Hello, Trailblazers. In this blog post, we will cover a very important topic that we use in our day-to-day Coding practices or Programming- CRON Expression.
CRON Expression
As you know, Salesforce provides a very great feature of Scheduling an Apex class. You can schedule classes in a standard way through customization with the frequency of weekly days or monthly dates with a preferred start time. If you want to know how to do this, go with this link.
But with the help of CRON expression, we can schedule our apex class more customized.
In this blog post, we’ll learn what is CRON and CRON Expression. What is its use in Salesforce, how to write it, and many more.
So, let’s get started…
What is CRON?
CRON is a time-based job scheduling system commonly used in Unix-like operating systems. It enables users to schedule jobs (commands or scripts) to run on a regular basis at predefined times, dates, or intervals.
What is CRON Expression?
A CRON expression is a string representing a schedule that determines when a task or job should run. It is a standard syntax used in many operating systems, applications, and services for scheduling recurring tasks.
Sample Code:
SampleSchedulableClass testobj = new SampleSchedulableClass(); String cronexp = ‘0 0 5 ? * * *’; System.schedule(‘Testing’, cronexp, testobj);
The above code executes the SampleSchedulableClass at 5.00 AM every day. Here, we stored the cron expression in the “cronexp” variable.
Use of Cron Expression in Salesforce:
In Salesforce, CRON expressions are primarily used for scheduling jobs, specifically in the context of scheduled Apex jobs. It allows developers to write a class that implements the Schedulable interface to run at specified intervals using CRON expressions.
For e.g. System.Schedule(JobName, CronExpression, SchedulableClassName);
Syntax of CRON Expression:
The syntax of a CRON expression consists of fields that represent different aspects of time. Each field specifies a unit of time, and a CRON expression typically consists of six or seven fields, representing seconds, minutes, hours, day of month, month, day of week, and an optional year.
Here’s a Breakdown of a Standard Six-Field CRON Expression:
| 0 | 30 | 5 | * | * | * |
| Sec | Mins | Hours | Day of Month | Month | Day of Week |
In the above example of CRON Expression, a job will run every day at 5.30 in the morning.
The following are the allowed values, special characters, and requirements of the expression:
| Name: | Allowed Values: | Special Characters: | Required: |
|---|---|---|---|
| Seconds | 0-59 | None | Yes |
| Minutes | 0-59 | None | Yes |
| Hours | 0-23 | None | Yes |
| Day of Month | 1-31 | , - * ? / L W | Yes |
| Month | 1-12 or JAN,FEB,MAR..DEC (Only Three Letters) | , - * ? / | Yes |
| Day of Week | 1-7 or SUN, MON, TUES...SAT | , - * ? / L # | Yes |
| Year (Optional) | Null or 1970-2099 | , - * ? / | No |
Special Characters in CRON Expression:
| Special Characters | Description |
|---|---|
| Comma (,) | Separates individual values. For example, 1,15 in the day-of-month field means "the 1st and 15th of the month." |
| Hyphen (-) | Specify a range of values. For example, JAN-AUG means more than one month, or 1-5 in the day-of-week field means "Monday to Friday". |
| Asterisk (*) | It specifies all values. E.g., * in the minutes field represents "every minute." |
| Question Mark (?) | It specifies no specific value. Only used for Day_of_Month and Day_of_Week. It allows you to specify the other field. For example, if you want to run a job every day, regardless of the day of the month, you can use 0 0 0 ? * *. |
| Forward Slash (/) | Specifies increments for ranges */5 in the minutes field, for example, signifies "every 5 minutes." |
| L | Represents the last day of the month or the last day of the week. In the day-of-month field, for example, L denotes "the last day of the month." When you use it with Day_of_Week, it means 7 or SAT. |
| W | It specifies the nearest weekday (Mon-Fri) of the given day of the month. For example, 5W in the day-of-month field means "the nearest weekday to the 5th day of the month." |
| # (nth day_of_week) | Specifies the nth occurrence of a day-of-week within a month. For example, 2#3 in the day-of-week field means "the third Tuesday of the month." |
Note: You can use the L and W together to specify the last weekday of the month.
Here are Some Examples of CRON Expression:
| Expression | Description |
|---|---|
| 0 30 7 * * ? * | Runs Every Day at 7:30 AM |
| 0 0 8 ? * MON-FRI * | Runs Every Weekday (Monday to Friday) at 8 AM |
| 0 0 * * * ? * | Runs Every Hour |
| 0 0/30 * * * ? * | Runs Every 30 Minutes |
| 0 0 0 1 * ? * | Runs on Every first day of the month, at Midnight |
| 0 0 16? * SUN * | Runs Every Sunday at 4 PM |
| 0 0 3,6,9,12 * * ? * | Runs Every Day at 3 AM, 6 AM, 9 AM, and 12 PM |
| 0 0/5 9-17 ? * MON-FRI * | Runs Every 5 Minutes Between 9 AM and 5 PM on Weekdays |
Conclusion:
With this, we learned what CRON expression is and covered all its aspects with the help of real-life examples.
Thus, I hope you enjoyed and understood CRON expression fairly. So, let’s take a reference from this post and build your Apex Schedulers according to your requirements. Stay Tuned!
