Spring Boot 2响应式项目启动时未用Netty，默认启动Tomcat问题

Hey there! I’ve run into this exact snag before—let’s break down what’s going on and how to fix it.

The Root Cause #

Spring Boot’s auto-configuration picks a server based on what’s in your classpath:

• When you use spring-boot-starter-webflux, Netty is the default reactive server out of the box.
• But if any Servlet-stack dependencies sneak into your classpath (like spring-boot-starter-web), Spring Boot will automatically switch to a Servlet container (Tomcat, by default) instead of Netty.

Looking at your dependencies, the culprit is almost certainly spring-boot-starter-hateoas. In Spring Boot 2.x, this starter automatically pulls in spring-boot-starter-web (the traditional Servlet stack) unless you explicitly exclude it. That’s why Tomcat is firing up instead of Netty.

Fix Steps #

• Exclude the Servlet stack from the HATEOAS starter
  Modify your spring-boot-starter-hateoas dependency in build.gradle to exclude the unwanted spring-boot-starter-web module:

  compile("org.springframework.boot:spring-boot-starter-hateoas:${springBootVersion}") {
      exclude group: 'org.springframework.boot', module: 'spring-boot-starter-web'
  }
  

• Verify your dependency tree
  Run this command to double-check that no Servlet-related dependencies are still lurking in your project:

  ./gradlew dependencies
  

  Look for entries like org.springframework.boot:spring-boot-starter-tomcat—if you still see them, trace back to which starter is bringing them in and repeat the exclusion step.

• Rebuild and restart
  After making the changes, rebuild your project and start the app. It should now use Netty as the default server for your reactive WebFlux application.

Quick Note #

If you’re using any other starters (like spring-boot-starter-data-jdbc or similar), make sure none of them are pulling in spring-boot-starter-web accidentally. The dependency tree check will help you spot those hidden dependencies.

内容的提问来源于stack exchange，提问作者Dina Bogdan