Replacing Tomcat with Undertow in Spring Boot: Benefits, Configuration, and Performance Comparison

Spring Boot uses Tomcat as its default embedded servlet container. For high‑concurrency scenarios, Undertow offers better performance and lower memory consumption.

1. Tomcat in Spring Boot

Spring Boot is a popular Java web framework that simplifies creating web services. Its default embedded container is Tomcat, which runs the application out of the box.

2. Introducing Undertow

Undertow is a flexible, high‑performance web server written in Java. It supports both blocking and non‑blocking I/O, Servlet 3.1, WebSocket, and is the default server for WildFly.

High performance under load

Supports Servlet 4.0

Full WebSocket support (JSR‑356)

Embedded‑only, no external container required

Modular and lightweight

3. Switching from Tomcat to Undertow

Remove the Tomcat starter from the Maven/Gradle dependencies and add the Undertow starter.

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </exclusion>
    </exclusions>
</dependency>

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

After rebuilding and starting the application, the embedded server will be Undertow.

4. Performance & Memory Comparison

Benchmark tests on the same hardware show that Undertow achieves higher QPS and lower memory usage than Tomcat under identical request loads.

5. Conclusion

For high‑concurrency Java backend services, replacing Tomcat with Undertow can significantly improve throughput and reduce resource consumption, making Undertow the preferred choice.