fixed problem when client id is not set

sdks/java/io/mqtt/src/main/java/org/apache/beam/sdk/io/mqtt/MqttIO.java

@@ -209,6 +209,10 @@ private MQTT createClient() throws Exception {
         String clientId = getClientId() + "-" + UUID.randomUUID().toString();
         LOG.debug("MQTT client id set to {}", clientId);
         client.setClientId(clientId);
+      } else {
+        String clientId = UUID.randomUUID().toString();
+        LOG.debug("MQTT client id set to random value {}", clientId);
+        client.setClientId(clientId);
       }
       return client;
     }

sdks/java/io/mqtt/src/test/java/org/apache/beam/sdk/io/mqtt/MqttIOTest.java

@@ -27,6 +27,7 @@
 
 import org.apache.activemq.broker.BrokerService;
 import org.apache.activemq.broker.Connection;
+import org.apache.beam.sdk.io.mqtt.MqttIO.Read;
 import org.apache.beam.sdk.testing.PAssert;
 import org.apache.beam.sdk.testing.RunnableOnService;
 import org.apache.beam.sdk.testing.TestPipeline;
@@ -77,6 +78,66 @@ public void startBroker() throws Exception {
     brokerService.waitUntilStarted();
   }
 
+  @Test(timeout = 60 * 1000)
+  @Category(RunnableOnService.class)
+  public void testReadNoClientId() throws Exception {
+    final String topicName = "READ_TOPIC_NO_CLIENT_ID";
+    Read mqttReader = MqttIO.read()
+    .withConnectionConfiguration(
+        MqttIO.ConnectionConfiguration.create(
+            "tcp://localhost:" + port,
+            topicName))
+  .withMaxNumRecords(10);
+    PCollection<byte[]> output = pipeline.apply(mqttReader);
+    PAssert.that(output).containsInAnyOrder(
+        "This is test 0".getBytes(),
+        "This is test 1".getBytes(),
+        "This is test 2".getBytes(),
+        "This is test 3".getBytes(),
+        "This is test 4".getBytes(),
+        "This is test 5".getBytes(),
+        "This is test 6".getBytes(),
+        "This is test 7".getBytes(),
+        "This is test 8".getBytes(),
+        "This is test 9".getBytes()
+    );
+
+    // produce messages on the brokerService in another thread
+    // This thread prevents to block the pipeline waiting for new messages
+    MQTT client = new MQTT();
+    client.setHost("tcp://localhost:" + port);
+    final BlockingConnection publishConnection = client.blockingConnection();
+    publishConnection.connect();
+    Thread publisherThread = new Thread() {
+      public void run() {
+        try {
+          LOG.info("Waiting pipeline connected to the MQTT broker before sending "
+              + "messages ...");
+          boolean pipelineConnected = false;
+          while (!pipelineConnected) {
+            Thread.sleep(1000);
+            for (Connection connection : brokerService.getBroker().getClients()) {
+              if (!connection.getConnectionId().isEmpty()) {
+                pipelineConnected = true;
+              }
+            }
+          }
+          for (int i = 0; i < 10; i++) {
+            publishConnection.publish(topicName, ("This is test " + i).getBytes(),
+                QoS.AT_LEAST_ONCE, false);
+          }
+        } catch (Exception e) {
+          // nothing to do
+        }
+      }
+    };
+    publisherThread.start();
+    pipeline.run();
+
+    publishConnection.disconnect();
+    publisherThread.join();
+  }
+
   @Test(timeout = 60 * 1000)
   @Category(RunnableOnService.class)
   public void testRead() throws Exception {