Agent skill

Java 21

by Gentleman-Programming in Gentleman-Programming/Gentleman-Skills

Java 21 language and runtime patterns for modern, safe code.

MITAuto-check passed

Install Java 21

skills CLI

$ npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI

$ gh skill install Gentleman-Programming/Gentleman-Skills java-21 --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy

$ git clone --depth 1 https://github.com/Gentleman-Programming/Gentleman-Skills.git skills-src && mkdir -p .claude/skills && cp -r skills-src/community/java-21 .claude/skills/java-21 && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Paste into Claude Code

Install the "java-21" agent skill from https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21 into .claude/skills/java-21/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "java-21", then confirm the skill loads.

Claude Code copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

$skill-installer

$skill-installer install https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21

Type this inside Codex. $skill-installer <name> installs a curated skill from openai/skills. The installer writes to $CODEX_HOME/skills (default ~/.codex/skills). Restart Codex if the skill does not show up.

skills CLI

$ npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a codex

Project install goes to .agents/skills/; add -g for ~/.codex/skills/.

GitHub CLI

$ gh skill install Gentleman-Programming/Gentleman-Skills java-21 --agent codex

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/Gentleman-Programming/Gentleman-Skills.git skills-src && mkdir -p .agents/skills && cp -r skills-src/community/java-21 .agents/skills/java-21 && rm -rf skills-src

Use ~/.agents/skills/ instead of .agents/skills for a personal install.

Codex skills documentation · loads skills from .agents/skills/

Paste into Codex

Install the "java-21" agent skill from https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21 into .agents/skills/java-21/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "java-21", then confirm the skill loads.

Codex copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a cursor

Project install goes to .agents/skills/; add -g for ~/.cursor/skills/.

GitHub CLI

$ gh skill install Gentleman-Programming/Gentleman-Skills java-21 --agent cursor

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/Gentleman-Programming/Gentleman-Skills.git skills-src && mkdir -p .cursor/skills && cp -r skills-src/community/java-21 .cursor/skills/java-21 && rm -rf skills-src

Use ~/.cursor/skills/ instead of .cursor/skills for a personal install.

Cursor skills documentation · loads skills from .cursor/skills/, .agents/skills/, .claude/skills/, .codex/skills/

Paste into Cursor

Install the "java-21" agent skill from https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21 into .cursor/skills/java-21/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "java-21", then confirm the skill loads.

Cursor copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

gemini skills install

$ gemini skills install https://github.com/Gentleman-Programming/Gentleman-Skills.git --path community/java-21

--scope user (default) or --scope workspace; --path is the subfolder of the repo that holds the skill; --consent skips the security confirmation prompt.

skills CLI

$ npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a gemini-cli

Project install goes to .agents/skills/; add -g for ~/.gemini/skills/.

GitHub CLI

$ gh skill install Gentleman-Programming/Gentleman-Skills java-21 --agent gemini-cli

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/Gentleman-Programming/Gentleman-Skills.git skills-src && mkdir -p .gemini/skills && cp -r skills-src/community/java-21 .gemini/skills/java-21 && rm -rf skills-src

Use ~/.gemini/skills/ instead of .gemini/skills for a personal install, then run /skills reload.

Gemini CLI skills documentation · loads skills from .gemini/skills/, .agents/skills/

Paste into Gemini CLI

Install the "java-21" agent skill from https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21 into .gemini/skills/java-21/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "java-21", then confirm the skill loads.

Gemini CLI copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

GitHub CLI

$ gh skill install Gentleman-Programming/Gentleman-Skills java-21

Installs for Copilot at project scope by default; add --scope user for a personal install. Preview a skill first with gh skill preview. Needs GitHub CLI 2.90.0 or later (public preview).

skills CLI

$ npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a github-copilot

Project install goes to .agents/skills/; add -g for ~/.copilot/skills/.

Manual copy

$ git clone --depth 1 https://github.com/Gentleman-Programming/Gentleman-Skills.git skills-src && mkdir -p .github/skills && cp -r skills-src/community/java-21 .github/skills/java-21 && rm -rf skills-src

Use ~/.copilot/skills/ instead of .github/skills for a personal install. Commit .github/skills so cloud agent and code review can use it.

GitHub Copilot skills documentation · loads skills from .github/skills/, .claude/skills/, .agents/skills/

Paste into GitHub Copilot

Install the "java-21" agent skill from https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21 into .github/skills/java-21/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "java-21", then confirm the skill loads.

