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Demystifying Daemons: Developing a Daemon in Java

Date: 14-11-2023

In the realm of software development, certain terms tend to evoke curiosity and intrigue. One such term is “daemon.” Derived from Greek mythology, where daemons were benevolent spirits, in the context of computer science and Java development, daemons are background processes that perform specific tasks independently. These hidden heroes play a crucial role in making our applications more responsive and efficient. In this blog post, we’ll explore what daemons are, their uses, the benefits of employing them, and how mastering them can empower full-stack Java developers.

What Are Daemons?

To fully grasp the significance of daemons in Java development, we must first understand what they are. In this context, It is background processes that operate independently of the main application. They run silently, performing tasks that don’t require direct user interaction. These tasks could range from managing database connections and handling email delivery to scheduled data cleanup.

The term “daemon” draws inspiration from Greek mythology, where it refers to benevolent spirits. Similarly, in the world of Java development, daemons are indeed benevolent, as they enhance our applications’ capabilities without causing disruptions.

The Uses of Daemons in Java Development

Daemons are versatile and find applications in various aspects of Java development:

Web Applications

In web applications, daemons can handle background tasks such as sending notifications, processing user uploads, or performing data synchronization.

Server-Side Development

For server-side development, daemons are invaluable for managing server resources, monitoring application health, and handling concurrent requests efficiently.

Task Automation

Developers often use daemons for automating routine tasks like data backups, log rotation, and database maintenance.


Daemons allow applications to multitask without blocking the main application thread. For example, a web server may use daemons to handle incoming requests concurrently.

Benefits of Using Daemons

Why do full-stack Java developers rely on daemons in their applications? Here are some key benefits:

Enhanced Performance

Daemons offload tasks to separate processes, ensuring the main application remains responsive and efficient, leading to a smoother user experience.

Resource Management

Daemons manage system resources effectively, preventing resource leaks and optimizing system performance.

Scheduled Tasks

They excel at scheduling tasks and automating repetitive operations, reducing the need for manual intervention.

Error Handling

Daemons can be programmed to monitor application health and respond to errors promptly.

Developing a Daemon in Java 

Certificate Expiry Notifications

Here, we present a skeleton of a daemon responsible for sending notifications to users about expiring certificates in the next 30, 60, or 90 days. This daemon keeps track of certificate expiration dates and sends timely reminders, enhancing user engagement and trust.

// Package and import statements
public class ExpiringCertNotificationDaemon extends ShedlockDaemon {
    // Class implementation
    // ...
    @Scheduled(cron = "0 0 2 ? * MON")
    @SchedulerLock(name = "ExpiringCertNotificationDaemon_run", 
                    lockAtLeastFor = "PT10M", lockAtMostFor = "PT30M")
    public void run() {
        // Method implementation
        // ...
    private void processActiveTenantExpiringCertificates() {
        // Method implementation
        // ...
    private void processTenantNotifications(Tenant tenant) {
        // Method implementation
        // ...
    // Additional methods and functionalities are defined in the daemon
    // ...

Class declaration

  • The class ExpiringCertNotificationDaemon is declared with the @Service annotation, indicating it as a service class in the Spring framework.
  • It extends ShedlockDaemon, indicating that it inherits or overrides methods from the ShedlockDaemon class, providing additional functionalities related to scheduling and task management.

Scheduling task execution

The run() method is annotated with various annotations:

  • @Async(“schedulerThreadPoolTaskExecutor”): Indicates that the method should be executed asynchronously using the specified thread pool executor.
  • @Scheduled(cron = “0 0 2 ? * MON”): Defines a cron expression for scheduling the method to run at 2 AM on Mondays.
// These are valid formats for cron expressions:
    // 0 0 * * * * = the top of every hour of every day.
    // */10 * * * * * = every ten seconds.
    // 0 0 8-10 * * * = 8, 9 and 10 o'clock of every day.
    // 0 0 6,19 * * * = 6:00 AM and 7:00 PM every day.
    // 0 0/30 8-10 * * * = 8:00, 8:30, 9:00, 9:30, 10:00 and 10:30 every day.
    // 0 0 9-17 * * MON-FRI = on the hour nine-to-five weekdays
    // 0 0 0 25 12 ? = every Christmas Day at midnight
    // The pattern is: second, minute, hour, day, month, weekday

  • @SchedulerLock: Utilizes ShedLock, a library for distributed locking, to ensure the method is only executed when a lock can be obtained and held for a specific duration.

Daemon functionality

  • processActiveTenantExpiringCertificates() is a private method within the daemon class that encapsulates the logic for finding and handling expiring certificates for the active tenants.
  • processTenantNotifications(Tenant tenant) is also a private method within the daemon class that is responsible for sending notifications to specific tenants /users on different channels, i.e., WebApp, System, or SMS.


Daemons, Once shrouded in mystique, are now revealed as valuable assets to Java developers. They improve application performance, automate tasks, and empower developers to create efficient, responsive, and reliable software. As you embark on your journey as a Java developer, remember that mastering daemons is one of the keys to success.

By understanding what daemons are, their applications, and the benefits they bring, you can harness their power to create exceptional Java applications that stand out in the world of software development.

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