Cron expressions for scheduling in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct cron expression syntax for scheduling a task every 5 minutes

Which of the following cron expressions correctly schedules a Spring Boot task to run every 5 minutes?

A 0 */5 * * * *

B 0 0/5 * * * ?

C */5 * * * *

D 0 5 * * * ?

💡 Hint

Spring Boot cron expressions use six fields: second, minute, hour, day of month, month, day of week.

Spring Boot cron expressions require six fields. Option B uses the correct six-field format with '0' seconds and '0/5' minutes to run every 5 minutes.

Option B has only five fields, which is invalid in Spring Boot.

Option B runs every 5 minutes but includes seconds as '0', which is correct, and the minute field is '*/5' which is also valid; however, the standard format is '0 0/5 * * * ?'.

Option B runs at 5 minutes past every hour, not every 5 minutes.

❓ component_behavior

intermediate

What is the output behavior of this scheduled method with given cron?

Given the Spring Boot scheduled method below, how often will it run? @Scheduled(cron = "0 15 10 * * ?") public void runTask() { System.out.println("Task executed"); }

Spring Boot

@Scheduled(cron = "0 15 10 * * ?")
public void runTask() { System.out.println("Task executed"); }

A Runs every minute during 10 AM hour

B Runs every hour at 15 minutes past

C Runs every day at 10:15 AM

D Runs every day at 15:10 (3:10 PM)

💡 Hint

Look at the hour and minute fields in the cron expression.

The cron expression '0 15 10 * * ?' means at second 0, minute 15, hour 10, every day, any month, any day of week.

This schedules the task to run once daily at 10:15 AM.

🔧 Debug

advanced

Why does this cron expression cause a runtime error in Spring Boot?

Consider this cron expression used in @Scheduled: @Scheduled(cron = "0 0 25 * * ?") public void task() { /*...*/ } Why will this cause an error when the application runs?

Spring Boot

@Scheduled(cron = "0 0 25 * * ?")
public void task() { /*...*/ }

A Hour field '25' is invalid because valid hours are 0-23

B Minute field '0' is invalid because it must be between 1-59

C Day of month '*' is invalid because it must be a number

D Second field '0' is invalid because it must be between 1-59

💡 Hint

Check the allowed ranges for each cron field in Spring Boot.

The hour field in a cron expression must be between 0 and 23.

Here, '25' is outside this range, causing a runtime error.

Minute and second fields with '0' are valid, and day of month '*' means every day, which is valid.

❓ state_output

advanced

What is the output of this scheduled task with a complex cron expression?

Given this Spring Boot scheduled method: @Scheduled(cron = "0 0 9-17/2 * * MON-FRI") public void workHoursTask() { System.out.println("Working..."); } How often and when will this method print "Working..."?

Spring Boot

@Scheduled(cron = "0 0 9-17/2 * * MON-FRI")
public void workHoursTask() { System.out.println("Working..."); }

A Runs every 2 hours starting at 9 AM until 5 PM, Monday to Friday

B Runs every hour from 9 AM to 5 PM every day

C Runs once daily at 9 AM on weekdays

D Runs every 2 hours all days including weekends

💡 Hint

Look at the hour field with range and step, and the day of week field.

The cron expression '0 0 9-17/2 * * MON-FRI' means:

• At second 0, minute 0
• Hours from 9 to 17 stepping every 2 hours (9,11,13,15,17)
• Every day of month
• Every month
• Only Monday to Friday

So the task runs at 9 AM, 11 AM, 1 PM, 3 PM, and 5 PM on weekdays.

🧠 Conceptual

expert

Which cron expression schedules a task to run at 8:30 AM on the last Friday of every month?

Select the correct Spring Boot cron expression that runs a task at 8:30 AM on the last Friday of each month.

A 0 30 8 ? * 6L

B 0 30 8 * * 5L

C 0 30 8 ? * 6#5

D 0 30 8 ? * 5L

💡 Hint

In cron, day of week 5 is Friday, and 'L' means last occurrence in the month.

In Spring Boot cron expressions:

• Day of week '5' means Friday.
• 'L' after day of week means last occurrence of that weekday in the month.
• Question mark '?' is used in day of month when day of week is specified.

Option D '0 30 8 ? * 5L' means at 8:30 AM, on the last Friday of every month.

Option D uses '6L' which is Saturday, not Friday.

Option D uses '*' in day of month which conflicts with day of week '5L'.

Option D uses '6#5' which means the 5th Saturday, which may not exist every month.

