Using Undertow as a Servlet Container

Spring Boot uses Tomcat as the Servlet container by default, but Undertowis gradually gaining more popularity for its excellent performance.

To use Undertow instead of Tomcat in Spring Boot, simply exclude spring-boot-starter-tomcat from the spring-boot-starter-web starter and add the spring-boot-starter-undertow dependency.

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<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>

After using Undertow as a Servlet container, start the application and you should see a warning log.

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  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::                (v2.7.4)

...
2022-10-17 11:24:33.234  WARN 13748 --- [           main] io.undertow.websockets.jsr               : UT026010: Buffer pool was not set on WebSocketDeploymentInfo, the default pool will be used
...

2022-10-17 11:24:33.234 WARN 13748 — [ main] io.undertow.websockets.jsr : UT026010: Buffer pool was not set on WebSocketDeploymentInfo, the default pool will be used

Apparently, the reason for this warning is that something is not set and the system uses the default settings.

Solution 1, set the WebSocketDeploymentInfo property

Just manually set a reasonable value for the io.undertow.websockets.jsr.WebSocketDeploymentInfo property via the WebServerFactoryCustomizer configuration class.

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import io.undertow.server.DefaultByteBufferPool;
import io.undertow.websockets.jsr.WebSocketDeploymentInfo;

import org.springframework.boot.web.embedded.undertow.UndertowServletWebServerFactory;
import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.context.annotation.Configuration;

@Configuration
public class UndertowServletWebServerFactoryConfiguration implements WebServerFactoryCustomizer<UndertowServletWebServerFactory> {

    @Override
    public void customize(UndertowServletWebServerFactory factory) {
        
        factory.addDeploymentInfoCustomizers(deploymentInfo -> {
            
            WebSocketDeploymentInfo webSocketDeploymentInfo = new WebSocketDeploymentInfo();
            webSocketDeploymentInfo.setBuffers(new DefaultByteBufferPool(false, 4096));

            deploymentInfo.addServletContextAttribute("io.undertow.websockets.jsr.WebSocketDeploymentInfo", webSocketDeploymentInfo);
        });
    }
}

Solution 2, directly exclude the undertow-websockets-jsr dependency

If your application doesn’t use websocket-related stuff, you can simply exclude the undertow-websockets-jsr dependency from spring-boot-starter-undertow.

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<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>
    <!-- Exclude undertow-websockets-jsr dependency-->
    <exclusions>
        <exclusion>
            <groupId>io.undertow</groupId>
            <artifactId>undertow-websockets-jsr</artifactId>
        </exclusion>
    </exclusions>
</dependency>

It has been verified that both of the above 2 methods work.