Comprehensive Guide to Understand Spring Bean Scopes

• Singleton: Per Spring IoC container, only one instance of a single bean definition will be generated, and the same object will be shared for each request made for that bean. The default scope of a bean is singleton.
• Prototype: Each time a request is made for a single bean definition, a new instance will be generated.
• Request: Each time an HTTP request for a single bean definition is made, a new instance is generated for that bean. However, this is only usable when using a web-aware Spring ApplicationContext.
• Session: A single bean definition is scoped to an HTTP Session's lifecycle. However, this is only usable when using a web-aware Spring ApplicationContext.
• Global-Session: A single bean definition is scoped to an global HTTP Session's lifecycle. However, this is only usable when using a web-aware Spring ApplicationContext.

// JAVA CODE
package bean;
public class Sample{
    public int value;
  
    //A setter method to set the value
    public void setValue(int data)
    {
        this.value = data;
    }
  
    //A getter method to get the value
    public String getValue()
    {
        return value;
    }
}

<!-- XML CODE -->
<!DOCTYPE beans PUBLIC
     "-//SPRING//DTD BEAN 2.0//EN"
     "http://www.springframework.org/dtd/spring-beans-2.0.dtd">
<beans>
     < bean
         id = "id1"
         class= "bean.Sample"
         scope = "singleton" / >
</beans>

// JAVA CODE
package driver;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import bean.Sample;
  
public class User {
  
    public static void main(String[] args)
    {
        // Loading the XML configuration file
        ApplicationContext appContext = new ClassPathXmlApplicationContext( "resources/SampleXML.xml");
        Sample obj1 = (Sample)appContext.getBean("id1");
        obj1.setValue(10);
        System.out.println("The value for object obj1 is :" + obj1.getValue());
        Sample obj2 = (Sample)appContext.getBean("id1");
        System.out.println("The value for object obj2 is :" + obj2.getValue());
        if(obj1 == obj2)
        {
            System.out.println("Both obj1 and obj2 are pointing to the same object.");
        }
        else
        {
            System.out.println("obj1 and obj2 are different objects.");
        }
        System.out.println("The address of object obj1 is :" + obj1);
        System.out.println("The address of object obj2 is :" + obj2);

    }
}

// JAVA CODE
package bean;
public class Sample{
    public int value;
  
    //A setter method to set the value
    public void setValue(int data)
    {
        this.value = data;
    }
  
    //A getter method to get the value
    public String getValue()
    {
        return value;
    }
}

<!-- XML CODE -->
<!DOCTYPE beans PUBLIC
     "-//SPRING//DTD BEAN 2.0//EN"
     "http://www.springframework.org/dtd/spring-beans-2.0.dtd">
<beans>
     < bean
         id = "id1"
         class= "bean.Sample"
         scope = "prototype" / >
</beans>

// JAVA CODE
package driver;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import bean.Sample;
  
public class User {
  
    public static void main(String[] args)
    {
        // Loading the XML configuration file
        ApplicationContext appContext = new ClassPathXmlApplicationContext( "resources/SampleXML.xml");
        Sample obj1 = (Sample)appContext.getBean("id1");
        obj1.setValue(10);
        System.out.println("The value for object obj1 is :" + obj1.getValue());
        Sample obj2 = (Sample)appContext.getBean("id1");
        System.out.println("The value for object obj2 is :" + obj2.getValue());
        if(obj1 == obj2)
        {
            System.out.println("Both obj1 and obj2 are pointing to the same object.");
        }
        else
        {
            System.out.println("obj1 and obj2 are different objects.");
        }
        System.out.println("The address of object obj1 is :" + obj1);
        System.out.println("The address of object obj2 is :" + obj2);

    }
}

Interview Questions on Spring Bean:

• Through XML: In this technique, we must register custom init() and destroy() methods for a bean in the Spring XML configuration file when defining the bean.
• Using Programmatic Approach: To give the generated bean, the ability to call a custom init() method on the start of a spring container and a custom destroy() method on the close of the container, we must incorporate our bean with two interfaces, InitializingBean and DisposableBean, and override the afterPropertiesSet() and destroy() methods. The afterPropertiesSet() method is called when the container first starts and the bean is formed, while the destroy() method is called when the container is closed.
• Using Annotation: To give the generated bean the ability to call a custom init() method on a spring container's startup and to call a custom destroy() method on the container's closure, we need to annotate the init() method with the @PostConstruct annotation and the destroy() method with the @PreDestroy annotation.

• Only one object is generated for a single bean definition per Spring IoC container. Same object is shared for each request made for that bean.
• The scope of a bean is singleton by default. As a result, we don't have to specifically declare a bean as a singleton.
• For stateless beans, singleton scope is used.

• A new object is generated for a single bean definition each time a request is made for that bean. For every new request, a new object is generated.
• Since scope is not defaulted to prototype, you must specifically declare the scope of a being as prototype.
• For stateful beans, prototype scope is used.

• no: This is the default setting, so no autowiring is allowed. Wiring should be done with explicit bean references.
• byName: It injects the object dependency based on the bean's name. It matches and connects its properties to the beans in the XML file with the same names.
• byType: It injects object dependencies based on their type. If the form of a property matches exactly one of the beans’ names in the XML file, it is matched and wired.
• constructor: It injects the dependency by invoking the class's constructor. It has a vast range of configurations to choose from.
• autodetect: The container attempts to connect using autowire by constructor first, then autowire by byType if that fails.

• Properties like primitives, strings, and classes cannot be autowired.
• Since autowiring is less accurate, it is often desirable to use explicit wiring.
• Use of <constructor-arg> and <property> settings to specify dependencies will override autowiring.