Spring Boot Component Scanning: @ComponentScan

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Component scanning in Spring Boot is a powerful feature that automatically detects and registers beans within the Spring application context. This simplifies the configuration and development process by eliminating the need for manual bean registration.

Prerequisites

JDK 17 or later
Maven or Gradle
IDE (IntelliJ IDEA, Eclipse, etc.)

What is Component Scanning?

Component scanning is the process of automatically discovering and registering beans in the Spring application context. It is typically enabled through annotations such as @Component, @Service, @Repository, and @Controller.

Step-by-Step Guide

Step 1: Set Up a Spring Boot Project

Use Spring Initializr to create a new project with the following configuration:

Project: Maven Project
Language: Java
Spring Boot: 3.2.x
Dependencies: Spring Web

Download and unzip the project, then open it in your IDE.

Step 2: Create Components

Create different components in various packages to demonstrate component scanning.

2.1 Create a Service Component

Create a class named MyService in the com.example.demo.service package.

package com.example.demo.service;

import org.springframework.stereotype.Service;

@Service
public class MyService {

    public String serve() {
        return "Service is working";
    }
}

Explanation:

@Service: Marks the class as a service component, making it eligible for component scanning and automatic registration.

2.2 Create a Repository Component

Create a class named MyRepository in the com.example.demo.repository package.

package com.example.demo.repository;

import org.springframework.stereotype.Repository;

@Repository
public class MyRepository {

    public String fetch() {
        return "Repository is working";
    }
}

Explanation:

@Repository: Marks the class as a repository component, making it eligible for component scanning and automatic registration.

2.3 Create a Controller Component

Create a class named MyController in the com.example.demo.controller package.

package com.example.demo.controller;

import com.example.demo.service.MyService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class MyController {

    @Autowired
    private MyService myService;

    @GetMapping("/test")
    public String test() {
        return myService.serve();
    }
}

Explanation:

@RestController: Marks the class as a REST controller component, making it eligible for component scanning and automatic registration.
@Autowired: Automatically injects the MyService bean into the controller.

Step 3: Configure Component Scanning

3.1 Default Component Scanning

By default, Spring Boot scans for components in the package where the @SpringBootApplication annotation is located and all its sub-packages. Therefore, if your DemoApplication class is located in the com.example.demo package, it will automatically scan for components in the com.example.demo package and its sub-packages.

package com.example.demo;

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

@SpringBootApplication
public class DemoApplication {

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

Explanation:

@SpringBootApplication: A convenience annotation that combines @Configuration, @EnableAutoConfiguration, and @ComponentScan.

3.2 Customizing Component Scanning

If your components are located outside the default package structure, you can customize the component scanning using the @ComponentScan annotation.

package com.example.demo;

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

@SpringBootApplication
@ComponentScan(basePackages = {"com.example.demo.service", "com.example.demo.repository", "com.example.demo.controller"})
public class DemoApplication {

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

Explanation:

@ComponentScan(basePackages = {...}): Specifies the packages to scan for components.

Step 4: Running and Testing the Application

4.1 Run the Application

Run the Spring Boot application using your IDE or the command line:

./mvnw spring-boot:run

4.2 Test the Component Scanning

Open a web browser or use a tool like Postman to send a GET request to http://localhost:8080/test. You should see the message "Service is working" returned from the MyService component, indicating that component scanning and dependency injection are working correctly.

Additional Configurations

Excluding Specific Beans

You can exclude specific beans from being scanned and registered using the @ComponentScan.Filter annotation.

@SpringBootApplication
@ComponentScan(basePackages = {"com.example.demo"},
    excludeFilters = @ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, classes = MyRepository.class))
public class DemoApplication {

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

Explanation:

@ComponentScan.Filter(type = FilterType.ASSIGNABLE_TYPE, classes = MyRepository.class): Excludes the MyRepository class from component scanning.

Using `@Component` for Custom Components

You can create custom components using the @Component annotation.

package com.example.demo.custom;

import org.springframework.stereotype.Component;

@Component
public class MyCustomComponent {

    public String custom() {
        return "Custom component is working";
    }
}

Explanation:

@Component: Marks the class as a generic component, making it eligible for component scanning and automatic registration.

Conclusion

In this comprehensive guide, you have learned how to use component scanning in Spring Boot. We covered:

Setting up a Spring Boot project.
Creating different components (@Service, @Repository, @RestController).
Configuring default and customized component scanning.
Running and testing the application.

By following these steps, you can effectively use component scanning to simplify the configuration and development process in your Spring Boot applications.

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