Method to exchange JWT for Google-signed OIDC token

GCPAuthenticationInterceptor.java

private DecodedJWT getGoogleIdToken() throws IOException {
    String jwt = getSignedJwt();
    final GenericData tokenRequest = new GenericData()
            .set("grant_type", JWT_BEARER_TOKEN_GRANT_TYPE)
            .set("assertion", jwt);
    final UrlEncodedContent content = new UrlEncodedContent(tokenRequest);

    final HttpRequestFactory requestFactory = httpTransport.createRequestFactory();

    final HttpRequest request = requestFactory
            .buildPostRequest(new GenericUrl(OAUTH_TOKEN_URI), content)
            .setParser(new JsonObjectParser(JacksonFactory.getDefaultInstance()));

    HttpResponse response = request.execute();
    GenericData responseData = response.parseAs(GenericData.class);
    String idToken = (String) responseData.get("id_token");
    return JWT.decode(idToken);
}

Is JWT_BEARER_TOKEN_GRANT_TYPE enum ?
Do you know what the value in here can be ? JWT_BEARER_TOKEN_GRANT_TYPE

See full example here: https://gist.github.com/tylertreat/b2f17f9d55ae55002b218a7b4cf778b5#file-gcpauthenticationinterceptor-java-L38

That's helpful. Thanks a ton.