How to update Gmail signature using Gmail API in Java

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1

Setting Gmail signature through Gmail API - Permission Issues

I’m working on a Java application that needs to modify Gmail signatures using the Gmail API. I’ve successfully implemented the basic authentication flow based on the official documentation, but I’m running into permission problems when trying to access signature settings.

The main issue is that I keep getting a 403 “Insufficient Permission” error when attempting to update the signature. Here’s my current implementation:

import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.extensions.java6.auth.oauth2.AuthorizationCodeInstalledApp;
import com.google.api.client.extensions.jetty.auth.oauth2.LocalServerReceiver;
import com.google.api.client.googleapis.auth.oauth2.GoogleAuthorizationCodeFlow;
import com.google.api.client.googleapis.auth.oauth2.GoogleClientSecrets;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.util.store.FileDataStoreFactory;
import com.google.api.services.gmail.GmailScopes;
import com.google.api.services.gmail.model.*;
import com.google.api.services.gmail.Gmail;

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

public class EmailSignatureManager {
    private static final String APP_NAME = "Email Signature Tool";
    private static final java.io.File CREDENTIAL_DIR = new java.io.File(
            System.getProperty("user.home"), ".auth/email-signature-app");
    private static FileDataStoreFactory credentialStore;
    private static final JsonFactory jsonFactory = JacksonFactory.getDefaultInstance();
    private static HttpTransport httpTransport;
    private static final List<String> REQUIRED_SCOPES = Arrays.asList(GmailScopes.MAIL_GOOGLE_COM);

    static {
        try {
            httpTransport = GoogleNetHttpTransport.newTrustedTransport();
            credentialStore = new FileDataStoreFactory(CREDENTIAL_DIR);
        } catch (Throwable error) {
            error.printStackTrace();
            System.exit(1);
        }
    }

    public static Credential createCredentials() throws IOException {
        InputStream secretStream = EmailSignatureManager.class.getResourceAsStream("/oauth_credentials.json");
        GoogleClientSecrets secrets = GoogleClientSecrets.load(jsonFactory, new InputStreamReader(secretStream));
        
        GoogleAuthorizationCodeFlow authFlow = new GoogleAuthorizationCodeFlow.Builder(
                httpTransport, jsonFactory, secrets, REQUIRED_SCOPES)
                .setDataStoreFactory(credentialStore)
                .setAccessType("offline")
                .build();
        
        Credential userCredential = new AuthorizationCodeInstalledApp(
                authFlow, new LocalServerReceiver()).authorize("user");
        return userCredential;
    }

    public static Gmail buildGmailClient() throws IOException {
        Credential credential = createCredentials();
        return new Gmail.Builder(httpTransport, jsonFactory, credential)
                .setApplicationName(APP_NAME)
                .build();
    }

    public static void main(String[] args) throws IOException {
        Gmail gmailService = buildGmailClient();
        String userId = "me";
        
        // This works fine
        ListLabelsResponse labelResponse = gmailService.users().labels().list(userId).execute();
        
        // Commented signature update code that causes 403 error
        // SendAs primaryEmail = null;
        // ListSendAsResponse emailAliases = gmailService.users().settings().sendAs().list("me").execute();
        // for (SendAs alias: emailAliases.getSendAs()) {
        //     if (alias.getIsPrimary()) {
        //         primaryEmail = alias;
        //         break;
        //     }
        // }
        // SendAs updatedSettings = new SendAs().setSignature("My custom signature");
        // SendAs response = gmailService.users().settings().sendAs().patch(
        //         "me",
        //         primaryEmail.getSendAsEmail(),
        //         updatedSettings)
        //         .execute();
        
        List<Label> allLabels = labelResponse.getLabels();
        if (allLabels.size() == 0) {
            System.out.println("No labels available.");
        } else {
            System.out.println("Available labels:");
            for (Label currentLabel : allLabels) {
                System.out.printf("- %s\n", currentLabel.getName());
            }
        }
    }
}

The error I’m encountering is:

Exception in thread "main" com.google.api.client.googleapis.json.GoogleJsonResponseException: 403 Forbidden
{
  "code" : 403,
  "errors" : [ {
    "domain" : "global",
    "message" : "Insufficient Permission",
    "reason" : "insufficientPermissions"
  } ],
  "message" : "Insufficient Permission"
}

What are the correct permissions or scopes needed to modify Gmail signatures? Is there a specific way to handle signature updates that I’m missing?

2

You’re using the wrong scope. GmailScopes.MAIL_GOOGLE_COM is too broad and often fails for settings. Switch to GmailScopes.GMAIL_SETTINGS_BASIC or GmailScopes.GMAIL_SETTINGS_SHARING. Don’t forget to clear your stored credentials after changing scopes - it needs to re-auth with the new permissions.

3

The scope isn’t your only problem - you need proper exception handling around the signature update code. I ran into the same thing and found the API throws 403s on the first few tries, even with correct permissions. Wrap your signature update in a retry loop with exponential backoff. Also double-check you’re grabbing the primary SendAs email right before the patch operation. Some accounts have multiple SendAs entries for the primary email, so you’ve got to match Gmail’s exact string format.

4

Had this exact issue last month building a signature management tool. The problem’s in your OAuth2 setup - you need Gmail API enabled in Google Cloud Console AND the OAuth consent screen configured properly. Most people miss that you have to add the specific Gmail scopes in the consent screen configuration, not just your code. After enabling the API, go to OAuth consent screen and add the Gmail settings scope explicitly. Then delete your stored credentials folder completely and re-authenticate. The 403 error usually means Google’s servers don’t recognize your app as having permission to modify user settings, even if the scope looks right in your code.