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.
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