docs(samples): Use an error listener instead of catching an exception…

… for the OptimisticSubscribeExample

samples/snippets/src/main/java/pubsub/OptimisticSubscribeExample.java

@@ -18,10 +18,12 @@
 
 // [START pubsub_optimistic_subscribe]
 
+import com.google.api.gax.rpc.NotFoundException;
 import com.google.cloud.pubsub.v1.AckReplyConsumer;
 import com.google.cloud.pubsub.v1.MessageReceiver;
 import com.google.cloud.pubsub.v1.Subscriber;
 import com.google.cloud.pubsub.v1.SubscriptionAdminClient;
+import com.google.common.util.concurrent.MoreExecutors;
 import com.google.pubsub.v1.ProjectSubscriptionName;
 import com.google.pubsub.v1.PubsubMessage;
 import com.google.pubsub.v1.PushConfig;
@@ -58,22 +60,35 @@ public static void optimisticSubscribeExample(
     Subscriber subscriber = null;
     try {
       subscriber = Subscriber.newBuilder(subscriptionName, receiver).build();
+
+      // Listen for resource NOT_FOUND errors and rebuild the  subscriber and restart subscribing
+      // when the current subscriber encounters these errors.
+      subscriber.addListener(
+          new Subscriber.Listener() {
+            public void failed(Subscriber.State from, Throwable failure) {
+              System.out.println(failure.getStackTrace());
+              if (failure instanceof NotFoundException) {
+                try (SubscriptionAdminClient subscriptionAdminClient =
+                    SubscriptionAdminClient.create()) {
+                  TopicName topicName = TopicName.of(projectId, topicId);
+                  // Create a pull subscription with default acknowledgement deadline of 10 seconds.
+                  // The client library will automatically extend acknowledgement deadlines.
+                  Subscription subscription =
+                      subscriptionAdminClient.createSubscription(
+                          subscriptionName, topicName, PushConfig.getDefaultInstance(), 10);
+                  System.out.println("Created pull subscription: " + subscription.getName());
+                  optimisticSubscribeExample(projectId, subscriptionId, topicId);
+                } catch (IOException err) {
+                  System.out.println("Failed to create pull subscription: " + err.getMessage());
+                }
+              }
+            }
+          },
+          MoreExecutors.directExecutor());
+
       subscriber.startAsync().awaitRunning();
       System.out.printf("Listening for messages on %s:\n", subscriptionName.toString());
       subscriber.awaitTerminated(30, TimeUnit.SECONDS);
-    } catch (IllegalStateException e) {
-      try (SubscriptionAdminClient subscriptionAdminClient = SubscriptionAdminClient.create()) {
-        TopicName topicName = TopicName.of(projectId, topicId);
-        // Create a pull subscription with default acknowledgement deadline of 10 seconds.
-        // Messages not successfully acknowledged within 10 seconds will get resent by the server.
-        Subscription subscription =
-            subscriptionAdminClient.createSubscription(
-                subscriptionName, topicName, PushConfig.getDefaultInstance(), 10);
-        System.out.println("Created pull subscription: " + subscription.getName());
-        optimisticSubscribeExample(projectId, subscriptionId, topicId);
-      } catch (IOException err) {
-        System.out.println("Failed to create pull subscription: " + err.getMessage());
-      }
     } catch (TimeoutException e) {
       subscriber.stopAsync();
     }