Bootstrap.scala

import cats.effect._
import cats.implicits._
import javax.servlet._
import javax.servlet.annotation._
import java.util.concurrent._
import org.http4s._
import org.http4s.server._
import org.http4s.servlet._
import scala.concurrent.ExecutionContext.global

abstract class Bootstrap[F[_]] extends ServletContextListener {
  implicit def F: ConcurrentEffect[F]
  implicit def cs: ContextShift[F]

  def appF: F[HttpApp[F]] = F.delay(HttpApp[F] { _ => Response[F](Status.Ok).withEntity("Hi!").pure[F] })

  var pool: ExecutorService = _

  override def contextInitialized(sce: ServletContextEvent): Unit = {
    pool = Executors.newCachedThreadPool()

    val ctx = sce.getServletContext
    val blocker = Blocker.liftExecutorService(pool)
    val f = appF.flatMap { app =>
      val servlet = new AsyncHttp4sServlet[F](
        app,
        servletIo = BlockingServletIo(4096, blocker),
        serviceErrorHandler = DefaultServiceErrorHandler
      )
      F.delay {
        val reg = ctx.addServlet("example", servlet)
        reg.addMapping("/")
        reg.setAsyncSupported(true)
      }
    }
    F.toIO(f).unsafeRunSync()
  }

  override def contextDestroyed(sce: ServletContextEvent): Unit = {
    if (pool != null)
      pool.shutdown()
  }
}

@WebListener
class BootstrapIO extends Bootstrap[IO] {
  val cs: ContextShift[IO] = IO.contextShift(global)
  val F: ConcurrentEffect[IO] = IO.ioConcurrentEffect(cs)
}