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.

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 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.

resources/logback.xml

<?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.

com/zetcode/service/WordService.java

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.

com/zetcode/Application.java

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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