Practice

1. What does a cron expression in Spring Boot primarily specify?

easy

A. The output format of the task

B. When a scheduled task should run

C. The name of the scheduled task

D. The priority of the task

Solution

1. Step 1: Understand the purpose of cron expressions

   Cron expressions define timing details for scheduling tasks.

2. Step 2: Identify what cron expressions control in Spring Boot

   They tell Spring Boot exactly when to run a method automatically.

3. Final Answer:

   When a scheduled task should run -> Option B

4. Quick Check:

   Cron expression = schedule time [OK]

Hint: Cron expressions set task run times, not names or outputs [OK]

Common Mistakes:

• Thinking cron sets task names
• Confusing cron with output formatting
• Assuming cron controls task priority

2. Which of the following is the correct syntax to schedule a method to run every minute using Spring Boot's @Scheduled annotation with a cron expression?

easy

A. @Scheduled(cron = "*/1 * * * *")

B. @Scheduled(cron = "* * * * *")

C. @Scheduled(cron = "0 0/1 * * ?")

D. @Scheduled(cron = "0 * * * * *")

Solution

1. Step 1: Recall Spring Boot cron format

   Spring Boot cron uses 6 fields: seconds, minutes, hours, day, month, weekday.

2. Step 2: Identify correct expression for every minute

   "0 * * * * *" means at 0 seconds every minute, every hour, every day.

3. Final Answer:

   @Scheduled(cron = "0 * * * * *") -> Option D

4. Quick Check:

   6-field cron with 0 seconds = every minute [OK]

Hint: Spring Boot cron needs 6 fields; seconds first [OK]

Common Mistakes:

• Using 5-field cron (missing seconds)
• Misplacing seconds field
• Using '?' in wrong position

3. Given the cron expression "0 15 10 * * ?" in a Spring Boot @Scheduled annotation, when will the scheduled method run?

medium

A. Every day at 10:15 AM

B. Every hour at 15 minutes past

C. Every day at 10:00 AM

D. Every minute at 15 seconds past

Solution

1. Step 1: Break down the cron fields

   Fields: seconds=0, minutes=15, hours=10, day=any, month=any, weekday=? (no specific day)

2. Step 2: Interpret the timing

   Means run at 10:15:00 every day, ignoring weekday.

3. Final Answer:

   Every day at 10:15 AM -> Option A

4. Quick Check:

   0 15 10 * * ? = daily 10:15 AM [OK]

Hint: Seconds first, then minutes, then hours in Spring cron [OK]

Common Mistakes:

• Confusing minutes and hours positions
• Ignoring '?' meaning in weekday
• Assuming it runs every hour

4. Identify the error in this Spring Boot cron expression: @Scheduled(cron = "0 0 25 * * ?")

medium

A. Seconds field missing

B. Day of month field is invalid

C. Hour value 25 is invalid

D. Weekday field cannot be '?'

Solution

1. Step 1: Check each cron field's valid range

   Seconds: 0-59, Minutes: 0-59, Hours: 0-23, Day: 1-31, Month: 1-12, Weekday: 1-7 or ?

2. Step 2: Identify invalid value

   Hour field is 25, which is outside 0-23 range, causing error.

3. Final Answer:

   Hour value 25 is invalid -> Option C

4. Quick Check:

   Hour must be 0-23, 25 is invalid [OK]

Hint: Hours max 23 in cron, check all field ranges [OK]

Common Mistakes:

• Using hour > 23
• Forgetting seconds field
• Misusing '?' in day or weekday

5. You want to schedule a Spring Boot task to run every Monday at 8:30 AM, but only during the months of June, July, and August. Which cron expression correctly achieves this?

hard

A. "0 30 8 ? 6,7,8 MON"

B. "0 30 8 * 6-8 *"

C. "0 30 8 ? 5-8 MON"

D. "0 30 8 ? 6-8 1"

Solution

1. Step 1: Understand cron fields needed

   Seconds=0, Minutes=30, Hours=8, Day=? (no specific day), Month=6,7,8 (June-Aug), Weekday=MON (Monday)

2. Step 2: Check options for correct syntax

   "0 30 8 ? 6,7,8 MON" uses '?' for day, lists months as 6,7,8, and weekday as MON, which is correct.

3. Final Answer:

   "0 30 8 ? 6,7,8 MON" -> Option A

4. Quick Check:

   Use '?' for day, list months, specify MON weekday [OK]

Hint: Use '?' for day when specifying weekday, list months with commas [OK]

Common Mistakes:

• Using '*' for day and weekday together
• Using range '-' instead of commas for months
• Using numeric weekday instead of MON