Spring Boot ApplicationRunner Example

1. Introduction

The ApplicationRunner interface in Spring Boot provides a way to run specific pieces of code once the Spring Boot application has fully started. This is particularly useful for tasks like running checks, preloading data, or initiating processes that need to occur right after the application is ready.

Key Points

1. ApplicationRunner is an interface used to execute code after the application startup.

2. It requires an implementation of the run method, which contains the code to be executed.

3. ApplicationRunner beans are automatically detected and run by Spring Boot.

4. It is one of the last steps in the startup sequence, ensuring the application context is fully initialized.

2. Implementation Steps

1. Implement the ApplicationRunner interface in a component class.

2. Define the run method to specify the actions to perform after startup.

3. Mark the implementation class with @Component to ensure it is detected by Spring Boot's auto-configuration.

3. Implementation Example

// Step 1: Create an ApplicationRunner implementation
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.stereotype.Component;

@Component
public class AppStartupRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("Hello, Spring Boot has started!");
    }
}

// Step 2: Spring Boot main application class
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

Explanation:

1. The class AppStartupRunner implements ApplicationRunner, specifically overriding the run method.

2. Within the run method, you can define any code that should execute post-launch; in this case, it simply prints a message.

3. The @Component annotation marks AppStartupRunner as a bean, ensuring it is detected and run by Spring Boot upon application start.

4. ApplicationRunner is called after the Spring context is loaded but before the Spring Boot application is fully up and running. This allows executing initialization code that needs access to all the beans configured in the application context.