Spring @Component Annotation Example

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In this quick article, we will discuss how to use @Component annotation in Spring (Spring Boot) based applications.

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Spring @Component Annotation Overview

The @Component annotation indicates that an annotated class is a "component". Such classes are considered candidates for auto-detection when using annotation-based configuration and classpath scanning.
 
In short, @Component is a class-level annotation. During the component scan, Spring Framework automatically detects classes annotated with @Component annotation and creates Spring beans for those classes.

Component Scanning

Spring can automatically scan a package for beans if component scanning is enabled.

 @ComponentScan configures which packages to scan for classes with annotation configuration. We can specify the base package names directly with one of the basePackages or value arguments (value is an alias for basePackages):
 
@Configuration
@ComponentScan(basePackages = "com.javaguides.annotations")
class UserConfig {}

Spring @Component Annotation Example

Let’s create a very simple Spring boot maven application to showcase the use of Spring @Component annotation and how Spring autodetects it with annotation-based configuration and classpath scanning.
 
Add below Spring boot starter web dependency:
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

Create Spring Component class - ComponentDemo.java

Let’s create a simple component class and mark it with @Component annotation.
@Component
class ComponentDemo{
    public String getValue() {
        return "Hello World";
    } 
}
Spring Container will automatically create and manage the spring bean for the above class because it is annotated with @Component annotation.

Running Spring Boot Application

Note that we have created ApplicationContext and retrieved ComponentDemo bean using getBean() Method.
package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.stereotype.Component;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);
        ComponentDemo componentDemo = (ComponentDemo) applicationContext.getBean("componentDemo");
        System.out.println(componentDemo.getValue());
    }
}


@Component
class ComponentDemo {
    public String getValue() {
        return "Hello World";
    }
}

Output:


By default, the bean instances of this class have the same name as the class name with a lowercase initial. On top of that, we can specify a different name using the optional value argument of this annotation. 
@Component("componentDemo")
class ComponentDemo {
    public String getValue() {
        return "Hello World";
    }
}

Related Spring and Spring Boot Annotations


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