How to make JVM scoped bean to be available for the Spring container in a Spring Boot application

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Bean Scopes refers to the lifecycle of Bean that means when the object of Bean will be instantiated, how long does that object live, and how many objects will be created for that bean throughout. Basically, it controls the instance creation of the bean and it is managed by the spring container.
Bean Scopes in Spring 
The spring framework provides five scopes for a bean. We can use three of them only in the context of web-aware Spring ApplicationContext and the rest of the two is available for both IoC container and Spring-MVC container. The following are the different scopes provided for a bean: 
 

1. Singleton: Only one instance will be created for a single bean definition per Spring IoC container and the same object will be shared for each request made for that bean.
2. Prototype: A new instance will be created for a single bean definition every time a request is made for that bean.
3. Request: A new instance will be created for a single bean definition every time an HTTP request is made for that bean. But Only valid in the context of a web-aware Spring ApplicationContext.
4. Session: Scopes a single bean definition to the lifecycle of an HTTP Session. But Only valid in the context of a web-aware Spring ApplicationContext.
5. Global-Session: Scopes a single bean definition to the lifecycle of a global HTTP Session. It is also only valid in the context of a web-aware Spring ApplicationContext.

Let’s us see some of them in detail:
 

Singleton Scope:

If the scope is a singleton, then only one instance of that bean will be instantiated per Spring IoC container and the same instance will be shared for each request. That is when the scope of a bean is declared singleton, then whenever a new request is made for that bean, spring IOC container first checks whether an instance of that bean is already created or not. If it is already created, then the IOC container returns the same instance otherwise it creates a new instance of that bean only at the first request. By default, the scope of a bean is a singleton. 
Let’s understand this scope with an example. 
 

• Step1: Lets first create a bean (i.e.), the backbone of the application in the spring framework. 

Java

package bean;

public class HelloWorld {

    public String name;

    public void setName(String name)

    {

        this.name = name;

    }

    public String getName()

    {

        return name;

    }

}

• Step 2: Now, we write a Spring XML configuration file “spring.xml” and configure the bean defined above. 

XML

• Step 3: Finally, write a driver class “Client.java” to request the above bean. 

Java

package driver;

import org.springframework

    .context.ApplicationContext;

import org.springframework

    .context.support

    .ClassPathXmlApplicationContext;

import bean.HelloWorld;

public class Client {

    public static void main(String[] args)

    {

        ApplicationContext

            ap

            = new ClassPathXmlApplicationContext(

                "resources/spring.xml");

        HelloWorld Geeks1

            = (HelloWorld)ap.getBean("hw");

        Geeks1.setName("Geeks1");

        System.out.println(

            "Hello object (hello1)"

            + " Your name is: "

            + Geeks1.getName());

        HelloWorld Geeks2

            = (HelloWorld)ap.getBean("hw");

        System.out.println(

            "Hello object (hello2)"

            + " Your name is: "

            + Geeks2.getName());

        System.out.println(

            "'Geeks1' and 'Geeks2'"

            + " are referring"

            + "to the same object: "

            + (Geeks1 == Geeks2));

        System.out.println(

            "Address of object Geeks1: "

            + Geeks1);

        System.out.println(

            "Address of object Geeks2: "

            + Geeks2);

    }

}

• Output: 

• Explanation: When we call the getName() method by using the reference of ‘Geeks1’ and ‘Geeks2’, then we are getting the same outputs. This means that both the reference is calling the getName() method of the same object. Furthermore, when we are comparing the reference ‘Geeks1’ and ‘Geeks2’ then output is “true” which means the same object is shared between ‘Geeks1’ and ‘Geeks2’. So it is clear that a new instance of bean (HelloWorld) is created when we made the request the first time and for each new request, the same object is being shared. 
   

Prototype Scope:

If the scope is declared prototype, then spring IOC container will create a new instance of that bean every time a request is made for that specific bean. A request can be made to the bean instance either programmatically using getBean() method or by XML for Dependency Injection of secondary type. Generally, we use the prototype scope for all beans that are stateful, while the singleton scope is used for the stateless beans.
Let’s understand this scope with an example: 
 

• Step 1: Let us first create a bean (i.e.), the backbone of the application in the spring framework.
   

Java

package bean;

public class HelloWorld {

    public String name;

    public void setName(String name)

    {

        this.name = name;

    }

    public String getName()

    {

        return name;

    }

}

• Step 2: Now, we write a Spring XML configuration file “spring.xml” and configure the bean defined above.
   

XML

• Step 3: Finally, write a driver class “Client.java” to request the above bean.
   

Java

package driver;

import org.springframework

    .context.ApplicationContext;

import org.springframework.context.support

    .ClassPathXmlApplicationContext;

import bean.HelloWorld;

public class Client {

    public static void main(String[] args)

    {

        ApplicationContext ap

            = new ClassPathXmlApplicationContext(

                "resources/spring.xml");

        HelloWorld Geeks1

            = (HelloWorld)ap.getBean("hw");

        Geeks1.setName("Geeks1");

        System.out.println(

            "Hello object (hello1)"

            + " Your name is: "

            + Geeks1.getName());

        HelloWorld Geeks2

            = (HelloWorld)ap.getBean("hw");

        System.out.println(

            "Hello object (hello2)"

            + "Your name is: "

            + Geeks2.getName());

        System.out.println(

            "'Geeks1' and 'Geeks2'"

            + "are referring "

            + "to the same object: "

            + (Geeks1 == Geeks2));

        System.out.println(

            "Address of object Geeks1: "

            + Geeks1);

        System.out.println(

            "Address of object Geeks2: "

            + Geeks2);

    }

}

• Output: 

• Explanation: When we call getName() method by using the reference ‘Geeks1’ and ‘Geeks2’, then we get different outputs that means both the reference is calling getName() method of a different object. Furthermore, when we are comparing the reference ‘Geeks1’ and ‘Geeks2’ then output is “false” which means both references is referring to a different object. So it is clear that a new instance of bean (HelloWorld) is being created at each request made for this bean.
  Difference between Singleton and Prototype