How to pool objects in Spring?

Question

I'm following this tutorial regarding how to pool objects in Spring. I've followed the instruction written on the tutorial but when I run my application, it always generates a new instance of the object. I'm expecting that since I'm pooling the objects, existing objects will be reuse. As such, no new instances should be created. Also, when I access of the getter method of the bean, a new instance of the bean is created again.

What could have I done wrong? Did I misunderstood the concept of pooling in Spring?

Below is my code:

Application Context: (This is just the body of my application context.)

<bean id="simpleBeanTarget" class="com.bean.SimpleBean" scope="prototype">

</bean>

<bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPoolTargetSource">
    <property name="targetBeanName" value="simpleBeanTarget" />
    <property name="maxSize" value="2" />
</bean>

<bean id="simpleBean" class="org.springframework.aop.framework.ProxyFactoryBean">
    <property name="targetSource" ref="poolTargetSource" />
</bean>

The controller: (This is just the body of my method)

@RequestMapping("/hello")
public ModelAndView helloWorld(HttpServletRequest request, HttpServletResponse response)
{
    String message = "Hello World, Spring 3.";
    try
    {
        System.out.println("Accessing Application Context");
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

        System.out.println("Getting Bean");
        SimpleBean simpleBean = (SimpleBean) context.getBean("simpleBean");
        //A new SimpleBean... is printed here.

        System.out.println("Displaying Hello World: " + simpleBean.getRandomNum());
        //After this line, A new SimpleBean... is printed again. I simply access the getter method. Why does it create a new instance of SimpleBean?

        return new ModelAndView("hello", "message", message);
    }catch(Exception e)
    {
        System.out.println("Error: " + e);
        e.printStackTrace();
        return new ModelAndView("hello", "message", "Error! " + e.getMessage());
    }
}

The bean I'm pooling:

package com.bean;

import java.util.Random;

public class SimpleBean
{
    int randomNum;
    String str;

    SimpleBean()
    {
        Random randomGenerator = new Random();
        randomNum = randomGenerator.nextInt(100);

        //I'm printing this line just to check if a instance of this bean is created.
        System.out.println("#####################A new SimpleBean was born: " + randomNum);

        str = "This is a string.";
    }

    public int getRandomNum()
    {
        return randomNum;
    }

    public void setRandomNum(int randomNum)
    {
        this.randomNum = randomNum;
    }

    public String getStr()
    {
        if (str == null)
            return "str is null";
        return str;
    }

    public void setStr(String str)
    {
        this.str = str;
    }
}

The body of my web.xml:

<display-name>Spring3MVC</display-name>

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

<welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
</welcome-file-list>

<servlet>
    <servlet-name>spring</servlet-name>
    <servlet-class>
        org.springframework.web.servlet.DispatcherServlet
    </servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>spring</servlet-name>
    <url-pattern>*.html</url-pattern>
</servlet-mapping>

Answer 1

On each request, you create a brand new Spring application context, then you get new object in a new applicatin context each action. so you should load your spring context use 'ContextLoaderListener' in web.xml.

Reference fragment in web.xml

<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>
        classpath*:spring/appContext.xml  classpath*:spring/appContext-security.xml
    </param-value>
</context-param>

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

See you code:

try
{
    System.out.println("Accessing Application Context");
    ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
   ...

For more knowledge about Spring context loading, see MKyong 's tutorial or Spring reference