performance-smell-detection

Performance Smell Detection Skill

Identify potential code-level performance issues - with nuance, not absolutes

What It Does

Helps notice potential performance smells in Java code:

• Stream API usage patterns
• Boxing/unboxing overhead
• Regex compilation costs
• Collection inefficiencies
• String operations

Philosophy: "Measure first, optimize second" - modern JVMs are highly optimized.

When to Use

• "Check for performance issues"
• "Review this hot path"
• "Is this code efficient?"
• Investigating measured slowness

Java Version Awareness

This skill accounts for modern Java optimizations:

Topic	Java 9+ Change
String +	Uses invokedynamic, well optimized outside loops
StringBuilder	Still best for loops
Virtual Threads	Java 21+ for I/O-bound work
String hashCode	Java 25 constant folding

Severity Levels

Level	Meaning	Action
🔴 High	Usually worth fixing	Fix proactively
🟡 Medium	Measure first	Profile before changing
🟢 Low	Nice to have	Only if critical path

What It Checks

1. Strings - Concatenation in loops (still valid concern)
2. Streams - Overhead in tight loops, parallel misuse
3. Boxing - Primitive wrappers in hot paths
4. Regex - Pattern.compile in loops
5. Collections - Wrong type, unbounded queries
6. Modern patterns - Virtual threads, structured concurrency

What It Does NOT Check

• JPA/Database - Use jpa-patterns skill
• Architecture - Use architecture-review skill
• JVM tuning - Out of scope (GC, heap, etc.)

Example Usage

You: Check this code for performance issues

Claude: [Identifies potential smells]
        [Rates severity: 🔴/🟡/🟢]
        [Recommends measuring before changing]
        [Suggests modern alternatives if applicable]

Related Skills

• jpa-patterns - Database performance (N+1, pagination)
• java-code-review - General code quality
• concurrency-review - Thread safety and async patterns

References

• Inside.java - JDK 25 Performance
• Java 25 Features - InfoQ
• Baeldung - Streams vs Loops
• Baeldung - String Concatenation