Undertow Concepts

Internal

• Undertow
• XNIO Concepts

Overview

Undertow is a web server written in Java. It provides both blocking and non-blocking APIs based on NIO. It has a composition-based architecture that allows assembling a web server combining small single purpose handlers.

Undertow is embeddable, its lifecycle is controlled by the embedding application and its configuration is controlled programmatically via API calls. Undertow is the default web server in WildFly since version 8, and in this case the configuration is exposed via XML in the server configuration file as:

        ...
        <subsystem xmlns="urn:jboss:domain:undertow:3.0">
            ...
        </subsystem>
        ...

It has support for Servlet 3.1.

Artifacts

• undertow-core
• undertow-servlet provides support for Servlet 3.1
• undertow-websockets-jsr provides support for the Java API for Websockets (JSR-356)

Container

There in no Undertow container. Undertow applications are assembled from Undertow handler classes and it is up to the embedding application to manage the lifecycle of all Undertow handlers.

Architecture

An Undertow server consists of one or more XNIO worker instances, one or more connectors and a handler chain.

XNIO Concepts

• Worker

Connector

A connector (also known as a Listener) is the Undertow part that handles incoming connections and the underlying wire protocol. By default, Undertow ships with 3 listeners: HTTP, HTTPS and AJP.

A connector comprises two XNIO Listeners: an open listener and a read listener:

Open Listener

The open listener is invoked when a connection is first received, and it will do any work necessary to set up the connection. Then, it will pass it to the read listener.

Read Listener

The read listener is responsible for parsing the incoming request into a HttpServerExchange instance.

Request Lifecycle

• http://undertow.io/undertow-docs/undertow-docs-1.3.0/index.html#request-lifecycle
• http://undertow.io/undertow-docs/undertow-docs-1.3.0/undertow-request-lifecycle.html

HttpServerExchange

An exchange can be in blocking or non-blocking mode, and it can be put in blocking mode.