Boost Java Concurrency with Project Loom: A Hands‑On Guide to Virtual Threads

What Is Project Loom?

Project Loom is an Oracle initiative aimed at dramatically reducing the effort required to write, maintain, and observe high‑throughput concurrent applications. It introduces lightweight virtual threads that are managed by the JVM instead of the operating system, making a one‑thread‑per‑request model feasible without sacrificing throughput. Virtual threads are available in Spring Boot 3.1.

How to Use Virtual Threads

Choosing the Java Version

Virtual threads were introduced in Java 19. The author uses the Azul Zulu distribution version 20.30.11 on a Mac M1.

Creating a Test Project

Generate a Spring Boot project via Spring Initializr or an IDE, add the

spring-web

dependency, and ensure you use Spring Boot 3.1 with Java 20.

Enabling Virtual Thread Support

Virtual threads are disabled by default in Java 19 and must be enabled via Maven compiler arguments.

<code>&lt;plugin&gt;
  &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
  &lt;artifactId&gt;spring-boot-maven-plugin&lt;/artifactId&gt;
&lt;/plugin&gt;
&lt;plugin&gt;
  &lt;groupId&gt;org.apache.maven.plugins&lt;/groupId&gt;
  &lt;artifactId&gt;maven-compiler-plugin&lt;/artifactId&gt;
  &lt;configuration&gt;
    &lt;compilerArgs&gt;--enable-preview&lt;/compilerArgs&gt;
  &lt;/configuration&gt;
&lt;/plugin&gt;</code>

Enable virtual threads via a bean

<code>@Bean
TomcatProtocolHandlerCustomizer<?> threadExecutorCustomizer() {
  return protocolHandler -> protocolHandler.setExecutor(
    Executors.newVirtualThreadPerTaskExecutor());
}</code>

This bean customizes the Tomcat protocol handler to use an executor backed by virtual threads, allowing many concurrent tasks with minimal resource cost.

Adding a Test Endpoint

<code>@Slf4j
@RequestMapping
@RestController
public class DemoController {
  @GetMapping("/")
  public String demo() {
    try {
      TimeUnit.MILLISECONDS.sleep(300);
    } catch (InterruptedException e) {
      log.error(e.getMessage());
    }
    return "Current Thread Name: " + Thread.currentThread();
  }
}
</code>

Getting Started

Use

Thread.currentThread()

to verify that the request is handled by a virtual thread.

Performance Test Comparison

100‑Thread Test

Without virtual threads

Throughput: 300 req/s

With virtual threads

Throughput: 300 req/s

300‑Thread Test

Without virtual threads

Throughput: 650 req/s

With virtual threads

Throughput: 950 req/s

500‑Thread Test

Without virtual threads

Throughput: 650 req/s

With virtual threads

Throughput: 1500 req/s

Chart Output

Conclusion

Virtual threads prove especially beneficial for blocking operations. As the number of concurrent requests grows, the performance advantage of virtual threads becomes increasingly evident. The tests were conducted without any additional Spring Boot tuning or optimization.

References

[1] Azul Zulu distribution 20.30.11: https://www.azul.com/downloads/?version=java-20-sts&os=macos&architecture=arm-64-bit&package=jdk#zulu