Monday, December 28, 2015

Spring - bean scopes example

In Spring, bean scope is used to decide which type of bean instance should be return from Spring container back to the caller.

5 types of bean scopes supported :
  1. singleton – Return a single bean instance per Spring IoC container
  2. prototype – Return a new bean instance each time when requested
  3. request – Return a single bean instance per HTTP request. *
  4. session – Return a single bean instance per HTTP session. *
  5. globalSession – Return a single bean instance per global HTTP session. *

In most cases, you may only deal with the Spring’s core scope – singleton and prototype, and the default scope is singleton.

Singleton vs Prototype

Here’s an example to show you what’s the different between bean scope : singleton andprototype.
package com.mkyong.customer.services;
 
publicclassCustomerService
{
        String message;
        
        public String getMessage(){
               return message;
        }
 
        publicvoidsetMessage(String message){
               this.message = message;
        }
}
            

1. Singleton example

If no bean scope is specified in bean configuration file, default to singleton.
<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-2.5.xsd">
 
<bean id="customerService" 
            class="com.mkyong.customer.services.CustomerService" />
               
</beans>
Run it
package com.mkyong.common;
 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
 
import com.mkyong.customer.services.CustomerService;
 
publicclassApp
{
publicstaticvoidmain( String[] args )
{
        ApplicationContext context =
        newClassPathXmlApplicationContext(newString[]{"Spring-Customer.xml"});
 
        CustomerService custA =(CustomerService)context.getBean("customerService");
        custA.setMessage("Message by custA");
        System.out.println("Message : "+ custA.getMessage());
        
        //retrieve it again
        CustomerService custB =(CustomerService)context.getBean("customerService");
        System.out.println("Message : "+ custB.getMessage());
}
}
 
Output
Message : Message by custA
Message : Message by custA
Since the bean ‘customerService’ is in singleton scope, the second retrieval by ‘custB’ will display the message set by ‘custA’ also, even it’s retrieve by a new getBean() method. In singleton, only a single instance per Spring IoC container, no matter how many time you retrieve it with getBean(), it will always return the same instance.

2. Prototype example

If you want a new ‘customerService’ bean instance, every time you call it, use prototype instead.
<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-2.5.xsd">
 
<bean id="customerService" class="com.mkyong.customer.services.CustomerService" 
         scope="prototype"/>
               
</beans>
Run it again
Message : Message by custA
Message : null
In prototype scope, you will have a new instance for each getBean() method called.


Request scope vs Prototype scope


Prototype creates a brand new instance everytime you call getBean on the ApplicationContext. Whereas for Request, only one instance is created for an HttpRequest. So in a single HttpRequest, I can call getBean twice on Application and there will ever be one bean instantiated, whereas that same bean scoped to Prototype in that same single HttpRequest would get 2 different instances. 

HttpRequest scope 
Mark mark1 = context.getBean("mark"); 
Mark mark2 = context.getBean("mark"); 
mark1 == mark2; //This will return true 

Prototype scope 

Mark mark1 = context.getBean("mark"); 
Mark mark2 = context.getBean("mark"); 
mark1 == mark2; //This will return false

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