Spring Boot: ComponentScan

In this post, I am going to explain

a.   What is component scan?

b.   How can we communicate component scan information to spring boot?

c.   Example Application using component scan

What is Component Scan?

Spring is a dependency injection framework, where objects define their dependencies using spring annotations. Suppose an object ‘obj1’ is using other objects obj2, obj3. In the source code, ‘obj1’ mention its dependencies obj2, obj3 through some annotations like @Bean constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method.

The basic step in spring dependency injection is, to add right annotations @Component or @Service or @Repository, and tell spring about the packages to be scanned to get these components.

package com.sample.model;

import org.springframework.stereotype.Component;

@Component

public class Employee {

}

For example, I added @Component annotation on class Employee, it indicates that Employee class is a "component". Such classes are considered as candidates for auto-detection, when using annotation-based configuration and class path scanning.

How can we communicate component scan information to spring boot?

By using '@ComponentScan' annotation, we can specify the packages to be scanned for components. Spring scan all the packages mentioned in @ComponentScan annotation and automatically create beans for every class annotated with @Component, @Service, @Controller, @RestController, @Repository.

Example

@ComponentScan({"com.sample.model", "com.sample.service"})

Example Application using component scan

Employee.java

package com.sample.model;

import org.springframework.stereotype.Component;

@Component
public class Employee {

}

EmployeeService.java

package com.sample.service;

import org.springframework.stereotype.Component;

@Component
public class EmployeeService {

}

Test.java

package com.sample.demo;

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

@SpringBootApplication
@ComponentScan({"com.sample.model", "com.sample.service"})
public class Test {

 public static void main(String args[]) {
  ApplicationContext applicationContext = SpringApplication.run(Test.class, args);

  for (String name : applicationContext.getBeanDefinitionNames()) {
   System.out.println(name);
  }
 }
}

Run the application Test.java, you can able to see the bean names, employee, employeeService in the output.

Note

If there is only one package to scan for components, you can specify like below.

@ComponentScan("com.sample.model")

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