[Java] Add killActor API in Java (#6728)

* Add killActor API in Java

* fix javadoc

* update test case

* Address comments

java/test/src/main/java/org/ray/api/TestUtils.java

@@ -28,8 +28,17 @@ public class TestUtils {
   }
 
   public static void skipTestIfDirectActorCallEnabled() {
-    if (ActorCreationOptions.DEFAULT_USE_DIRECT_CALL) {
-      throw new SkipException("This test doesn't work when direct actor call is enabled.");
+    skipTestIfDirectActorCallEnabled(true);
+  }
+
+  public static void skipTestIfDirectActorCallDisabled() {
+    skipTestIfDirectActorCallEnabled(false);
+  }
+
+  private static void skipTestIfDirectActorCallEnabled(boolean enabled) {
+    if (enabled == ActorCreationOptions.DEFAULT_USE_DIRECT_CALL) {
+      throw new SkipException(String.format("This test doesn't work when direct actor call is %s.",
+          enabled ? "enabled" : "disabled"));
     }
   }
 

java/test/src/main/java/org/ray/api/test/KillActorTest.java

@@ -0,0 +1,40 @@
+package org.ray.api.test;
+
+import com.google.common.collect.ImmutableList;
+import org.ray.api.Ray;
+import org.ray.api.RayActor;
+import org.ray.api.RayObject;
+import org.ray.api.TestUtils;
+import org.ray.api.annotation.RayRemote;
+import org.ray.api.exception.RayActorException;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+@Test(groups = { "directCall" })
+public class KillActorTest extends BaseTest {
+
+  @RayRemote
+  public static class HangActor {
+
+    public boolean alive() {
+      return true;
+    }
+
+    public boolean hang() throws InterruptedException {
+      while (true) {
+        Thread.sleep(1000);
+      }
+    }
+  }
+
+  public void testKillActor() {
+    TestUtils.skipTestUnderSingleProcess();
+    TestUtils.skipTestIfDirectActorCallDisabled();
+    RayActor<HangActor> actor = Ray.createActor(HangActor::new);
+    Assert.assertTrue(Ray.call(HangActor::alive, actor).get());
+    RayObject<Boolean> result = Ray.call(HangActor::hang, actor);
+    Assert.assertEquals(0, Ray.wait(ImmutableList.of(result), 1, 500).getReady().size());
+    Ray.killActor(actor);
+    Assert.expectThrows(RayActorException.class, result::get);
+  }
+}