A really well done Singleton??? I think it is :-)

MongoDBManager.java

/**
 * <a href="https://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom">
 *  Initialization on-demand Holder idiom.
 * </a>
 */
public class MongoDBManager {
    private static final String MONGODB_CONNECTION_URL = "mongodb.connection.url";

    public static MongoDBManager getInstance() {
        return LazyHolder.INSTANCE;
    }

    private static final class LazyHolder {
        static final MongoDBManager INSTANCE = new MongoDBManager();
    }

    private final MongoClient client;

    private MongoDBManager() {
        final String connection = getMongoDBProperties().getProperty(MONGODB_CONNECTION_URL);
        client = MongoClients.create(new ConnectionString(connection));
    }

    public MongoClient mongoClient() {
        return client;
    }

    private static Properties getMongoDBProperties() {
        try {
            InputStream stream = MongoDBManager.class.getResourceAsStream("/data/mongodb.properties");
            Properties properties = new Properties();
            properties.load(stream);
            return properties;
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

}

MongoDBManagerTest.java

public class MongoDBManagerTest {
    private static final int THREADS = 50;

    @Test
    public void testConcurrentInstantiation() throws Exception {
        ExecutorService executorService = Executors.newFixedThreadPool(THREADS);
        long count = Stream.generate(this::mongoDBManagerInstanceTask)
            .limit(THREADS)
            .parallel()
            .map(executorService::submit)
            .map(this::completedTask)
            .distinct()
            .count();

        assertThat(count, equalTo(1L));
    }

    private Callable<MongoDBManager> mongoDBManagerInstanceTask() {
        return MongoDBManager::getInstance;
    }

    private <T> T completedTask(Future<T> future) {
        try {
            return future.get();
        } catch (InterruptedException | ExecutionException e) {
            throw new RuntimeException(e);
        }
    }
}