# Virtual Threads (Java 21) — Advanced Java: Streams, Concurrency, JVM and Spring

Source: https://www.geekswithgeeks.com/en/advanced-java/adv-virtual-threads

> Handle massive concurrency with lightweight virtual threads that make blocking code cheap, and know when platform threads still fit.

## Threads that are cheap

A **virtual thread** is scheduled by the JVM, not the OS, so you can run millions of them. When one blocks on I/O, the JVM parks it and reuses the carrier thread. This keeps the simple thread-per-request style scalable.

## Creating virtual threads

Use `Thread.ofVirtual()` or an executor that starts one virtual thread per task.

```java
Thread.ofVirtual().name("worker-1").start(() -> fetch(url));

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (String u : urls) {
        executor.submit(() -> fetch(u));   // one cheap thread per task
    }
}   // close() waits for all tasks
```

Virtual threads help I/O-bound work, not CPU-bound loops. Do not pool them, avoid long `synchronized` blocks around blocking calls, and use `ReentrantLock` instead.

Quick check

**Quiz:** What are virtual threads best suited for?

- [x] Many concurrent blocking I/O tasks
- [ ] CPU-bound number crunching only
- [ ] Replacing the garbage collector
- [ ] Compiling code

*Answer:* Many concurrent blocking I/O tasks. They make blocking I/O cheap so you can scale thread-per-task code.