Working with Beans in Spring Framework

A JavaBean is a standard Java class with getter/setter methods, a no-arg constructor, and is serializable. And a Spring Bean is any Java object managed by Spring and can be automatically injected into other components. The Spring Dependency injection allows Spring to manage and inject beans where needed.

Creating a bean

We can create beans with Spring Framework as follows:

Using `@Component` Annotation

import org.springframework.stereotype.Component;

@Component
public class UserService {
    public String getUserName() {
        return "john.doe";
    }
}

Using `@Bean` inside a `@Configuration` class

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Bean
    public UserService userService() {
        return new UserService();
    }
}

Using XML-based bean definition

`<bean id="userService" class="com.example.UserService"/>`

This approached was used before the annotations implementation.

Using a bean

Once a class is defined as a bean, it can be injected into other components/beans using the @Autowired annotation.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class Service {

    private final UserService userService;

    @Autowired
    public Service(UserService userService) {
        this.userService = userService;
    }

    public void printUserName() {
        System.out.println(userService.getUserName());
    }
}

Bean scopes

By default, all beans in Spring Framework have a Singleton scope (one instance per spring container), but we can change their scope.

The following are the scopes a bean can has:

Scope	Description
Singleton	Default. A single instance is created and shared.
Prototype	A new instance is created every time the bean is requested.
Request	A new instance is created per HTTP request (for web apps).
Session	A new instance is created per user session (for web apps).

The scope can be change with the @Scope annotation:

import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Component;

@Component
@Scope("prototype")
public class PrototypeBean {
    // Bean logic here
}