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Spring @Autowired tutorial

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>11</maven.compiler.source>
        <maven.compiler.target>11</maven.compiler.target>
        <spring-version>5.1.3.RELEASE</spring-version>

    </properties>

    <dependencies>

        <dependency>
            <groupId>ch.qos.logback</groupId>
            <artifactId>logback-classic</artifactId>
            <version>1.2.3</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>1.6.0</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 tutorial, we have injected dependencies in Spring with @Autowired.

You might also be interested in these related tutorials: Spring @Bean annotation tutorial, Spring @ComponentScan tutorial, Java tutorial, or list all Spring tutorials.