Java Claude Skills — Java 8 → Java 21 Modernization for Claude Code

AI-powered Java code modernization skill for Claude Code.
Analyzes legacy Java, generates a prioritized upgrade report, scores it 1–100, and applies every change — all inside your editor.

---

Why this skill?

Migrating Java 8 code to Java 21 is high-value but tedious — every codebase has the same problems:

Problem	Impact
double for monetary values	Silent precision loss on every FX calculation
Unguarded chained .get() calls	NPE crashes in production settlement flows
synchronized HashMap	Race conditions on position updates under load
Fixed thread pools	Bottleneck at 200–500 concurrent trades
Boilerplate DTOs with 60+ lines	Noise that obscures domain logic

This skill acts as a Senior Java Architect in your Claude Code session. It knows every Java feature from 8 through 21, enforces financial-domain safety rules, and produces code you can ship — not just advice.

---

Skills

1. java-modernization-review

Analyzes legacy Java code and produces a full modernization report saved to analysis.md. Stops before touching any code — you review first.

Covers 7 upgrade priority tracks:

Priority	Track	Java Version
1	Streams, Optional, method references	8+
2	var, helpful NPE messages, built-in HTTP client	9–11
3	Records, sealed classes, pattern matching, text blocks	12–16
4	Virtual threads, structured concurrency, lock-free atomics	17–21
5	String templates, unnamed classes, type inference	18–21
6	Legacy removal — Date→java.time, reactive→virtual threads	8+
7	Module system (module-info.java)	9+

Every review produces:

• Critical issues (NPE chains, thread safety, double money) flagged first
• Quick wins checklist with occurrence counts
• Before/After code examples for the top 3 changes
• A 4-phase modernization roadmap
• A testing strategy

---

2. java-modernization-implement

Reads analysis.md (produced by the review skill), applies every recommended change, and writes a *Modern.java file alongside the original. The original is never modified.

---

Modernization Score (1–100)

Every review and implementation produces a Before / After score across 9 dimensions:

Dimension	Max Points
NPE prevention	15
Monetary precision (BigDecimal)	15
Thread safety	15
Streams & collections	10
Exception handling	10
Modern data carriers (Records)	10
Concurrency model (virtual threads)	10
Modern Java features	10
Financial domain rules	5
Total	100

Real result on the included TradeProcessor.java example:

| Dimension              | Before  | After  |
|------------------------|---------|--------|
| NPE prevention         |  0/15   | 15/15  |
| Monetary precision     |  0/15   | 15/15  |
| Thread safety          |  0/15   | 15/15  |
| Streams / collections  |  0/10   | 10/10  |
| Exception handling     |  0/10   |  6/10  |
| Modern data carriers   |  0/10   | 10/10  |
| Concurrency model      |  2/10   | 10/10  |
| Modern Java features   |  1/10   |  8/10  |
| Financial domain rules |  0/5    |  5/5   |
| TOTAL                  |  3/100  | 95/100 |

Improvement: +92 points

---

Two-Step Workflow

Step 1 — Review                          Step 2 — Implement
───────────────────                      ──────────────────────────────
/java-modernization-review    ────────►  /java-modernization-implement

Analyzes the file.                       Reads analysis.md.
Generates analysis.md.                   Applies every change.
Stops. Waits for approval.               Writes *Modern.java alongside original.
                                         Reports final score + delta.

The original file is never modified.

---

Install (Claude Code)

Step 1 — Add this repo as a marketplace:

/plugin marketplace add Santoshrt999/Java-Claude-Skills

Step 2 — Install both skills:

/plugin install java-claude-skills@java-modernization-review
/plugin install java-claude-skills@java-modernization-implement

Step 3 — Reload plugins:

/reload-plugins

---

Usage

Run the review:

/java-modernization-review

Review src/main/java/com/yourcompany/TradeProcessor.java

Opens analysis.md with the full report and score. Review it, then:

Apply the changes:

/java-modernization-implement

Writes TradeProcessorModern.java and prints the final score with improvement delta.

---

Financial Domain Rules (always enforced)

• BigDecimal for all monetary amounts — never double or float
• ZonedDateTime for timestamps — never Date
• ConcurrentHashMap.merge() for atomic position/balance updates
• Immutable domain objects via Records with compact constructor validation
• Idempotent message handlers for settlement and clearing
• Audit trail via event sourcing

---

Example

Side-by-side comparison included in this repo:

File	Description
TradeProcessor.java	Legacy Java 8 — NPE chains, double money, synchronized HashMap, swallowed exceptions, rigid DTO
TradeProcessorModern.java	Java 21 — Optional, streams, ConcurrentHashMap.merge(), virtual threads, Record, BigDecimal
analysis.md	Full review report with checklist and final score

---

Who is this for?

• Java developers modernizing legacy financial systems, trading platforms, or microservices to Java 21
• Teams migrating from Java 8, 11, or 17 to Java 21 LTS
• Anyone using Claude Code who wants AI-assisted code review with measurable results
• Engineers working with Kafka, Solace, RabbitMQ messaging systems on legacy threading models

---

License

MIT — free to use, modify, and share.

---

Built with Claude Code · Skill format compatible with Claude Code plugin marketplace