Is there a way to actually append the current timestamp onto this json file?

public static String FILE_LOCATION = "/fitness"+File.separator
+"customers_" + "yyyymmddmmss" + ".json";

this is what i have so far.

Your format String includes mm twice and no HH for hours. But yes, using a DateTimeFormatter and something like

public static String FILE_LOCATION = "/fitness/customers_"
			+ DateTimeFormatter.ofPattern("yyyyMMddHHmmss")
			.format(LocalDateTime.now()) + ".json";

If you want the local date-time (which is different in different time-zones) to be captured as part of the name of the file, the answer by Elliott Frisch is what you need.

However, if you want the date-time to be independent of a time-zone, you can append the value of Instant.now() which gives you current moment at UTC time-line.

public static String FILE_LOCATION = "/fitness/customers_"
            + Instant.now().toString() + ".json";

or in the custom format as

public static String FILE_LOCATION = "/fitness/customers_"
            + DateTimeFormatter.ofPattern("yyyyMMddHHmmss")
            .format(LocalDateTime.now(ZoneOffset.UTC)) + ".json";

Note that LocalDateTime.now() is equivalent to LocalDateTime.now(ZoneId.systemDefault()) which gives you the date-time in the JVM's time-zone.