undertow-io/undertow

Undertow is a Java web server built around non-blocking input/output, which is a design approach that lets the server handle many simultaneous connections without dedicating a separate thread to each one. That matters for performance: traditional blocking servers tie up a thread while waiting for data to arrive from a client, whereas a non-blocking design lets the same thread switch to other work while it waits. The project is made up of three main components. The core HTTP server can operate in both blocking and non-blocking modes, giving developers the option to write straightforward sequential request-handling code or to use the more efficient but more complex non-blocking style depending on their needs. The second component is a Servlet implementation that covers versions 4.0, 5.0, and 6.0 of the Servlet specification, which is the standard API that Java web applications have used for decades to define how HTTP requests are processed. The third component is a WebSocket implementation that follows the JSR-356 and Jakarta 2.0 specifications, covering the protocol that allows a browser and server to hold a persistent two-way connection open for real-time communication. Undertow is a Red Hat project and serves as the web server underneath WildFly, Red Hat's application server. Development discussion happens on a Google Group and a Zulip chat stream. Bug reports go through Red Hat's issue tracker. Security-relevant bugs have a dedicated reporting path: email to Red Hat's security team rather than the public issue tracker, to avoid disclosing vulnerabilities before a fix is ready.