Remove Undertow-specific support and testing

Undertow does not support Servlet 6.1, we need to remove compatibility
tests as well as Undertow-specific classes for WebSocket and reactive
support.

Closes gh-35354

Author: bclozel

framework-docs/modules/ROOT/pages/core/databuffer-codec.adoc

@@ -3,8 +3,8 @@
 
 Java NIO provides `ByteBuffer` but many libraries build their own byte buffer API on top,
 especially for network operations where reusing buffers and/or using direct buffers is
-beneficial for performance. For example Netty has the `ByteBuf` hierarchy, Undertow uses
-XNIO, Jetty uses pooled byte buffers with a callback to be released, and so on.
+beneficial for performance. For example Netty has the `ByteBuf` hierarchy,
+Jetty uses pooled byte buffers with a callback to be released, and so on.
 The `spring-core` module provides a set of abstractions to work with various byte buffer
 APIs as follows:
 

framework-docs/modules/ROOT/pages/overview.adoc

@@ -73,7 +73,7 @@ As of Spring Framework 6.0, Spring has been upgraded to the Jakarta EE 9 level
 traditional `javax` packages. With EE 9 as the minimum and EE 10 supported already,
 Spring is prepared to provide out-of-the-box support for the further evolution of
 the Jakarta EE APIs. Spring Framework 6.0 is fully compatible with Tomcat 10.1,
-Jetty 11 and Undertow 2.3 as web servers, and also with Hibernate ORM 6.1.
+Jetty 11 as web servers, and also with Hibernate ORM 6.1.
 
 Over time, the role of Java/Jakarta EE in application development has evolved. In the
 early days of J2EE and Spring, applications were created to be deployed to an application

framework-docs/modules/ROOT/pages/web-reactive.adoc

@@ -3,7 +3,7 @@
 
 This part of the documentation covers support for reactive-stack web applications built
 on a {reactive-streams-site}/[Reactive Streams] API to run on non-blocking servers,
-such as Netty, Undertow, and Servlet containers. Individual chapters cover
+such as Netty and Servlet containers. Individual chapters cover
 the xref:web/webflux.adoc#webflux[Spring WebFlux] framework,
 the reactive xref:web/webflux-webclient.adoc[`WebClient`],
 support for xref:web/webflux-test.adoc[testing],

framework-docs/modules/ROOT/pages/web/webflux-websocket.adoc

@@ -367,7 +367,7 @@ subsequently use `DataBufferUtils.release(dataBuffer)` when the buffers are cons
 `WebSocketHandlerAdapter` delegates to a `WebSocketService`. By default, that is an instance
 of `HandshakeWebSocketService`, which performs basic checks on the WebSocket request and
 then uses `RequestUpgradeStrategy` for the server in use. Currently, there is built-in
-support for Reactor Netty, Tomcat, Jetty, and Undertow.
+support for Reactor Netty, Tomcat, and Jetty.
 
 `HandshakeWebSocketService` exposes a `sessionAttributePredicate` property that allows
 setting a `Predicate<String>` to extract attributes from the `WebSession` and insert them
@@ -446,7 +446,7 @@ specify CORS settings by URL pattern. If both are specified, they are combined b
 === Client
 
 Spring WebFlux provides a `WebSocketClient` abstraction with implementations for
-Reactor Netty, Tomcat, Jetty, Undertow, and standard Java (that is, JSR-356).
+Reactor Netty, Tomcat, Jetty, and standard Java (that is, JSR-356).
 
 NOTE: The Tomcat client is effectively an extension of the standard Java one with some extra
 functionality in the `WebSocketSession` handling to take advantage of the Tomcat-specific

framework-docs/modules/ROOT/pages/web/webflux.adoc

@@ -8,7 +8,7 @@ The original web framework included in the Spring Framework, Spring Web MVC, was
 purpose-built for the Servlet API and Servlet containers. The reactive-stack web framework,
 Spring WebFlux, was added later in version 5.0. It is fully non-blocking, supports
 {reactive-streams-site}/[Reactive Streams] back pressure, and runs on such servers as
-Netty, Undertow, and Servlet containers.
+Netty, and Servlet containers.
 
 Both web frameworks mirror the names of their source modules
 ({spring-framework-code}/spring-webmvc[spring-webmvc] and

framework-docs/modules/ROOT/pages/web/webflux/http2.adoc

@@ -4,6 +4,6 @@
 
 [.small]#xref:web/webmvc/mvc-http2.adoc[See equivalent in the Servlet stack]#
 
-HTTP/2 is supported with Reactor Netty, Tomcat, Jetty, and Undertow. However, there are
+HTTP/2 is supported with Reactor Netty, Tomcat, and Jetty. However, there are
 considerations related to server configuration. For more details, see the
 {spring-framework-wiki}/HTTP-2-support[HTTP/2 wiki page].

framework-docs/modules/ROOT/pages/web/webflux/new-framework.adoc

@@ -127,7 +127,7 @@ You have maximum choice of libraries, since, historically, most are blocking.
 
 * If you are already shopping for a non-blocking web stack, Spring WebFlux offers the same
 execution model benefits as others in this space and also provides a choice of servers
-(Netty, Tomcat, Jetty, Undertow, and Servlet containers), a choice of programming models
+(Netty, Tomcat, Jetty, and Servlet containers), a choice of programming models
 (annotated controllers and functional web endpoints), and a choice of reactive libraries
 (Reactor, RxJava, or other).
 
