JAVA EE: Spring Auto Scanning Components | Spring Component Scan

Wednesday, 29 August 2018

Spring Auto Scanning Components | Spring Component Scan | Spring Tutorial

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Overview: Automatic Component Scanning in Spring

In traditional Spring applications, every bean had to be explicitly declared in XML configuration files using <bean> tags. As projects expand, managing extensive XML configurations becomes cumbersome. Auto Component Scanning solves this by allowing the Spring Framework to automatically scan designated packages, identify annotated classes, and register them directly as beans in the ApplicationContext.

In this guide, you will learn the fundamentals of Spring component scanning, how it discovers beans, and how to configure it to simplify your application setup.

How Spring Component Scan Works

Component scanning streamlines bean management through automated detection:

Stereotype Detection: Spring inspects the classpath for classes marked with stereotype annotations such as @Component, @Repository, @Service, and @Controller.
Base Package Traversal: By specifying a base package, Spring recursively scans that package and all of its sub-packages.
Automated Lifecycle & Injection: Discovered components are instantiated, wired, and registered into the container without manual XML declarations.
Combines Annotation Configurations: Enabling component scanning automatically activates annotation-based configuration features (such as @Autowired).

Step-by-Step Configuration

Annotate Java Classes: Mark your target classes with @Component or its specialized stereotypes:
@Component public class CustomerService { ... }
Configure Component Scanning: In your Spring XML configuration file, include the context namespace and declare the base package:
<context:component-scan base-package="com.ramnjava" />
Load Application Context: When starting the container, Spring scans the packages, registers the components, and makes them available for injection or lookup.

Declares Components Manually

pom.xml

<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%20http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.ram.core</groupId>

    <artifactId>SpringDemo</artifactId>

    <packaging>jar</packaging>

    <version>1.0-SNAPSHOT</version>

    <name>SpringDemo</name>

    <url>http://maven.apache.org</url>

    <properties>

        <spring.version>5.0.2.RELEASE</spring.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>junit</groupId>

            <artifactId>junit</artifactId>

            <version>3.8.1</version>

            <scope>test</scope>

        </dependency>

        <!-- Spring 5 dependencies -->

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-core</artifactId>

            <version>${spring.version}</version>

        </dependency>

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-context</artifactId>

            <version>${spring.version}</version>

        </dependency>

    </dependencies>

</project>

EmployeeDAO.java

package com.ram.dao;

public class EmployeeDAO

{

    public void addEmployee()

    {

        System.out.println("inside EmployeeDAO addEmployee()");

    }

}

EmployeeService.java

package com.ram.service;

import com.ram.dao.EmployeeDAO;

public class EmployeeService

{

    EmployeeDAO employeeDAO;

    public void setEmployeeDAO(EmployeeDAO employeeDAO)

    {

        this.employeeDAO = employeeDAO;

    }

    public void addEmployee()

    {

        System.out.println("inside EmployeeService addEmployee()");

        employeeDAO.addEmployee();

    }

    @Override

    public String toString()

    {

        return "EmployeeService [employeeDAO=" + employeeDAO + "]";

    }

}

applicationContext.xml

<beans xmlns="http://www.springframework.org/schema/beans"

    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 

    xmlns:context="http://www.springframework.org/schema/context"

    xsi:schemaLocation="http://www.springframework.org/schema/beans

    http://www.springframework.org/schema/beans/spring-beans-4.3.xsd

    http://www.springframework.org/schema/context

    http://www.springframework.org/schema/context/spring-context-4.3.xsd">

    <bean id="employeeService" class="com.ram.service.EmployeeService">

        <property name="employeeDAO" ref="employeeDAO" />

    </bean>

    <bean id="employeeDAO" class="com.ram.dao.EmployeeDAO" />

</beans>

App.java

package com.ram.core;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.ram.service.EmployeeService;

public class App

{

    public static void main(String[] args)

    {

        ApplicationContext context = new ClassPathXmlApplicationContext(

                "applicationContext.xml");

        System.out.println("---------------------------------------");

        EmployeeService employeeService = context.getBean("employeeService",

                EmployeeService.class);

        System.out.println(

                "Got employeeService object from the ApplicationContext(Spring Container)");

        System.out.println(employeeService);

        employeeService.addEmployee();

    }

}

Output

Aug 14, 2018 9:36:02 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Tue Aug 14 09:36:02 IST 2018]; root of context hierarchy

Aug 14, 2018 9:36:02 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [applicationContext.xml]

---------------------------------------

Got employeeService object from the ApplicationContext(Spring Container)

EmployeeService [employeeDAO=com.ram.dao.EmployeeDAO@1794d431]

inside EmployeeService addEmployee()

inside EmployeeDAO addEmployee()

Auto Components Scanning