CRON Expression in Salesforce

Hello to all the Salesforce developers, here in this blog I am covering a very crucial topic that we all use in our coding skills - CRON expression. As we all know that Salesforce provide us a very great feature of Scheduling an apex class. In standard way through customization part, we can schedule an apex class with frequency of weekly days or monthly dates with a preferred start time. But through CRON expression we can schedule an apex with our customized date time or occurrence through code. That’s an interesting feature that Salesforce allows developers to customize according to their own need and scenario.

What is CRON?

CRON is  a software utility that is a time-based job scheduler in Unix-like computer operating systems. Developers who want to set up and maintain software environments, use this cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.

What is CRON expression?

A CRON expression is basically a string of five or six fields separated by white spaces that represents a set of times, normally as a schedule to execute some routine.

Use in Salesforce

Use schedule with an Apex class that implements the Schedulable interface to schedule the class to run at the time specified by a Cron expression.

System.Schedule(JobName, CronExpression, SchedulableClass);

The System.Schedule method takes three arguments: a name for the job, an expression used to represent the time and date the job is scheduled to run, and the name of the class. 

Sample Code

TestSchedulableClass testobj = new TestSchedulableClass();

String cronexpression = ‘0 0 0 ? * * *’

System.schedule(‘Testing’, cronexpression, testobj);

The Above Code Executes TestSchedulableClass At 12:00 AM Every Day .

CRON expression has the following syntax:

0 0 5 ? * 1,2,3,4,5,6,7

{1} {2} {3} {4} {5} {6}

{1} Seconds - so 0 here i.e. start of the minute.

{2} Minutes - 0 again so start of the hour.

{3} Hours -  5 so 5 am. Uses 24 hour notation so 21 = 9pm

{4} Day_of_month - ? means no specific value, only available for day of the month and day of the week.

{5} Month - * indicates all values, i.e. every month. (if we only want to run on 1st Jan say, this would be 1)

{6} Day_of_week - 1,2,3,4,5,6,7 here specifies days 1,2,3,4,5,6,7 in the week. We could also write this string as MON-FRI or preferably as * to indicate all values.

So this job reads to run at "0 seconds past 0 minutes of the 5th hour on no specified day of the month for every month of the year for every day of the week".

The following are the values for the expression:

Name	Values	Special Characters
{1} Seconds	0–59	None
{2} Minutes	0–59	None
{3} Hours	0–23	None
{4} Day_of_month	1–31	, - * ? / L W
{5} Month	1–12 or the following: JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC	, - * /
{6} Day_of_week	1–7 or the following: SUN MON TUE WED THU FRI SAT	, - * ? / L #
optional_year	null or 1970–2099	, - * /

The special characters are defined as follows:

Special Character	Description
,	Delimits values. For example, use JAN, MAR, APR to specify more than one month.
-	Specifies a range. For example, use JAN-MAR to specify more than one month.
*	Specifies all values. For example, if Month is specified as *, the job is scheduled for every month.
?	Specifies no specific value. This is only available for Day_of_month and Day_of_week, and is generally used when specifying a value for one and not the other.
/	Specifies increments. The number before the slash specifies when the intervals will begin, and the number after the slash is the interval amount. For example, if you specify 1/5 for Day_of_month, the Apex class runs every fifth day of the month, starting on the first of the month.
L	Specifies the end of a range (last). This is only available for Day_of_month and Day_of_week. When used with Day of month, L always means the last day of the month, such as January 31, February 29 for leap years, and so on. When used with Day_of_week by itself, it always means 7 or SAT. When used with a Day_of_week value, it means the last of that type of day in the month. For example, if you specify 2L, you are specifying the last Monday of the month. Do not use a range of values with L as the results might be unexpected.
W	Specifies the nearest weekday (Monday-Friday) of the given day. This is only available for Day_of_month. For example, if you specify 20W, and the 20th is a Saturday, the class runs on the 19th. If you specify 1W, and the first is a Saturday, the class does not run in the previous month, but on the third, which is the following Monday.
#	Specifies the nth day of the month, in the format weekday#day_of_month. This is only available for Day_of_week. The number before the # specifies weekday (SUN-SAT). The number after the # specifies the day of the month. For example, specifying 2#2 means the class runs on the second Monday of every month.

NOTE: Use the L and W together to specify the last weekday of the month.

Cron Expression Examples

Expression	Description
0 0 0 ? * * *	at 12:00 AM every day
0 0 12 * * ?	at 12:00 PM every day
0 0 10 ? * *	at 10.00 AM every day
0 0 10 * * ?	at 10.00 AM every day
0 0 10 * * ? *	at 10.00 AM every day
0 0 15 ? * * *	at 3:00 PM every day
0 0-5 15 * * ?	Every minute starting at 3:00 PM and ending at 3:05 PM, every day
0 15 17 ? * MON-FRI	at 5:15 PM every Monday, Tuesday, Wednesday, Thursday and Friday
0 25 5 15 * ?	at 5:25 AM on the 15th day of every month
0 15 17 ? * 6#3	at 5:15 PM on the third Friday of every month
0 0 17 ? * 6L	runs the last Friday of every month at 5:00 PM.
‘0 30 * * * *’;	every 30 minutes
0 0 23 * * ? 2018	runs every day at 11:00 PM during the year 2016.

Example to schedule a class for every 5 min

System.schedule(Schedule Job Name 1',  '0 00 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 2',  '0 05 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 3',  '0 10 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 4',  '0 15 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 5',  '0 20 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 6',  '0 25 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 7',  '0 30 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 8',  '0 35 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 9',  '0 40 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 10', '0 45 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 11', '0 50 * * * ?', new testScheduleFiveMinutes());

System.schedule(Schedule Job Name 12', '0 55 * * * ?', new testScheduleFiveMinutes());

Hope you all get a quick idea of CRON expression. Have fun with CRON and built your schedulers
as per your requirements.