This is a sample Android application demonstrating token refreshing using Retrofit Authenticator and Kotlin Mutex. It integrates with the DummyJSON API to simulate login and protected requests.

• Kotlin
• Retrofit
• Coroutine Mutex
• Jetpack ViewModel
• MVVM Architecture
• DummyJSON API

• Login with DummyJSON API
• Stores and uses access & refresh tokens
• Automatically refreshes expired tokens using Retrofit Authenticator
• Two authenticator implementations:
  • TokenAuthenticatorMutex: Uses Kotlin Mutex to safely handle concurrent refresh attempts
  • TokenAuthenticator: Basic token refresh without synchronization (for comparison/demo)

This project includes two authenticator classes to demonstrate token refreshing:

• TokenAuthenticator – Basic implementation
• TokenAuthenticatorMutex – Uses Mutex to prevent multiple refresh calls

You can switch between them in the NetworkModule inside the di package:

object NetworkModule {

    @Provides
    @Singleton
    fun provideOkhttpClient(
        headerInterceptor: HeaderInterceptor,
        tokenAuthenticator: TokenAuthenticatorMutex // ← Change this to TokenAuthenticator to test without Mutex
    ): OkHttpClient {
        return RetrofitHelper.getOkHttpClient()
            .addInterceptor(headerInterceptor)
            .authenticator(tokenAuthenticator)
            .build()
    }
}

• Retrofit Interceptor: Adds Authorization header
• Authenticator: Automatically refreshes token on 401 errors
• Mutex: Prevents multiple token refreshes at the same time

1. When the app launches, it shows a Login screen.
2. Press the Login button to authenticate using default credentials:
   • username: emilys
   • password: emilyspass
   • expiresInMins: 1 → Token expires in 1 minute
3. After login, you are redirected to the Profile screen.
4. On the Profile screen, press the Load button to trigger 3 parallel API calls.
5. If the token is expired, all 3 calls will receive 401, and:
   • Using TokenAuthenticator: May trigger 3 refresh calls (undesirable).
   • Using TokenAuthenticatorMutex: Only 1 refresh call happens, others wait for it (thread-safe).