I have an ApplicationRunner implementation named RefAppRunner which performs some logic.

I would like to write a SpringBoot based unit test to validate that ApplicationRunner behaves as expected based on the ApplicationArguments it gets.

The implementation also has some @Autowired fields.

What is the recommended way of unit testing such a component in a Spring Boot application?

I want all the Spring based context to be initialized as though the app is launched properly, but I want to supply variations of the ApplicationArguments

Is there a recommended way to write such tests?

I have commented out @SpringBootTest because if it is enabled, then the custom ApplicationRunner will have been invoked before the run test method is invoked .. meaning there is no control over the values passed to the impl.

Here is my current Java unit test in a Spring Boot project:

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.DefaultApplicationArguments;
import org.springframework.boot.test.context.SpringBootTest;

//@SpringBootTest
public class RefAppRunnerTest
{
  @Autowired
  private RefAppRunner refAppRunner;

  @Test
  public void run() throws Exception {
    refAppRunner.run(new DefaultApplicationArguments(new String[]{}));
  }
}

RefAppRunner is an ApplicationRunner implementation:

@Component
public class RefAppRunner implements ApplicationRunner {

  @Override
  public void run(ApplicationArguments args) {
    // ..
  }
}

But I’m getting this error:

java.lang.NullPointerException: Cannot invoke "com.apps.referenceapp.RefAppRunner.run(org.springframework.boot.ApplicationArguments)" because "this.refAppRunner" is null
  at com.apps.referenceapp.RefAppRunnerTest.run(RefAppRunnerTest.java:16)