performance-smell-detection

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

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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

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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

// 🔴 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);