Scheduled tasks with @Scheduled in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding fixedRate in @Scheduled

What is the behavior of a method annotated with @Scheduled(fixedRate = 5000) in a Spring Boot application?

Spring Boot

@Scheduled(fixedRate = 5000)
public void task() {
    System.out.println("Task executed");
}

A The method runs continuously without any delay.

B The method runs every 5 seconds, counting from the start of the previous execution.

C The method runs once after 5 seconds delay and then stops.

D The method runs every 5 seconds, counting from the end of the previous execution.

💡 Hint

Think about how fixedRate schedules tasks relative to the start time of the previous run.

The fixedRate attribute schedules the method to run at a fixed interval measured from the start time of the previous execution, regardless of how long the method takes to run.

📝 Syntax

intermediate

Correct usage of cron expression in @Scheduled

Which of the following @Scheduled annotations uses a valid cron expression to run a task every day at 3:30 AM?

A @Scheduled(cron = "0 3 30 * * ?")

B @Scheduled(cron = "30 3 * * *")

C @Scheduled(cron = "0 30 3 * * *")

D @Scheduled(cron = "0 30 3 * * ?")

💡 Hint

Remember the cron format: second, minute, hour, day of month, month, day of week, and optional year.

The correct cron expression for 3:30 AM daily is 0 30 3 * * ?. It means at second 0, minute 30, hour 3, every day, every month, any day of week.

🔧 Debug

advanced

Why does @Scheduled method not run?

Given the following Spring Boot component, why does the scheduled method never execute?

Spring Boot

import org.springframework.scheduling.annotation.Scheduled;
import org.springframework.stereotype.Component;

@Component
public class MyTask {

    @Scheduled(fixedDelay = 1000)
    public void run() {
        System.out.println("Running task");
    }
}

A The method must be static to be scheduled.

B The fixedDelay value must be in milliseconds, but 1000 is invalid.

C The @EnableScheduling annotation is missing in the configuration.

D The @Component annotation is not enough; @Service is required.

💡 Hint

Check if scheduling is enabled in the Spring Boot application.

Scheduling requires enabling with @EnableScheduling on a configuration class. Without it, @Scheduled methods won't run.

❓ state_output

advanced

Effect of long-running @Scheduled method with fixedRate

Consider a method annotated with @Scheduled(fixedRate = 2000) that takes 5 seconds to complete. What happens when the application runs?

Spring Boot

@Scheduled(fixedRate = 2000)
public void longTask() throws InterruptedException {
    Thread.sleep(5000);
    System.out.println("Task done");
}

A The method executions overlap; a new run starts every 2 seconds even if the previous is still running.

B The method waits for the previous execution to finish before starting the next, so it runs every 5 seconds.

C The method runs only once because the fixedRate is less than the execution time.

D The scheduler throws an exception due to overlapping executions.

💡 Hint

Think about how fixedRate schedules tasks regardless of method duration.

With fixedRate, the scheduler triggers the method every 2 seconds from the start time, so if the method takes longer, executions overlap.

🧠 Conceptual

expert

Choosing between fixedDelay and fixedRate

Which statement best describes the difference between fixedDelay and fixedRate in @Scheduled annotations?

A fixedDelay schedules the next execution after the previous finishes; fixedRate schedules based on start time regardless of duration.

B fixedDelay schedules tasks at fixed intervals ignoring execution time; fixedRate waits for task completion before next run.

C fixedDelay and fixedRate behave identically but fixedDelay is deprecated.

D fixedDelay schedules tasks only once; fixedRate schedules repeated tasks.

💡 Hint

Consider when the next task starts relative to the previous task's start or end.

fixedDelay waits until the previous execution finishes before scheduling the next. fixedRate schedules tasks at fixed intervals from the start time, regardless of how long the task takes.

Practice

1. What is the main purpose of the @Scheduled annotation in Spring Boot?

easy

A. To run a method automatically at fixed intervals or times

B. To mark a method as a REST endpoint

C. To inject dependencies into a class

D. To configure database connections

Solution

1. Step 1: Understand the role of @Scheduled

   The @Scheduled annotation is used to mark methods that should run automatically on a schedule without manual calls.

2. Step 2: Compare with other options

   Options A, C, and D relate to other Spring features like REST, dependency injection, and database setup, not scheduling.