GitHub Copilot copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

skills CLI

$ npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a opencode

OpenCode documents no install command of its own. Project install goes to .agents/skills/; add -g for ~/.config/opencode/skills/.

GitHub CLI

$ gh skill install Gentleman-Programming/Gentleman-Skills java-21 --agent opencode

Project scope by default (.agents/skills/); add --scope user for a personal install.

Manual copy

$ git clone --depth 1 https://github.com/Gentleman-Programming/Gentleman-Skills.git skills-src && mkdir -p .opencode/skills && cp -r skills-src/community/java-21 .opencode/skills/java-21 && rm -rf skills-src

Use ~/.config/opencode/skills/ instead of .opencode/skills for a personal install.

OpenCode skills documentation · loads skills from .opencode/skills/, .claude/skills/, .agents/skills/

Paste into OpenCode

Install the "java-21" agent skill from https://github.com/Gentleman-Programming/Gentleman-Skills/tree/main/community/java-21 into .opencode/skills/java-21/ in this project. Copy the whole folder (SKILL.md and every file beside it), keep the folder name "java-21", then confirm the skill loads.

OpenCode copies the folder itself, the same result as the manual copy. Check what it changed before you commit it.

Facts

Skill name: java-21
GitHub stars: 657
Token cost: ~693 tokens
SKILL.md length: 150 words
Files: 1
Skills in repo: 23
Repo updated: Mar 28, 2026
First seen: Oct 7, 2026
Licence: MIT

At a glance

Java 21 language and runtime patterns for modern, safe code.

SKILL.md covers When to Use, Critical Patterns, Code Examples and Anti-Patterns, plus 2 more sections
Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Java 21 is an agent skill from Gentleman-Programming/Gentleman-Skills. Java 21 language and runtime patterns for modern, safe code. Trigger: When writing Java 21 code using records, sealed types, or virtual threads.

Its SKILL.md is about 690 tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts.

It works with Java. The repository describes itself as: Community-driven AI agent skills for Claude Code, OpenCode, and other AI assistants. Curated patterns and community contributions. The licence is MIT.

Example prompts

“/java-21”

What it can do on your machine

Read from SKILL.md and the folder at commit c8036a3. It shows what the files ask for, not the result of running them.

Tool permissions
Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.
From allowed-tools in the SKILL.md frontmatter.
Runs code
No scripts in the folder and no shell commands in SKILL.md (its code samples are java).
From the folder's file list and the shell code blocks in SKILL.md.
Network
Links to these hosts (documentation or services it may open):
- oracle.com
- openjdk.org
From URLs in SKILL.md, links to its own repository left out.
Credentials
Names no API keys, tokens, secrets or passwords.
From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Java 21 loads about 693 tokens when it runs. Until then it costs about 38 tokens; SKILL.md has 150 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it: ~38
When it runs · the whole SKILL.md, loaded when a task matches: ~693

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from Gentleman-Programming/Gentleman-Skills at commit c8036a3, republished under its MIT licence (© Gentleman-Programming). 150 words, ~693 tokens.

Download SKILL.mdSave it as .claude/skills/java-21/SKILL.md (or your agent's skills folder).

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
metadata.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

Files

Just SKILL.md in community/java-21 of Gentleman-Programming/Gentleman-Skills.

Open the folder on GitHubat commit c8036a3

Compare with similar skills

Java 21 next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Java 21 compared with similar skills
Skill	Stars	Used in	Tokens	Auto-check	Licence	Repo updated
Java 21 this skillGentleman-Programming/Gentleman-Skills	657	—	~693	Automated safety check: Pass	MIT	6 mo ago
Brainstormingxpinjection/test-driven-spring-boot	112	54 repos	~2.6k	Automated safety check: Pass	MIT	1 mo ago
Android API Diffgkd-kit/gkd	43k	—	~796	Automated safety check: Pass	GPL-3.0	yesterday
Video Cover Imageitwanger/toBeBetterJavaer	18k	—	~3.3k	Automated safety check: Pass	None	today
Lancedb Update Lance Dependencylancedb/lancedb	12k	—	~1.1k	Automated safety check: Pass	Apache-2.0	today
Java SDK E2E Test with Replay Snapshotgithub/copilot-sdk	11k	—	~1.8k	Automated safety check: Pass	MIT	today

