Java 21 LTS: Complete Guide to New Features and Performance Improvements

Java 21 marks another significant milestone as the latest Long Term Support (LTS) release, bringing revolutionary features that will reshape how we write Java code. With virtual threads, enhanced pattern matching, and numerous performance improvements, Java 21 sets the stage for the next era of Java development.

Why Java 21 Matters

As an LTS release, Java 21 will be supported for years to come, making it the perfect foundation for enterprise applications. The combination of performance improvements, developer productivity enhancements, and new language features makes this one of the most impactful Java releases in recent history.

Virtual Threads: Revolutionizing Concurrency

Virtual threads are perhaps the most game-changing feature in Java 21. They enable applications to handle millions of concurrent tasks with minimal overhead.

Traditional Threading Limitations

// Traditional approach - limited by OS threads
ExecutorService executor = Executors.newFixedThreadPool(100);

for (int i = 0; i < 10000; i++) {
    executor.submit(() -> {
        // This approach is limited by thread pool size
        performIOOperation();
    });
}

Virtual Threads Solution

// Virtual threads - handle millions of concurrent tasks
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (int i = 0; i < 1_000_000; i++) {
        executor.submit(() -> {
            // Each task gets its own virtual thread
            performIOOperation();
        });
    }
} // Auto-close executor

Real-World Performance Impact

In benchmarks, applications using virtual threads show:

• 10-100x improvement in concurrent request handling
• Reduced memory usage compared to traditional thread pools
• Better resource utilization for I/O-bound operations

Pattern Matching Enhancements

Java 21 significantly expands pattern matching capabilities, making code more readable and less error-prone.

Switch Expressions with Patterns

public String processShape(Object shape) {
    return switch (shape) {
        case Circle(var radius) -> 
            "Circle with radius " + radius;
        case Rectangle(var width, var height) -> 
            "Rectangle " + width + "x" + height;
        case Square(var side) -> 
            "Square with side " + side;
        case null -> 
            "No shape provided";
        default -> 
            "Unknown shape: " + shape.getClass().getSimpleName();
    };
}

Record Patterns

Record patterns allow destructuring of record instances directly in pattern matching:

public record Point(int x, int y) {}
public record Line(Point start, Point end) {}

public double calculateLength(Line line) {
    return switch (line) {
        case Line(Point(var x1, var y1), Point(var x2, var y2)) -> 
            Math.sqrt(Math.pow(x2 - x1, 2) + Math.pow(y2 - y1, 2));
    };
}

String Templates (Preview)

String templates provide a safer and more readable way to compose strings:

// Traditional string concatenation
String message = "Hello " + name + ", you have " + count + " messages";

// String templates (preview)
String message = STR."Hello \{name}, you have \{count} messages";

// With expressions
String formatted = STR."Total: \{price * quantity} (tax: \{price * quantity * 0.1})";

Sequenced Collections

New collection interfaces provide consistent access to first and last elements:

// SequencedList interface
List<String> list = new ArrayList<>();
list.addFirst("first");
list.addLast("last");
String first = list.getFirst();
String last = list.getLast();

// SequencedSet interface
LinkedHashSet<String> set = new LinkedHashSet<>();
set.addFirst("alpha");
set.addLast("omega");

// SequencedMap interface
LinkedHashMap<String, Integer> map = new LinkedHashMap<>();
map.putFirst("start", 1);
map.putLast("end", 100);

Performance Improvements

Generational ZGC

Java 21 introduces Generational ZGC, combining the benefits of generational garbage collection with ZGC's low-latency characteristics:

# Enable Generational ZGC
java -XX:+UseZGC -XX:+UseGenerationalZGC MyApplication

Benefits include:

• Lower allocation rates through better young generation handling
• Reduced GC overhead for typical application patterns
• Maintained low latency characteristics of ZGC

Vector API Improvements

Enhanced vector operations for better performance in computational tasks:

// Vector API for SIMD operations
var species = FloatVector.SPECIES_256;
var a = FloatVector.fromArray(species, arrayA, 0);
var b = FloatVector.fromArray(species, arrayB, 0);
var result = a.mul(b).add(c);
result.intoArray(resultArray, 0);

