Fix server worker executor initialization.

Author: krisztiankocsis

jooby-jetty/src/main/java/org/jooby/internal/jetty/JettyServer.java

@@ -71,13 +71,11 @@ public class JettyServer implements org.jooby.spi.Server {
   private final Logger log = LoggerFactory.getLogger(org.jooby.spi.Server.class);
 
   private Server server;
-  private Executor executor;
 
   @Inject
   public JettyServer(final HttpHandler handler, final Config conf,
       final Provider<SSLContext> sslCtx) {
     this.server = server(handler, conf, sslCtx);
-    this.executor = this.server.getThreadPool();
   }
 
   private Server server(final HttpHandler handler, final Config conf,
@@ -201,7 +199,7 @@ public void stop() throws Exception {
   @Override
   public Optional<Executor> executor()
   {
-    return Optional.of(executor);
+    return Optional.ofNullable(server.getThreadPool());
   }
 
   private void tryOption(final Object source, final Config config, final Method option) {

jooby-netty/src/main/java/org/jooby/internal/netty/NettyServer.java

@@ -157,7 +157,7 @@ public void join() throws InterruptedException {
   @Override
   public Optional<Executor> executor()
   {
-    return Optional.of(executor);
+    return Optional.ofNullable(executor);
   }
 
   @SuppressWarnings({"rawtypes", "unchecked" })

jooby-undertow/src/main/java/org/jooby/internal/undertow/UndertowServer.java

@@ -56,7 +56,6 @@ private interface SetOption {
   private static final Logger log = LoggerFactory.getLogger(org.jooby.spi.Server.class);
 
   private Undertow server;
-  private Executor executor;
 
   private final GracefulShutdownHandler shutdown;
 
@@ -88,8 +87,6 @@ public UndertowServer(final org.jooby.spi.HttpHandler dispatcher, final Config c
 
     this.server = ubuilder.setHandler(shutdown)
         .build();
-
-    this.executor = server.getWorker();
   }
 
   private String host(final String host) {
@@ -214,7 +211,7 @@ public void stop() throws Exception {
   @Override
   public Optional<Executor> executor()
   {
-    return Optional.of(executor);
+    return Optional.ofNullable(server.getWorker());
   }
 
 }