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>

Firearm

Did you check for more than 2 requests? – Arrival

Yes. I tried to issue more than 2 requests even the maxSize of my pool is only 2. All of the requests I made created a new instance of SimpleBean which is not what I expected. I'm expecting that previous instances of SimpleBean will be reuse. – Firearm

I just reviewed the spring source code of bean context. I don't find out the code where ClassPathXmlApplicationContext cache the singleton. but I know this class SingletonBeanFactoryLocator do cache application context by factoryKey, i wrap and use it often in my system. – Erosion

You could print out the hashcode of your application context in your controller method to check if it is singleton. I bet it will print out different hashcode each call. – Erosion

@HenryLeu: Thanks! Yup, I checked the hashcode of my application context and it indeed return a different hashcode every request. How do I use the ContextLoaderListener? Sorry, I'm also currently researching about it. Maybe you could point me to a quick tutorial on how to use it? – Firearm

See MKyong 's tutorial or Spring reference – Erosion

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

Erosion

Hi, Henry! Thanks for pointing that out! I will check this. I thought that ClassPathXmlApplicationContext will only give me a new reference to the ApplicationContext not a new instance of ApplicationContext. – Firearm

Answer 2

When you mention it means you are saying to service provider of urs pooling that you want to create maximum of two objects with object state provided via urs constructor.

  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.";

    }

so SimpleBean simpleBean = (SimpleBean) context.getBean("simpleBean"); is a pooled object. iF two execution path want to get service from simpleBean then that object instance is provided via Pooling.If more thean two execution path then it is provided via prototype scope.

Layby

Hi. Thanks for your reply! Yes, I understand that new instance of the bean will be created if I exceeded the size of my pool. However, I'm only making a request one at a time. So it is impossible for me to exceed the number of my pool. – Firearm

Use thread pooling for urs service if you want to exceed urs pool size. – Layby

give scope='request' if you are operating on http test.If non http then give scope='thread' if you want to create more than one execution path. – Layby

Answer 3

Try this will work...

<property name="targetSource" ref="poolTargetSource" />

<!-- Added so that different instance of object is created -->
<property name="singleton" value="false" />

Camelopardus