What are Virtual Threads? How do they differ from Green Threads? When is it better to use them?

Virtual Threads are lightweight threads implemented at the virtual machine level (e.g., in Project Loom for Java), allowing the creation of a large number of concurrent tasks with minimal resource overhead. They are managed by the JVM rather than the operating system, reducing context switch costs.

Green Threads are also lightweight threads, but implemented in user space rather than the OS. However, green threads are usually not integrated into the standard OS thread model and may have limitations regarding system call interactions.

Differences:

• Virtual Threads are integrated into the JVM and support the standard thread API, simplifying migration of existing code.
• Green Threads often require special support in the runtime and may not always be compatible with system calls.

When to use:

• Virtual Threads are suitable for applications with many parallel tasks, such as high-load servers, where efficient resource utilization is important.
• Green Threads can be used in specific environments or languages that do not support native threads.

Example of using Virtual Threads in Java (Project Loom):

public class VirtualThreadExample {
    public static void main(String[] args) {
        Thread.startVirtualThread(() -> {
            System.out.println("Virtual thread is running");
        });
    }
}

This allows creating thousands of such threads with minimal overhead compared to traditional OS threads.