@@ -165,7 +165,7 @@ unsure what benefits to look for, start by learning about how non-blocking I/O w
 == Servers
 
 Spring WebFlux is supported on Tomcat, Jetty, Servlet containers, as well as on
-non-Servlet runtimes such as Netty and Undertow. All servers are adapted to a low-level,
+non-Servlet runtimes such as Netty. All servers are adapted to a low-level,
 xref:web/webflux/reactive-spring.adoc#webflux-httphandler[common API] so that higher-level
 xref:web/webflux/new-framework.adoc#webflux-programming-models[programming models] can be supported across servers.
 
@@ -175,7 +175,7 @@ xref:web/webflux/dispatcher-handler.adoc#webflux-framework-config[WebFlux infras
 lines of code.
 
 Spring Boot has a WebFlux starter that automates these steps. By default, the starter uses
-Netty, but it is easy to switch to Tomcat, Jetty, or Undertow by changing your
+Netty, but it is easy to switch to Tomcat, or Jetty by changing your
 Maven or Gradle dependencies. Spring Boot defaults to Netty, because it is more widely
 used in the asynchronous, non-blocking space and lets a client and a server share resources.
 
@@ -188,8 +188,6 @@ adapter. It is not exposed for direct use.
 NOTE: It is strongly advised not to map Servlet filters or directly manipulate the Servlet API in the context of a WebFlux application.
 For the reasons listed above, mixing blocking I/O and non-blocking I/O in the same context will cause runtime issues.
 
-For Undertow, Spring WebFlux uses Undertow APIs directly without the Servlet API.
-
 
 [[webflux-performance]]
 == Performance

framework-docs/modules/ROOT/pages/web/webflux/reactive-spring.adoc

@@ -7,7 +7,7 @@ applications:
 * For server request processing there are two levels of support.
 ** xref:web/webflux/reactive-spring.adoc#webflux-httphandler[HttpHandler]: Basic contract for HTTP request handling with
 non-blocking I/O and Reactive Streams back pressure, along with adapters for Reactor Netty,
-Undertow, Tomcat, Jetty, and any Servlet container.
+Tomcat, Jetty, and any Servlet container.
 ** xref:web/webflux/reactive-spring.adoc#webflux-web-handler-api[`WebHandler` API]: Slightly higher level, general-purpose web API for
 request handling, on top of which concrete programming models such as annotated
 controllers and functional endpoints are built.
@@ -40,10 +40,6 @@ The following table describes the supported server APIs:
 | Netty API
 | {reactor-github-org}/reactor-netty[Reactor Netty]
 
-| Undertow
-| Undertow API
-| spring-web: Undertow to Reactive Streams bridge
-
 | Tomcat
 | Servlet non-blocking I/O; Tomcat API to read and write ByteBuffers vs byte[]
 | spring-web: Servlet non-blocking I/O to Reactive Streams bridge
@@ -67,10 +63,6 @@ The following table describes server dependencies (also see
 |io.projectreactor.netty
 |reactor-netty
 
-|Undertow
-|io.undertow
-|undertow-core
-
 |Tomcat
 |org.apache.tomcat.embed
 |tomcat-embed-core
@@ -104,30 +96,6 @@ Kotlin::
 ----
 ======
 
-*Undertow*
-[tabs]
-======
-Java::
-+
-[source,java,indent=0,subs="verbatim,quotes"]
-----
-	HttpHandler handler = ...
-	UndertowHttpHandlerAdapter adapter = new UndertowHttpHandlerAdapter(handler);
-	Undertow server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build();
-	server.start();
-----
-
-Kotlin::
-+
-[source,kotlin,indent=0,subs="verbatim,quotes"]
-----
-	val handler: HttpHandler = ...
-	val adapter = UndertowHttpHandlerAdapter(handler)
-	val server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build()
-	server.start()
-----
-======
-
 *Tomcat*
 [tabs]
 ======

framework-platform/framework-platform.gradle

@@ -54,9 +54,6 @@ dependencies {
 		api("io.r2dbc:r2dbc-spi:1.0.0.RELEASE")
 		api("io.reactivex.rxjava3:rxjava:3.1.10")
 		api("io.smallrye.reactive:mutiny:1.10.0")
-		api("io.undertow:undertow-core:2.3.18.Final")
-		api("io.undertow:undertow-servlet:2.3.18.Final")
-		api("io.undertow:undertow-websockets-jsr:2.3.18.Final")
 		api("io.vavr:vavr:0.10.4")
 		api("jakarta.activation:jakarta.activation-api:2.1.3")
 		api("jakarta.annotation:jakarta.annotation-api:3.0.0")

spring-web/spring-web.gradle

@@ -28,7 +28,6 @@ dependencies {
 	optional("io.netty:netty-transport")
 	optional("io.projectreactor.netty:reactor-netty-http")
 	optional("io.reactivex.rxjava3:rxjava")
-	optional("io.undertow:undertow-core")
 	optional("jakarta.el:jakarta.el-api")
 	optional("jakarta.faces:jakarta.faces-api")
 	optional("jakarta.json.bind:jakarta.json.bind-api")