Java

Java 21: Virtual Threads Standard and Modern LTS Era

Ravinder··7 min read
JavaLTSVirtual ThreadsPattern MatchingJava 21
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Java 21: Virtual Threads Standard and Modern LTS Era

Java 21: Virtual Threads Finalized and the Modern LTS Era

Java 21 (September 2023) is a Long-Term Support release with support until September 2031. It finalizes virtual threads, pattern matching, and structured concurrency - marking a new era in Java development.

1. Virtual Threads - Now Standard

Project Loom features are finalized and production-ready.

// Standard API - no preview needed!
import java.util.concurrent.*;
 
public class VirtualThreadsStandard {
    
    public static void main(String[] args) throws Exception {
        // Create executor for virtual threads
        ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
        
        // Simple usage - same as traditional threads
        for (int i = 0; i < 10_000; i++) {
            final int id = i;
            executor.submit(() -> {
                System.out.println("Virtual thread " + id);
                try {
                    Thread.sleep(1000); // Blocking I/O simulation
                } catch (InterruptedException e) {
                    Thread.currentThread().interrupt();
                }
            });
        }
        
        executor.close(); // Auto-shutdown
    }
    
    // Creating individual virtual threads
    public static void createIndividual() {
        Thread vthread = Thread.ofVirtual()
            .name("worker")
            .start(() -> System.out.println("Running on virtual thread"));
    }
    
    // Virtual thread builder for customization
    public static void builderApproach() {
        Thread.Builder.OfVirtual builder = Thread.ofVirtual()
            .name("task-", 0);
        
        Thread vt = builder.unstarted(() -> {
            System.out.println("Custom virtual thread");
        });
        vt.start();
    }
}

2. Pattern Matching - Now Complete

Record patterns and comprehensive pattern matching are finalized.

// Full pattern matching support
sealed interface Result<T> permits Success, Failure {}
record Success<T>(T value) implements Result<T> {}
record Failure<T>(Exception error) implements Result<T> {}
 
// Pattern matching in switch is now standard
public void handleResult(Result<?> result) {
    switch (result) {
        case Success(var value) -> System.out.println("Success: " + value);
        case Failure(var error) -> System.out.println("Error: " + error.getMessage());
    }
}
 
// Guarded patterns
public String analyzeValue(Object value) {
    return switch (value) {
        case Integer i when i < 0 -> "Negative integer";
        case Integer i when i == 0 -> "Zero";
        case Integer i when i > 0 -> "Positive integer";
        
        case String s when s.isBlank() -> "Empty string";
        case String s when s.length() > 100 -> "Long string";
        case String s -> "Normal string";
        
        case Double d when Double.isNaN(d) -> "Not a number";
        case Double d when Double.isInfinite(d) -> "Infinity";
        case Double d -> "Normal double";
        
        default -> "Other type";
    };
}
 
// Record pattern destructuring
record Point(int x, int y) {}
record Rectangle(Point topLeft, Point bottomRight) {}
 
public void analyzeGeometry(Object shape) {
    if (shape instanceof Rectangle(Point(var x1, var y1), Point(var x2, var y2))) {
        int width = x2 - x1;
        int height = y2 - y1;
        System.out.printf("Rectangle: %dx%d%n", width, height);
    }
}
 
// Array patterns (future)
// if (value instanceof int[] { 1, 2, 3 }) { }

3. Structured Concurrency - Now Standard

Proper coordination of concurrent tasks is finalized.

// Structured Concurrency API finalized
import java.util.concurrent.StructuredTaskScope;
 
public class ConcurrentDataFetcher {
    
    // Graceful failure handling
    public record UserData(String user, String permissions, String roles) {}
    
    public UserData fetchUserData(String userId) throws Exception {
        try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
            
            Future<String> userFuture = scope.fork(() -> fetchUser(userId));
            Future<String> permFuture = scope.fork(() -> fetchPermissions(userId));
            Future<String> roleFuture = scope.fork(() -> fetchRoles(userId));
            
            scope.join();
            
            return new UserData(
                userFuture.resultNow(),
                permFuture.resultNow(),
                roleFuture.resultNow()
            );
        }
    }
    
    // Success-first variant
    public String getFirstValidEmail(List<String> userIds) throws Exception {
        try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {
            
            for (String id : userIds) {
                scope.fork(() -> fetchEmail(id));
            }
            
            return scope.join(); // Returns first successful result
        }
    }
    
    static String fetchUser(String id) { return "user-" + id; }
    static String fetchPermissions(String id) { return "admin"; }
    static String fetchRoles(String id) { return "developer"; }
    static String fetchEmail(String id) { return id + "@example.com"; }
}

4. String Templates (Preview)

Runtime string interpolation.

