How to create CRON expressions & what are the components of a typical CRON expression? Usage of CRON expression in @Scheduled

CRON expressions are mainly used for scheduling tasks, which should run frequently at a fixed rate, lets say, running a task every day at mid night 12'0 Clock, running a task every hour & like wise.

For Example, An Online shopping cart application might have many schedulers that schedule the tasks, for instance, once an order is placed successfully by a customer,  a shipping order scheduler might processes these successful orders placed in previous hour or before some time.

A typical CRON expression: "0/5 * * ? * *"

CRON expression basically consists of 7 components. Each component will have range of allowed values, Allowed special characters. For example, "Seconds" component will have range of values allowed from 0 to 59.

CRON expression components:

"Seconds" "Minutes" "Hours" "Day of month" "month" "Day of week" "Year"

All are mandatory parameters, except "Year".

"Seconds" - a mandatory parameter

     Range of values:         0-59

     Allowed characters     , - * /

Examples: 

    10,20      executes at 10th second and 20th second

    *             executes at every second

    0/5          executes every 5 seconds starting from 0th second

"Minutes" - a mandatory parameter
     Range of values:         0-59
     Allowed characters     , - * /
Examples: 
    10,20      executes at 10th minute and 20th minute 
    *             executes at every minute
    0/5          executes every 5 minutes starting from 0th minute
"Hours" - a mandatory parameter
     Range of values:         0-23
     Allowed characters     , - * /
Examples: 
    10,20      executes at 10th hour and 20th hour 
    *             executes at every hour
    0/5          executes every 5 hours starting from mid night 12:00 AM
"Day of month" - a mandatory parameter
     Range of values:         1-31
     Allowed characters    , - * ? / L W C
Examples: 
    10,20      executes at 10th day and 20th day 
    *             executes at every day
    1/5          executes every 5 days starting from first day
    L represents the last day of the month. L is allowed only with "Day of month" 
    & "Day of week".
    L represents 31st in Jan, 28th in Feb (non-leap years)
    W represents the nearest week-day. Only allowed to use when it is a single day, 
    not allowed with range of values.
    12W 
    say if 12th day of the month is Saturday, it will trigger on 11th day (Friday), 
    if 12th day is Sunday, it will trigger on 13th day (Monday), 
    if 12th day is a Monday, as it is weekday, it will trigger on 12th day (Monday)
    C represents the calendar. Only allowed for "Day of month" & 
    "Day of week" components.
    Values are calculated against an associated calendar if any. 
    If no calendar associated, if it is declared like
    5C represents the first day included by the calendar on or after the 5th.
"Month" - a mandatory parameter
     Range of values:         0-11 or JAN-DEC
     Allowed characters    , - * /
Examples: 
    1,2                      executes at FEB & MAR months 
   FEB,MAR          executes at FEB & MAR months 
    *                         executes at every month
"Day of Week" - a mandatory parameter
     Range of values:         0-11 or JAN-DEC
     Allowed characters    , - * ? / L C #
  L represents the last day of the month. L is allowed only with "Day of month" 
    & "Day of week"
    1-7 (1 - sun, 2- mon, 3-tues, 4-wed, 5- thur, 6-fri, 7-sat)
    6L represents last Friday of the month.
    C represents the calendar. Only allowed for "Day of month" 
    & "Day of week" components.
    Values are calculated against an associated calendar if any. 
    If no calendar associated, if it is declared like
    1C represents the first day included by the calendar on or after Sunday.
    7C represents the first day included by the calendar on or after Saturday.
   # is used to represent a single day, like nth XXX day. 
    Only allowed for "Day of week".
   7#2 represents the 2nd Saturday of the month.
 "Year" - an optional parameter
     Range of values:         empty or 1970-2099
     Allowed characters    , - * /
Examples: 
    2000-2010         executes for range of years from 2000 to 2010
   2000,2006          executes for years 2000 and 2006
    *                         executes at every year
Examples with all components combined:
0 0 12 * * ?
Fire at 12:00 PM (noon) every day
0 20 10 ? * *
Fire at 10:20 AM every day
0 15 11 * * ?
Fire at 11:15 AM every day
0 15 10 * * ? *
Fire at 10:15 AM every day
0 20 20 * * ? 2010
Fire at 08:20 PM every day during the year 2010
0 * 14 * * ?
Fire every minute starting at 2:00 PM and ending at 2:59 PM, every day
0 0/5 14 * * ?
Fire every 5 minutes starting at 2:00 PM and ending at 2:55 PM, every day
0 0/5 14,18 * * ?
Fire every 5 minutes starting at 2:00 PM and ending at 2:55 PM, 
AND fire every 5 minutes starting at 6:00 PM and ending at 6:55 PM, 
every day
0 0-5 16 * * ?
Fire every minute starting at 4:00 PM and ending at 4:05 PM, every day
0 10,22 14 ? 3 FRI
Fire at 2:10 PM and at 2:22 PM every Friday in the month of March
0 15 10 ? * MON-FRI
Fire at 10:15 AM every Monday, Tuesday, Wednesday, Thursday and Friday
0 15 10 15 * ?
Fire at 10:15 AM on the 15th day of every month
0 15 10 L * ?
Fire at 10:15 AM on the last day of every month
0 15 10 ? * 7L
Fire at 10:15 AM on the last Saturday of every month
0 15 10 ? * 7L
Fire at 10:15 AM on the last Saturday of every month
0 15 10 ? * 7L 2002-2004
Fire at 10:15 AM on every last Saturday of every month 
during the years 2002, 2003 and 2004
0 15 10 ? * 6#3
Fire at 10:15 AM on the third Friday of every month
0 0 12 1/5 * ?
Fire at 12 PM (noon) every 5 days every month, 
starting on the first day of the month
0 6 6 6 6 ?
Fire every June 6 at 6:06 AM
How to use CRON expression in spring scheduling @Scheduled annotation?
As we know that, firstly, we need to enable scheduling using @EnableScheduling 
and use @Scheduled annotation on tasks.
Full example: Spring boot scheduling example
Example:
    @Scheduled(cron = "0/5 * * ? * *")
    public void reportCurrentTime() throws InterruptedException {
        //Thread.sleep(6000l);
        logger.info(formatter.format(new Date()));
    }