Spring Boot @Component

last modified May 20, 2022

Spring Boot @Component tutorial shows how to use the @Component annotation in a Spring application. In the example, we create a Spring Boot console application.

Spring is a popular Java/Kotlin application framework and Spring Boot is an evolution of Spring that helps create stand-alone, production-grade Spring based applications easily.

@Component

@Component is the most generic Spring annotation. A Java class decorated with @Component is found during classpath scanning and registered in the context as a Spring bean. @Service, @Repository, and @Controller are specializations of @Component, which are used for more specific cases.

@ComponentScan ensures that the classes decorated with @Component are found and registered as Spring beans. @ComponentScan is automatically included with @SpringBootApplication.

@Bean servers a similar purpose as @Component. It is not autodetected. Methods decorated with @Bean produce a bean to be managed by the Spring container during configuration stage.

Spring Boot @Component example

The following application demonstrates the usage of @Component. It uses the annotation to create a bean that randomly generates names.

build.gradle 
...
src
├── main
│   ├── java
│   │   └── com
│   │       └── zetcode
│   │           ├── Application.java
│   │           ├── MyRunner.java
│   │           └── service
│   │               ├── NowService.java
│   │               └── RandomNameService.java
│   └── resources
└── test
    ├── java
    └── resources

This is the project structure.

plugins {
    id 'org.springframework.boot' version '2.6.7'
    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
    id 'java'
}

group = 'com.zetcode'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '17'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter'
}

This is the Gradle build file. The spring-boot-starter is a core starter, including auto-configuration support, logging, and YAML.

spring.main.banner-mode=off
logging.level.org.springframework=ERROR
logging.pattern.console=%d{dd-MM-yyyy HH:mm:ss} %magenta([%thread]) %highlight(%-5level) %logger.%M - %msg%n

The application.properties is the main configuration file in Spring Boot. We turn off the Spring banner, reduce the amount of logging of the Spring framework by selecting only error messages, and set the console logging pattern.

package com.zetcode.service;

import org.springframework.stereotype.Service;

import java.util.List;
import java.util.Random;

@Service
public class RandomNameService {

    private List<String> names = List.of("Paul", "Peter", "Lucia", 
        "Martin", "Robert", "Svetlana");

    public String getName() {

        var rand = new Random();
        return names.get(rand.nextInt(names.size()));
    }
}

The RandomNameService is a Java class decorated with @Service, which is a special case of @Component. It will be detected during component scan process and registered as a Spring bean.

package com.zetcode.service;

import org.springframework.stereotype.Service;

import java.time.Instant;

@Service
public class NowService {

    public String now() {

        return Instant.now().toString();
    }
}

This is another service class.

package com.zetcode;

import com.zetcode.service.RandomNameService;
import com.zetcode.service.NowService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;

@Component
public class MyRunner implements CommandLineRunner {

    private static final Logger logger = LoggerFactory.getLogger(MyRunner.class);

    private final RandomNameService randomNameService;
    private final NowService nowService;

    public MyRunner(RandomNameService randomNameService, NowService nowService) {
        this.randomNameService = randomNameService;
        this.nowService = nowService;
    }

    @Override
    public void run(String... args) throws Exception {

            logger.info("Random name: {}", randomNameService.getName());
            logger.info("Now: {}", nowService.now());
    }
}

By implementing the CommandLineRunner interface, the run method of the MyRunner class will be executed after the application starts.

@Component
public class MyRunner implements CommandLineRunner {

MyRunner is decorated with @Component, so it will be autodetected and registered.

private final RandomNameService randomNameService;
private final NowService nowService;

public MyRunner(RandomNameService randomNameService, NowService nowService) {
    this.randomNameService = randomNameService;
    this.nowService = nowService;
}

We inject two service beans.

@Override
public void run(String... args) throws Exception {

        logger.info("Random name: {}", randomNameService.getName());
        logger.info("Now: {}", nowService.now());
}

In the run method, we call the service methods.

package com.zetcode;

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

@SpringBootApplication
public class Application  {

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

Application is the entry point which sets up Spring Boot application. The @SpringBootApplication annotation enables auto-configuration and component scanning. It is a convenience annotation for @Configuration, @EnableAutoConfiguration, and @ComponentScan annotations.

$ ./gradlew bootRun
... 
20-05-2022 13:54:18 [main] INFO  com.zetcode.MyRunner.run - Random name: Svetlana
20-05-2022 13:54:18 [main] INFO  com.zetcode.MyRunner.run - Now: 2022-05-20T11:54:18.594606276Z

After the application is run, we can see the log messages in the console.

In this tutorial, we have shown how to use @Component annotation in a Spring application.

List Spring Boot tutorials.