java-21
Java 21 language and runtime patterns for modern, safe code. Trigger: When writing Java 21 code using records, sealed types, or virtual threads.
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- 2026년 2월 2일 12:36
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소스 지침 · 읽기 전용 미리보기- 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
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