How to get started with OAuth2 for Google Contacts API

Question

I am trying to make use of Google Contacts API with OAuth 2 authentication. But some of the old sample codes are now not working for this API.I found a sample code which is makes request for Access Token and in response Google servers would give me the Access Token for ~1 hour limit and a Refresh Token with. BUT this code has some issue

HttpTransport httpTransport = GoogleNetHttpTransport.newTrustedTransport();
            JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();

            String APPLICATION_NAME = "PROJECT_NAME";
            String SERVICE_ACCOUNT_EMAIL = "NUMERCALS-ALPHANUMERICALS@developer.gserviceaccount.com";
            java.io.File p12File = new java.io.File("PROJECT_NAME-NUMERICALS.p12");

            GoogleCredential credential = 
                    new GoogleCredential.Builder()                                                                    
                  .setTransport(httpTransport)                                                     
                  .setJsonFactory(jsonFactory)                                                   
                  .setServiceAccountId(SERVICE_ACCOUNT_EMAIL)   
                  .setServiceAccountScopes(
                        Collections.singleton("https://www.google.com/m8/feeds"))                                                          
                  .setServiceAccountPrivateKeyFromP12File(p12File)                                                  
                  .setServiceAccountUser("user@example.com")
                  .build();

            if (!credential.refreshToken()) {
                throw new RuntimeException("Failed OAuth to refresh the token");
            }

            ContactsService service = new ContactsService(APPLICATION_NAME);
            service.setOAuth2Credentials(credential);

            Query gQuery = new Query(new java.net.URL("https://www.google.com/m8/feeds/groups/user@example.com/full"));
            gQuery.setMaxResults(32767);
            ContactGroupFeed groupFeed = service.query(gQuery, ContactGroupFeed.class);

            for (ContactGroupEntry group : groupFeed.getEntries()) {

And I am getting some issues with it

com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
{
  "error" : "invalid_grant"
}
    at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:105)
    at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:287)
    at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:307)
    at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:269)
    at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:489)
    at javaCode.FinalCode.main(FinalCode.java:68)

I am new to use OAuth2 for Google APIs so guide me if I am mistaken somewhere or provide a working code snippet.It would be great help.Thanks in advance.

Answer 1

In you example you are trying to use Service account to impersonate the user. While this is possible, it can only be achieved under Google for work or Google for education accounts. It would not work for normal gmail accounts.

If this is the case for your use case, then remember that the domain administrator should grant access to your application and also should grant Domain wide delegation of authority.

But if you are just trying to make some tests with the API, then it would be better if you use normal OAuth.

Check this tutorial, I know is for Drive, but there you can see the basic steps to get a authorization token. After you get it, the call to the different Google Apis is very similar.

I would recommend to check the OAuth Playground. There you can check the complete OAuth process and you can play with the API.