java-21

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha cfd30384f54c · 22 Oct 2026Report an issue or request a re-scan

• Publishergentleman-programming GitHub organizationOwner of the repository this skill is read from
• Source foldergentleman-programming/gentleman-skills/community/java-21The exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdictcfd30384f54cA new audit runs when the source changes
• Repository statusNot archivedLast push 207 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Java 21 language and runtime patterns for modern, safe code. Trigger: When writing Java 21 code using records, sealed types, or virtual threads.

---
name: java-21
description: >
  Java 21 language and runtime patterns for modern, safe code.
  Trigger: When writing Java 21 code using records, sealed types, or virtual threads.
metadata:
  author: diegnghrmr
  version: "1.0"
---

## When to Use

Load this skill when:
- Writing Java 21 application or library code
- Designing immutable DTOs or value objects
- Modeling closed hierarchies with sealed types
- Using virtual threads for blocking I/O

## Critical Patterns

### Pattern 1: Records for immutable data

Use records for DTOs and value objects, validate in compact constructors.

### Pattern 2: Sealed types + pattern matching

Use sealed interfaces/classes and switch pattern matching for exhaustiveness.

### Pattern 3: Virtual threads for I/O

Use virtual threads to handle blocking I/O without large thread pools.

## Code Examples

### Example 1: Record with validation

```java
package com.acme.user;

public record Email(String value) {
  public Email {
    if (value == null || !value.contains("@")) {
      throw new IllegalArgumentException("Invalid email");
    }
  }
}
```

### Example 2: Sealed hierarchy + switch pattern matching

```java
package com.acme.payment;

public sealed interface Payment permits Card, BankTransfer { }

public record Card(String last4) implements Payment { }
public record BankTransfer(String iban) implements Payment { }

public final class PaymentPrinter {
  public String describe(Payment payment) {
    return switch (payment) {
      case Card card -> "card-" + card.last4();
      case BankTransfer bank -> "iban-" + bank.iban();
    };
  }
}
```

### Example 3: Virtual threads for blocking calls

```java
package com.acme.io;

import java.util.concurrent.Executors;

public final class Fetcher {
  public void fetchAll(java.util.List<String> urls) throws Exception {
    try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
      for (String url : urls) {
        executor.submit(() -> blockingFetch(url));
      }
    }
  }

  private void blockingFetch(String url) {
    // perform blocking I/O here
  }
}
```

## Anti-Patterns

### Don't: Use mutable data carriers

```java
// BAD: mutable DTO
public class UserDto {
  public String name;
  public String email;
}
```

### Don't: Spin up raw platform threads per request

```java
// BAD: expensive and unbounded
new Thread(() -> blockingFetch("https://api" )).start();
```

## Quick Reference

| Task | Pattern |
|------|---------|
| Immutable DTOs | Use records with validation |
| Closed hierarchies | Use sealed interfaces + switch |
| Blocking I/O scale | Use virtual threads executor |

## Resources

- Java 21 Release Notes: https://www.oracle.com/java/technologies/javase/21-relnote-issues.html
- JEP Index: https://openjdk.org/jeps/0

Excerpt, first 2,770 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/gentleman-programming/gentleman-skills
git -C gentleman-skills checkout cfd30384f54c

Works in

1. 2026-10-22
   ✓SAFE B 89 / 100

   first audit · source cfd30384f54c

Questions about java-21

• github.com/gentleman-programming/gentleman-skills/tree/main/comm — the audited source at commit cfd30384f54c and every finding aboveObserved · 22 Oct 2026

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