When using Spring Cloud GCP you will need to authenticate at some point in order to use the GCP services. In this example below using a GCP Cloud SQL instance you really only need to do 3 things to access it externally from your Spring Boot application as follows.
1. Enable the Google Cloud SQL API which is detailed here
2. Ensure that your GCP SDK can login to your Google Cloud SQL. This command will take you to a web page asking which google account you want to use
$ gcloud auth application-default login
3. Finally some application properties in your Spring Boot application detailing the Google Cloud SQL instance name and database name as shown below.
Now when you do that and your application starts up you will see a log message as follows below clearly warning you this this method of authentication can have implications at some point.
2019-02-07 09:10:26.700 WARN 2477 --- [ main] c.g.a.oauth2.DefaultCredentialsProvider : Your application has authenticated using end user credentials from Google Cloud SDK. We recommend that most server applications use service accounts instead. If your application continues to use end user credentials from Cloud SDK, you might receive a "quota exceeded" or "API not enabled" error. For more information about service accounts, see https://cloud.google.com/docs/authentication/.
Clearly that's something we have to resolve. To do that we simply can add another Spring Boot application property pointing to a service account JSON file for us to authenticate against to remove the warning.
Note: You can also use an ENV variable as follows
You can get a JSON key generated from the GCP console "IAM and Admin -> Service Accounts" page
For more information on authentication visit this link https://cloud.google.com/docs/authentication/getting-started