Estou com vergonha de estar simplesmente falhando com um exemplo de código, mas vou culpar o fato de que é tarde ...

Tirei uma cópia e cole de:https://developers.google.com/gmail/api/quickstart/quickstart-java e baixou as bibliotecas do cliente:https://code.google.com/p/google-api-java-client/ ehttps://developers.google.com/api-client-library/java/apis/gmail/v1

Quando executo a amostra, recebo a seguinte exceção:

Exception in thread "main" java.lang.IllegalArgumentException
    at com.google.api.client.repackaged.com.google.common.base.Preconditions.checkArgument(Preconditions.java:76)
    at com.google.api.client.util.Preconditions.checkArgument(Preconditions.java:37)
    at com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets.getDetails(GoogleClientSecrets.java:82)
    at com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow$Builder.<init>(GoogleAuthorizationCodeFlow.java:195)
    at com.emailreply.musterion.GmailApiQuickstart.main(GmailApiQuickstart.java:40)

No Google, não consigo encontrar uma resposta simples, por isso estou assumindo estupidez ou falta de uma biblioteca / incorreta.

As bibliotecas que eu tenho são:

/libs/commons-logging-1.1.1.jar
/libs/google-api-client-1.19.0.jar
/libs/google-api-client-android-1.19.0.jar
/libs/google-api-client-appengine-1.19.0.jar
/libs/google-api-client-gson-1.19.0.jar
/libs/google-api-client-jackson2-1.19.0.jar
/libs/google-api-client-java6-1.19.0.jar
/libs/google-api-client-servlet-1.19.0.jar
/libs/google-http-client-1.19.0.jar
/libs/google-http-client-android-1.19.0.jar
/libs/google-http-client-appengine-1.19.0.jar
/libs/google-http-client-gson-1.19.0.jar
/libs/google-http-client-jackson2-1.19.0.jar
/libs/google-http-client-jdo-1.19.0.jar
/libs/google-oauth-client-1.19.0.jar
/libs/google-oauth-client-appengine-1.19.0.jar
/libs/google-oauth-client-java6-1.19.0.jar
/libs/google-oauth-client-jetty-1.19.0.jar
/libs/google-oauth-client-servlet-1.19.0.jar
/libs/gson-2.1.jar
/libs/httpclient-4.0.1.jar
/libs/httpcore-4.0.1.jar
/libs/jackson-core-2.1.3.jar
/libs/jdo2-api-2.3-eb.jar
/libs/jetty-6.1.26.jar
/libs/jetty-util-6.1.26.jar
/libs/jsr305-1.3.9.jar
/libs/transaction-api-1.1.jar
google-api-services-gmail-v1-rev10-1.19.0.jar

O exemplo mencionado acima:

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.List;

import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.client.googleapis.auth.oauth2.GoogleOAuthConstants;
import com.google.api.client.googleapis.auth.oauth2.GoogleTokenResponse;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.services.gmail.Gmail;
import com.google.api.services.gmail.model.ListThreadsResponse;
import com.google.api.services.gmail.model.Thread;


public class GmailApiQuickstart {

  // Check https://developers.google.ciom/gmail/api/auth/scopes for all available scopes
  private static final String SCOPE = "https://www.googleapis.com/auth/gmail.readonly";
  private static final String APP_NAME = "Gmail API Quickstart";
  // Email address of the user, or "me" can be used to represent the currently authorized user.
  private static final String USER = "me";
  // Path to the client_secret.json file downloaded from the Developer Console
  private static final String CLIENT_SECRET_PATH = "./client_secret.json";


  public static void main (String [] args) throws IOException {
    HttpTransport httpTransport = new NetHttpTransport();
    JsonFactory jsonFactory = new JacksonFactory();

    GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(jsonFactory,  new BufferedReader(new InputStreamReader(GmailApiQuickstart.class.getResourceAsStream(CLIENT_SECRET_PATH))));

    // Allow user to authorize via url.
    GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
        httpTransport, jsonFactory, clientSecrets, Arrays.asList(SCOPE))
        .setAccessType("online")
        .setApprovalPrompt("auto").build();

    String url = flow.newAuthorizationUrl().setRedirectUri(GoogleOAuthConstants.OOB_REDIRECT_URI).build();


    System.out.println("Please open the following URL in your browser then type the authorization code:\n" + url);

    // Read code entered by user.
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String code = br.readLine();

    // Generate Credential using retrieved code.
    GoogleTokenResponse response = flow.newTokenRequest(code)
        .setRedirectUri(GoogleOAuthConstants.OOB_REDIRECT_URI).execute();
    GoogleCredential credential = new GoogleCredential()
        .setFromTokenResponse(response);

    // Create a new authorized Gmail API client
    Gmail service = new Gmail.Builder(httpTransport, jsonFactory, credential)
        .setApplicationName(APP_NAME).build();

    // Retrieve a page of Threads; max of 100 by default.
    ListThreadsResponse threadsResponse = service.users().threads().list(USER).execute();
    List<Thread> threads = threadsResponse.getThreads();

    // Print ID of each Thread.
    for (Thread thread : threads) {
        System.out.println("Thread ID: " + thread.getId());
    }
  }

}

Substituí a referência aCLIENT_SECRET_PATH com:

new BufferedReader(new InputStreamReader(GmailApiQuickstart.class.getResourceAsStream(CLIENT_SECRET_PATH)))

por nenhuma outra razão senão tentar algo diferente. Ele funciona e lê o arquivo corretamente.

Alguma ideia?