Java 21 Features Every Backend Developer Should Know

Introduction

Java 21 LTS brings revolutionary features that fundamentally change how we write backend services. Let's explore the most impactful features for backend developers.

Virtual Threads: Concurrency Made Simple

Virtual threads (Project Loom) make writing concurrent code as easy as writing sequential code:

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i -> {
        executor.submit(() -> {
            Thread.sleep(Duration.ofSeconds(1));
            return processData(i);
        });
    });
}

Why It Matters

• Lightweight: Create millions of virtual threads without overhead
• Simpler code: No need for reactive programming complexity
• Better resource utilization: One platform thread handles many virtual threads

Real-World Example

@GetMapping("/users/{id}")
public User getUser(@PathVariable Long id) {
    // Runs on virtual thread with Spring Boot 3.2+
    var user = userService.findById(id);
    var orders = orderService.findByUserId(id);
    var preferences = preferenceService.findByUserId(id);
    
    return enrichUser(user, orders, preferences);
}

Pattern Matching for Switch

Write cleaner type-checking code:

String formatted = switch (obj) {
    case Integer i -> String.format("int %d", i);
    case Long l    -> String.format("long %d", l);
    case String s  -> String.format("String %s", s);
    default        -> obj.toString();
};

Record Patterns

Destructure records in pattern matching:

record Point(int x, int y) {}

if (obj instanceof Point(int x, int y)) {
    System.out.println("x: " + x + ", y: " + y);
}

Real-World Impact

• 40% reduction in server count (virtual threads)
• Cleaner codebase (pattern matching)
• Better performance (optimized JVM)

Conclusion

Java 21 makes backend development more productive and performant. Virtual threads alone justify the upgrade.