Spring Boot CommandLineRunner with Example

Introduction

Spring Boot provides various ways to initialize and execute code when the application starts up. One of the convenient ways to run custom initialization logic is by implementing the CommandLineRunner interface. In this tutorial, we’ll explore what CommandLineRunner is, how to use it in a Spring Boot application, and provide examples to demonstrate its usage.

What is CommandLineRunner?

CommandLineRunner is a functional interface provided by Spring Boot. It is part of the org.springframework.boot package and contains a single method run(). This interface is designed to be implemented by Spring Boot components that need to execute custom logic when the application starts up. When the Spring Boot application starts, the run() method of all beans implementing CommandLineRunner will be called in the order of their priority.

Implementing CommandLineRunner

To use CommandLineRunner in your Spring Boot application, you need to follow these steps:

1. Create a Spring Bean: Create a Spring Bean and implement the CommandLineRunner interface. Override the run() method to include the initialization logic you want to execute when the application starts.
2. Annotate the Bean: Annotate the bean class with @Component or any other stereotype annotation such as @Service, @Repository, etc., to make it a Spring-managed bean.
3. (Optional) Specify Order: If you have multiple CommandLineRunner beans and you want to control the order of their execution, you can annotate them with @Order and specify the desired order.

Example: Creating a CommandLineRunner Bean

Let’s create a simple example to demonstrate the usage of CommandLineRunner. Suppose we have a Spring Boot application that needs to print a welcome message when it starts up.

In this example, we’ve created a StartupRunner class that implements the CommandLineRunner interface. Inside the run() method, we print a welcome message to the console.

Ordering CommandLineRunner Beans

If you have multiple CommandLineRunner beans and you want to specify the order in which they should be executed, you can use the @Order annotation.

In this example, we’ve created two CommandLineRunner beans (FirstRunner and SecondRunner) and specified their order using the @Order annotation.

Conclusion

In this tutorial, we’ve explored the CommandLineRunner interface in Spring Boot and learned how to use it to execute custom initialization logic when the application starts up. We’ve created an example to demonstrate how to implement and annotate a CommandLineRunner bean, as well as how to specify the order of execution for multiple CommandLineRunner beans. CommandLineRunner provides a convenient way to perform startup tasks in Spring Boot applications, making it easier to manage application initialization.