How to get milliseconds from LocalDateTime in Java 8

I'm not entirely sure what you mean by "current milliseconds" but I'll assume it's the number of milliseconds since the "epoch," namely midnight, January 1, 1970 UTC.

If you want to find the number of milliseconds since the epoch right now, then use System.currentTimeMillis() as Anubian Noob has pointed out. If so, there's no reason to use any of the new java.time APIs to do this.

However, maybe you already have a LocalDateTime or similar object from somewhere and you want to convert it to milliseconds since the epoch. It's not possible to do that directly, since the LocalDateTime family of objects has no notion of what time zone they're in. Thus time zone information needs to be supplied to find the time relative to the epoch, which is in UTC.

Suppose you have a LocalDateTime like this:

LocalDateTime ldt = LocalDateTime.of(2014, 5, 29, 18, 41, 16);

You need to apply the time zone information, giving a ZonedDateTime. I'm in the same time zone as Los Angeles, so I'd do something like this:

ZonedDateTime zdt = ldt.atZone(ZoneId.of("America/Los_Angeles"));

Of course, this makes assumptions about the time zone. And there are edge cases that can occur, for example, if the local time happens to name a time near the Daylight Saving Time (Summer Time) transition. Let's set these aside, but you should be aware that these cases exist.

Anyway, if you can get a valid ZonedDateTime, you can convert this to the number of milliseconds since the epoch, like so:

long millis = zdt.toInstant().toEpochMilli();

What I do so I don't specify a time zone is,

System.out.println("ldt " + LocalDateTime.now().atZone(ZoneId.systemDefault()).toInstant().toEpochMilli());
System.out.println("ctm " + System.currentTimeMillis());

gives

ldt 1424812121078 
ctm 1424812121281

As you can see the numbers are the same except for a small execution time.

Just in case you don't like System.currentTimeMillis, use Instant.now().toEpochMilli()