Spring Boot application development Opinionated rules plus a router to deeper references. Code samples in the references were compiled and run on Spring Boot 4.1.1 (Spring Framework 7) and, where they differ, on Boot 3.5.16 (Framework 6.2). Boot 3 vs 4 differences are marked "Boot 3" / "Boot 4" in each reference. Before writing code 1. Detect, do not assume: read / for the Spring Boot version, Java version, starters in use ( is Boot 3, is Boot 4), persistence (JPA, JDBC), the migration tool, the test setup, and the package layout and naming conventions of the existing code. Match them. 2. Nev…