Is SpringBootApplication annotation trigger component scan?

In this post, you are going to learn

a.   What is component scan?

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

c.   Is SpringBootApplication annotation trigger component scan?

d.   Example Application using SpringBootApplication annotation

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 on what 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 an annotated 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"})

Is SpringBootApplication annotation trigger component scan?

Yes, SpringBootApplication trigger component scan. When you see the definition of SpringBootApplication annotation, it looks like below.

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@SpringBootConfiguration

@EnableAutoConfiguration

@ComponentScan(excludeFilters = {

                  @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

                  @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

public @interface SpringBootApplication {

}

As you observe, SpringBootApplication annotation is annotated with ComponentScan annotation.

If the package hierarchies are below your main application class (@SpringBootApplication annotated class), then you’re covered by implicit components scan.

For example,

package com.sample;

@SpringBootApplication

public class Test {

}

‘Test’ class is annotated with ‘SpringBootApplication’ annotation and it is defined under the package com.sample. So Spring scan all the components under the package com.sample, and in all the sub packages (com.sample.*).

Example Application using SpringBootApplication annotation

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;

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

@SpringBootApplication
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.

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