Spring @Autowired Annotation Example

In this section we will learn about @Autowired Annotation. 

We can use the @Autowired to mark a dependency which Spring is going to resolve and inject.

If @Autowired is applied to

• a field: then the dependency is stored in this field.
• a setter: then the setter is invoked, with the parameter that is determined by the same algorithm like for the field dependency injection.
• a constructor: then the constructor is invoked with the parameters determined by the same algorithm like for the field dependency injection.  

Constructor Injection

@RestController
public class UserController {
    private UserService userService;
 
    @Autowired
    public UserController(UserService userService) {
        this.userService = userService;
    }
}

Setter Injection

@RestController
public class UserController {
    private UserService userService;
 
    @Autowired
    public void setUserService(UserService userService) {
        this.userService = userService;
    }
}

Field Injection

@RestController
public class UserController {
    @Autowired
    private UserService userService;
}

The following example creates a Spring Boot web application which uses @Autowired annotation.

Project Directory

Pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
   https://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-parent</artifactId>
      <version>3.0.1</version>
      <relativePath/> <!-- lookup parent from repository -->
   </parent>
   <groupId>com.knf.dev.demo</groupId>
   <artifactId>spring-autowired-example</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <name>spring-autowired-example</name>
   <description>Demo project for Spring Boot</description>
   <properties>
      <java.version>17</java.version>
   </properties>
   <dependencies>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-web</artifactId>
      </dependency>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-test</artifactId>
         <scope>test</scope>
      </dependency>
   </dependencies>

   <build>
      <plugins>
         <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
         </plugin>
      </plugins>
   </build>

</project>

UserService.java

package com.knf.dev.demo.service;

import org.springframework.stereotype.Component;

@Component
public class UserService {

    public String getUser()
    {
        return "I am the User";
    }
}

UserController.java

package com.knf.dev.demo.controller;

import com.knf.dev.demo.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/users")
public class UserController {

    @Autowired
    UserService userService;

    @GetMapping
    public String getUser()
    {
        return userService.getUser();
    }
}

Spring Boot Main - Application.java

package com.knf.dev.demo;

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

@SpringBootApplication
public class Application {

   public static void main(String[] args) {
      SpringApplication.run(Application.class, args);
   }

}

Run Application - Application.java

Application is the entry point that sets up the Spring Boot application. The @SpringBootApplication annotation enables auto-configuration and component scanning.

Let's run this Spring boot application from either IntelliJ IDEA IDE by right click - Run 'Application.main()'

Or you can use the below maven command to run:

mvn spring-boot:run

Try to send a GET request to the http://localhost:8080/users endpoint using Postman.

Download Source Code

$ git clone https://github.com/knowledgefactory4u/spring-annotations

More related topics,

• Spring @RequestHeader Annotation Example 

• Spring @RestController Annotation Example

• Spring @RequestParam Annotation Example

• Spring @PathVariable Annotation Example

• Spring @ResponseBody Annotation Example

• Spring @RequestBody Annotation Example

• Spring @ControllerAdvice Annotation Example