Implementing GCP IAM Authentication in Spring Boot
Fast recall
Quick revision
Core answer
gcloud auth application-default login; on GCP infrastructure it automatically uses the attached service account. For service-to-service authentication, use identity tokens or the GoogleCredentials class to generate bearer tokens for HTTP calls.Understanding of GCP's credential chain, service account design, and how to implement secure service-to-service calls without managing long-lived keys.
Downloading JSON service account key files and committing them to repositories or baking them into Docker images — this is a significant security anti-pattern GCP's Workload Identity was designed to eliminate.
Explain Workload Identity Federation for GKE, the difference between access tokens and identity tokens, and how to use `google-auth-library-java` to obtain tokens programmatically for custom HTTP clients.
Interview-ready guide
Interview answer
GCP authentication for Spring Boot follows one principle: let the platform provide credentials, never manage keys manually. Application Default Credentials (ADC) implements this by resolving credentials from the environment automatically.
Application Default Credentials — the resolution chain
- ADC resolves credentials in order: GOOGLE_APPLICATION_CREDENTIALS environment variable (points to a service account key file), then gcloud user credentials (for local development), then the metadata server (for GKE, Cloud Run, Compute Engine)
- Which means on Cloud Run and GKE, your Spring Boot code calls
GoogleCredentials.getApplicationDefault()or uses any GCP client library, and credentials are resolved from the platform metadata server — no key files, no environment variables
Service accounts
- A service account is a non-human GCP identity — create one per application with only the IAM roles it needs; grant
roles/cloudsql.clientfor Cloud SQL access,roles/storage.objectCreatorfor writes to a specific bucket - On GKE, use Workload Identity to bind a Kubernetes service account to a GCP service account — pods that use that Kubernetes service account automatically authenticate as the GCP service account
Local development with gcloud
gcloud auth application-default loginstores user credentials locally; ADC picks them up when running locally; the same code that uses ADC in production works locally without any credential configuration changes- For testing with a specific service account locally: download a service account key (JSON file), set
GOOGLE_APPLICATION_CREDENTIALS=/path/to/key.json— but delete the key file after testing; never commit it
Least-privilege IAM
- GCP IAM supports granular conditions — grant access only to specific resources:
roles/storage.objectCreatoronprojects/my-project/buckets/uploads-bucketrestricts the role to one bucket, not all Cloud Storage - Use
gcloud projects get-iam-policyandgcloud asset search-all-iam-policiesto audit which service accounts have which roles
Implementing GCP IAM Authentication in Spring Boot
Direct Answer
GCP IAM authentication in Spring Boot is primarily handled through Application Default Credentials (ADC) and service accounts. Locally, ADC uses the credentials from gcloud auth application-default login; on GCP infrastructure it automatically uses the attached service account. For service-to-service authentication, use identity tokens or the GoogleCredentials class to generate bearer tokens for HTTP calls.
Common Mistakes & Pitfalls
Downloading JSON service account key files and committing them to repositories or baking them into Docker images — this is a significant security anti-pattern GCP's Workload Identity was designed to eliminate.
Check your understanding
Answer these without looking back at the article.
- What is the difference between an IAM access token and an OIDC identity token in GCP?
- How do you use Workload Identity Federation to authenticate from GitHub Actions to GCP?
- How do you implement service account impersonation in Java for cross-project access?
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