An Interview Question on Spring Singleton

Spring Singleton

While taking interview on Spring core, Often I ask a question

What do you mean by Spring Singleton scope?

Most of the time I got an answer like Spring singleton scope manages only one object in the container.

Then after getting this answer I will ask the next question,

Please tell me what will be the output of the following program

Spring.xml file

<?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="scopeTest" class="com.example.scope.Scope" scope="singleton">

    <property name="name" value="Shamik Mitra"/>    

    </bean>    

    <bean id="scopeTestDuplicate" class="com.example.scope.Scope" scope="singleton">

          <property name="name" value="Samir Mitra"/>    

    </bean>

</beans>

Scope.java

package com.example.scope;

public class Scope {

   private String name;

   public String getName() {

      return name;

   }

   public void setName(String name) {

      this.name = name;

   }

   @Override

   public String toString() {

      return "Scope [name=" + name + "]";

   }

}

Main class

package com.example.scope;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {

   public static void main(String[] args) {

      ApplicationContext ctx = new ClassPathXmlApplicationContext(

              "configFiles/Scope.xml");

      Scope scope = (Scope) ctx.getBean("scopeTest");

      Scope scopeDuplicate = (Scope) ctx.getBean("scopeTestDuplicate");      

      System.out.println(scope==scopeDuplicate);

      System.out.println(scope  + "::"+ scopeDuplicate);

     

   }

}

Here I create two beans of Scope class and make spring scope as singleton now checking the references.

Now Interviewee got confused, and I will get three types of answer

This code will not compile throw error at runtime, as you can not define two spring beans of the same class with scope singleton in XML. (Very rare).

References check will return true, as container maintains one object so both bean definition will return the same object so memory location would be same.(Often)

They said References check will return false and Spring singleton is not worked as they have told earlier.(few)

The third answer is the correct answer, Spring singleton is not worked as Java Singleton.

If we see the output of the program we will understand that it will return two different instances, So in a  container, there may be more than one object in spite of the scope is the singleton.

Output:

Reference Check ::false

Scope [name=Shamik Mitra]::Scope [name=Samir Mitra]

So again Question is What do you mean by Spring Singleton Scope?

According to Spring documentation

When a bean is a singleton, only one shared instance of the bean will be managed, and all requests for beans with an id or ids matching that bean definition will result in that one specific bean instance being returned by the Spring container.

To put it another way, when you define a bean definition and it is scoped as a singleton, then the Spring IoC container will create exactly one instance of the object defined by that bean definition. This single instance will be stored in a cache of such singleton beans, and all subsequent requests and references for that named bean will result in the cached object being returned.

So it is clear that for a given id Spring container maintains only one shared instance in singleton cache.

In my example, I use two different ids (scopeTest, ScopeTestDuplicate) so Spring container creates two instances of the same class and bound it with respective ids and stores it in singleton cache.

You can think Spring Container manage Key-value pair where Key is the id or name of the bean and value is bean itself , so for a given key, it maintains singleton. So if we use that key as a reference of other beans the same bean will be injected to other beans.

In a one words , Spring guarantees exactly one shared bean instance for the given Id per IoC container unlike Java Singleton where Singleton hard codes the scope of an object such that one and only one instance of a particular class will ever be created per ClassLoader.

Picture taken from Spring docs