Red Hat


Undertow is a flexible performant web server written in java, providing both blocking and non-blocking API’s based on NIO.

Undertow has a composition based architecture that allows you to build a web server by combining small single purpose handlers. The gives you the flexibility to choose between a full Java EE servlet 4.0 container, or a low level non-blocking handler, to anything in between.

Undertow is designed to be fully embeddable, with easy to use fluent builder APIs. Undertow’s lifecycle is completely controlled by the embedding application.

Undertow is sponsored by JBoss and is the default web server in the Wildfly Application Server.

Why Undertow

HTTP/2 Support

Undertow supports HTTP/2 out of the box, no overriding the boot class path required.

HTTP Upgrade Support

Support for HTTP upgrade, to allow multiple protocols to be multiplexed over the HTTP port.

Web Socket Support

Undertow provides full support for Web Sockets, including JSR-356 support.

Servlet 4.0

Undertow provides support for Servlet 4.0, including support for embedded servlet. It is also possible to mix both Servlets and native undertow non-blocking handlers in the same deployment.


Undertow can be embedded in an application or run standalone with just a few lines of code.


An Undertow server is configured by chaining handlers together. It is possible to add as much or as little functionality as you need, so you don’t pay for what you are not using.

Show me the code

Ok then. Here is a simple Hello World server using Async IO:

public class HelloWorldServer {

    public static void main(final String[] args) {
        Undertow server = Undertow.builder()
                .addHttpListener(8080, "localhost")
                .setHandler(new HttpHandler() {
                    public void handleRequest(final HttpServerExchange exchange) throws Exception {
                        exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
                        exchange.getResponseSender().send("Hello World");