokens(tokens: TokenAuth)
    fun clear()
    fun isExpired() = getTokens()?.expiresAt?.isBefore(Instant.now()) ?: true
    fun isNotExpired() = !isExpired()
}