Remove singleton instances once the client is finished

This can prevent unwanted behavior to happen because of missing
initialization

RISE-V2G-EVCC/src/main/java/com/v2gclarity/risev2g/evcc/transportLayer/TCPClient.java

@@ -139,6 +139,7 @@ public class TCPClient extends StatefulTransportLayerClient {
 				getInStream().close();
 				getOutStream().close();
 				getTcpSocketToServer().close();
+				releaseInstance();
 				Thread.currentThread().interrupt();
 			} catch (IOException e) {
 				getLogger().error("Error occurred while trying to close TCP socket to server", e);
@@ -148,6 +149,10 @@ public class TCPClient extends StatefulTransportLayerClient {
 		}
 	}
 	
+	private static void releaseInstance() {
+		uniqueTCPClientInstance = null;
+	}
+	
 	public Socket getTcpSocketToServer() {
 		return tcpSocketToServer;
 	}

RISE-V2G-EVCC/src/main/java/com/v2gclarity/risev2g/evcc/transportLayer/TLSClient.java

@@ -211,6 +211,7 @@ public class TLSClient extends StatefulTransportLayerClient {
 				getInStream().close();
 				getOutStream().close();
 				getTlsSocketToServer().close();
+				releaseInstance();
 				Thread.currentThread().interrupt();
 			} catch (IOException e) {
 				getLogger().error("Error occurred while trying to close TCP socket to server", e);
@@ -220,6 +221,10 @@ public class TLSClient extends StatefulTransportLayerClient {
 		}
 	}
 	
+	private static void releaseInstance() {
+		uniqueTLSClientInstance = null;
+	}
+	
 	
 	public SSLSocket getTlsSocketToServer() {
 		return tlsSocketToServer;

RISE-V2G-EVCC/src/main/java/com/v2gclarity/risev2g/evcc/transportLayer/UDPClient.java

@@ -51,7 +51,7 @@ public class UDPClient {
 	 *  access the instance variable -> thread safe.
 	 */
 	private Logger logger = LogManager.getLogger(this.getClass().getSimpleName());
-	private static final UDPClient uniqueUDPClientInstance = new UDPClient();
+	private static UDPClient uniqueUDPClientInstance = new UDPClient();
 	private int multicastSocketPort;
 	private Inet6Address multicastAddress;
 	private MulticastSocket socketToUDPServer;
@@ -106,6 +106,13 @@ public class UDPClient {
 	
 
 	public static UDPClient getInstance() {
+		if (uniqueUDPClientInstance == null) {
+			synchronized (UDPClient.class) {
+				if (uniqueUDPClientInstance == null) {
+					uniqueUDPClientInstance = new UDPClient();
+				}
+			}
+		}
 		return uniqueUDPClientInstance;
 	}
 	
@@ -144,6 +151,11 @@ public class UDPClient {
 	public void stop() {
 		getSocketToUDPServer().close();
 		getLogger().debug("UDP client stopped");
+		releaseInstance();
+	}
+	
+	private static void releaseInstance() {
+		uniqueUDPClientInstance = null;
 	}