// Text blocks with expressions (Preview)
String name = "World";
int value = 42;
 
// Traditional concatenation
String old = "Hello " + name + ", value: " + value;
 
// String templates (Preview)
String modern = STR."Hello \{name}, value: \{value}";
 
// More complex expressions
String complex = STR."""
    Name: \{name}
    Value: \{value}
    Doubled: \{value * 2}
    Uppercase: \{name.toUpperCase()}
    """;
 
// SQL template
String userId = "user123";
String sqlQuery = EXEC."SELECT * FROM users WHERE id = \{userId}";
 
// JSON template
var json = EXEC."""
    {
      "name": "\{name}",
      "value": \{value}
    }
    """;

5. Finalized Features

Many preview features now standard.

Pattern Matching for instanceof (Complete):

// Now fully standard
Object obj = "Hello";
 
if (obj instanceof String str &&
    str.length() > 0 &&
    !str.isBlank()) {
    System.out.println("Valid string: " + str);
}

Record Patterns:

// Fully standard
public record User(String name, Email email) {}
public record Email(String address) {}
 
public void printEmail(Object user) {
    if (user instanceof User(var name, Email(var addr))) {
        System.out.println(name + ": " + addr);
    }
}

6. Foreign Function & Memory API (Preview 3)

Getting closer to finalization.

// FFM API improvements
import java.lang.foreign.*;
 
public class FFMExample {
    public static void main(String[] args) throws Throwable {
        // Arena for memory management
        try (Arena arena = Arena.ofConfined()) {
            
            // Allocate memory
            MemorySegment segment = arena.allocate(
                ValueLayout.JAVA_INT
            );
            
            // Write value
            segment.set(ValueLayout.JAVA_INT, 0, 42);
            
            // Read value
            System.out.println(segment.get(ValueLayout.JAVA_INT, 0));
        }
    }
}

Developer Impact

Paradigm Shift:

  • Virtual threads make I/O-bound services trivial to scale
  • Pattern matching makes code more expressive
  • Structured concurrency eliminates threading bugs
  • Simple blocking code now scales to millions

Enterprise Impact:

  • Can retire complex reactive frameworks
  • Simpler, more maintainable code
  • 10x-100x better throughput for I/O
  • Debugging becomes straightforward

Pros and Cons

Pros ✅

  • Virtual Threads Standard: Revolutionary scalability
  • Pattern Matching Complete: Much more expressive code
  • Structured Concurrency: Better concurrent programming
  • LTS Support: 8 years of support (until Sept 2031)
  • FFM API Stable: Improving native code interop
  • String Templates: Cleaner string operations
  • Finalized Features: Everything stable and production-ready
  • Best LTS Since Java 11: Massive improvements

Cons ❌

  • Large Learning Curve: Many new concepts
  • Ecosystem Adaptation: Libraries need updates
  • String Templates Preview: Still evolving
  • Migration Effort: From older Java versions is significant

Real-World Architecture

// Modern microservice with Java 21
public class OrderServiceAPI {
    
    // Virtual thread executor
    private final ExecutorService executor = 
        Executors.newVirtualThreadPerTaskExecutor();
    
    // Structured concurrency for data fetching
    public record OrderDetails(
        Order order,
        User customer,
        InventoryStatus inventory
    ) {}
    
    public OrderDetails getOrderWithDetails(String orderId) throws Exception {
        try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
            Future<Order> orderFuture = scope.fork(() -> 
                fetchOrder(orderId)
            );
            Future<User> userFuture = scope.fork(() ->
                fetchCustomer(orderId)
            );
            Future<InventoryStatus> invFuture = scope.fork(() ->
                checkInventory(orderId)
            );
            
            scope.join();
            
            return new OrderDetails(
                orderFuture.resultNow(),
                userFuture.resultNow(),
                invFuture.resultNow()
            );
        }
    }
    
    // Simple blocking API thanks to virtual threads
    public void handleRequest(Order order) {
        executor.submit(() -> {
            var details = switch (order) {
                case Order(String id, _, _) -> 
                    getOrderWithDetails(id); // No callback hell!
            };
            
            processOrder(details);
        });
    }
    
    // Helper methods
    Order fetchOrder(String id) { return new Order(id, "", 0); }
    User fetchCustomer(String id) { return new User(""); }
    InventoryStatus checkInventory(String id) { return new InventoryStatus(); }
    void processOrder(OrderDetails details) { }
}
 
record Order(String id, String status, double total) {}
record User(String name) {}
record InventoryStatus() {}

Conclusion

Java 21 LTS represents a watershed moment for Java. Virtual threads fundamentally change how developers write concurrent code. Pattern matching makes code more expressive and type-safe. Structured concurrency brings order to concurrent programming. This is the best LTS release since Java 11.

Recommendation:

  • Adopt Java 21 immediately for all new projects
  • Plan migration from 8/11/17 to Java 21
  • Support until September 2031
  • Expect to stay on Java 21 for many years
  • This is a long-term strategic choice for enterprises

Key Benefits:

  • 100x throughput improvement for I/O-bound services
  • Simple, readable code replaces complex reactive patterns
  • Better platform for concurrent applications
  • Modern feature set with long-term stability
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