code-quality

Check for chained method calls without null checks, Optional.get() without isPresent(), and methods returning null instead of Optional or empty collections. Prefer Optional.map chains or early returns, and annotate public APIs with @Nullable or @NonNull.

Verify HTTP verbs match semantics (GET for retrieval, POST for creation, PUT for replacement), confirm URL path versioning like /api/v1, check correct status codes such as 201 for creation, and ensure endpoints return DTOs rather than JPA entities.

The N+1 problem occurs when a loop executes one query per entity, such as fetching orders for each user individually. Fix it with a JOIN FETCH query in JPA or batch loading, and add pagination instead of loading entire tables.

Use @Transactional whenever a method performs multiple related writes, such as saving a user and their roles together. Without it, each repository call commits separately, leaving data inconsistent if a later step fails.

Catching Exception or Throwable hides unexpected errors and makes debugging harder, especially with empty catch blocks that silently swallow failures. Catch specific exceptions, log with context, and rethrow wrapped exceptions preserving the original cause.