Spring 5 Functional Bean Registration

1. Overview

Spring 5 comes with support for functional bean registration in the application context.

Simply put, this can be done through overloaded versions of a new registerBean() method defined in the GenericApplicationContext class.

Let's have a look at a few examples of this functionality in action.

2. Maven Dependencies

The quickest way to setup a Spring 5 project is to use Spring Boot by adding the spring-boot-starter-parent dependency to the pom.xml:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
</parent>

We also need the spring-boot-starter-web and spring-boot-starter-test for our example, to use a web application context in a JUnit test:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <scope>test</scope>
</dependency>

Of course, Spring Boot is not necessary in order to use the new functional way to register a bean. We could also just add the spring-core dependency directly:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-core</artifactId>
    <version>5.3.3</version>
</dependency>

3. Functional Bean Registration

The registerBean() API can receive two types of functional interfaces as parameters:

• a Supplier argument used to create the object
• a BeanDefinitionCustomizer vararg which can be used to provide one or more lambda expressions to customize the BeanDefinition; this interface has a single customize() method

First, let's create a very simple class definition that we will use to create beans:

public class MyService {
    public int getRandomNumber() {
        return new Random().nextInt(10);
    }
}

Let's also add a @SpringBootApplication class that we can use to run a JUnit test:

@SpringBootApplication
public class Spring5Application {
    public static void main(String[] args) {
        SpringApplication.run(Spring5Application.class, args);
    }
}

Next, we can set up our test class using the @SpringBootTest annotation to create a GenericWebApplicationContext instance:

@RunWith(SpringRunner.class)
@SpringBootTest(classes = Spring5Application.class)
public class BeanRegistrationIntegrationTest {
    @Autowired
    private GenericWebApplicationContext context;
   
    //...
}

We're using the GenericWebApplicationContext type in our example, but any type of application context can be used in the same way to register a bean.

Let's see how we can register a bean using a lambda expression for creating the instance:

context.registerBean(MyService.class, () -> new MyService());

Let's verify that we can now retrieve the bean and use it:

MyService myService = (MyService) context.getBean("com.baeldung.functional.MyService"); 
 
assertTrue(myService.getRandomNumber() < 10);

We can see in this example that if the bean name isn't explicitly defined, it will be determined from the lower-case name of the class. The same method above can also be used with an explicit bean name:

context.registerBean("mySecondService", MyService.class, () -> new MyService());

Next, let's see how we can register a bean by adding a lambda expression to customize it:

context.registerBean("myCallbackService", MyService.class, 
  () -> new MyService(), bd -> bd.setAutowireCandidate(false));

This argument is a callback that we can use to set bean properties such as autowire-candidate flag or primary flag.

4. Conclusion

In this quick tutorial, we've seen how we can use the functional way of registering a bean.

The source code for the example can be found over on GitHub.

res – REST with Spring (eBook) (everywhere)

Learning to build your API
with Spring?

Download the E-book