java-21 Skill | Agent Skills

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

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

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

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

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

Don't: Spin up raw platform threads per request

// 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

Agent Skills: java-21

Install this agent skill to your local

Skill Files