Similar skills

Brainstorming
xpinjection/test-driven-spring-boot
You MUST use this before any creative work - creating features, building components, adding functionality, or modifying behavior.
112 GitHub starsUsed in 54 repos~2.6k tokens
Agent WorkflowsAuto-check passed
Android API Diff
gkd-kit/gkd
Looks up Android framework Java and AIDL APIs across versions with the android-api-diff CLI: signatures, availability, source files and hidden-API access code.
43k GitHub stars~796 tokensUpdated yesterday
MobileAuto-check passed
Video Cover Image
itwanger/toBeBetterJavaer
Generate matched 3:4, 16:9, and 4:3 short-video cover images from toBeBetterJavaer video scripts or AI/Java technical topics.
18k GitHub stars~3.3k tokensUpdated today
Media & CreativeAuto-check passed
Lancedb Update Lance Dependency
lancedb/lancedb
Update LanceDB to a specific Lance release or tag. An agent skill from lancedb/lancedb.
12k GitHub stars~1.1k tokensUpdated today
DatabasesAuto-check passed
Java SDK E2E Test with Replay Snapshot
github/copilot-sdk
Official
Creates a Java SDK end-to-end test for the Copilot SDK that runs against a recorded YAML snapshot through a replay proxy, so CI needs no real authentication.
11k GitHub stars~1.8k tokensUpdated today
Testing & QAAuto-check passed
Fory Release
apache/fory
Prepare an Apache Fory release candidate from a clean release branch, including the version bump, RC tag, JVM staging, ASF source artifacts, SVN upload, and vote email.
4.6k GitHub stars~2.9k tokensUpdated today
Auto-check passed

Questions about Java 21

What does Java 21 do?

Java 21 language and runtime patterns for modern, safe code. Java 21 is an agent skill from Gentleman-Programming/Gentleman-Skills. Java 21 language and runtime patterns for modern, safe code.

How do I install Java 21 in Claude Code?

Run `npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a claude-code`. Or copy the skill folder (community/java-21 in Gentleman-Programming/Gentleman-Skills) into .claude/skills/java-21 in your project. Claude Code loads it when a task matches its description.

How do I install Java 21 in Codex?

Run `npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a codex`. Or copy the skill folder (community/java-21 in Gentleman-Programming/Gentleman-Skills) into .agents/skills/java-21 in your project. Codex loads it when a task matches its description.

Can I use Java 21 in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add Gentleman-Programming/Gentleman-Skills --skill java-21 -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/java-21, .gemini/skills/java-21, .github/skills/java-21 and .opencode/skills/java-21 in your project.

What does Java 21 need to run?

SKILL.md names no scripts, command-line tools or credentials: Java 21 is instructions for the agent only.

Does Java 21 access the network?

SKILL.md names 2 domains. As links in the text: oracle.com and openjdk.org. This is read from the text; nothing was executed.

Is Java 21 safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Java 21 use?

Java 21 is published under the MIT licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Java 21 use?

About 693 tokens (SKILL.md is roughly 2.8k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Java 21?

Skills that share tags, products or a category with Java 21: Brainstorming (xpinjection/test-driven-spring-boot, 112 stars), Android API Diff (gkd-kit/gkd, 43k stars), Video Cover Image (itwanger/toBeBetterJavaer, 18k stars) and Lancedb Update Lance Dependency (lancedb/lancedb, 12k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Java 21?

Gentleman-Programming (a GitHub organization) maintains it in Gentleman-Programming/Gentleman-Skills, which has 657 GitHub stars. The repository holds 23 skills in this directory. The repository was last updated on March 28, 2026.

Source: Gentleman-Programming/Gentleman-Skills on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.

Install Java 21

Facts

At a glance

What it does

Example prompts

What it can do on your machine

Tool permissions

Runs code

Network

Credentials

Context cost

Safety

SKILL.md

When to Use

Critical Patterns

Pattern 1: Records for immutable data

Pattern 2: Sealed types + pattern matching

Pattern 3: Virtual threads for I/O

Code Examples

Example 1: Record with validation

Example 2: Sealed hierarchy + switch pattern matching

Example 3: Virtual threads for blocking calls

Anti-Patterns

Don't: Use mutable data carriers

Don't: Spin up raw platform threads per request

Quick Reference

Resources

Files

Compare with similar skills

Similar skills

More from Gentleman-Programming/Gentleman-Skills

Related

Questions about Java 21