A simple Interceptor to include a Firebase user's id token in all requests. Useful for authenticating Firebase users with a custom backend. See: https://firebase.google.com/docs/auth/admin/verify-id-tokens#retrieve_id_tokens_on_clients)

FirebaseUserIdTokenInterceptor.java

public class FirebaseUserIdTokenInterceptor implements Interceptor {

    // Custom header for passing ID token in request.
    private static final String X_FIREBASE_ID_TOKEN = "YOUR-CUSTOM-HEADER";

    @Override
    public Response intercept(Chain chain) throws IOException {
        Request request = chain.request();

        try {
            FirebaseUser user = FirebaseAuth.getInstance().getCurrentUser();
            if (user == null) {
                throw new Exception("User is not logged in.");
            } else {
                Task<GetTokenResult> task = user.getToken(true);
                GetTokenResult tokenResult = Tasks.await(task);
                String idToken = tokenResult.getToken();

                if (idToken == null) {
                    throw new Exception("idToken is null");
                } else {
                    Request modifiedRequest = request.newBuilder()
                            .addHeader(X_FIREBASE_ID_TOKEN, idToken)
                            .build();
                    return chain.proceed(modifiedRequest);
                }
            }
        } catch (Exception e) {
            throw new IOException(e.getMessage());
        }
    }
}

Great solution, thank you! ;)

getIdToken parameter should be false, else this will force fetch token again even if they are not expired.

Agreed! as mentioned by @gowthamgts
For generic usage keep parameter as false because,

fireBaseUser!!.getIdToken(true)
will always fetch a new token (adds time as it needs a n/w call)

fireBaseUser!!.getIdToken(false)
will only fetch a new token if the old one is expired (saves time as it fetches cached token)

if the token is expired it will automatically fetch a new one for you.

here's an example for kotlin fam

fun getJwtToken(): String {
    val result = Tasks.await(fireBaseUser!!.getIdToken(false))
    return result!!.token!!
}

Interesting. I've been tried on this method for hours. It didn't fix my problem. I was so confused.

After checkoutfirebase auth

Finally, I went through.

class AuthInterceptor : Interceptor { @Throws(IOException::class) override fun intercept(chain: Interceptor.Chain): okhttp3.Response { return try { val user = FirebaseAuth.getInstance().currentUser if (user == null) { throw Exception("User is not logged in.") } else { val task: Task<GetTokenResult> = user.getIdToken(false) val tokenResult = Tasks.await(task) val idToken = tokenResult.token if (idToken == null) { throw Exception("idToken is null") } else { val original = chain.request() val originalHttpUrl = original.url val url = originalHttpUrl.newBuilder() .addQueryParameter("auth", "$idToken") .build() val requestBuilder = original.newBuilder() .url(url) val request = requestBuilder.build() chain.proceed(request) } } } catch (e: Exception) { throw IOException(e.message) } } }

Adding addQueryParameter worked for.