Englishspring-dotenv, which in turn is based on dotenv-java is a library that enables Spring applications to access and use environment variables from .env files.
.env files are a convention that enables us to store environment variables locally per directory / project instead of either doing so system-wide or in an ad-hoc manner (e.g., using the export shell command), thus allowing use to more strictly follow the storing config in the environment principle from The Twelve-Factor App in Spring (Boot) applications.