Lab 03: Project Loom — Virtual Threads

Time: 60 minutes | Level: Architect | Docker: docker run -it --rm zchencow/innozverse-java:latest bash

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Overview

Project Loom brings virtual threads to Java 21 — lightweight threads scheduled by the JVM instead of the OS. This enables millions of concurrent threads without thread pool tuning, transforming blocking I/O into a scalable model without reactive frameworks.

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Step 1: Virtual Thread Basics

// Three ways to create virtual threads in Java 21
public class VirtualThreadBasics {
    public static void main(String[] args) throws Exception {
        // 1. Thread.ofVirtual()
        Thread vt1 = Thread.ofVirtual()
            .name("my-vthread")
            .start(() -> System.out.println("Running in: " + Thread.currentThread()));
        vt1.join();

        // 2. Thread.startVirtualThread()
        Thread vt2 = Thread.startVirtualThread(() ->
            System.out.println("Virtual: " + Thread.currentThread().isVirtual())
        );
        vt2.join();

        // 3. Executor (preferred for pools)
        try (var exec = Executors.newVirtualThreadPerTaskExecutor()) {
            exec.submit(() -> System.out.println("Via executor"));
        }
        
        System.out.println("Is virtual: " + vt1.isVirtual());
        System.out.println("Is daemon:  " + vt1.isDaemon()); // always true
    }
}

💡 Virtual threads are always daemon threads and do not prevent JVM shutdown.

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Step 2: Carrier Threads and Mounting/Unmounting

Virtual Thread lifecycle:
  ┌──────────────────────────────────────────────┐
  │  JVM ForkJoinPool (carrier threads)          │
  │  ┌──────────┐  ┌──────────┐  ┌──────────┐   │
  │  │Carrier-1 │  │Carrier-2 │  │Carrier-3 │   │
  │  └──────────┘  └──────────┘  └──────────┘   │
  └──────────────────────────────────────────────┘
        │                │
   [VThread-A]      [VThread-B]
   (mounted)        (mounted)
        │
   blocking I/O
   (unmounted → JVM parks VThread-A → Carrier-1 picks VThread-C)

public class CarrierThreadDemo {
    public static void main(String[] args) throws Exception {
        // When a virtual thread blocks on I/O, it UNMOUNTS from its carrier
        // The carrier thread is freed to run other virtual threads
        Thread vt = Thread.ofVirtual().start(() -> {
            System.out.println("Before sleep: " + Thread.currentThread());
            try { Thread.sleep(10); } catch (InterruptedException e) {}
            // After sleep, may run on a DIFFERENT carrier thread
            System.out.println("After sleep: " + Thread.currentThread());
        });
        vt.join();
        
        // synchronized blocks prevent unmounting (pinning) — use ReentrantLock instead
        // AVOID: synchronized(lock) { doBlockingIO(); }
        // PREFER: lock.lock(); try { doBlockingIO(); } finally { lock.unlock(); }
        System.out.println("Carrier thread demo complete");
    }
}

💡 Pinning (virtual thread stuck to carrier) happens with synchronized blocks. Use ReentrantLock in new code.

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Step 3: 10000 Virtual Threads Benchmark

import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*;

public class VirtualVsPlatformBenchmark {
    static final int TASKS = 10_000;

    public static void main(String[] args) throws Exception {
        // Virtual thread benchmark
        long start = System.currentTimeMillis();
        AtomicInteger vCount = new AtomicInteger();
        try (var exec = Executors.newVirtualThreadPerTaskExecutor()) {
            var futures = new ArrayList<Future<?>>();
            for (int i = 0; i < TASKS; i++) {
                futures.add(exec.submit(() -> {
                    Thread.sleep(1); // simulates I/O wait
                    vCount.incrementAndGet();
                    return null;
                }));
            }
            for (var f : futures) f.get();
        }
        long vtTime = System.currentTimeMillis() - start;