Key Bindings and Structured Concurrency

Scoped Values

A better alternative to ThreadLocal for sharing data across thread boundaries:

public class UserContext {
    private static final ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();
    
    public static void runWithUser(User user, Runnable task) {
        ScopedValue.where(CURRENT_USER, user).run(task);
    }
    
    public static User getCurrentUser() {
        return CURRENT_USER.get();
    }
}

Structured Concurrency

Better management of concurrent operations:

try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future<String> user = scope.fork(() -> fetchUser(userId));
    Future<List<Order>> orders = scope.fork(() -> fetchOrders(userId));
    
    scope.join();           // Wait for all tasks
    scope.throwIfFailed();  // Propagate any failures
    
    return new UserProfile(user.resultNow(), orders.resultNow());
}

Migration Considerations

Compatibility

Java 21 maintains strong backward compatibility, but consider:

• Deprecated API removals: Some old APIs have been removed
• Security manager deprecation: Plan for alternative security approaches
• Module system: Ensure your modules are properly configured

Performance Testing

When migrating to Java 21:

// Benchmark virtual threads vs traditional threads
@Benchmark
public void traditionalThreads() {
    try (var executor = Executors.newFixedThreadPool(100)) {
        // Traditional thread pool benchmark
    }
}

@Benchmark
public void virtualThreads() {
    try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
        // Virtual threads benchmark
    }
}

Best Practices for Java 21

Virtual Threads Usage

// Good: Use virtual threads for I/O-bound tasks
public CompletableFuture<String> fetchDataAsync(String url) {
    return CompletableFuture.supplyAsync(() -> {
        // I/O operation - perfect for virtual threads
        return httpClient.get(url);
    }, Executors.newVirtualThreadPerTaskExecutor());
}

// Avoid: CPU-intensive tasks with virtual threads
// Use traditional thread pools for CPU-bound work

Pattern Matching Best Practices

// Good: Exhaustive pattern matching
public int processValue(Object value) {
    return switch (value) {
        case Integer i -> i * 2;
        case String s -> s.length();
        case null -> 0;
        default -> throw new IllegalArgumentException("Unsupported type");
    };
}

Real-World Applications

Web Applications

@RestController
public class UserController {
    
    @GetMapping("/users/{id}")
    public CompletableFuture<User> getUser(@PathVariable String id) {
        // Virtual threads handle concurrent requests efficiently
        return CompletableFuture.supplyAsync(() -> {
            return userService.findById(id);
        }, virtualThreadExecutor);
    }
}

Data Processing

public class DataProcessor {
    
    public void processLargeDataset(List<DataItem> items) {
        try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
            var futures = items.stream()
                .map(item -> scope.fork(() -> processItem(item)))
                .toList();
            
            scope.join();
            scope.throwIfFailed();
            
            // All items processed successfully
        }
    }
}

Performance Benchmarks

Early adopters report significant improvements:

• Web applications: 2-5x increase in concurrent user handling
• Microservices: 30-50% reduction in memory usage
• Data processing: 20-40% improvement in throughput
• Startup time: 10-20% faster application startup

Looking Ahead

Java 21's features lay the groundwork for future developments:

• Project Loom: Virtual threads are just the beginning
• Project Panama: Better native interop coming
• Project Valhalla: Value types on the horizon
• Pattern matching: Continued evolution and expansion

Conclusion

Java 21 LTS represents a pivotal moment in Java's evolution. The introduction of virtual threads alone makes it worth upgrading, but combined with enhanced pattern matching, improved performance, and better developer experience, Java 21 sets a new standard for what modern Java development should look like.

For enterprise applications, the LTS status provides the stability needed for long-term projects, while the performance improvements and new features ensure your applications remain competitive and maintainable.

The migration to Java 21 isn't just an upgrade—it's an investment in the future of your Java applications. Start planning your migration today to take advantage of these revolutionary improvements.

Ready to upgrade? Begin with non-critical services, measure the performance improvements, and gradually roll out across your entire application portfolio. Java 21 is not just the present of Java development—it's the foundation for the next decade of innovation.