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I am trying to use GMAIL API to download attachments from my email account using OAuth 2.0 server to server Application. Here is my code:

 private static final String USER = "[email protected]"; private static String emailAddress = "[email protected]"; { try { httpTransport = GoogleNetHttpTransport.newTrustedTransport(); credential = new GoogleCredential.Builder() .setTransport(httpTransport) .setJsonFactory(JSON_FACTORY) .setServiceAccountId(emailAddress) .setServiceAccountPrivateKeyFromP12File(new File("myfile.p12")) .setServiceAccountScopes(Collections.singleton(GmailScopes.GMAIL_READONLY)) .build(); } catch (IOException | GeneralSecurityException e) { throw new RuntimeException(e); } } // Create a new authorized Gmail API client Gmail service = new Gmail.Builder(httpTransport, JSON_FACTORY, credential) .setApplicationName(APP_NAME).build(); // Retrieve a page of Threads 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()); }

I get the error at the threadsResponse call. These are the errors:

Exception in thread "main" com.google.api.client.googleapis.json.GoogleJsonResponseException: 500 Internal Server Error.

{ "code" : 500, "errors" : [ { "domain" : "global", "message" : "Backend Error", "reason" : "backendError" } ], "message" : "Backend Error" } at com.google.api.client.googleapis.json.GoogleJsonResponseException.from(GoogleJsonResponseException.java:145) at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:113) at com.google.api.client.googleapis.services.json.AbstractGoogleJsonClientRequest.newExceptionOnError(AbstractGoogleJsonClientRequest.java:40) at com.google.api.client.googleapis.services.AbstractGoogleClientRequest$1.interceptResponse(AbstractGoogleClientRequest.java:312) at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:1049) at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:410) at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:343) at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:460) at core.crescent.gmail.GmailApiQuickstart.main(GmailApiQuickstart.java:192)

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  • If you want to access a single user, you can use authentication for single user instead of service account. I don't think gmail API supports service account. stackoverflow.com/questions/24523003/… Commented Jan 5, 2015 at 22:39
  • I am using OAuth2 Server to server application link because it seems like it's the only way to get authorization to access a gmail account without a user consent (without having to open browsers and do manual consent). Is there another way to accomplish this. Thank you Commented Jan 12, 2015 at 19:06

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You could retry the request, with exponential backoff. Worked for me. See https://developers.google.com/drive/web/handle-errors

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So I figured out the issue and fixed it!

When setting up "Credential" (in the code I posted above). I needed to add .setServiceAccountUser(USER) sub-routine. By doing this, you make the link between the service account created and the GMAIL account you want to access. I saw it earlier in https://developers.google.com/accounts/docs/OAuth2ServiceAccount, but I was under the impression that it was only if you wanted to link with an account under a different domain name.

This fixed my issues. Thank you all for your responses

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