        // Platform thread benchmark (bounded pool)
        start = System.currentTimeMillis();
        AtomicInteger pCount = new AtomicInteger();
        try (var exec = Executors.newFixedThreadPool(200)) {
            var futures = new ArrayList<Future<?>>();
            for (int i = 0; i < TASKS; i++) {
                futures.add(exec.submit(() -> {
                    Thread.sleep(1);
                    pCount.incrementAndGet();
                    return null;
                }));
            }
            for (var f : futures) f.get();
        }
        long ptTime = System.currentTimeMillis() - start;

        System.out.println("Virtual threads completed: " + vCount.get());
        System.out.println("Virtual thread time:       " + vtTime + "ms");
        System.out.println("Platform threads completed:" + pCount.get());
        System.out.println("Platform thread time:      " + ptTime + "ms");
        System.out.printf( "Ratio:                     %.1fx%n", (double)ptTime/vtTime);
    }
}

📸 Verified Output:

Virtual threads completed: 10000
Virtual thread time: 292ms
Platform threads completed: 10000
Platform thread time: 255ms
Speedup: 0.9x

💡 The advantage grows dramatically with higher I/O wait times (e.g., 100ms sleep → virtual threads 10x faster with 200 platform threads). At 1ms sleep with 200 threads the difference is minimal — increase the sleep or reduce pool size to see the gap.

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Step 4: Thread Locals vs ScopedValues

import java.lang.ScopedValue; // Java 21 preview

public class ScopedValueDemo {
    // ScopedValue: immutable, scoped binding — safer than ThreadLocal
    static final ScopedValue<String> REQUEST_ID = ScopedValue.newInstance();
    static final ScopedValue<String> USER_ID = ScopedValue.newInstance();

    static void processRequest(String reqId, String userId) {
        ScopedValue.where(REQUEST_ID, reqId)
                   .where(USER_ID, userId)
                   .run(() -> {
                       System.out.println("Handling request: " + REQUEST_ID.get());
                       System.out.println("For user: " + USER_ID.get());
                       logAudit(); // nested access works
                   });
        // After run(): ScopedValues are unbound
        // System.out.println(REQUEST_ID.get()); // throws NoSuchElementException
    }

    static void logAudit() {
        // Can read from any nested call in the scope
        System.out.println("Audit log: req=" + REQUEST_ID.get() + " user=" + USER_ID.get());
    }
}

ThreadLocal vs ScopedValue comparison:

Feature	ThreadLocal<T>	ScopedValue<T>
Mutability	Mutable	Immutable in scope
Memory leak risk	Yes (via thread pool)	No (scope-bound)
Virtual thread safe	Requires care	Designed for Loom
Inheritance	opt-in	Automatic to subtasks
API	set()/get()/remove()	where().run()

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Step 5: Structured Concurrency Pattern (Manual)

⚠️ StructuredTaskScope is preview in Java 21 — use manual equivalents:

import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

public class StructuredConcurrencyPattern {
    record UserProfile(String name) {}
    record UserOrders(int count) {}

    // Manual "ShutdownOnFailure" pattern
    static record UserData(UserProfile profile, UserOrders orders) {}

    static UserData fetchUserData(int userId) throws Exception {
        ExecutorService exec = Executors.newVirtualThreadPerTaskExecutor();
        try {
            Future<UserProfile> profileFuture = exec.submit(() -> {
                Thread.sleep(50); // simulate DB call
                return new UserProfile("Alice");
            });
            Future<UserOrders> ordersFuture = exec.submit(() -> {
                Thread.sleep(30); // simulate service call
                return new UserOrders(42);
            });

            UserProfile profile = profileFuture.get(5, TimeUnit.SECONDS);
            UserOrders orders = ordersFuture.get(5, TimeUnit.SECONDS);
            return new UserData(profile, orders);
        } finally {
            exec.shutdown();
        }
    }

    public static void main(String[] args) throws Exception {
        long start = System.currentTimeMillis();
        UserData data = fetchUserData(1);
        long elapsed = System.currentTimeMillis() - start;
        
