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

Spring Boot uses Tomcat as its default embedded servlet container, but it also supports Undertow, a high‑performance, lightweight web server.

Undertow, developed by Red Hat, offers non‑blocking NIO, full Servlet 3.1/4.0 support, WebSocket, and can be embedded directly into Java applications.

Key features of Undertow include high concurrency performance, Servlet 4.0 support, full WebSocket (JSR‑356) support, embedded operation without a separate container, flexible handler chains, and a small footprint consisting of two core JARs.

To switch Spring Boot from Tomcat to Undertow, remove the Tomcat starter dependency and add the Undertow starter dependency in pom.xml:

<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 changes to Undertow.

Benchmark tests on the same hardware show that Undertow achieves higher QPS and lower memory consumption than Tomcat under high concurrency, making it a better choice for performance‑critical services.

In conclusion, while Spring Boot can run with either Tomcat or Undertow, Undertow provides superior performance and resource efficiency for high‑traffic applications.