/java-perf-check — Java Performance Quick Scan

/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 — recommend ExecutorService or @Async
• Shared HashMap — recommend ConcurrentHashMap
• Java 21+: synchronized inside virtual-thread code — pinning risk; recommend ReentrantLock

Transaction Scope

• @Transactional without readOnly = true on read-only service methods
• HTTP calls, file I/O, or Thread.sleep() inside a @Transactional method

Step 3 — Output

## Performance Scan — [scope]

🔴 HIGH    [count]   (likely production impact)
🟡 MEDIUM  [count]   (noticeable under load)
🔵 LOW     [count]   (minor optimisation)

### Findings

[For each finding:]
[Severity] [Category] — [ClassName]:[line]
Problem: [one sentence + estimated impact]
Fix:
  Before: [code]
  After:  [code]

### Top 3 Impact Actions
1. [highest gain fix]
2. [second]
3. [third]

Step 4 — Next Steps

• For a full performance deep-dive → use the java-performance-reviewer agent
• For JPA-specific issues → run /java-jpa
• To measure real hotspots → enable Hibernate stats: spring.jpa.properties.hibernate.generate_statistics=true
• For production profiling → Spring Boot Actuator + Micrometer: /actuator/metrics