Use ExecutorService variables to assign ExecutorService Instances (ap…

…ache#11373)

* Added inspection rule to prohibit ExecutorService assignment to Executor

* Use ExecutorService type variable to assign ExecutorService

* Changed : Variable => variable

* Removed unused Executor import

extensions-core/druid-kerberos/src/main/java/org/apache/druid/security/kerberos/KerberosHttpClient.java

@@ -39,15 +39,15 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
-import java.util.concurrent.Executor;
+import java.util.concurrent.ExecutorService;
 
 public class KerberosHttpClient extends AbstractHttpClient
 {
   private static final Logger log = new Logger(KerberosHttpClient.class);
 
   private final HttpClient delegate;
   private final CookieManager cookieManager;
-  private final Executor exec = Execs.singleThreaded("test-%s");
+  private final ExecutorService exec = Execs.singleThreaded("test-%s");
   private final String internalClientPrincipal;
   private final String internalClientKeytab;
 

server/src/test/java/org/apache/druid/server/initialization/JettyTest.java

@@ -76,7 +76,7 @@
 import java.util.Locale;
 import java.util.Map;
 import java.util.concurrent.CountDownLatch;
-import java.util.concurrent.Executor;
+import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 import java.util.concurrent.ThreadLocalRandom;
 import java.util.concurrent.TimeUnit;
@@ -268,7 +268,7 @@ public EnumSet<DispatcherType> getDispatcherType()
   public void testTimeouts() throws Exception
   {
     // test for request timeouts properly not locking up all threads
-    final Executor executor = Executors.newFixedThreadPool(100);
+    final ExecutorService executor = Executors.newFixedThreadPool(100);
     final AtomicLong count = new AtomicLong(0);
     final CountDownLatch latch = new CountDownLatch(1000);
     for (int i = 0; i < 10000; i++) {