Keycloak and CAS for example are popular in the Java ecosystem. But generally any authentication server that can generate JWTs for authenticated users. And th

Keycloak and CAS for example are popular in the Java ecosystem. But generally any authentication server that can generate JWTs for authenticated users. And then with any JWT lib you check the validity (expiration, signature, ...) of the token. See https://jwt.io https://jwt.io for a list of libs.