Cron Expressions for Spring's CronTrigger

What is Cron ?

Cron is a unix based job scheduler tool.
Can be configured to execute tasks at a specified time.
We can use our java programs to make use of the Cron tool by configuring cron expressions.
Here we focus on writing cron expressions which is compatible with the Spring's CronTrigger.
Configuring these cron expressions with Spring is coming soon !!!

Cron Expression Format

Field Name	Mandatory	Allowed Values	Allowed Special Characters
Seconds	YES	0-59	, - * /
Minutes	YES	0-59	, - * /
Hours	YES	0-23	, - * /
Day of month	YES	1/31/2015	, - * ? / L W
Month	YES	1-12 or JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC	, - * /
Day of week	YES	1-7 or SUN, MON, TUE, WED, THU, FRI, SAT	, - * ? / L #
Year	NO	empty, 1970-2099	, - * /

source : quartz tutorials

Here's an old school trick to help you to memorize this ;) If this is too dumb for you, make your own one :D :D

Sexy Minions, Our DOMbAsses, Mouthed Dow's Yearbook :D :D :D

Seconds Minutes Hours Day Of Month Month Day of Week Year

Anyway, Spring's CronTrigger accept only 6 fields starting with Seconds, up to Day of Week.
So, a cron format could be "5/10 10-30 10 ? * MON,TUE". Once caught, this is really easy.

Special Characters

* - all values. In the above example ,the star corresponds to every month.
- - a range of values. In the above example, 0-15 means, it should start in 10th minute and end in 30th minute.
x/y - starts at value x. Run again when another y number of time units passed. In the above example, the cron starts at the 5th second, and thereafter run when each 10 seconds passed. means, it runs in 15, 25, 35, 45, 55 th seconds.
, - specifies individual values. In the above example, MON,TUE means, this cron runs in Monday and Tuesday only
With above four characters we can write something like this "0-5,10,15,20/10 * * ? * SAT,MON,SUN". The section "0-5,10,15,20/10 " means, the cron runs from 0th seconds to 5th second, then at 10th and 15th seconds, then each 10 seconds after starting from 20th second.
? - Allowed only in Day of Month and Day of Week fields. There's a small catch here. In above example;

Say, we put 15 to the Day of Month's position and the value MON to the Day of Week's position. Then it is a huge ambiguity.
Putting 15 implies that the cron will be run in every 15th day of the month. Putting WED implies that cron will be run on every Monday of the week.
So, will this cron fire on 15th of the month if it falls on a Monday, Or should it fire on both the 15th day of the month and every Monday. (drunk)
Here's the solution. You cannot put two values there. That's where "?" comes to play
If you specify the Day of month, you put "?" to the Day of week and vice versa. That's it , no more questions. ;)

Executing Cron Jobs with Java

To make java programs to execute these cron jobs we use Spring's org.springframework.scheduling.support.CronTrigger and it seems to support for Vixie Cron, where the special characters L, W and # are not supported.
We can use org.springframework.scheduling.quartz.CronTriggerBean to support the above mentioned characters, but that is not our focus here.
How to do this ? I will post next and will update here.