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Spring @ComponentScan tutorial

Spring @ComponentScan tutorial shows how to enable component scanning in a Spring application. Component scanning enables auto-detection of beans by Spring container.

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

Spring @ComponentScan

@ComponentScan annotation enables component scanning in Spring. Java classes that are decorated with stereotypes such as @Component, @Configuration, @Service are auto-detected by Spring. With @ComponentScan's basePackages attribute specifies which packages should be scanned for decorated beans.

The @ComponentScan annotation is an alternative to <context:component-scan> XML tag.

Spring @ComponentScan example

The application enables component scanning with @ComponentScan. We have one service bean that returns the current time.

pom.xml
src
├───main
│   ├───java
│   │   └───com
│   │       └───zetcode
│   │           │   Application.java
│   │           └───service
│   │                   TimeService.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>componentscan</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/TimeService.java
package com.zetcode.service;

import org.springframework.stereotype.Service;

import java.time.LocalTime;

@Service
public class TimeService {

    public LocalTime getTime() {

        var now = LocalTime.now();

        return now;
    }
}

The TimeService class is annotated with the @Service annotation. It is registered by Spring as a managed bean with the help of component scanning.

com/zetcode/Application.java
package com.zetcode;

import com.zetcode.service.TimeService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@ComponentScan(basePackages = "com.zetcode")
@Configuration
public class Application {

    private static final Logger logger = LoggerFactory.getLogger(Application.class);

    public static void main(String[] args) {

        var ctx = new AnnotationConfigApplicationContext(Application.class);

        var timeService = (TimeService) ctx.getBean("timeService");
        logger.info("The time is {}", timeService.getTime());

        ctx.close();
    }
}

The application is annotated with @ComponentScan. The basePackages option tells Spring to look for components in the com/zetcode package and its subpackages.

var ctx = new AnnotationConfigApplicationContext(Application.class);

AnnotationConfigApplicationContext is a Spring standalone application context. It accepts the annotated Application as an input; thus the scanning is enabled.

var timeService = (TimeService) ctx.getBean("timeService");
logger.info("The time is {}", timeService.getTime());

We get the registered service bean and call its method.

$ mvn -q exec:java
10:57:01.912 INFO  com.zetcode.Application - The time is 10:57:01.912235800

We run the application.

In this tutorial, we have enabled component scanning with @ComponentScan.

You might also be interested in these related tutorials: Spring BeanDefinitionBuilder tutorial, Spring AnnotationConfigApplicationContext, Spring Singleton scope bean, Spring @Bean annotation tutorial, Spring @Configuration annotation tutorial, Spring inject List XML tutorial, Spring BeanDefinitionBuilder tutorial, Spring HikariCP tutorial, and Java tutorial.