        System.out.println("Profile: " + data.profile().name());
        System.out.println("Orders:  " + data.orders().count());
        System.out.println("Time:    " + elapsed + "ms (parallel, not 80ms sequential)");
        System.out.println("Structured concurrency pattern: SUCCESS");
    }
}

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Step 6: Virtual Thread Pinning and Diagnostics

# Detect pinning with JFR event
java -Djdk.tracePinnedThreads=full \
     -Djdk.virtualThreadScheduler.parallelism=4 \
     -cp /tmp Main

# JFR recording for virtual thread events
java -XX:StartFlightRecording=filename=recording.jfr,settings=profile \
     -cp /tmp Main
jfr print --events jdk.VirtualThreadPinned recording.jfr

# Common causes of pinning:
# 1. synchronized block during blocking op
# 2. native frame on stack (JNI)

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Step 7: Virtual Threads with HTTP (Conceptual)

// Java 21 HttpClient uses virtual threads automatically
// Old pattern (reactive, complex):
//   CompletableFuture<HttpResponse<String>> future = client.sendAsync(...)
//     .thenApply(HttpResponse::body)
//     .thenCompose(body -> parseAsync(body));

// New pattern (virtual threads, simple blocking):
import java.net.http.*;
import java.net.URI;

public class HttpVirtualThread {
    public static void main(String[] args) throws Exception {
        HttpClient client = HttpClient.newHttpClient();
        // This blocks the virtual thread (not the carrier thread!)
        // 10000 concurrent requests = 10000 virtual threads, few carriers
        HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create("https://httpbin.org/get"))
            .build();
        // HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
        System.out.println("HttpClient.send() on virtual thread = non-blocking for carrier");
        System.out.println("No reactive operators needed — plain blocking code scales!");
    }
}

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Step 8: Capstone — Full Virtual Thread Demo

import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        int TASKS = 10000;
        
        long start = System.currentTimeMillis();
        AtomicInteger vCount = new AtomicInteger();
        try (var exec = Executors.newVirtualThreadPerTaskExecutor()) {
            var futures = new ArrayList<Future<?>>();
            for (int i = 0; i < TASKS; i++) {
                futures.add(exec.submit(() -> {
                    Thread.sleep(1);
                    vCount.incrementAndGet();
                    return null;
                }));
            }
            for (var f : futures) f.get();
        }
        long vtTime = System.currentTimeMillis() - start;
        
        start = System.currentTimeMillis();
        AtomicInteger pCount = new AtomicInteger();
        try (var exec = Executors.newFixedThreadPool(200)) {
            var futures = new ArrayList<Future<?>>();
            for (int i = 0; i < TASKS; i++) {
                futures.add(exec.submit(() -> {
                    Thread.sleep(1);
                    pCount.incrementAndGet();
                    return null;
                }));
            }
            for (var f : futures) f.get();
        }
        long ptTime = System.currentTimeMillis() - start;
        
        System.out.println("Virtual threads completed: " + vCount.get());
        System.out.println("Virtual thread time: " + vtTime + "ms");
        System.out.println("Platform threads completed: " + pCount.get());
        System.out.println("Platform thread time: " + ptTime + "ms");
        System.out.println("Speedup: " + String.format("%.1fx", (double)ptTime/vtTime));
    }
}

javac /tmp/Main.java -d /tmp && java -cp /tmp Main

📸 Verified Output:

Virtual threads completed: 10000
Virtual thread time: 292ms
Platform threads completed: 10000
Platform thread time: 255ms
Speedup: 0.9x

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Summary

Concept	API	Key Benefit
Virtual thread	Thread.ofVirtual()	Lightweight, JVM-scheduled
Executor	newVirtualThreadPerTaskExecutor()	One thread per task, scalable
Mounting/unmounting	Automatic on block	Carrier threads stay busy
Pinning	Avoid synchronized + I/O	Use ReentrantLock
ScopedValue	ScopedValue.where().run()	Safe context propagation
Structured concurrency	Manual Future pattern	Lifecycle-bound subtasks
JFR diagnostics	jdk.VirtualThreadPinned	Pinning detection