Spring @Autowired tutorial

last modified October 18, 2023

Spring @Autowired tutorial shows how to inject dependencies in a Spring application with @Autowired annotation.

Spring is a popular Java application framework for creating enterprise applications.

Spring @Autowired

@Autowired annotation marks a constructor, field, setter method or config method to be autowired by Spring's dependency injection facilities. It is an alternative to the JSR-330 @Inject annotation.

Spring @Autowired example

The application injects a dependency with @Autowired. The dependency is a service object that returns words.

pom.xml
src
├───main
│ ├───java
│ │ └───com
│ │ └───zetcode
│ │ │ Application.java
│ │ └───service
│ │ WordService.java
│ └───resources
│ logback.xml
└───test
 └───java

This is the project structure.

<?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
 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.zetcode</groupId>
 <artifactId>springautowired</artifactId>
 <version>1.0-SNAPSHOT</version>
 <properties>
 <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 <maven.compiler.source>17</maven.compiler.source>
 <maven.compiler.target>17</maven.compiler.target>
 <spring-version>5.3.23</spring-version>
 </properties>
 <dependencies>
 <dependency>
 <groupId>ch.qos.logback</groupId>
 <artifactId>logback-classic</artifactId>
 <version>1.4.0</version>
 </dependency>
 <dependency>
 <groupId>org.springframework</groupId>
 <artifactId>spring-context</artifactId>
 <version>${spring-version}</version>
 </dependency>
 <dependency>
 <groupId>org.springframework</groupId>
 <artifactId>spring-core</artifactId>
 <version>${spring-version}</version>
 </dependency> 
 
 </dependencies>
 <build>
 <plugins>
 <plugin>
 <groupId>org.codehaus.mojo</groupId>
 <artifactId>exec-maven-plugin</artifactId>
 <version>3.1.0</version>
 <configuration>
 <mainClass>com.zetcode.Application</mainClass>
 </configuration>
 </plugin>
 </plugins>
 </build>
</project>

In the pom.xml file, we have basic Spring dependencies spring-core, spring-context, and logging logback-classic dependency.

The exec-maven-plugin is used for executing Spring application from the Maven on the command line.

<?xml version="1.0" encoding="UTF-8"?>
<configuration>
 <logger name="org.springframework" level="ERROR"/>
 <logger name="com.zetcode" level="INFO"/>
 <appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender">
 <encoder>
 <Pattern>%d{HH:mm:ss.SSS} %blue(%-5level) %magenta(%logger{36}) - %msg %n
 </Pattern>
 </encoder>
 </appender>
 <root>
 <level value="INFO" />
 <appender-ref ref="consoleAppender" />
 </root>
</configuration>

The logback.xml is a configuration file for the Logback logging library.

package com.zetcode.service;
import org.springframework.stereotype.Service;
import java.util.List;
import java.util.Random;
@Service
public class WordService {
 private final List<String> words = List.of("pen", "sky",
 "rock", "forest", "falcon", "eagle");
 public List<String> all() {
 return words;
 }
 public String randomWord() {
 return words.get(new Random().nextInt(words.size()));
 }
}

WordService class is annotated with the @Service annotation. It is registered by Spring as a managed bean with the help of component scanning. This service object is later injected into the Application with @Autowired.

package com.zetcode;
import com.zetcode.service.WordService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.stereotype.Component;
@Component
@ComponentScan(basePackages="com.zetcode")
public class Application {
 private static final Logger logger = LoggerFactory.getLogger(Application.class);
 @Autowired
 private WordService wordService;
 public static void main(String[] args) {
 var ctx = new AnnotationConfigApplicationContext(Application.class);
 var bean = ctx.getBean(Application.class);
 bean.run();
 ctx.close();
 }
 public void run() {
 logger.info("{}", wordService.randomWord());
 logger.info("{}", wordService.randomWord());
 var words = wordService.all();
 words.stream().forEach(word -> logger.info("{}", word));
 }
}

The application is annotated with outputs words using the WordService. The service dependency is injected into the Application with @Autowired.

@Autowired
private WordService wordService;

This is called field injection.

Note: While field injection is short and sweet, in general, it is recommended to use constructor or setter injection.

$ mvn -q exec:java
17:15:34.504 INFO com.zetcode.Application - falcon
17:15:34.507 INFO com.zetcode.Application - eagle
17:15:34.508 INFO com.zetcode.Application - pen
17:15:34.508 INFO com.zetcode.Application - sky
17:15:34.509 INFO com.zetcode.Application - rock
17:15:34.509 INFO com.zetcode.Application - forest
17:15:34.510 INFO com.zetcode.Application - falcon
17:15:34.510 INFO com.zetcode.Application - eagle

We run the application.

In this article we have injected dependencies in Spring with @Autowired.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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