providedRuntime `spring-boot-starter-undertow` in `build.gradle` but `:bootRun` still using Tomcat instead of undertow

Question

I have just initialised a new Spring Boot application and my build.gradle has the following dependencies:

dependencies {
    implementation 'org.springframework.boot:spring-boot-starter-web'
    providedRuntime 'org.springframework.boot:spring-boot-starter-undertow'
    testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

When I run ./gradlew bootRun , it is using Tomcat. I understand starter-web includes Tomcat, but isn't providedRuntime there to override that?

How do I actually use undertow to run my Spring controllers?

EDIT:

I just realised my ServletInitializer.java looks like this:

import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.boot.web.servlet.support.SpringBootServletInitializer;

public class ServletInitializer extends SpringBootServletInitializer {

    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
        return application.sources(AccountServiceApplication.class);
    }

}

This tells me it is initialising a servlet, which I assumed is undertow based on the dependencies, but was I wrong?

Answer 1

You need to exclude tomcat from the build:

dependencies {
  implementation ('org.springframework.boot:spring-boot-starter-web') {
      exclude module: 'spring-boot-starter-tomcat'
}
  providedRuntime 'org.springframework.boot:spring-boot-starter-undertow'
  testImplementation 'org.springframework.boot:spring-boot-starter-test'
}