Scheduled tasks with @Scheduled in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The @Scheduled annotation is used to mark a method to be executed automatically at fixed intervals or specific times, enabling scheduled tasks in a Spring Boot application.

beginner

How do you enable scheduling support in a Spring Boot application?

You enable scheduling by adding the @EnableScheduling annotation to a configuration class or the main application class.

intermediate

What are the common parameters of the @Scheduled annotation?

Common parameters include fixedRate (runs method at fixed intervals), fixedDelay (runs method after a fixed delay from last completion), and cron (runs method based on a cron expression).

intermediate

Explain the difference between fixedRate and fixedDelay in @Scheduled.

fixedRate runs the method at regular intervals regardless of how long the method takes, while fixedDelay waits until the previous execution finishes and then waits the specified delay before running again.

advanced

What is a cron expression in the context of @Scheduled?

A cron expression is a string that defines a schedule using fields for seconds, minutes, hours, day of month, month, day of week, and optionally year, allowing precise control over when a task runs.

Which annotation must be added to enable scheduling in Spring Boot?

A @EnableScheduling

B @Scheduled

C @EnableAsync

D @EnableTask

You must add @EnableScheduling to enable scheduling support. @Scheduled marks the methods to run on schedule.

What does fixedRate in @Scheduled do?

A Runs the method at fixed intervals regardless of execution time

B Runs the method after a fixed delay from last completion

C Runs the method only once

D Runs the method based on a cron expression

fixedRate runs the method at regular intervals, ignoring how long the method takes.

Which parameter of @Scheduled allows scheduling with a cron expression?

A fixedDelay

B delay

C fixedRate

D cron

The cron parameter accepts a cron expression to schedule tasks precisely.

If you want a method to run 5 seconds after the last execution finishes, which @Scheduled parameter do you use?

A fixedRate = 5000

B fixedDelay = 5000

C cron = "*/5 * * * * *"

D delay = 5000

fixedDelay waits 5 seconds after the last execution finishes before running again.

Where do you place the @Scheduled annotation?

A On a configuration class

B On the main application class

C On a method to be scheduled

D On a field variable

@Scheduled is placed on the method that should run on a schedule.

Describe how to set up a scheduled task in Spring Boot using @Scheduled.

Think about enabling scheduling and marking methods.

Explain the difference between fixedRate and fixedDelay in scheduling tasks.

Consider timing relative to method execution.

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