Bean Configuration

Objects are the key element in Java programming and the Spring Framework handles them in a very special way. Unlike a regular Java Object that is created with the new operator and then used, a Spring Object must be registered with the ApplicationContext first. When an Object is registered with the ApplicationContext it is referred to as a Bean (or “Managed Bean” or “Component”). When Spring manages an Object as a Bean it is creating a Proxy around your object and can do very interesting things with it.

How to configure a Bean

Spring Framework: Bean overview

Beans can be configured in three primary ways: XML, Java, or Annotations. We will focus on Annotation based configuration.

Read: spring.io: Annotation-based container configuration

Spring provides a few different annotations to drive the creation of beans. Ultimately Spring will use @Bean or @Component to create a Bean. Here are the most common ways to define Beans.

Annotation	Location	Description
@Configuration	class	Indicates that this class defines @Bean’s to be
@Bean	method	Creates a Bean from the return value of the method
@Component	class	Instruct Spring to create a Bean out of the class
@Service	class	Is a @Component but Stereotyped as a Service
@Repository	class	Is a @Component but Stereotyped as a Repository
@Controller	class	Is a @Component but Stereotyped as a Controller
@RestController	class	Same as @Controller but meant REST end-points

Read: Advantages of using spring stereotypes (stackoverflow.com)
Read: Difference between @Component, @Repository, @Service (stackoverflow.com)
Read: Difference between @Controller and @RestController (stackoverflow.com)
Read: @RestController vs @Controller (genuitec.com)

// Declare a Singleton Bean - one instance for the entire application
@Component
class UserTransformer { }

// Declare a Request Scope Bean - one instance per Web Request
@Component
@Scope("request", proxyMode = ScopedProxyMode.INTERFACES)
class UserWebRequestContext : UserRequestContext { }

// Declare a class that defines definition of Beans
@Configuration
class MyConfig {

  // Declare a Session Scope Bean - one instance per Web Session
  @Bean 
  @Scope("session")
  fun userSessionContext() = UserSessionContext()

}

// Declares a Bean with the Service stereotype
@Service
class UserService { }

// Declares a Bean with the Repository stereotype
@Repository
class UserRepository { }

@RestController
class HelloController {
  @RequestMapping("/hello")
  fun hello() = "Hello, World!"
}

Bean Lifecycle

Spring Framework: Lifecycle callbacks

As Spring Beans are created they follow a Lifecycle where you can hook-in callback listeners to do interesting things when the Beans are first created and destroyed.

Read: thejavaprogrammer.com: Spring Bean Life Cycle

A common use-case is to validate that certain properties are set on your Bean after it is created.

@Component
class ConnectionManager(
    @Value("\${connection.url}") var url: String = "") {

    @PostConstruct
    fun init(){
        Assert.isTrue(!url.isNullOrBlank(), "url must be set")
        Assert.isTrue(url.startsWith("http://"), "http is the only support protocol")
    }
}