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Lightrun – Third Party Code
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Flakiness in REST requests is a common issue. A request can get a 200 OK in one scenario and a 409 next time. Sometimes a request can even succeed and fail intermittently on the same exact request. In short, working over HTTP can be a bit of a mess without solid tooling.

Also, while it’s easy enough to debug these issues locally when developing the application, we’re talking about production here - we can’t afford the downtime while you’re stepping in and out of code. Uptime is kind of the whole point.

With Lightrun, you can get the same level of access you get with a local debugger or profiler - no downtime required. You can add logs, metrics, and snapshots (think breakpoints, but without stopping the running service), in a safe and read-only manner - without redeploying, restarting, or even stopping the running service. Performance and security are maintained throughout the process.

Learn how to debug a live REST API (built with Spring, of course), using Lightrun, in this 5-minute tutorial:

>> Debugging REST Requests in Spring-Based applications using the Lightrun Platform

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>
    <version>2.7.2</version>
</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.

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