How do you create a cron job in Spring Boot?

Table of Contents

• Introduction
• What is a Cron Expression?
• How to Create a Cron Job in Spring Boot?
  • 1. Enable Scheduling in Your Spring Boot Application
  • 2. Use @Scheduled to Create a Cron Job
    • Example 1: Simple Cron Job Running at Midnight
    • Example 2: Running a Cron Job Every Hour
    • Example 3: Running a Cron Job Every Monday at 9 AM
  • 3. Run Cron Jobs Using Fixed Intervals (Optional)
    • Example 4: Running a Job Every 5 Minutes
  • 4. Customize the Cron Expression via Application Properties (Optional)
    • Example: Using Application Properties for Cron Expression
• Conclusion

Introduction

In Spring Boot, creating a cron job allows you to schedule and automate the execution of tasks at specific intervals or times. Using cron expressions, you can configure tasks to run periodically (e.g., every day at midnight, every first Monday of the month) or at fixed intervals. The **@Scheduled** annotation in Spring provides an easy way to define cron jobs and manage task scheduling.

This guide explains how to create cron jobs in Spring Boot, leveraging cron expressions and the @Scheduled annotation to handle repetitive tasks like backups, cleanup jobs, notifications, and more.

What is a Cron Expression?

A cron expression is a string of five or six fields that represent the schedule for a recurring job. It defines the exact time and frequency at which the job should run. Cron expressions can represent complex schedules, such as:

• Every day at midnight
• Every Monday at 9 AM
• The 1st day of every month

A cron expression consists of the following five fields:

For example:

• 0 0 0 * * ? – Cron expression for running a task at midnight every day.
• 0 15 10 * * ? – Runs at 10:15 AM every day.
• 0 0/5 * * * ? – Runs every 5 minutes.

How to Create a Cron Job in Spring Boot?

In Spring Boot, creating a cron job can be done easily using the **@Scheduled** annotation. Here’s how you can set up and configure a cron job in your application.

1. Enable Scheduling in Your Spring Boot Application

Before you can create cron jobs, you need to enable scheduling in your Spring Boot application by adding the @EnableScheduling annotation to your main class or a configuration class.

The @EnableScheduling annotation ensures that Spring Boot can detect and execute tasks annotated with @Scheduled.

2. Use @Scheduled to Create a Cron Job

Once scheduling is enabled, you can use the @Scheduled annotation on any method to schedule it as a cron job. The @Scheduled annotation supports the cron attribute, where you can specify your cron expression.

Example 1: Simple Cron Job Running at Midnight

In this example:

• The method runAtMidnight() is scheduled to run every day at 12:00 AM (midnight) using the cron expression "0 0 0 * * ?".
• The @Scheduled annotation triggers the task based on the cron expression.

Example 2: Running a Cron Job Every Hour

In this case:

• The method runEveryHour() runs every hour on the hour (e.g., 1:00, 2:00, etc.).

Example 3: Running a Cron Job Every Monday at 9 AM

Here, the task will run every Monday at 9 AM. The cron expression "0 0 9 ? * MON" is interpreted as:

• 0: Seconds field, meaning "at second 0".
• 0: Minutes field, meaning "at minute 0".
• 9: Hours field, meaning "at 9 AM".
• ?: Day-of-month field, used when the day of the week is specified.
• *: Month field, meaning "every month".
• MON: Day-of-week field, meaning "every Monday".

3. Run Cron Jobs Using Fixed Intervals (Optional)

If you need to execute a task at fixed intervals (e.g., every 5 minutes), you can use the fixedRate or fixedDelay attributes of the @Scheduled annotation. This will execute the job repeatedly at fixed intervals, without needing to use a cron expression.

Example 4: Running a Job Every 5 Minutes

• The fixedRate attribute specifies that the task should be executed every 300,000 milliseconds (or 5 minutes).
• The @Scheduled(fixedDelay = 300000) attribute works similarly, but the task will execute after a fixed delay after the completion of the previous task.

4. Customize the Cron Expression via Application Properties (Optional)

You can also externalize cron expressions to make your cron job configuration more flexible. This way, you can change the schedule without modifying the code.

Example: Using Application Properties for Cron Expression

In your **application.properties** or **application.yml**, define the cron expression:

Then, use the @Value annotation in your class to inject the value into the scheduled method:

This way, you can change the cron schedule by simply modifying the application.properties file, making your cron jobs more configurable without requiring code changes.

Conclusion

Creating a cron job in Spring Boot is a straightforward process, thanks to the **@Scheduled** annotation and cron expressions. With just a few lines of code, you can schedule periodic tasks like database backups, email notifications, data cleanup, and more. The flexibility of cron expressions enables you to run tasks at any specific time or interval, providing precise control over your application's background processes.

By using cron expressions, you can automate your tasks efficiently and keep your application running smoothly without manual intervention. Additionally, Spring's scheduling support allows you to integrate cron jobs seamlessly with the rest of your application, making it an essential tool for automating routine operations.