Project Loom (Virtual Threads)

November 26, 2025

What It Is

Project Loom is an initiative by the OpenJDK community that introduces lightweight, virtual threads to the Java platform. Virtual threads are designed to simplify concurrent programming by providing a much larger number of threads than traditional platform threads, without the high resource costs. They are managed by the JVM rather than the operating system.

Why It's Used

Virtual threads aim to make high-concurrency applications easier to build and maintain. Traditional thread pools require careful tuning and management, while virtual threads eliminate much of this complexity by enabling applications to create millions of threads without exhausting system resources. This makes them ideal for scenarios such as web servers, message brokers, and microservices.

Example

Here is a simple example of how to use a virtual thread in Java 21:

Runnable task = () -> System.out.println(“Running in: ” + Thread.currentThread());

Thread.startVirtualThread(task);

Traditional:

Thread thread = new Thread();

Thread.start()

Virtual:

Thread.startVirtualThread(task)

Use Case

A practical use case for virtual threads is handling thousands (or even millions) of concurrent user requests in a web application or microservice. With traditional threads, this level of concurrency would overwhelm the operating system. Virtual threads solve this by being lightweight and more efficient.

Requirements

Java 21 or later (Virtual threads are part of the standard JDK since Java 21).
Spring Boot 3.2 or later (to support native integration with virtual threads).

Benefits

– Simplifies concurrent programming.
– Eliminates the need for complex thread pool management.
– Enables millions of concurrent operations.
– Reduces resource consumption compared to traditional threads.
– Seamlessly integrates with existing Java APIs.

Limitations & Considerations

– Still relatively new, so tooling and monitoring support are evolving.
– Performance benefits are workload-dependent (I/O-heavy workloads benefit most).
– Requires developers to understand structured concurrency and cooperative cancellation.

Integration with Spring Boot

Spring Boot 3.2+ offers built-in support for Project Loom. You can configure Spring applications to run tasks using virtual threads, making it easier to build reactive-like applications without rewriting code using reactive programming frameworks like Reactor. For example, configuring Tomcat or Jetty in a Spring Boot application can leverage virtual threads for request handling.

Spring Boot Example

@RestController

public class DemoController {

@GetMapping(“/test”)

public String handle() throws InterruptedException {

Thread.sleep(100); // Safe with virtual threads

return “Handled by ” + Thread.currentThread();

}

Benefits in Spring:

Use imperative code (no need for WebFlux).
Avoid thread pool tuning.
Gain scalability without complexity.

Performance Benefits

Lower memory use: Each virtual thread uses a tiny memory footprint.
Higher throughput: JVM can schedule many virtual threads efficiently.
Simplified code: Maintain synchronous programming style without async complexity.