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cron is a UNIX tool that has been around for a long time, so its scheduling capabilities are powerful
and proven. The CronTrigger class is based on the scheduling capabilities of cron.
CronTrigger uses “cron expressions”, which are able to create firing schedules such as: “At 8:00am every
Monday through Friday” or “At 1:30am every last Friday of the month”.
Cron expressions are powerful, but can be pretty confusing. This tutorial aims to take some of the mystery out of
creating a cron expression, giving users a resource which they can visit before having to ask in a forum or mailing
list.
Format
A cron expression is a string comprised of 6 or 7 fields separated by white space. Fields can contain any of the
allowed values, along with various combinations of the allowed special characters for that field. The fields are as
follows:
* (“all values”) - used to select all values within a field. For example, “*”
in the minute field means “every minute”.
? (“no specific value”) - useful when you need to specify something in one of the
two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a
particular day of the month (say, the 10th), but don’t care what day of the week that happens to be, I would put
“10” in the day-of-month field, and “?” in the day-of-week field. See the examples below for clarification.
- - used to specify ranges. For example, “10-12” in the hour field means “the
hours 10, 11 and 12”.
, - used to specify additional values. For example, “MON,WED,FRI” in the day-of-week
field means “the days Monday, Wednesday, and Friday”.
/ - used to specify increments. For example, “0/15” in the seconds field means “the
seconds 0, 15, 30, and 45”. And “5/15” in the seconds field means “the seconds 5, 20, 35, and 50”. You can
also specify ‘/’ after the ‘’ character - in this case ‘’ is equivalent to having ‘0’ before the ‘/’. ‘1/3’
in the day-of-month field means “fire every 3 days starting on the first day of the month”.
L (“last”) - has different meaning in each of the two fields in which it is
allowed. For example, the value “L” in the day-of-month field means “the last day of the month” - day
31 for January, day 28 for February on non-leap years. If used in the day-of-week field by itself, it simply means
“7” or “SAT”. But if used in the day-of-week field after another value, it means “the last xxx day of the
month” - for example “6L” means “the last friday of the month”. You can also specify an offset
from the last day of the month, such as “L-3” which would mean the third-to-last day of the calendar month.
When using the ‘L’ option, it is important not to specify lists, or ranges of values, as you’ll get
confusing/unexpected results.
W (“weekday”) - used to specify the weekday (Monday-Friday) nearest the given day.
As an example, if you were to specify “15W” as the value for the day-of-month field, the meaning is: “the
nearest weekday to the 15th of the month”. So if the 15th is a Saturday, the trigger will fire on Friday the 14th.
If the 15th is a Sunday, the trigger will fire on Monday the 16th. If the 15th is a Tuesday, then it will fire on
Tuesday the 15th. However if you specify “1W” as the value for day-of-month, and the 1st is a Saturday, the trigger
will fire on Monday the 3rd, as it will not ‘jump’ over the boundary of a month’s days. The ‘W’ character can only
be specified when the day-of-month is a single day, not a range or list of days.
The L and W characters can also be combined in the day-of-month field to yield LW, which
translates to *"last weekday of the month"*.
# - used to specify “the nth” XXX day of the month. For example, the value of “6#3”
in the day-of-week field means “the third Friday of the month” (day 6 = Friday and “#3” = the 3rd one in
the month). Other examples: “2#1” = the first Monday of the month and “4#5” = the fifth Wednesday of the month. Note
that if you specify “#5” and there is not 5 of the given day-of-week in the month, then no firing will occur that
month.
The legal characters and the names of months and days of the week are not case sensitive. MON
is the same as mon.
0 0/5 14,18 * * ?
Fire every 5 minutes starting at 2pm and ending at 2:55pm, AND fire every 5
minutes starting at 6pm and ending at 6:55pm, every day
0 10,44 14 ? 3 WED
Fire at 2:10pm and at 2:44pm every Wednesday in the month of March.
0 15 10 ? * 6L 2002-2005
Fire at 10:15am on every last friday of every month during the years 2002,
2003, 2004 and 2005
0 0 12 1/5 * ?
Fire at 12pm (noon) every 5 days every month, starting on the first day of the
month.
Pay attention to the effects of ? and * in the day-of-week and day-of-month fields!
Support for specifying both a day-of-week and a day-of-month value is not complete (you must currently use
the ‘?’ character in one of these fields).
Be careful when setting fire times between the hours of the morning when “daylight savings” changes occur
in your locale (for US locales, this would typically be the hour before and after 2:00 AM - because the time
shift can cause a skip or a repeat depending on whether the time moves back or jumps forward. You may find
this wikipedia entry helpful in determining the specifics to your locale:
https://secure.wikimedia.org/wikipedia/en/wiki/Daylight_saving_time_around_the_world
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