What is a Bean ?

A bean is nothing but an object that is instantiated and managed by a Spring IoC(Inversion of Control) container.

Scope of a Bean

Scope refers to the instance creation of that bean. i.e., number of object creation, duration.

1. Singleton: One instance per Spring Context. This is default scope of every bean.

2. Prototype: New bean whenever requested.

   @Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
   

3. request: One bean per HTTP request

4. session: One bean per HTTP session

5. global

Example of Scopes

In this example we create one bean with singleton scope while other with prototype. Then we request 2 beans from each class, and we can see that singleton doesn't create a new instance while prototype one does.

singletonclass.java

@Component
public class singletonclass {
    public void method() {
        System.out.println(this);
    }
}

prototypeclass.java

@Component
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
public class prototypeclass {
    public void method() {
        System.out.println(this);
    }
}

ScopeExampleApplication.java

@SpringBootApplication
public class ScopeExampleApplication {

    public static void main(String[] args) {
    ApplicationContext apx=SpringApplication.run(ScopeExampleApplication.class, args);
    singletonclass s1 = apx.getBean(singletonclass.class);
    singletonclass s2 = apx.getBean(singletonclass.class);
    s1.method();
    s2.method();

    prototypeclass p1 = apx.getBean(prototypeclass.class);
    prototypeclass p2 = apx.getBean(prototypeclass.class);
    p1.method();
    p2.method();

    }
}

OUTPUT

Scope_Example.singletonclass@2fb69ff6 // returns same hashcode
Scope_Example.singletonclass@2fb69ff6
Scope_Example.prototypeclass@6a078481 // returns different hashcode
Scope_Example.prototypeclass@79c5636f

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Lifecycle of a Bean

First thing what happens when we run the program is, that spring container gets started. Then container creates the instance of a bean as per the request and after that dependencies are injected. Beans are destroyed when the spring container is closed.

We are provided with @PostConstruct and @PreDestroy annotations to run just after bean creation or just before bean deletion.

Example

random.java

@Component
public class random {
    public void method() {
        System.out.println("Method of random class");
    }

    @PostConstruct
    public void before() {
        System.out.println("PostConstruct - This runs just after bean is created");
    }

    @PreDestroy
    public void after() {
        System.out.println("PreDestroy - Runs just before being destroyed");
    }
}

LifecycleExampleApplication.java

@SpringBootApplication
public class LifecycleExampleApplication {

    public static void main(String[] args) {
    ApplicationContext apx=SpringApplication.run(LifecycleExampleApplication.class, args);
    random r = apx.getBean(random.class);
    r.method();
    }
}

OUTPUT

PostConstruct - This runs just after bean is created
Method of random class
PreDestroy - Runs just before being destroyed

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Code link
References - Spring Master Class
Article by - Arjit Sharma