Why do you need @ComponentScan in Spring Boot ?

The @ComponentScan annotation in Spring Boot is used to automatically detect and register beans (components, services, repositories, etc.) from the specified package(s) into the Spring Application Context.

Why is @ComponentScan Needed?

By default, Spring Boot scans for components only in the package where the main application class is located and its sub-packages.
If your components are in different packages, you need @ComponentScan to explicitly specify the packages to scan.

1. Define Components in Different Packages

Package: com.example.service

package com.example.service;

import org.springframework.stereotype.Service;

@Service
public class MyService {
    public String getMessage() {
        return "Hello from MyService!";
    }
}

2. Enable Component Scanning in @SpringBootApplication

Package: com.example.app

package com.example.app;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
@ComponentScan(basePackages = "com.example.service")  // Scanning external package
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

Here, we explicitly tell Spring Boot to scan com.example.service for beans.

How @ComponentScan Works

• By default, @SpringBootApplication implicitly includes @ComponentScan with the package of the main class.
• If your beans are in different packages, you must specify @ComponentScan(basePackages = "...").
• It scans and registers beans annotated with:
  • @Component
  • @Service
  • @Repository
  • @Controller
  • @RestController

Multiple Packages Example

If your components are in multiple packages, specify them like this:

@ComponentScan(basePackages = {"com.example.service", "com.example.repository"})

Using @ComponentScan with @Configuration

You can also use @ComponentScan in a configuration class:

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan(basePackages = "com.example.service")
public class AppConfig {
}

Here, Spring Boot will scan com.example.service for components.

Key Takeaways

• Auto-scans components to avoid manual bean registration.
• Required if components are outside the default package structure.
• Works with multiple base packages.
• Implicitly included in @SpringBootApplication.