Spring @Component

In layman terms, a Component is responsible for some operations. Spring framework provides three other specific annotations to be used when marking a class as Component.

1. Service: Denotes that the class provides some services. Our utility classes can be marked as Service classes.
2. Repository: This annotation indicates that the class deals with CRUD operations, usually it’s used with DAO implementations that deal with database tables.
3. Controller: Mostly used with web applications or REST web services to specify that the class is a front controller and responsible to handle user request and return appropriate response.

Note that all these four annotations are in package org.springframework.stereotype and part of spring-context jar. Most of the time our component classes will fall under one of its three specialized annotations, so you may not use @Component annotation a lot.

Let’s create a very simple Spring maven application to showcase the use of Spring Component annotation and how Spring autodetects it with annotation-based configuration and classpath scanning. Create a maven project and add following spring core dependency.

<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-context</artifactId>
	<version>5.0.6.RELEASE</version>
</dependency>

That’s all we need to get the spring framework core features. Let’s create a simple component class and mark it with @Component annotation.

package com.journaldev.spring;

import org.springframework.stereotype.Component;

@Component
public class MathComponent {

	public int add(int x, int y) {
		return x + y;
	}
}

Now we can create an annotation based spring context and get the MathComponent bean from it.

package com.journaldev.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class SpringMainClass {

	public static void main(String[] args) {
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
		context.scan("com.journaldev.spring");
		context.refresh();

		MathComponent ms = context.getBean(MathComponent.class);

		int result = ms.add(1, 2);
		System.out.println("Addition of 1 and 2 = " + result);

		context.close();
	}

}

Just run the above class as normal java application and you should get following output in the console.

Jun 05, 2018 12:49:26 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@ff5b51f: startup date [Tue Jun 05 12:49:26 IST 2018]; root of context hierarchy
Addition of 1 and 2 = 3
Jun 05, 2018 12:49:26 PM org.springframework.context.support.AbstractApplicationContext doClose
INFO: Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@ff5b51f: startup date [Tue Jun 05 12:49:26 IST 2018]; root of context hierarchy

Did you realized the power of Spring, we didn’t have to do anything to inject our component to spring context. Below image shows the directory structure of our Spring Component example project. We can also specify the component name and then get it from spring context using the same name.

@Component("mc")
public class MathComponent {
}

MathComponent ms = (MathComponent) context.getBean("mc");

Although I have used @Component annotation with MathComponent, it’s actually a service class and we should use @Service annotation. The result will still be the same.

You can checkout the project from our GitHub Repository.

Reference: API Doc