public class SingleMessageService {
    
    ...
    private final String kUrl = "activeMQ.url";
    private final String kUsername = "activeMQ.username";
    private final String kPassword = "activeMQ.password";
    private final String kQueue = "activeMQ.queue";
    private final String kMode = "activeMQ.mode";

    ...
    private final String exConnectionStarted = "Connection has already started";
    private final String exConnect = "Failed connect Message Service provider";
    private final String exProducer = "Failed create Producer";
    private final String exNoMode = "Mode is absent";
    private final String exNoQueue = "Queue name is absent";
    private final String exStarted = "Message Service is not connected";
    private final String exClose = "Failed disconnect Message Service provider";
    private final String exSend = "Failed send message";
    private final String exProducerIsNull = "Producer is null";

    ...
    public void connect(MessageServiceConfig messageServiceConfig) {
        if (connection == null) {
            if (messageServiceConfig.getProperties()
                    .containsKey(kUrl)) {
                connectionFactory = new ActiveMQConnectionFactory(
                        (String) messageServiceConfig.getProperties()
                                .get(kUsername),
                        (String) messageServiceConfig.getProperties()
                                .get(kPassword),
                        (String) messageServiceConfig.getProperties()
                                .get(kUrl));
            } else {
                throw new MessageServiceException(exUrlAbsent);
            }
        ...