I want to read my server gmail account email from java application - spring-boot

Question

I need help in understanding the exact path I should follow to solve my problem.
I hava a gmail address(I can say this gmail is used by my java application) Whenever a mail comes to this gmail address. I want my application to know and get the email from my gmail inbox and proceed with the further logic.
So far Going through the google docs I have created a topic and a subscribe which has subscribed to that topic. Using push mechanism I'm also able to call my endpoint whenever a message get published manually from google console to that topic.
Now I want if any email comes to my gmail address, it should notify my endpoint and after I can just fetch the email from the gmail and proceed with the logic.
Here As I have only mine gmail address which I will be accessing I don't think I need to go for Oauth2 mechanism for gmail authentication purpose.
I went through the documentation which says. I should create a watch request, then maybe if any gmail inbox receives a mail it will notify my endpoint and then I can fetch the mail and proceed further. But I'm not sure what and how exactly I should proceed Do I need a service account if yes I have already created a service account and downloaded my private key.
GoogleCredential.fromStream(new FileInputStream("file.json"))
.createScoped(Collections.singleton(GmailScopes.MAIL_GOOGLE_COM)).createDelegated("xxxxx.iam.gserviceaccount.com");
then I'm calling watch request as :
Gmail gm = new Gmail.Builder(GoogleNetHttpTransport.newTrustedTransport(), JSON_FACTORY, googleCredBuilder()).build();
WatchResponse res = gm.users().watch("me", request).execute();
I got this error.
POST https://www.googleapis.com/gmail/v1/users/me/watch
{
"code": 400,
"errors": [
{
"domain": "global",
"message": "Precondition check failed.",
"reason": "failedPrecondition"
}
],
"message": "Precondition check failed.",
"status": "FAILED_PRECONDITION"
}
Can someone please help how exactly I should proceed to achieve my goal.
Thank you.

Answer 1

"Precondition check failed.",
Means that you are trying to use a service account with a standard google gmail address.
Service account authorization only works with gmail api and google workspace accounts once domain wide delegation has been configured.
You have two options
swtich to using a google workspace domain account.
Use Oauth2, set your application to production and not test so that the refresh token wont expire. Authorize your application once store the refresh token, then upload it to the server. It will be able to use the refresh token to request a new access token whenever it needs.