Getting Started on Undertow Server

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Undertow is a high performing web server which can be used for both blocking and non-blocking tasks. It is extermely flexible as application can assemble the parts in whatever way it would make sense. It also supports Servlet 4.0, JSR-356 compliant web socket implementation. Undertow is licensed under Apache License, Version 2.0.
Undertow source code is available in github. It can either downloaded and configure the server manually. Other easy option would be directly embedded into the project using the maven dependency as below. Undertow recent version can be checked in github releases page.
It has three components
  1. undertow-core: Undertow core, which provides support for non blocking handlers and web sockets
  2. undertow-servlet: Support for Servlet 3.1
  3. undertow-websockets-jsr: for the Java API for Websockets (JSR-356) standard
Bootstrapping Undertow
Undertow can be started using io.undertow.Undertow builder API. It doesn't have any global container it just assemble the handlers and start the server. Undertow applications has to manage the lifecycle of handlers and resource utitlized by the handlers.
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/html");
                        StringBuilder output = new StringBuilder("<h1>Checking Status Undertow</h1>");
                        output.append("<h3 style='color:green'>I am up and running at port 8080!!!</h3>");
Undertow server can be instantiated with builder having hostname, port number and handler details. In the HttpHandler, using HttpServerExchange parameter to get the response object. In this case, we are sending status of the server in html format in the response sender.
Undertow bootstrap within milliseconds (log for reference) whereas other microservice frameworks takes seconds to start.
Mar 12, 2019 3:38:10 AM org.xnio.Xnio <clinit>
INFO: XNIO version 3.3.8.Final
Mar 12, 2019 3:38:10 AM org.xnio.nio.NioXnio <clinit>
INFO: XNIO NIO Implementation Version 3.3.8.Final
Architecture Overview
An undertow server is basically composed of three things like XNIO Worker Instance, one or more connectors and handler chain to handle the incoming requests.  XNIO Worker is based on the Jboss XNIO project which is simplified low-level I/O layer built on top java nio library. Connectors are tied to XNIO worker with listeners for HTTP/1.1, HTTPS, AJP will generally do all IO operations. After connection, XNIO invokes the HttpOpenListener which creates the server connection(HttpServerConnection) to hold the state associated with the connection. Then, it invokes HttpReadListener which is responsible for parsing the request, they will create HttpServerExchange object populated with request data and then hand it to handler chain.
Root handler chain will be invoked by the connector where built in handlers are available and also custom handler can be created and added.
File Server
Undertow is shipped with many built in handlers like access log handlers, allowed methods, request dumping handler and so on. Resource handler is for listing and finding the files as ftp server. Now we will add the base path for the directory listing using resource manager and it can be handled for exception like file not found.
 HttpHandler handle404Example = new HttpHandler() {
  public void handleRequest(HttpServerExchange exchange) throws Exception {
   exchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
   exchange.getResponseSender().send("File Not Found");

 ResourceHandler resourceHandler = new ResourceHandler(
                 new PathResourceManager(Paths.get(System.getProperty("user.home")), 100), handle404Example)
 RequestDumpingHandler requestDumpingHandler = new RequestDumpingHandler(resourceHandler);
 Undertow server = Undertow.builder()
                 .addHttpListener(8080, "localhost")
                 .addPrefixPath("/", requestDumpingHandler)))
Now resourceHandler will list all the files from user home directory, if we try to hit the url with non existing files like http://localhost:8080/test, it will invoke the handle404Example which shows the content like "File not found".

Rest API
We can create rest api for retrieving and presisting the users with routing handler. Routing handler will invoke the handlers based on the api url and methods. In our case, we will route GET method of /users path to retrieve the users and POST method of /users path to save the data. Below example, will have class member holding all the "users", so post is adding and get is retrieving it from the users.
   RoutingHandler routes = new RoutingHandler().get("/users", new HttpHandler() {
           public void handleRequest(HttpServerExchange httpServerExchange) throws Exception {
                 httpServerExchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
       .post("/users", new RequestDumpingHandler(new HttpHandler() {
            public void handleRequest(HttpServerExchange httpServerExchange) throws Exception {
                 FormParserFactory.Builder builder = FormParserFactory.builder();

                 final FormDataParser formDataParser =;

                if (formDataParser != null) {
                     FormData formData = formDataParser.parseBlocking();
                     Integer id = Integer.parseInt(formData.get("id").peek().getValue());
                     User user1 = new User(id, formData.get("name").peek().getValue());
                     httpServerExchange.getResponseHeaders().put(Headers.CONTENT_TYPE, "text/plain");
                     httpServerExchange.getResponseSender().send("added successfully");


  Undertow undertow = Undertow.builder()
                     .addHttpListener(8080, "localhost")
For invoking the POST request, send the data as x-www-form-url-encoded through postman(chrome browser app) with keys as id and name. Likewise the GET request,  http://localhost:8080/users will retrieve the data.

There are more working examples provided as part of undertow project. clone the project and inside the project there is a folder examples. Once the project is build, move to target folder and execute the jar, which shows all the  examples as below.

  $ java -jar undertow-examples.jar
     Welcome to the Undertow Examples
     Please select an example:
     a) Basic Authentication
     b) Chat
     c) File Serving
     d) HTTP2
     e) Hello World
     f) JSR Web Sockets
     g) ModCluster Proxy Server
     h) Reverse Proxy
     i) Server Sent Events
     j) Servlet
    k) Session Handling
    l) Web Socket Extensions
   m) Web Sockets
choose the one and execute to see how that works.


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Nagappan is a techie-geek and a full-stack senior developer having 10+ years of experience in both front-end and back-end. He has experience on front-end web technologies like HTML, CSS, JAVASCRIPT, Angular and expert in Java and related frameworks like Spring, Struts, EJB and RESTEasy framework. He hold bachelors degree in computer science and he is very passionate in learning new technologies.

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