How to lazy initialize Spring beans?

The way of loading Spring Beans is one of the most important difference between BeanFactory and  ApplicationContext. The BeanFactory by default lazy loads the beans, it creates the bean only when the getBean() method is called. whereas ApplicationContext preloads all the singleton beans upon start-up.

We have two simple beans (Bean1 & Bean2), each having a no argument constructor.
Bean1.java

package com.javainterviewpoint;

public class Bean1 
{
    public Bean1()
    {
        System.out.println("Creating bean bean1");
    }
}

Bean2.java

package com.javainterviewpoint;

public class Bean2 
{
    public Bean2()
    {
        System.out.println("Creating bean bean2");
    }
}

SpringConfig.xml

<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="bean1" class="com.javainterviewpoint.Bean1"></bean>
     <bean id="bean2" class="com.javainterviewpoint.Bean2"></bean>
</beans>

we have defined our two beans in the SpringConfig.xml

ClientLogic.java

With BeanFactory

package com.javainterviewpoint;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.ClassPathResource;
import org.springframework.core.io.Resource;

public class ClientLogic 
{
    public static void main(String args[])
    {
        //Read the configuration file
        Resource resource = new ClassPathResource("SpringConfig.xml");
        //Read all the bean definition
        BeanFactory bf = new XmlBeanFactory(resource);
    }
}

Here we have read our configurations with a BeanFactory, upon running. When we look into the console, we could see that nothing happens just loading up of xml bean definition.

Apr 16, 2015 4:58:28 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [SpringConfig.xml]

Only when we add the below code, the beans will be called.

public class ClientLogic 
{
    public static void main(String args[])
    {
        //Read the configuration file
        Resource resource = new ClassPathResource("SpringConfig.xml");
        //Read all the bean definition
        BeanFactory bf = new XmlBeanFactory(resource);
        //Get the bean1 instance
        Bean1 bean1=(Bean1)bf.getBean("bean1");
        //Get the bean2 instance
        Bean2 bean2=(Bean2)bf.getBean("bean2");
    }
}

At console it will be

Apr 16, 2015 4:58:28 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [SpringConfig.xml]
Creating bean bean1
Creating bean bean2

With ApplicationContext

package com.javainterviewpoint;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class ClientLogic 
{
    public static void main(String args[])
    {
        ApplicationContext appContext = new ClassPathXmlApplicationContext("SpringConfig.xml");    
    }
}

Upon running itself bean will be getting loaded. Even no need to call.

Apr 16, 2015 5:08:36 PM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@3a1ec6: startup date [Thu Apr 16 17:08:36 IST 2015]; root of context hierarchy
Apr 16, 2015 5:08:36 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [SpringConfig.xml]
Apr 16, 2015 5:08:37 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@10e6817: defining beans [bean1,bean2]; root of factory hierarchy
Creating bean bean1
Creating bean bean2

In order to lazy initialize the bean loading we will use “lazy-init” attribute set to true, so that bean will get loaded only when called.

 <bean lazy-init="true" id="bean1" class="com.javainterviewpoint.Bean1"></bean>
 <bean lazy-init="true" id="bean2" class="com.javainterviewpoint.Bean2"></bean>

Once we run our Client Logic class beans will not get loaded unless and until called explicitly, as you can see the constructor is not called.

Apr 16, 2015 5:16:25 PM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@3a1ec6: startup date [Thu Apr 16 17:16:25 IST 2015]; root of context hierarchy
Apr 16, 2015 5:16:25 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [SpringConfig.xml]
Apr 16, 2015 5:16:25 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1e01885: defining beans [bean1,bean2,bean3]; root of factory hierarchy

Note : One more point to be noted is that ApplicationContext will pre-load Singleton scoped beans only.