Scheduled tasks with @Scheduled in Spring Boot - Step-by-Step Execution

Concept Flow - Scheduled tasks with @Scheduled

Application starts

↓

Spring scans for @Scheduled

↓

Schedule task setup

↓

Wait for trigger time

↓

Run scheduled method

↓

Task completes

↓

Wait for next trigger

↩Loop back to Run scheduled method

Spring Boot starts, finds methods marked with @Scheduled, sets timers, and runs those methods repeatedly at set times.

Execution Sample

Spring Boot

@Scheduled(fixedRate = 2000)
public void sayHello() {
  System.out.println("Hello every 2 seconds");
}

This method prints a message every 2 seconds automatically.

Execution Table

Step	Time (ms)	Action	Output	Next Trigger
1	0	Application starts, schedules sayHello()		2000
2	2000	sayHello() runs	Hello every 2 seconds	4000
3	4000	sayHello() runs	Hello every 2 seconds	6000
4	6000	sayHello() runs	Hello every 2 seconds	8000
5	8000	sayHello() runs	Hello every 2 seconds	10000
6	10000	sayHello() runs	Hello every 2 seconds	12000
7	12000	Application stops or continues scheduling

💡 Application stops or user interrupts; scheduled tasks stop running.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
Next Trigger (ms)	2000	4000	6000	8000	10000	12000	Stopped or continues

Key Moments - 3 Insights

Why does the scheduled method run automatically without being called?

Because Spring detects @Scheduled and sets timers to call the method at fixed intervals (see execution_table steps 2-6).

What happens if the method takes longer than the fixedRate to run?

Spring will still trigger the next run based on fixedRate timing, possibly causing overlap or delays depending on configuration.

How does Spring know when to run the method next?

Spring calculates the next trigger time after each run, shown in the 'Next Trigger' column in execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 4?

A Hello every 2 seconds

B No output

C Error message

D Hello every 4 seconds

💡 Hint

Check the 'Output' column at step 4 in the execution_table.

At which step does the next trigger time become 8000 ms?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Next Trigger' column in the execution_table for the value 8000.

If fixedRate is changed to 1000 ms, how would the 'Next Trigger' values change?

A They would stay the same

B They would increase by 1000 ms each step

C They would increase by 4000 ms each step

D They would decrease

💡 Hint

Compare the current fixedRate of 2000 ms with the new 1000 ms in variable_tracker.

Concept Snapshot

@Scheduled annotation runs methods automatically on a schedule.
Use fixedRate to run repeatedly every set milliseconds.
Spring sets timers and calls the method without manual calls.
Next trigger time updates after each run.
Useful for tasks like cleanup, notifications, or polling.

Full Transcript

When a Spring Boot application starts, it looks for methods marked with @Scheduled. It sets timers to run these methods automatically at fixed intervals, like every 2 seconds. The method runs without needing to be called manually. After each run, Spring calculates the next time to run the method again. This continues until the application stops or the scheduling is disabled. This helps automate repeated tasks easily.