Replacing Tomcat with Undertow in Spring Boot: Performance and Memory Benefits

Spring Boot uses Tomcat as its default embedded servlet container, but it also supports the Undertow container, which can provide better performance and lower memory consumption. This article demonstrates how to switch from Tomcat to Undertow and why you might want to do so.

Tomcat, an Apache‑licensed lightweight servlet container, supports Servlet and JSP specifications, includes an HTTP server, and is widely adopted for Java web applications. It offers management interfaces and is completely free.

Undertow, an open‑source project from Red Hat, is written entirely in Java, supports both blocking and non‑blocking I/O, and can be embedded directly into Java applications. It fully supports Servlet and WebSocket APIs and performs exceptionally well under high‑concurrency loads.

To replace Tomcat with Undertow in a Spring Boot project, simply add the Undertow starter dependency (e.g., <dependency>...<artifactId>undertow-spring-boot-starter</artifactId>...</dependency> ) to your build file. Spring Boot will automatically configure Undertow as the embedded server, and after restarting the application the container changes to Undertow.

Benchmark tests conducted on identical hardware show that Undertow achieves higher QPS (queries per second) and consumes less memory than Tomcat. The results indicate that Undertow delivers superior throughput and efficiency, especially in high‑traffic scenarios.

In conclusion, while Spring Boot can run with either Tomcat or Undertow, Undertow is the preferred choice for high‑concurrency business systems due to its better performance and lower resource usage.