Java 20: Foreign Function & Memory API Refinement

// Calling native C functions safely
import java.lang.foreign.*;
 
public class NativeCodeInterop {
    
    // Improved FFM API
    public static void main(String[] args) throws Throwable {
        Linker linker = Linker.nativeLinker();
        SymbolLookup stdlib = linker.defaultLookup();
        
        // Looking up strlen function
        MemorySegment strlenAddress = stdlib.find("strlen").get();
        
        // Function descriptor and handle
        FunctionDescriptor strlen = FunctionDescriptor.of(
            ValueLayout.JAVA_LONG,    // return type
            ValueLayout.ADDRESS       // parameter
        );
        
        MethodHandle strlenHandle = linker.downcallHandle(
            strlenAddress,
            strlen
        );
        
        // Calling native function
        Arena arena = Arena.ofAuto();
        MemorySegment nativeString = arena.allocateUtf8String("Hello World");
        
        long length = (long) strlenHandle.invoke(nativeString);
        System.out.println("String length: " + length); // 11
    }
    
    // Creating callbacks for C functions
    public static class CallbackExample {
        public static void registerCallback() throws Throwable {
            // Java callback that can be called from native code
            int javaCallback(int x) {
                return x * 2;
            }
            
            // Create native function pointer to Java method
            // Advanced usage for library integration
        }
    }
}

// More sophisticated pattern matching
public sealed class Shape {}
public record Circle(double radius) extends Shape {}
public record Rectangle(double width, double height) extends Shape {}
public record Line(double length) extends Shape {}
 
public String describeShape(Shape shape) {
    return switch (shape) {
        case Circle(double r) when r > 10 -> "Large circle: " + r;
        case Circle(double r) -> "Small circle: " + r;
        
        case Rectangle(double w, double h) when w == h -> 
            "Square: " + w;
        case Rectangle(double w, double h) -> 
            "Rectangle: " + w + "x" + h;
        
        case Line(double len) -> "Line: " + len;
    };
}
 
// Complex nested patterns
record Person(String name, Address address) {}
record Address(String city, String country) {}
 
public boolean isLocalResident(Object obj) {
    return obj instanceof Person(var name, Address(var city, "USA")) &&
           city.equals("New York");
}

// Record pattern destructuring
public record UserProfile(String username, int age, String email) {}
 
public void processUser(Object user) {
    if (user instanceof UserProfile(String name, int age, _)) {
        System.out.println(name + " is " + age + " years old");
    }
}
 
// Practical example
public void validateEmail(Object obj) {
    if (obj instanceof UserProfile(_, _, String email) &&
        email.contains("@")) {
        System.out.println("Valid email");
    }
}

// Virtual threads improvements
public class EnhancedVirtualThreads {
    
    public static void main(String[] args) throws Exception {
        int NUM_TASKS = 100_000;
        List<String> results = new ArrayList<>();
        
        // Better structured concurrency
        try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {
            
            for (int i = 0; i < NUM_TASKS; i++) {
                scope.fork(() -> performTask());
            }
            
            String firstSuccess = scope.join(); // Wait for first success
            System.out.println("Got result: " + firstSuccess);
        }
    }
    
    static String performTask() {
        // Work runs on virtual thread
        return "Result";
    }
}

// Scoped values - cleaner than ThreadLocal
import java.lang.ScopedValue;
 
final static ScopedValue<String> REQUEST_ID = ScopedValue.newInstance();
final static ScopedValue<User> CURRENT_USER = ScopedValue.newInstance();
 
public void handleRequest(String requestId, User user) {
    ScopedValue.callWhere(REQUEST_ID, requestId,
        () -> ScopedValue.callWhere(CURRENT_USER, user,
            () -> processRequest()
        )
    );
}
 
private void processRequest() {
    // Access scoped values anywhere in call chain
    String reqId = REQUEST_ID.get();
    User usr = CURRENT_USER.get();
    
    System.out.println("Processing: " + reqId + " for " + usr.name());
}
 
// Key difference from ThreadLocal:
// - Works perfectly with virtual threads
// - No cleanup needed
// - No memory leaks
// - Better performance

# G1GC improvements
java -XX:+UseG1GC \
     -XX:MaxGCPauseMillis=50 \
     -XX:+ParallelRefProcEnabled \
     MyApp
 
# ZGC improvements (sub-millisecond pauses)
java -XX:+UnlockExperimentalVMOptions \
     -XX:+UseZGC \
     -XX:ZCollectionInterval=120 \
     MyApp
 
# Shenandoah improvements
java -XX:+UnlockExperimentalVMOptions \
     -XX:+UseShenandoahGC \
     MyApp

// Modern web service with Java 20
public class RestAPI {
    static final ScopedValue<RequestContext> CONTEXT = 
        ScopedValue.newInstance();
    
    public void handleRequest(HttpExchange exchange) throws IOException {
        var context = new RequestContext(
            UUID.randomUUID().toString(),
            exchange.getRemoteAddress().getHostName()
        );
        
        ScopedValue.callWhere(CONTEXT, context, () -> {
            String path = exchange.getRequestURI().getPath();
            String response = switch (path) {
                case "/users" -> handleUsers();
                case "/orders" -> handleOrders();
                default -> "{\"error\":\"Not found\"}";
            };
            
            exchange.sendResponseHeaders(200, response.length());
            exchange.getResponseBody().write(response.getBytes());
            exchange.close();
        });
    }
    
    private String handleUsers() {
        RequestContext ctx = CONTEXT.get();
        logRequest("Fetching users for " + ctx.clientIP);
        return "[{\"id\":1,\"name\":\"John\"}]";
    }
    
    private String handleOrders() {
        RequestContext ctx = CONTEXT.get();
        logRequest("Fetching orders for " + ctx.clientIP);
        return "[{\"id\":1,\"total\":99.99}]";
    }
    
    private void logRequest(String msg) {
        RequestContext ctx = CONTEXT.get();
        System.out.println("[" + ctx.requestId + "] " + msg);
    }
}
 
record RequestContext(String requestId, String clientIP) {}