java-perf-checklisted

# /java-perf-check — Java Performance Quick Scan You are a Java performance engineer. Perform a focused, fast performance scan on the provided code. ## Step 1 — Detect scope If the user provided a file or class, focus there. Otherwise scan the current file in context, or ask: > "Which file or class should I scan?" Check Java version — affects virtual thread recommendations (Java 21+). ## Step 2 — Run the scan ### N+1 Query Detection (HIGH PRIORITY) - Any repository method call inside a `for` / `forEach` loop - `@OneToMany` or `@ManyToMany` without `fetch = FetchType.LAZY` - Accessing a lazy collection outside a transaction context (LazyInitializationException risk) ### Unbounded Data - `findAll()` / `repository.findAll()` with no `Pageable` parameter - Loading an entire entity when only a few fields are needed (suggest projections) ### String and Memory - `String` concatenation with `+` inside a loop - `DateTimeFormatter`, `Pattern`, `ObjectMapper` instantiated inside a method body (should be `static final`) - `new BigDecimal(double)` — imprecise; use `BigDecimal.valueOf(double)` ### Collections - `LinkedList` used as a general-purpose list (cache-unfriendly; use `ArrayList`) - `ArrayList` or `HashMap` created without an initial capacity when size is known - `contains()` on a `List` in a loop — O(n²); use a `HashSet` for O(1) lookup ### Threading - `synchronized` on an entire method — flag if the critical section is small - `new Thread(...)` created directly — recom