Embedded Server Configuration (Tomcat, Jetty, Undertow)

Q: Embedded Server Configuration (Tomcat, Jetty, Undertow)

Spring Boot defaults to **embedded Tomcat** but supports Jetty and Undertow by excluding `spring-boot-starter-tomcat` and adding the desired alternative — a single dependency swap, no code changes. **Tomcat is the most battle-tested**; **Undertow offers the lowest memory footprint and highest servlet throughput**; **Jetty is a solid middle ground** with strong WebSocket and SSE support. All three are configured through the `server.*` property namespace. For reactive WebFlux apps the default is **Netty** — an event-loop server that suits non-blocking I/O.

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Core answer

Spring Boot defaults to embedded Tomcat but supports Jetty and Undertow by excluding spring-boot-starter-tomcat and adding the desired alternative — a single dependency swap, no code changes. Tomcat is the most battle-tested; Undertow offers the lowest memory footprint and highest servlet throughput; Jetty is a solid middle ground with strong WebSocket and SSE support. All three are configured through the server.* property namespace. For reactive WebFlux apps the default is Netty — an event-loop server that suits non-blocking I/O.

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Interview answer

Spring Boot embeds the web server inside the JAR — Tomcat by default, with Jetty, Undertow, and Netty as alternatives. Choosing and configuring the right server affects throughput, memory usage, and the concurrency model.

Switching servers

Exclude spring-boot-starter-tomcat from spring-boot-starter-web and add spring-boot-starter-jetty or spring-boot-starter-undertow
Spring Boot auto-detects the server implementation on the classpath — no code changes required
WebFlux applications use Netty by default; Netty is non-blocking and handles thousands of concurrent connections on a small thread pool; Tomcat uses one thread per request

Key Tomcat settings

server.tomcat.threads.max=200 (default) — the maximum number of simultaneous requests; increase for high-concurrency workloads, but each thread consumes memory
server.tomcat.accept-count — the queue depth when all threads are busy; requests beyond this are rejected

Common configuration

server.port=8080, server.compression.enabled=true, server.ssl.key-store — configured via application.properties without touching code
server.shutdown=graceful — allows in-flight requests to complete before the JVM exits

Programmatic customization

WebServerFactoryCustomizer exposes server-specific knobs that have no property equivalent
Guard with @ConditionalOnClass so the bean activates only when the matching server is on the classpath

The decision between Tomcat and Netty is architectural: blocking I/O with Tomcat suits traditional request-response Spring MVC; non-blocking event-loop Netty suits reactive WebFlux. Do not mix — using a blocking call inside a Netty event loop blocks the loop and kills throughput.

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