Explain @ComponentScan annotation

The `@ComponentScan` annotation is used in Spring to automatically detect and register beans within the Spring application context. It is typically used along with `@Configuration` and `@SpringBootApplication` annotations to specify the base packages to scan for annotated components (e.g., `@Component`, `@Service`, `@Repository`, `@Controller`).

By default, `@ComponentScan` scans the package of the class that declares this annotation. However, you can specify a different base package to scan by providing a `basePackages` attribute.

Example

Step 1: Create a Component

Define a simple Spring component using the `@Component` annotation.

```java
package com.example.service;

import org.springframework.stereotype.Component;

@Component
public class MyComponent {

    public String getMessage() {
        return "Hello from MyComponent!";
    }
}
```

Explanation:

• @Component: Marks the class as a Spring component, making it eligible for component scanning and registration as a Spring bean.

Step 2: Create a Configuration Class with @ComponentScan

Create a configuration class that uses the @ComponentScan annotation to specify the base package to scan for components.

```java
package com.example.config;

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

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

Explanation:

• @Configuration: Indicates that the class can be used by the Spring IoC container as a source of bean definitions.
• @ComponentScan(basePackages = "com.example.service"): Specifies the base package to scan for Spring components.

Step 3: Using the Component in Application

In your main application class or any other class, you can now inject the scanned component.

```java
package com.example.demo;

import com.example.service.MyComponent;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication implements CommandLineRunner {

    @Autowired
    private MyComponent myComponent;

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

    @Override
    public void run(String... args) throws Exception {
        System.out.println(myComponent.getMessage());
    }
}
```

Explanation:

• @Autowired: Used to inject the MyComponent bean that was scanned and registered by the @ComponentScan annotation.
• The run method prints the message returned by the getMessage method of MyComponent.

Conclusion

• @ComponentScan: Used to specify the base packages to scan for annotated Spring components. It helps in automatically detecting and registering beans within the Spring application context.
• Example: Demonstrated how to use @ComponentScan to scan for components and inject them into a Spring Boot application.

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