Michael Bien's Weblog

A quick guide about how to configure Jetty to use Project Loom's virtual threads instead of plain old java threads.

Jetty's default thread pool implementation can be swapped out by implementing Jetty's ThreadPool interface and passing an instance to the Server constructor. If you are using jetty stand alone, everything is initialized by xml files.

Assuming you are using the recommended jetty home / jetty base folder structure, all what is needed is to create jetty-threadpool.xml in [jetty-base]/etc containing the following:

<Configure>
<New id="threadPool" class="dev.mbien.virtualthreads4jetty.VirtualThreadExecutor"/>
</Configure>

and put a jar containing the custom VirtualThreadExecutor into [jetty-base]/lib/ext. I uploaded a build to the release section of the vt4jetty github project.

If you don't have an lib/ext folder yet you can enable it with:

java -jar $JETTY_HOME/start.jar --add-to-start=ext

here the code:

package dev.mbien.virtualthreads4jetty;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import org.eclipse.jetty.util.thread.ThreadPool;

/**
 * Executes each task in a new virtual thread.
 * 
 * <p>Java's default ForkJoinPool is used as scheduler. To influence carrier
 * thread count use -Djdk.defaultScheduler.parallelism=N. Default is
 * {@link Runtime#availableProcessors()}.
 * 
 * @author mbien
 */
public class VirtualThreadExecutor implements ThreadPool {
    
    private final ExecutorService executor;

    public VirtualThreadExecutor() {
        executor = Executors.newThreadExecutor(
                Thread.builder().virtual().name("jetty-vt#", 0).factory());
        // too early for logging libs
        System.out.println("VirtualThreadExecutor is active.");
    }
    
    @Override
    public void execute(Runnable command) {
        executor.execute(command);
    }

    @Override
    public void join() throws InterruptedException {
        executor.shutdown();
        executor.awaitTermination(3, TimeUnit.SECONDS);
    }

    // those are hopefully only used for stats/dashboards etc
    @Override
    public int getThreads() { return -1; }

    @Override
    public int getIdleThreads() { return -1; }

    @Override
    public boolean isLowOnThreads() { return false; }
    
}

Tested with JDK16-loom+4-56 (2020/7/25) early access build from here and latest Jetty.

I encountered some JVM crashes while load testing Apache Roller with virtual threads enabled - keep in mind this is still all very much work in progress.