code: package io.ray.test;
  
  import io.ray.api.ActorHandle;
  import io.ray.api.ObjectRef;
  import io.ray.api.Ray;
  import java.util.concurrent.CountDownLatch;
  import org.testng.Assert;
  import org.testng.annotations.Test;
  
  @Test
  public class ActorConcurrentCallTest extends BaseTest {
  
    public static class ConcurrentActor {
      private final CountDownLatch countDownLatch = new CountDownLatch(3);
  
      public String countDown() {
        countDownLatch.countDown();
        try {
          countDownLatch.await();
          return "ok";
        } catch (InterruptedException e) {
          throw new RuntimeException(e);
        }
      }
    }
  
    public void testConcurrentCall() {
      ActorHandle<ConcurrentActor> actor =
          Ray.actor(ConcurrentActor::new).setMaxConcurrency(3).remote();
      ObjectRef<String> obj1 = actor.task(ConcurrentActor::countDown).remote();
      ObjectRef<String> obj2 = actor.task(ConcurrentActor::countDown).remote();
      ObjectRef<String> obj3 = actor.task(ConcurrentActor::countDown).remote();
  
      Assert.assertEquals(obj1.get(), "ok");
      Assert.assertEquals(obj2.get(), "ok");
      Assert.assertEquals(obj3.get(), "ok");
    }
  }
  