Switching Spring Boot 3 from Tomcat to Undertow: Step‑by‑Step Configuration and Performance Tuning

Optimization Background

When an application must support sudden high concurrency, Tomcat is no longer the optimal choice; Undertow offers a lightweight, high‑performance, non‑blocking I/O server developed by JBoss.

Switching Spring Boot 3 from Tomcat to Undertow

In pom.xml exclude the default Tomcat starter 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>

Multithread Optimisation

Add thread‑pool settings in application.yml (or application.properties):

server:
  undertow:
    threads:
      # I/O threads (recommended 1‑2× CPU cores)
      io: 16
      # Worker threads (recommended 8‑16× CPU cores)
      worker: 256
    buffer-size: 1024
    direct-buffers: true

Request Timeout Limits

server:
  undertow:
    no-request-timeout: 30000   # request timeout in ms
    idle-timeout: 60000        # idle connection timeout in ms

Request Body Size Limit

server:
  undertow:
    max-http-post-size: 2048   # 2 KB limit for POST bodies

Response Compression

Enable Gzip/Deflate compression to reduce payload size and improve bandwidth usage:

server:
  compression:
    enabled: true
    min-response-size: 1024   # trigger compression for responses ≥ 1 KB
    mime-types: application/json, text/html, text/xml, text/plain, text/css, application/javascript
    excluded-user-agents: IE6, IE7

Enable HTTP/2 (requires SSL)

server:
  http2:
    enabled: true

Undertow Access Log

server:
  undertow:
    accesslog:
      enabled: true
      dir: ./logs
      prefix: access_log.
      suffix: .log
      pattern: '%t %a "%r" %s (%D ms)'
      rotate: true

Monitoring and Tuning Recommendations

Monitor thread‑pool status via JMX or Spring Actuator.

Conduct load testing with JMeter or wrk to observe thread‑pool behaviour.

Adjust JVM heap and GC parameters based on observed load.

If a database is used, align connection‑pool size with the server’s thread count.

Appendix: Undertow Configuration Properties

Spring Boot exposes many properties prefixed with server.undertow. for fine‑grained control. The following diagrams summarise the hierarchy: