Discover the causes behind the unexpected behavior of schedulers in Spring Boot applications and learn how to resolve issues when using lazy initialization.
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Understanding Weird Spring Boot Scheduler Behavior with Lazy Initialization

When developing applications using Spring Boot, you may encounter unexpected behavior from scheduled tasks, especially when lazy initialization is in play. As you might have experienced, the scheduler might not function as intended without additional code interventions. In this guide, we're going to dive deep into this issue and explore a solution to ensure that scheduled tasks operate correctly in your application.

The Problem: Scheduler Not Working with Lazy Initialization

As you described, the problem arises in your Spring Boot application, version 2.6.7, running on Java 17. You have implemented a scheduling configuration like this:

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In addition, you created a service that runs scheduled tasks:

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Even with this configuration, the scheduled tasks do not execute unless you manually initialize the service via context.getBean(NoPrimaryServiceTasks.class);. This workaround indicates a deeper issue related to the lazy initialization of beans in the Spring context.

The Root Cause: Lazy Initialization in Spring Context

What is Lazy Initialization?

In Spring, lazy initialization means that beans are not instantiated until they are needed. This can enhance performance by delaying the creation of beans that may never be used. However, when implementing scheduled tasks, it can lead to situations where necessary beans are not ready when the application starts, resulting in the scheduler not functioning correctly.

The Addition of Lazy Initialization Code

In your codebase, there was an inadvertent addition of the following snippet:

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This forced all beans in your application context to be lazily initialized, including the scheduled tasks, preventing them from being executed on application startup.

The Solution: Correcting Lazy Initialization Settings

To tackle the issue of schedulers not working when lazy initialization is enabled, here's what you can do:

1. Review Bean Initialization Settings

You need to identify whether lazy initialization is genuinely necessary for your application's architecture. If it's inadvertently set for all beans, consider removing the previous snippet or customizing which beans should be lazily initialized.

2. Use @Lazy Annotation Wisely

If certain services genuinely need to be initialized lazily, use the @Lazy annotation on specific beans rather than on all beans globally:

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3. Enable Eager Initialization for Scheduled Beans

If your scheduled tasks require quick access, you can exclude them from lazy initialization by specifying which beans should initialize eagerly. Utilize the following property in your application.properties file:

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This adjustment will ensure that all beans, including scheduled tasks, are initialized at startup.

4. Check Related Configuration Classes

Make sure there are no other configuration classes affecting your bean initialization behavior. This includes reviewing your ScheduledBeanLazyInitializationExcludeFilter.

Conclusion

By understanding how lazy initialization affects scheduled tasks in Spring Boot and making the necessary adjustments, you can ensure that your application performs as expected without requiring manual initializations. Remember to strike a balance between lazy initialization for performance reasons and eager loading for components that are critical for application functionality.

If you have further questions or encounter related issues, feel