Understanding Spring Boot's Runner Classes
In Spring Boot, there are two critical interfaces: Application Runner and Command-Line Runner. These interfaces play a pivotal role in executing code once a Spring Boot application starts. Let's delve into these interfaces and how to implement them.
What is Application Runner?
The Application Runner interface is used to execute code after a Spring Boot application has started.
package com.techieuncle.app;
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class ExampleApplication implements ApplicationRunner {
public static void main(String[] args) {
SpringApplication.run(ExampleApplication .class, args);
}
@Override
public void run(ApplicationArguments arg0) throws Exception {
System.out.println("Let's Print Hello World in Application Runner");
}
}
What is the Command Line Runner in Spring Boot?
In Spring Boot, the Command Line Runner is an interface that allows developers to create components that run specific code when a Spring Boot application starts up. It is part of the Spring Framework's support for bootstrapping and initializing an application.
Here's how it works:
1. **Implementing the CommandLineRunner Interface:**
To use the Command Line Runner, you create a class that implements the `CommandLineRunner` interface, which defines a single method called `run`. This `run` method is where you place the code that you want to run when the Spring Boot application starts.
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 initialization or startup code goes here
System.out.println("Application started. Running command line tasks...");
}
}
2. **Spring Component Scanning:**
By annotating your class with `@Component` or a related stereotype annotation (such as `@Service` or `@Repository`), Spring Boot will automatically detect and register it as a bean in the Spring application context during component scanning.
3. **Execution on Application Startup:**
When your Spring Boot application starts, Spring Boot will execute the `run` method of all the beans that implement `CommandLineRunner`. This happens after the Spring application context is fully initialized, making it suitable for tasks like database initialization, setting up default configurations, or running any custom logic needed at startup.
You can also access the command-line arguments passed to your application as `args` in the `run` method, which can be useful for customizing the behavior based on command-line input.
By using the Command Line Runner interface, you can easily add custom startup logic to your Spring Boot application without having to rely on external scripts or manual intervention, making your application more self-contained and maintainable.
The Command-Line Runner is an interface used to execute code after a Spring Boot application has started. Here's how you can implement the Command-Line Runner interface in your main class:
package com.techieuncle.app;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class ExampleApplication implements CommandLineRunner {
public static void main(String[] args) {
SpringApplication.run(ExampleApplication.class, args);
}
@Override
public void run(String... arg0) throws Exception {
System.out.println("Let's Print Hello World in Command Line Runner");
}
}
