# performance-smell-detection

Flags code-level performance smells in Java, covering streams, collections, boxing, and regex compiled inside loops. Helps during code reviews of performance-critical paths.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 310 installs · ★ 760
- Author: decebals
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/decebals/claude-code-java/tree/HEAD/skills/performance-smell-detection/SKILL.md
- Updated: 2026-09-06
- HTML: https://affpapa.org/en/nevibecode/skills/performance-smell-detection

## What it does

The skill inspects Java source code to highlight patterns that strain memory or CPU time. It spots regex compilation inside loops and string concatenation via `+` in iterations. Repeated boxing in hot execution paths is also detected. When code creates streams inside tight loops, it suggests pre-computing lookups. Unbounded queries without pagination get flagged as memory risks.

Its scope stays strictly at the code level. For database bottlenecks like N+1 queries, use the `jpa-patterns` skill instead. Micro-optimizing without profiling data is a mistake, because modern JVMs optimize simple patterns reliably. Measure with JMH or profilers before refactoring.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill performance-smell-detection -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/performance-smell-detection/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/performance-smell-detection/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill performance-smell-detection -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/performance-smell-detection/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill performance-smell-detection -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/performance-smell-detection/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install decebals/claude-code-java performance-smell-detection
```

2. **Manually: put the files in place**
- File: `.github/skills/performance-smell-detection/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill performance-smell-detection -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/decebals/claude-code-java --path skills/performance-smell-detection
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/performance-smell-detection/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add decebals/claude-code-java --skill performance-smell-detection -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/decebals/claude-code-java/tree/HEAD/skills/performance-smell-detection/SKILL.md

## When to use it
- spot repeated regex compilation via `Pattern.compile` inside loops
- detect excessive object boxing in hot execution paths
- replace stream allocations inside tight loops with precomputed lookups
- identify unpaginated database queries that risk memory exhaustion

## Example prompts
- Check this service for Java performance smells: loops, boxing, and stream overhead
- Review the hot path in OrderProcessor.java to find unnecessary memory allocations
- Inspect this method for regex compilation inside iterations and string concatenation

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 00b21aef0ab0b5c5.

BEGIN THIRD-PARTY FILE 00b21aef0ab0b5c5
````text
---
name: performance-smell-detection
description: Detect potential code-level performance smells in Java - streams, collections, boxing, regex, object creation. Provides awareness, not absolutes - always measure before optimizing. For JPA/database performance, use jpa-patterns instead.
license: MIT
---

# Performance Smell Detection Skill

Identify **potential** code-level performance issues in Java code.

## Philosophy

> "Premature optimization is the root of all evil" - Donald Knuth

This skill helps you **notice** potential performance smells, not blindly "fix" them. Modern JVMs (Java 21/25) are highly optimized. Always:

1. **Measure first** - Use JMH, profilers, or production metrics
2. **Focus on hot paths** - 90% of time spent in 10% of code
3. **Consider readability** - Clear code often matters more than micro-optimizations

## When to Use
- Reviewing performance-critical code paths
- Investigating measured performance issues
- Learning about Java performance patterns
- Code review with performance awareness

## Scope

**This skill:** Code-level performance (streams, collections, objects)
**For database:** Use `jpa-patterns` skill (N+1, lazy loading, pagination)
**For architecture:** Use `architecture-review` skill

---

## Quick Reference: Potential Smells

| Smell | Severity | Context |
|-------|----------|---------|
| Regex compile in loop | 🔴 High | Always worth fixing |
| String concat in loop | 🟡 Medium | Still valid in Java 21/25 |
| Stream in tight loop | 🟡 Medium | Depends on collection size |
| Boxing in hot path | 🟡 Medium | Measure first |
| Unbounded collection | 🔴 High | Memory risk |
| Missing collection capacity | 🟢 Low | Minor, measure if critical |

---

## String Operations (Java 9+ / 21 / 25)

### What Changed

Since **Java 9** (JEP 280), string concatenation with `+` uses `invokedynamic`, not StringBuilder. The JVM optimizes simple concatenation well.

**Java 25** adds String::hashCode constant folding for additional optimization in Map lookups with String keys.

### Still Valid: StringBuilder in Loops

```java
// 🔴 Still problematic - new String each iteration
String result = "";
for (String s : items) {
    result += s;  // O(n²) - creates n strings
}

// ✅ StringBuilder for loops
StringBuilder sb = new StringBuilder();
for (String s : items) {
    sb.append(s);
}
String result = sb.toString();

// ✅ Or use String.join / Collectors.joining
String result = String.join("", items);
```

### Now Fine: Simple Concatenation

```java
// ✅ Fine in Java 9+ - JVM optimizes this
String message = "User " + name + " logged in at " + timestamp;

// ✅ Also fine
return "Error: " + code + " - " + description;
```

### Avoid in Hot Paths: String.format

```java
````
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