Use the Executor interface when specifying where to handle incoming messages

This is done so that one can simply specify a direct executor.
Change-Id: I1c3ea977dd7c2d604588d587fd67f7012355eedf

@@ -17,6 +17,7 @@ package org.onosproject.store.cluster.messaging;
 
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.function.Consumer;
 import java.util.function.Function;
@@ -173,7 +174,7 @@ public interface ClusterCommunicationService {
                               Function<byte[], M> decoder,
                               Function<M, R> handler,
                               Function<R, byte[]> encoder,
-                              ExecutorService executor);
+                              Executor executor);
 
     /**
      * Adds a new subscriber for the specified message subject.
@@ -187,7 +188,7 @@ public interface ClusterCommunicationService {
     <M> void addSubscriber(MessageSubject subject,
                            Function<byte[], M> decoder,
                            Consumer<M> handler,
-                           ExecutorService executor);
+                           Executor executor);
 
     /**
      * Removes a subscriber for the specified message subject.

@@ -43,6 +43,7 @@ import com.google.common.util.concurrent.SettableFuture;
 import java.io.IOException;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.function.Consumer;
 import java.util.function.Function;
@@ -270,7 +271,7 @@ public class ClusterCommunicationManager
             Function<byte[], M> decoder,
             Function<M, R> handler,
             Function<R, byte[]> encoder,
-            ExecutorService executor) {
+            Executor executor) {
         messagingService.registerHandler(subject.value(),
                 new InternalMessageResponder<>(decoder, encoder, handler),
                 executor);
@@ -280,7 +281,7 @@ public class ClusterCommunicationManager
     public <M> void addSubscriber(MessageSubject subject,
             Function<byte[], M> decoder,
             Consumer<M> handler,
-            ExecutorService executor) {
+            Executor executor) {
         messagingService.registerHandler(subject.value(),
                 new InternalMessageConsumer<>(decoder, handler),
                 executor);

@@ -54,6 +54,7 @@ import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.Executor;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
@@ -780,13 +781,13 @@ public class EventuallyConsistentMapImplTest {
         @Override
         public <M, R> void addSubscriber(MessageSubject subject,
                 Function<byte[], M> decoder, Function<M, R> handler,
-                Function<R, byte[]> encoder, ExecutorService executor) {
+                Function<R, byte[]> encoder, Executor executor) {
         }
 
         @Override
         public <M> void addSubscriber(MessageSubject subject,
                 Function<byte[], M> decoder, Consumer<M> handler,
-                ExecutorService executor) {
+                Executor executor) {
         }
 
         @Override

@@ -17,7 +17,7 @@ package org.onlab.netty;
 
 import java.io.IOException;
 import java.util.concurrent.CompletableFuture;
-import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executor;
 
 /**
  * Interface for low level messaging primitives.
@@ -48,7 +48,7 @@ public interface MessagingService {
      * @param handler message handler
      * @param executor executor to use for running message handler logic.
      */
-    public void registerHandler(String type, MessageHandler handler, ExecutorService executor);
+    public void registerHandler(String type, MessageHandler handler, Executor executor);
 
     /**
      * Registers a new message handler for message type.

@@ -42,7 +42,7 @@ import java.net.UnknownHostException;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
-import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executor;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicLong;
@@ -202,11 +202,11 @@ public class NettyMessagingService implements MessagingService {
     }
 
     @Override
-    public void registerHandler(String type, MessageHandler handler, ExecutorService executor) {
+    public void registerHandler(String type, MessageHandler handler, Executor executor) {
         handlers.put(type, new MessageHandler() {
             @Override
             public void handle(Message message) throws IOException {
-                executor.submit(() -> {
+                executor.execute(() -> {
                     try {
                         handler.handle(message);
                     } catch (Exception e) {