Use Another Web Server

Many Spring Boot starters include default embedded containers.

For servlet stack applications, the spring-boot-starter-web includes Tomcat by including spring-boot-starter-tomcat, but you can use spring-boot-starter-jetty or spring-boot-starter-undertow instead.
For reactive stack applications, the spring-boot-starter-webflux includes Reactor Netty by including spring-boot-starter-reactor-netty, but you can use spring-boot-starter-tomcat, spring-boot-starter-jetty, or spring-boot-starter-undertow instead.

When switching to a different HTTP server, you need to swap the default dependencies for those that you need instead. To help with this process, Spring Boot provides a separate starter for each of the supported HTTP servers.

The following Maven example shows how to exclude Tomcat and include Jetty for Spring MVC:

<properties>
	<servlet-api.version>3.1.0</servlet-api.version>
</properties>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<exclusions>
		<!-- Exclude the Tomcat dependency -->
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<!-- Use Jetty instead -->
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-jetty</artifactId>
</dependency>

The version of the servlet API has been overridden as, unlike Tomcat 9 and Undertow 2, Jetty 9.4 does not support servlet 4.0.

If you wish to use Jetty 10, which does support servlet 4.0, you can do so as shown in the following example:

<properties>
	<jetty.version>10.0.8</jetty.version>
</properties>
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-web</artifactId>
	<exclusions>
		<!-- Exclude the Tomcat dependency -->
		<exclusion>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-tomcat</artifactId>
		</exclusion>
	</exclusions>
</dependency>
<!-- Use Jetty instead -->
<dependency>
	<groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-jetty</artifactId>
	<exclusions>
		<!-- Exclude the Jetty-9 specific dependencies -->
		<exclusion>
			<groupId>org.eclipse.jetty.websocket</groupId>
			<artifactId>websocket-server</artifactId>
		</exclusion>
		<exclusion>
			<groupId>org.eclipse.jetty.websocket</groupId>
			<artifactId>javax-websocket-server-impl</artifactId>
		</exclusion>
	</exclusions>
</dependency>

Note that along with excluding the Tomcat starter, a couple of Jetty9-specific dependencies also need to be excluded.

The following Gradle example configures the necessary dependencies and a module replacement to use Undertow in place of Reactor Netty for Spring WebFlux:

dependencies {
	implementation "org.springframework.boot:spring-boot-starter-undertow"
	implementation "org.springframework.boot:spring-boot-starter-webflux"
	modules {
		module("org.springframework.boot:spring-boot-starter-reactor-netty") {
			replacedBy("org.springframework.boot:spring-boot-starter-undertow", "Use Undertow instead of Reactor Netty")
		}
	}
}

spring-boot-starter-reactor-netty is required to use the WebClient class, so you may need to keep a dependency on Netty even when you need to include a different HTTP server.