In Spring, component scanning is a feature that allows the framework to detect and register Beans (annotated with @Component, @Service, @Repository, @Controller, or any custom stereotype annotations) automatically during application startup.

Here’s how you can enable and use this feature effectively:

1. Enable Component Scanning in a Java Configuration Class

To enable automatic scanning, use the @ComponentScan annotation in your configuration class. This is commonly used to define the base packages to scan for Spring-managed components.

Example:

package org.kodejava.spring;

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

@Configuration
@ComponentScan(basePackages = "org.kodejava.spring") // Specify base package to scan
public class AppConfig {
}

Here, Spring will scan the org.kodejava.spring package and its sub-packages for any classes annotated with @Component, @Service, @Repository, or @Controller.

2. Use Stereotype Annotations

Add one of the following annotations to your classes to mark them as Spring-managed components:

• @Component: Generic for any Spring-managed component.
• @Service: Specifically for service layer components.
• @Repository: For DAO components (adds exception translation).
• @Controller/@RestController: For web controllers in a Spring MVC application.

Example:

package org.kodejava.spring.service;

import org.springframework.stereotype.Service;

@Service
public class MyService {
    public String getMessage() {
        return "Hello, Spring Component Scanning!";
    }
}

3. Shortcut with @SpringBootApplication

If you’re using Spring Boot, the @SpringBootApplication annotation already includes component scanning. It automatically scans the package where the main application class resides and its sub-packages.

Example:

package org.kodejava.spring;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

In this case, Spring Boot will scan all components in com.example and its sub-packages automatically.

4. Advanced Configuration (Optional)

a. Scanning Multiple or Specific Packages

You can specify multiple packages to scan:

@ComponentScan(basePackages = {"org.kodejava.spring.service", "org.kodejava.spring.repository"})

b. Filter Components

You can filter which types of components to include or exclude using the includeFilters or excludeFilters attributes of @ComponentScan.

Example:

@ComponentScan(
    basePackages = "org.kodejava.spring",
    includeFilters = @ComponentScan.Filter(MyCustomAnnotation.class),
    excludeFilters = @ComponentScan.Filter(RestController.class)
)

This will scan the org.kodejava.spring package but include only components annotated with @MyCustomAnnotation and exclude all @RestController components.

c. Scan by Custom Annotation

You can create your custom annotation and use it as a filter:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Component // Marks it as a Spring component
public @interface MyCustomAnnotation {
}

Then annotate classes using @MyCustomAnnotation and configure the scanner accordingly.

5. XML-based Configuration (Legacy Approach)

If you’re using XML-based configuration (rare in modern Spring apps), you can configure component scanning like this:

<context:component-scan base-package="org.kodejava.spring"/>

6. Tips

• Place your configuration class or main application class at a high-level base package to ensure all sub-packages are scanned automatically.
• Avoid scanning unnecessary packages outside your application (e.g., third-party libraries or system packages) to improve performance.
• Use @Lazy with components for lazy initialization if needed.

By using these approaches, you can enable automatic detection and registration of Spring beans with ease!

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Maven Dependencies

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>6.2.6</version>
</dependency>