I’m deploying a Spring Boot (Java 21) backend with PostgreSQL on Railway, but the application is failing during startup due to environment variable resolution issues. The backend uses a custom HikariCP DataSourceConfig and Flyway, and it expects PostgreSQL credentials via environment variables (PGHOST, PGPORT, PGDATABASE, PGUSER, PGPASSWORD) or Railway reference variables from the Postgres service. Although I have configured both Shared Variables and Reference Variables in Railway, the values are not being resolved correctly at runtime and sometimes appear as null or unresolved placeholders like ${{PostgresDB.PGHOST}}.

Because of this, the application fails with errors such as Driver org.postgresql.Driver claims to not accept jdbcUrl and Failed to initialize JPA EntityManagerFactory, preventing the backend from starting successfully. I need help understanding the correct and reliable way to connect Railway PostgreSQL variables to a Spring Boot backend and why the environment variables are not being injected properly at runtime.