What is Command Line Runner Interface in Spring Boot?

In Spring Boot, the CommandLineRunner interface is used to run a specific piece of code when the application is fully started. This can be useful for tasks like database initialization, data seeding, triggering certain actions immediately after startup, or any other startup-related task.

Implementing the CommandLineRunner interface requires you to override its run method, which gets executed after the application context is loaded.

How to use `CommandLineRunner`:

Implement CommandLineRunner in your component:

@Component
public class StartupRunner implements CommandLineRunner {

    @Override
    public void run(String... args) throws Exception {
        System.out.println("This code runs on application startup");
    }
}

In the example above, the code inside the run method gets executed when the application starts.

Accessing Application Arguments:

You can also access the application arguments passed in the command line using the args parameter:

@Override
public void run(String... args) throws Exception {
    for (String arg : args) {
        System.out.println("Arg: " + arg);
    }
}

Ordering Multiple CommandLineRunner Beans:

If you have multiple beans implementing CommandLineRunner, you can control the order of their execution using the Order annotation or by implementing the Ordered interface:

@Component
@Order(1)
public class FirstStartupRunner implements CommandLineRunner {
    // ...
}

@Component
@Order(2)
public class SecondStartupRunner implements CommandLineRunner {
    // ...
}

In the example above, FirstStartupRunner will execute before SecondStartupRunner because of the order specified.

Conclusion:

The CommandLineRunner interface is a simple but powerful feature in Spring Boot, allowing developers to execute specific actions after the Spring application context has been fully initialized. It's great for tasks that you want to be done every time the application starts, but be cautious to not overload it with heavy operations that can significantly delay the application's startup time.

How to use Command Line Runner in Spring Boot:

To use CommandLineRunner in Spring Boot, implement the interface in a class and override the run method. Spring Boot will automatically detect and execute it during startup. Example:

import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class MyCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        // Your startup logic here
    }
}

Spring Boot CommandLineRunner example:

Here's a simple example of a CommandLineRunner that prints a message during application startup:

@Component
public class MyCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("Application started with command-line arguments: " + Arrays.toString(args));
    }
}

Custom CommandLineRunner in Spring Boot:

Create a custom CommandLineRunner by implementing the interface and defining your logic. For instance:

@Component
public class CustomCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        // Custom logic here
    }
}

What is Command Line Runner Interface in Spring Boot?

How to use CommandLineRunner:

Conclusion:

How to use `CommandLineRunner`: