In this tutorial we will discuss about ApplicationContext instantiation for web applications and how to access it. There are numerous different ways to get a task done in Spring framework and that is one advantage of it.
In web application, ApplicationContext is created using ContextLoaderListener and ContextLoaderServlet.
ContextLoaderListener : It is listener implementation that is added to web.xml file.
ContextLoaderServlet : It is servlet implementation that is configured with load-on-startup tag in web.xml.


Here is the complete source code example demonstrating ApplicationContext instantiation for web applications.

InputService.java

import java.util.Random;
import java.util.Scanner;
public class InputService {
    public int getIntValue() {
        Random r=new Random();
        int val=r.nextInt();
        System.out.println("Returning value = "+val);
        return val;
    }
}

OutputService.java

public class OutputService{
    public int write(int x) {
        System.out.println("Output is "+x);
        return x;
    }
}

CalServlet .java

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.context.ApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;

public class CalServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        ApplicationContext ac = WebApplicationContextUtils.getRequiredWebApplicationContext(getServletContext());
        CalcMachine cal = (CalcMachine) ac.getBean("cal");
        response.getWriter().print("Adding of numbers is = " + cal.doAdd());
        response.getWriter().close();
    }
}

services.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

    <bean id="in" class="InputService"/>
    <bean id="out" class="OutputService"/>
    
    <bean id="cal" class="CalcMachine">
        <property name="inputService">
            <ref bean="in" />
        </property>
        <property name="outputService">
            <ref bean="out" />
        </property>
    </bean>
</beans>

 web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
    id="WebApp_ID" version="2.5">
    <display-name>DI2_web</display-name>

    <servlet>
        <description></description>
        <display-name>CalServlet</display-name>
        <servlet-name>CalServlet</servlet-name>
        <servlet-class>CalServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>CalServlet</servlet-name>
        <url-pattern>/CalServlet</url-pattern>
    </servlet-mapping>

    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/services.xml</param-value>
    </context-param>

     <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener> 
   
</web-app>