Spring Boot provides a range of functionalities that allow developers to run specific code blocks during the application's lifecycle. One such feature is the CommandLineRunner interface. This interface is specifically designed to execute a code block just once, right after the Spring Boot application has initialized. This can be particularly useful for tasks like database seeding, running sanity checks, or any other initialization logic you want to run before your application begins processing.
There are a few ways to utilize the CommandLineRunner in a Spring Boot application. Let's explore them:
Using @Component Annotation
Quiz: What's the role of the @Component annotation in Spring?
The `@Component` annotation is used in Spring to indicate that a class should be considered as a Spring-managed component. When the application context is being created, classes annotated with `@Component` are auto-detected and a bean instance is created for them in the Spring container.
Integrating with @SpringBootApplication
Quick Quiz: Explain the syntax String.... How is it different than String[] args in Java?
The String... syntax in Java denotes a varargs (variable number of arguments). It allows you to pass any number of arguments when invoking a method. Internally, varargs are treated as an array. The key difference between String... args and String[] args is in how you call the method. With varargs (String... args), you can call the method with multiple string arguments directly, without having to create an array. With String[] args, you'd need to pass an array of strings. However, inside the method body, args behaves like an array in both cases.