3. Final Answer:

   To run a method automatically at fixed intervals or times -> Option A

4. Quick Check:

   @Scheduled = automatic method run [OK]

Hint: Remember: @Scheduled means automatic timed method run [OK]

Common Mistakes:

• Confusing @Scheduled with REST or dependency injection annotations
• Thinking @Scheduled configures databases
• Assuming @Scheduled requires manual method calls

2. Which of the following is the correct way to schedule a method to run every 5 seconds using @Scheduled?

easy

A. @Scheduled(fixedDelay = 5000)

B. @Scheduled(rate = 5)

C. @Scheduled(cron = "0 */5 * * * *")

D. @Scheduled(fixedRate = 5000)

Solution

1. Step 1: Understand fixedRate vs fixedDelay

   fixedRate runs the method every specified milliseconds regardless of method duration. fixedDelay waits after method finishes before next run.

2. Step 2: Check the options

   @Scheduled(fixedRate = 5000) uses fixedRate = 5000 which means run every 5 seconds. @Scheduled(fixedDelay = 5000) uses fixedDelay which waits after method ends. @Scheduled(rate = 5) uses invalid attribute rate. @Scheduled(cron = "0 */5 * * * *") uses a cron expression which runs every 5 minutes.

3. Final Answer:

   @Scheduled(fixedRate = 5000) -> Option D

4. Quick Check:

   fixedRate = 5000 means every 5 seconds [OK]

Hint: Use fixedRate for exact interval runs in milliseconds [OK]

Common Mistakes:

• Using 'rate' instead of 'fixedRate'
• Confusing fixedDelay with fixedRate
• Incorrect cron syntax for simple intervals

3. Given the code below, what will be printed and how often?

@Scheduled(fixedDelay = 3000)
public void printMessage() {
    System.out.println("Hello World");
}

medium

A. Method runs only once after 3 seconds delay

B. "Hello World" prints every 3 seconds regardless of method duration

C. "Hello World" prints every 3 seconds after method finishes

D. Code will cause a compilation error

Solution

1. Step 1: Understand fixedDelay behavior

   fixedDelay = 3000 means the method runs, then waits 3 seconds after it finishes before running again.

2. Step 2: Analyze the print output frequency

   Since the method prints "Hello World" each time it runs, it will print every 3 seconds after the previous run completes.

3. Final Answer:

   "Hello World" prints every 3 seconds after method finishes -> Option C

4. Quick Check:

   fixedDelay = wait after method ends [OK]

Hint: fixedDelay waits after method ends before next run [OK]

Common Mistakes:

• Confusing fixedDelay with fixedRate
• Thinking method runs only once
• Assuming compilation error without reason

4. Identify the error in this scheduled task code:

@Scheduled(fixedRate = "5000")
public void task() {
    System.out.println("Running task");
}

medium

A. fixedRate value should be a number, not a string

B. Method must return a value

C. Missing @EnableScheduling annotation on class

D. Method name cannot be 'task'

Solution

1. Step 1: Check fixedRate attribute type

   The fixedRate attribute expects a long number (milliseconds), not a string. Using quotes causes a syntax error.

2. Step 2: Verify other options

   Method can be void, so no return needed. Missing @EnableScheduling is a configuration issue but not a syntax error here. Method name 'task' is valid.

3. Final Answer:

   fixedRate value should be a number, not a string -> Option A

4. Quick Check:

   fixedRate expects number, not string [OK]

Hint: fixedRate and fixedDelay need numbers, no quotes [OK]

Common Mistakes:

• Putting quotes around fixedRate or fixedDelay values
• Assuming method must return a value
• Confusing configuration errors with syntax errors

5. You want a method to run every Monday at 8:30 AM using @Scheduled. Which cron expression is correct?

hard

A. @Scheduled(cron = "30 8 * * 1")

B. @Scheduled(cron = "0 30 8 ? * MON")

C. @Scheduled(cron = "0 8 30 * * MON")

D. @Scheduled(cron = "0 30 8 * * 1")

Solution

1. Step 1: Understand cron format in Spring

   Spring cron has 6 fields: second, minute, hour, day of month, month, day of week.

2. Step 2: Analyze each option

   @Scheduled(cron = "0 30 8 ? * MON"): "0 30 8 ? * MON" means at second 0, minute 30, hour 8, any day of month (?), any month (*), on Monday (MON). This matches 8:30 AM Monday.
   @Scheduled(cron = "30 8 * * 1") is missing seconds field and uses 5 fields.
   @Scheduled(cron = "0 8 30 * * MON") has hour and minute swapped.
   @Scheduled(cron = "0 30 8 * * 1") uses day of week as 1 which is Sunday in some systems, MON is clearer.

3. Final Answer:

   @Scheduled(cron = "0 30 8 ? * MON") -> Option B

4. Quick Check:

   Spring cron: sec min hour day month weekday [OK]

Hint: Spring cron has 6 fields; use '?' for day of month if using day of week [OK]

Common Mistakes:

• Using 5-field cron instead of 6-field in Spring
• Swapping hour and minute positions
• Using numeric day of week without knowing system mapping