Replacing Default Tomcat Server With Jetty or Undertow in Spring Boot 3: A Guide

One of the key features of the Spring Boot framework is the built-in support for embedded servers. It includes support for embedded Tomcat, Jetty, and Undertow servers. This means you don’t need any external web servers and no need to deploy WAR files anymore.

The embedded Tomcat server is available through the spring-boot-starter-web dependency.

However, if you want Jetty or Undertow servers then you can include spring-boot-starter-jetty or spring-boot-starter-undertow dependencies.

This article explains how to replace the default embedded Tomcat with Jetty or Undertow servers.

https://youtu.be/1gEoiMVULt4?si=RVOx-P3o65KPN2-p&embedable=true

Replace Tomcat With the Jetty in Spring Boot

To use the embedded Jetty server instead of Tomcat, you need to exclude the spring-boot-starter-tomcat from spring-boot-starter-web and include the spring-boot-starter-jetty dependency. Also, you will need to exclude the default added spring-boot-starter-tomcat dependency.

For Spring Boot Version 2.x.x

For Gradle, make the following changes to your build.gradle file.

dependencies {
    implementation('org.springframework.boot:spring-boot-starter-web') {
        exclude group:'org.springframework.boot', module:'spring-boot-starter-tomcat'
    }
    implementation 'org.springframework.boot:spring-boot-starter-jetty'
}

If you’re using Maven build tools, you need to add this to your pom.xml file.

<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-jetty</artifactId>
</dependency>

For Spring Boot Version 3.x.x

The above configuration works for Spring Boot version 2.x, but it will throw the following error if you’re using the latest Spring Boot version 3.x.

java.lang.ClassNotFoundException: jakarta.servlet.http.HttpSessionContext 
with Spring Boot 3 and Jetty server

This is because Jetty does not yet support Servlet 6.0. To use Jetty with Spring Boot 3.0, you will have to downgrade the Servlet API to 5.0. You can use the jakarta-servlet.version property to do so.

Ref: Spring-Boot-3.0-Migration-Guide

ext['jakarta-servlet.version'] = '5.0.0'
dependencies {
    implementation('org.springframework.boot:spring-boot-starter-web')
    implementation 'org.springframework.boot:spring-boot-starter-jetty'
    modules {
        module("org.springframework.boot:spring-boot-starter-tomcat") {
            replacedBy("org.springframework.boot:spring-boot-starter-jetty")
        }
    }
}

If you’re using Maven build tools, you need to add this to your pom.xml file.

<properties>
    <java.version>17</java.version>
    <jakarta-servlet.version>5.0.0</jakarta-servlet.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
        <exclusions>
            <exclusion>
                <artifactId>spring-boot-starter-tomcat</artifactId>
                <groupId>org.springframework.boot</groupId>
            </exclusion>
        </exclusions>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-jetty</artifactId>
    </dependency>
</dependencies>

Replace Tomcat With Undertow in Spring Boot

To use Undertow instead of Tomcat, first, you need to exclude the spring-boot-starter-tomcat from spring-boot-starter-web. Then you should add the undertow starter as shown here.

For Gradle, make the following changes to your build.gradle file.

dependencies {
    implementation('org.springframework.boot:spring-boot-starter-web') {
        exclude group:'org.springframework.boot', module:'spring-boot-starter-tomcat'
    }
    implementation 'org.springframework.boot:spring-boot-starter-undertow'
}

If you’re using Maven build tools, you need to add this to your pom.xml file.

<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>

Also published here