As of JDK 21 there is no way to set up a custom Executor/scheduler for VirtualThread class: this class along with ThreadBuilders.VirtualThreadBuilder and VirtualThreadFactory are java.lang package-private and the only way to do so is to use Reflection. The solution is wrapped below in an implementation of Executor interface:

public class CustomVirtualThreadPoolExecutor implements Executor { private final Object builderInstance; private final Class<?> builderClass; private UncaughtExceptionHandler uhe; public CustomVirtualThreadPoolExecutor() throws ReflectiveOperationException {  this(Executors.newCachedThreadPool()); } public CustomVirtualThreadPoolExecutor(Executor executor) throws ReflectiveOperationException {  try {   builderClass = Class.forName("java.lang.ThreadBuilders$VirtualThreadBuilder");   final Constructor<?> ctor = builderClass.getDeclaredConstructor(Executor.class);   ctor.setAccessible(true); builderInstance = ctor.newInstance(executor); } catch (ClassNotFoundException | NoSuchMethodException | SecurityException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) { throw new ReflectiveOperationException(e); } } public void setUncaughtExceptionHandler(UncaughtExceptionHandler ueh) { this.uhe = Objects.requireNonNull(ueh); } @Override public void execute(final Runnable task) { try { final Method startMethod = builderClass.getMethod("start", Runnable.class);   startMethod.setAccessible(true); startMethod.invoke(builderInstance, task); } catch (NoSuchMethodException | SecurityException | IllegalAccessException | InvocationTargetException e) { if (uhe != null)  uhe.uncaughtException(null, e);  } } } 

As access to java.lang classes is attempted it might be necessary to explicitly allow it in parameters

@Configuration public class AppConfig { @Bean  public TomcatProtocolHandlerCustomizer<?> protocolHandlerVirtualThreadExecutorCustomizer() {   return protocolHandler -> { try { protocolHandler.setExecutor(new CustomVirtualThreadPoolExecutor());   } catch (ReflectiveOperationException e) {   throw new BeanInitializationException("Cannot create " + CustomVirtualThreadPoolExecutor.class.getSimpleName(), e); } } }   } 

@Configuration @EnableAsync public class AppConfig { @Bean public TaskExecutor taskExecutor() { try { return new TaskExecutorAdapter(new CustomVirtualThreadPoolExecutor()); } catch (ReflectiveOperationException e) { throw new BeanInitializationException("Cannot create " + CustomVirtualThreadPoolExecutor.class.getSimpleName(), e); } } } 

As of JDK 21 there is no way to set up a custom Executor/scheduler for VirtualThread class: this class along with ThreadBuilders.VirtualThreadBuilder and VirtualThreadFactory are java.lang package-private and the only way to do so is to use Reflection.

public static Thread.Builder customVirtualThreadPool(Executor executor) throws ReflectiveOperationException { var ctor = Class.forName("java.lang.ThreadBuilders$VirtualThreadBuilder") .getDeclaredConstructor(Executor.class); ctor.setAccessible(true); return (Thread.Builder) ctor.newInstance(executor); } 

As access to java.lang classes is attempted it might be necessary to explicitly allow it in application parameters

@Bean public TomcatProtocolHandlerCustomizer<?> protocolHandlerVirtualThreadExecutorCustomizer() { return protocolHandler -> { try {   protocolHandler.setExecutor(  customVirtualThreadPool(Executors.newCachedThreadPool())::start); } catch (ReflectiveOperationException e) { throw new BeanInitializationException("Cannot create custom Virtual Thread Executor", e); } }; } 

@Configuration @EnableAsync public class AppConfig { @Bean public TaskExecutor taskExecutor() { try { return new TaskExecutorAdapter(customVirtualThreadPool(Executors.newCachedThreadPool())::start); } catch (ReflectiveOperationException e) { throw new BeanInitializationException("Cannot create custom Virtual Thread Executor", e); } } } 

As of JDK 21 there is no way to set up a custom Executor/scheduler for VirtualThread class: this class along with ThreadBuilders.VirtualThreadBuilder and VirtualThreadFactory are java.lang package-private and the only way to do so is to use Reflection. The solution is wrapped below in an implementation of Executor interface:

Note that this can be used to set up custom ForkJoinPool instance as an actual pool in order to get an access to work-stealing count.

According to a request of the OP to configure VirtualThread with a custom thread pool, the following solution is discussed below.

As of JDK 21 there is no way to set up a custom Executor/scheduler for VirtualThread class: this class along with ThreadBuilders.VirtualThreadBuilder and VirtualThreadFactory are java.lang package-private and the only way to do so is to use Reflection. The solution is wrapped below in an implementation of Executor interface:

Note that this can be used to set up custom ForkJoinPool instance as an actual pool in order to get an access to work-stealing count.

For Spring Boot/Tomcat application further configuration does not differ from configuration of any other such Executor.

As usual, to run Tomcat worker threads on this Excecutor, the ones that executes @Controllers methods, annotated with @RequestMapping and analogous, customize Tomcat Protocol Handler:

@Configuration public class AppConfig { @Bean public TomcatProtocolHandlerCustomizer<?> protocolHandlerVirtualThreadExecutorCustomizer() { return protocolHandler -> { try { protocolHandler.setExecutor(new CustomVirtualThreadPoolExecutor()); } catch (ReflectiveOperationException e) { throw new BeanInitializationException("Cannot create " + CustomVirtualThreadPoolExecutor.class.getSimpleName(), e); } } } } 

Similarly, to run Spring-controlled asynchronous methods on this Executor, configuration will look like this:

@Configuration @EnableAsync public class AppConfig { @Bean public TaskExecutor taskExecutor() { try { return new TaskExecutorAdapter(new CustomVirtualThreadPoolExecutor()); } catch (ReflectiveOperationException e) { throw new BeanInitializationException("Cannot create " + CustomVirtualThreadPoolExecutor.class.getSimpleName(), e); } } } 

and the asynchronous method like this:

@Service @Async // will use `taskExecutor` by default, if there are more than one in the app, then Bean name can be specified public class AsyncService { public void doSomething() { ... } } 

Note that presence of spring.threads.virtual.enabled = true in application property file is not required in both cases.

The above is applicable to Spring Boot 3.2.0; be cautious with older Tomcats, they may or may not support the configuration above.