Java Spring Boot - SpringBootApplication, Component , Component and Autowired

 src/ 

└── main/ 

     └── java/ 

         └── com/ 

             └── example/ 

├── MyApp.java              # Main class with @SpringBootApplication 

├── GreetingService.java    # @Component class (business logic) 

└── GreetingController.java # @Component with @Autowired field 

 

✅ 1. MyApp.java — Main Class 

package com.example; 
 
import org.springframework.boot.SpringApplication; 
import org.springframework.boot.autoconfigure.SpringBootApplication; 
 
@SpringBootApplication 
public class MyApp { 
   public static void main(String[] args) { 
       SpringApplication.run(MyApp.class, args); 
   } 
} 
 

 

✅ 2. GreetingService.java — Component (Bean) 

package com.example; 
 
import org.springframework.stereotype.Component; 
 
@Component 
public class GreetingService { 
 
   public String getMessage() { 
       return "Hello testuser"; 
   } 
} 
 

 

✅ 3. GreetingController.java — REST API Controller 

package com.example; 
 
import org.springframework.beans.factory.annotation.Autowired; 
import org.springframework.web.bind.annotation.GetMapping; 
import org.springframework.web.bind.annotation.RestController; 
 
@RestController 
public class GreetingController { 
 
   @Autowired 
   private GreetingService greetingService;  // Injected automatically 
 
   @GetMapping("/greet") 
   public String greet() { 
       return greetingService.getMessage();  // Returns "Hello testuser" 
   } 
} 
 

 

▶️ How to Run It 

1. Build the project 

mvn clean install 
 

2. Run the Spring Boot app 

mvn spring-boot:run 
 

3. Test the endpoint 

Open in browser or use curl: 

http://localhost:8080/greet 
 

Expected Output: 

Hello testuser