sample script, service metjod, public access

java/src/jmri/jmrix/mqtt/MqttAdapter.java

@@ -143,6 +143,10 @@ public void publish(String topic, byte[] payload) {
         }
     }
 
+    public void publish(String topic, String payload) {
+        publish(topic, payload.getBytes());
+    }
+
     public MqttClient getMQttClient() {
         return (mqttClient);
     }

java/src/jmri/jmrix/mqtt/MqttSystemConnectionMemo.java

@@ -62,7 +62,7 @@ void setMqttAdapter(MqttAdapter ma) {
         mqttAdapter = ma;
     }
 
-    MqttAdapter getMqttAdapter() {
+    public MqttAdapter getMqttAdapter() {
         return mqttAdapter;
     }
 }

jython/SendMqttMessage.py

@@ -0,0 +1,22 @@
+# Example of how to define how to send an arbitrary Mqtt message
+#
+# To see the result e.g. (your broker and channel may vary)
+#     mosquitto_sub -v -h 'test.mosquitto.org' -t '/trains/#'
+#
+# Author: Bob Jacobsen, copyright 2020
+
+import jmri
+import java
+import jarray
+from org.python.core.util import StringUtil
+
+# Find the MqttAdapter
+mqqtAdapter = jmri.InstanceManager.getDefault( jmri.jmrix.mqtt.MqttSystemConnectionMemo ).getMqttAdapter()
+
+
+# create some content
+topic = "jmri/test/topic"  # note this will be prefixed by configured channel, i.e. "/trains/"
+payload = "message content"
+
+# send
+mqqtAdapter.publish(topic, payload)