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Java Concepts

# Java 11 to 21 + Spring Boot Concepts and Migration Guide

This guide covers essential **Java features (11 to 21)** and **Spring Boot updates** relevant to modern Spring Boot applications, including REST APIs, microservices, security, and performance optimization.

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## 🟢 Java 11 to 21: Key Concepts for Spring Boot Projects

### 🔹 Java 11 (LTS)
- `var` in lambda:
  ```java
  list.forEach((var item) -> System.out.println(item));

  • New HttpClient API:

    HttpClient client = HttpClient.newHttpClient();
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://example.com"))
    .build();

  • String API improvements:

    • isBlank(), lines(), strip(), repeat()

🔹 Java 12–14

  • Switch Expressions (preview):

    var result = switch (day) {
    case MONDAY, FRIDAY -> "Work";
    case SATURDAY, SUNDAY -> "Rest";
    default -> throw new IllegalStateException();
};

  • Records (Preview in Java 14) – Great for DTOs.

🔹 Java 15–16

  • Records (Stable in Java 16):

    public record UserDto(String name, int age) {}

  • Pattern Matching for instanceof:

    if (obj instanceof String s) {
    System.out.println(s.toLowerCase());
}

🔹 Java 17 (LTS)

  • Sealed Classes:

    public sealed class Shape permits Circle, Square {}

  • GC and performance improvements

🔹 Java 18–20

  • Mostly preview features:

    • Pattern matching for switch
    • Record patterns
    • Virtual Threads (Preview in Java 19/20)

🔹 Java 21 (LTS)

  • Virtual Threads (Stable):

    try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> dbCall());
}

  • Structured Concurrency

  • Record Patterns

  • Scoped Values (alternative to ThreadLocal)

  public final static ScopedValue<User> LOGGED_IN_USER = ScopedValue.newInstance();
  ScopedValue.where(LOGGED_IN_USER, user.get())
                .run(() -> controller.processRequest(request, response));
  User loggedInUser = Server.LOGGED_IN_USER.get();

🟢 Spring Boot Upgrades (2.5 → 3.x)

🔸 Spring Boot 3.x + Spring Framework 6

  • Java 17+ required
  • Migration from javax.* → jakarta.*
  • Native compilation support with GraalVM
  • Virtual Thread compatibility
  • Observability: Micrometer, tracing
  • RFC 7807 support for error responses

🔸 Spring Boot Relevant Concepts

Area Key Concepts
🔌 REST APIs @RestController, @RequestBody, @ControllerAdvice
💾 Data Layer Spring Data JPA, QueryDSL, @Transactional
🛡 Security Spring Security (OAuth2, JWT, RBAC), @PreAuthorize
🛠 Dev Tools Spring DevTools, Actuator
🧪 Testing @SpringBootTest, MockMVC, Testcontainers
⚙️ Config application.yml, Profiles, Config Server
🌐 HTTP Clients WebClient, RestTemplate
🧵 Async & Concurrency @Async, Virtual Threads (Java 21)
☁️ Deployment Docker, K8s, Layered Jars

🔧 Migration Advice

✅ Java 11 → 17 or 21:

  • Update build configuration (sourceCompatibility)
  • Migrate javax.* → jakarta.*
  • Use records for DTOs
  • Introduce virtual threads for high I/O workloads

✅ Spring Boot 2.x → 3.x:

  • Replace all javax.* with jakarta.*
  • Ensure all dependencies are 3.x compatible
  • Update to Spring Security 6 (new DSL)
  • Review and replace deprecated actuator endpoints

🧰 Need More?

Want a migration checklist or code upgrade examples? Just ask.


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