How to Run Batch Class for Last 3 mondays of every month

Question

I can not find any cron expression that allows me to run batch job for last 3 Mondays of every month.

Lets say if current month has 4 weeks then I have to run the batch job for 2nd, 3rd and 4th Week of the month.

If Current month has 5 weeks then I have to run batch job for 3rd,4th & 5th Week.

As per my understanding I have to run my batch class for every monday(as there is no such cron expression for last 3 mondays)

But How can I filter it out so that batch class recognize the week and run only last 3 weeks?

Answer 1

All you need to check is add 21 days in today date.

so your code will be:-

DateTime today = System.now(); 
DateTime after21 = today.addDays(21); 
if(after21.month() != today.month()){    
    //run batch 
}

Lets say today is 10th sep and today is Monday. So when I add 21 days to day date next date will be 1st of oct. So this Monday is one of last 3 Mondays. I have added

"!=" and not ">"

in checking month condition because if you are checking for December month then adding 21 days will take you to next month which is January.