Gets the number of milliseconds from at 0:00:00 on January 1, 1970 to the present

import java.time.Clock;
import java.util.Calendar;

public class Test {
public static void main(String[] args) {

    Calendar calendar=Calendar.getInstance();
    //方法一：
    System.out.println(calendar.getTimeInMillis());
    //方法二：
    System.out.println(System.currentTimeMillis());
    //方法三：
    System.out.println(Clock.systemDefaultZone().millis());

}

}