Virtual threads

Create millions of lightweight virtual threads instead of heavy OS threads.

Code Comparison

✕ Java 8

Thread thread = new Thread(() -> {
    System.out.println("hello");
});
thread.start();
thread.join();

✓ Java 21+

Thread.startVirtualThread(() -> {
    IO.println("hello");
}).join();

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Why the modern way wins

⚡

Lightweight

Virtual threads use KB of memory, platform threads use MB.

♾️

Scalable

Create millions of threads — no pool sizing needed.

🧹

Simple model

Write blocking code that scales like async code.

Old Approach

Platform Threads

Modern Approach

Virtual Threads

Since JDK

Difficulty

Beginner

JDK Support

Virtual threads

Available

Widely available since JDK 21 LTS (Sept 2023)

How it works

Virtual threads are lightweight threads managed by the JVM, not the OS. You can create millions of them without tuning thread pools. They're ideal for I/O-bound tasks like HTTP calls and database queries.

Thread.sleep with Duration

Java 8

// What unit is 5000? ms? us?
Thread.sleep(5000);

// 2.5 seconds: math required
Thread.sleep(2500);

Java 19+

Thread.sleep(
    Duration.ofSeconds(5)
);
Thread.sleep(
    Duration.ofMillis(2500)
);

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JDK 19+ →

Concurrency Advanced

Scoped values

Java 8

static final ThreadLocal<User> CURRENT =
    new ThreadLocal<>();
void handle(Request req) {
    CURRENT.set(authenticate(req));
    try { process(); }
    finally { CURRENT.remove(); }
}

Java 25

static final ScopedValue<User> CURRENT =
    ScopedValue.newInstance();
void handle(Request req) {
    ScopedValue.where(CURRENT,
        authenticate(req)
    ).run(this::process);
}

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JDK 25+ →

Concurrency Intermediate

CompletableFuture chaining

Pre-Java 8

Future<String> future =
    executor.submit(this::fetchData);
String data = future.get(); // blocks
String result = transform(data);

Java 8+

CompletableFuture.supplyAsync(
    this::fetchData
)
.thenApply(this::transform)
.thenAccept(IO::println);

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JDK 8+ →