Spring @Bean Annotation with Example

Author:
In this article, we will discuss Spring Java configuration-based @Bean annotation with examples. We will also discuss different scenarios of how and when to use @Bean annotation.

@Bean Annotation Overview

@Bean annotation indicates that a method produces a bean to be managed by the Spring container. The @Bean annotation is usually declared in the Configuration class to create Spring Bean definitions.

The below diagram shows an internal implementation of @Bean Annotation:

Declaring a Bean and Example

To declare a bean, simply annotate a method with the @Bean annotation. You use this method to register a bean definition within an ApplicationContext of the type specified as the method’s return value.
 
By default, the bean name will be the same as the method name.
 
The following is a simple example of a @Bean method declaration: 
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import com.companyname.projectname.customer.CustomerService;
import com.companyname.projectname.order.OrderService;

@Configuration
public class Application {

    @Bean
    public CustomerService customerService() {
        return new CustomerService();
    }
 
    @Bean
    public OrderService orderService() {
       return new OrderService();
    }
}
The preceding configuration is exactly equivalent to the following Spring XML: 
<beans>
        <bean id="customerService" class="com.companyname.projectname.CustomerService"/>
        <bean id="orderService" class="com.companyname.projectname.OrderService"/>
</beans>
Note that the method name and bean name in XML is exactly the same.
 
Let's look at different scenarios in which @Bean annotation is used.

Injecting Bean Dependencies

A @Bean annotated method can have an arbitrary number of parameters describing the dependencies required to build that bean. For instance, if our CustomerController requires a CustomerService we can materialize that dependency via a method parameter: 
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import com.companyname.projectname.customer.CustomerController;
import com.companyname.projectname.customer.CustomerService;

@Configuration
public class Application {

    private CustomerService customerService;  
    @Bean
    public CustomerService customerService() {
        customerService = new CustomerService();
        return customerService;
    }
 
    @Bean
    public CustomerController customerController(CustomerService customerService) {
        return new CustomerController(customerService);
    }
}
The resolution mechanism is pretty much identical to constructor-based dependency injection. 

Bean life cycle methods

@Bean annotation provides initMethod and destroyMethod attributes to perform certain actions after bean initialization or before bean destruction by a container.
public class Foo {
        public void init() {
                // initialization logic via xml config
        }
}

public class Bar {
        public void cleanup() {
                // destruction logic via xml config
        }
}

@Configuration
public class AppConfig {

        @Bean(initMethod = "init")
        public Foo foo() {
                return new Foo();
        }

        @Bean(destroyMethod = "cleanup")
        public Bar bar() {
                return new Bar();
        }

}

Specifying Bean Scope Using the @Scope Annotation

You can specify that your beans defined with the @Bean annotation should have a specific scope. You can use any of the standard scopes specified in the Bean Scopes. The default scope is a singleton, but you can override this with the @Scope annotation: 
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Scope;

import com.companyname.projectname.customer.CustomerService;
import com.companyname.projectname.order.OrderService;

@Configuration
public class Application {

    @Bean
    @Scope("prototype")
    public CustomerService customerService() {
        return new CustomerService();
    }
 
    @Bean
    @Scope("prototype")
    public OrderService orderService() {
        return new OrderService();
    }
}

Customizing Bean Naming

By default, the bean name is the same as the method name. We can specify the bean name using the name attribute of @Bean annotation:
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import com.companyname.projectname.customer.CustomerService;
import com.companyname.projectname.order.OrderService;

@Configuration
public class Application {

    @Bean(name = "cService")
    public CustomerService customerService() {
        return new CustomerService();
    }
 
    @Bean(name = "oService")
    public OrderService orderService() {
        return new OrderService();
    }
}

Bean Aliasing

As discussed in Naming beans, it is sometimes desirable to give a single bean multiple names, otherwise known as bean aliasing. The name attribute of the @Bean annotation accepts a String array for this purpose. 
@Configuration
public class AppConfig {

        @Bean(name = { "dataSource", "subsystemA-dataSource", "subsystemB-dataSource" })
        public DataSource dataSource() {
                // instantiate, configure and return DataSource bean...
        }

}

Injecting inter-bean dependencies

When @Bean have dependencies on one another, expressing that dependency is as simple as having one bean method call another: 
@Configuration
public class AppConfig {
        @Bean
        public Foo foo() {
                return new Foo(bar());
        }

        @Bean
        public Bar bar() {
                return new Bar();
        }
}
That's all about @Bean annotation.

Related Spring and Spring Boot Annotations

Comments

Post a Comment

Leave Comment