add libraries server project

libraries-server/src/main/java/com/baeldung/mqtt/EngineTemperatureSensor.java

@@ -0,0 +1,49 @@
+package com.baeldung.mqtt;
+
+import java.util.Random;
+import java.util.concurrent.Callable;
+
+import org.eclipse.paho.client.mqttv3.IMqttClient;
+import org.eclipse.paho.client.mqttv3.MqttMessage;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class EngineTemperatureSensor implements Callable<Void> {
+    
+    private static final Logger log = LoggerFactory.getLogger(EngineTemperatureSensor.class);
+    public static final String TOPIC = "engine/temperature";
+
+    private IMqttClient client;
+    private Random rnd = new Random();
+
+    public EngineTemperatureSensor(IMqttClient client) {
+        this.client = client;
+    }
+
+    @Override
+    public Void call() throws Exception {
+        
+        if ( !client.isConnected()) {
+            log.info("[I31] Client not connected.");
+            return null;
+        }
+            
+        MqttMessage msg = readEngineTemp();
+        msg.setQos(0);
+        msg.setRetained(true);
+        client.publish(TOPIC,msg);        
+        
+        return null;        
+    }
+    
+    /**
+     * This method simulates reading the engine temperature
+     * @return
+     */
+    private MqttMessage readEngineTemp() {             
+        double temp =  80 + rnd.nextDouble() * 20.0;        
+        byte[] payload = String.format("T:%04.2f",temp).getBytes();        
+        MqttMessage msg = new MqttMessage(payload); 
+        return msg;
+    }
+}