Chapter 10. Undertow

Undertow is a web server designed to be used for both blocking and non-blocking tasks. It replaces JBoss Web in JBoss EAP 7. Some of its main features are:

Request Lifecycle

When a client connects to the server, Undertow creates a io.undertow.server.HttpServerConnection. When the client sends a request, it is parsed by the Undertow parser, and then the resulting io.undertow.server.HttpServerExchange is passed to the root handler. When the root handler finishes, one of four things can happen:

By far the most common use of HttpServerExchange.dispatch() is to move execution from an IO thread where blocking is not allowed into a worker thread, which does allow for blocking operations. This pattern generally looks like:

public void handleRequest(final HttpServerExchange exchange) throws Exception {
    if (exchange.isInIoThread()) {
      exchange.dispatch(this);
      return;
    }
    //handler code
}

Because exchange is not actually dispatched until the call stack returns, you can be sure that more that one thread is never active in an exchange at once. The exchange is not thread safe. However it can be passed between multiple threads as long as both threads do not attempt to modify it at once.

Ending the Exchange

There are two ways to end an exchange, either by fully reading the request channel, and calling shutdownWrites() on the response channel and then flushing it, or by calling HttpServerExchange.endExchange(). When endExchange() is called, Undertow will check if the content has been generated yet. If it has, then it will simply drain the request channel and close and flush the response channel. If not and there are any default response listeners registered on the exchange, then Undertow will give each of them a chance to generate a default response. This mechanism is how default error pages are generated.

For more information on configuring the Undertow, see Configuring the Web Server in the JBoss EAP Configuration Guide.

Undertow provides a default set of handlers that you can use with any application deployed to JBoss EAP. You can find a full list of the available handlers as well as their attributes here.

To use a handler with a deployment, you need to add a WEB-INF/undertow-handlers.conf file.

allowed-methods(methods='GET')

All handlers may also take an optional predicate to apply that handler in specific cases.

path('/my-path') -> allowed-methods(methods='GET')

The above example will only apply the allowed-methods handler to the path /my-path.

Some handlers have a default parameter, which allows you to specify the value of that parameter in the handler definition without using the name.

path('/a') -> redirect('/b')

You also may update the WEB-INF/jboss-web.xml file to include the definition of one or more handlers but using WEB-INF/undertow-handlers.conf is preferred.

<jboss-web>
    <http-handler>
        <class-name>io.undertow.server.handlers.AllowedMethodsHandler</class-name>
        <param>
            <param-name>methods</param-name>
            <param-value>GET</param-value>
        </param>
    </http-handler>
</jboss-web>

A full list of provided Undertow handlers can be found here.

<jboss-web>
    <http-handler>
        <class-name>org.jboss.example.MyHttpHandler</class-name>
    </http-handler>
</jboss-web>

package org.jboss.example;

import io.undertow.server.HttpHandler;
import io.undertow.server.HttpServerExchange;

public class MyHttpHandler implements HttpHandler {
    private HttpHandler next;

    public MyHttpHandler(HttpHandler next) {
        this.next = next;
    }

    public void handleRequest(HttpServerExchange exchange) throws Exception {
        // do something
        next.handleRequest(exchange);
    }
}

<jboss-web>
    <http-handler>
        <class-name>org.jboss.example.MyHttpHandler</class-name>
        <param>
            <param-name>myParam</param-name>
            <param-value>foobar</param-value>
        </param>
    </http-handler>
</jboss-web>

For these parameters to work, the handler class needs to have corresponding setters.

package org.jboss.example;

import io.undertow.server.HttpHandler;
import io.undertow.server.HttpServerExchange;

public class MyHttpHandler implements HttpHandler {
    private HttpHandler next;
    private String myParam;

    public MyHttpHandler(HttpHandler next) {
        this.next = next;
    }

    public void setMyParam(String myParam) {
        this.myParam = myParam;
    }

    public void handleRequest(HttpServerExchange exchange) throws Exception {
        // do something, use myParam
        next.handleRequest(exchange);
    }
}