ComponentScan

@ComponentScan

If you understand component scan, you understand Spring.

Spring is a dependency injection framework. It is all about beans and wiring in dependencies.

The first step of defining Spring Beans is by adding the right annotation - @Component or @Service or @Repository.

However, Spring does not know about the bean unless it knows where to search for it.

This part of “telling Spring where to search” is called a Component Scan.

You define the packages that have to be scanned.

Once you define a Component Scan for a package, Spring would search the package and all its sub packages for components/beans.

Defining a Component Scan

• If you are using Spring Boot, check configuration in Approach 1.
• If you are doing a JSP/Servlet or a Spring MVC application without using Spring Boot use Approach 2.

Approach 1 : Component Scan in a Spring Boot Project

Executive Summary

• If your other packages hierarchies are below your main app with the @SpringBootApplication annotation, you’re covered by implicit components scan.
• If there are beans/components in other packages which are not sub packages of the main package, you should manually add them as @ComponentScan

Approach 2: Non Spring Boot Project

In a non Spring Boot Project, we would typically define the component scan explicitly in an XML application context or a Java Application Context.

Reference : http://www.springboottutorial.com/spring-boot-and-component-scan