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Discover how to effectively utilize the `CommandLineRunner` interface in Spring Boot to execute specific tasks at startup.
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Understanding How CommandLineRunner Works in Spring Boot

When working with Spring Boot applications, you may have encountered the CommandLineRunner interface. But how does it function, and how can you effectively use it in your applications? This guide will break down the concept of CommandLineRunner and offer a clear solution to a common confusion surrounding its usage.

What is CommandLineRunner?

The CommandLineRunner interface is a simple way to execute code after your Spring Boot application has started. It is particularly useful for running tasks that you want to initiate at startup, such as data initialization or executing specific routines. When you define a CommandLineRunner bean, it allows you to run specific code with the command line arguments passed to your Spring Boot application.

Example Code

Consider the following example:

[[See Video to Reveal this Text or Code Snippet]]

In the above code snippet, the demo method provides a CommandLineRunner implementation in which you can write the code you want to execute after your application starts.

How it Actually Works

Invoking the demo method

One of the common points of confusion is understanding who invokes the demo method. In Spring Boot, when the application context is created, all the beans are initialized, and for each CommandLineRunner bean defined in the context, the run method is automatically invoked. This means that you do not need to call it explicitly; Spring handles it for you when the application starts.

Accessing Command Line Arguments

The args parameter provided to the run method of CommandLineRunner is essential for receiving command line arguments. When you start your Spring Boot application, you can pass arguments that can be accessed within the run method. However, in the original code you shared, the command line arguments were not printed inside the demo method because the wrong method of passing arguments was used when calling SpringApplication.run().

Correct Usage of Command Line Arguments

To correctly access the command line arguments in the demo method, update the main method as follows:

[[See Video to Reveal this Text or Code Snippet]]

Running the Application

When you execute your application with command line arguments, you should do so like this:

[[See Video to Reveal this Text or Code Snippet]]

This will pass userName=Rahul_Kumar as an argument which can now be accessed inside your demo method.

Common Issues and Solutions

Command Line Arguments Not Accessed: Make sure to pass the args in the SpringApplication.run() call. If you do not include the args parameter, the run method of your CommandLineRunner will not receive any command line arguments.

Understanding Bean Invocation: Remember that Spring manages the lifecycle of the beans, so you don’t have to worry about manually invoking CommandLineRunner beans.

Conclusion

The CommandLineRunner interface in Spring Boot is a powerful feature that allows you to execute code once your application starts. By setting it up correctly and managing the command line arguments properly, you can utilize this interface efficiently to perform initial tasks in your Spring Boot applications.

Feel free to try it out and see how it enhances your application startup routines!