Drop Undertow support

Spring Framework 7.0 requires a Servlet 6.1 baseline for Servlet containers.

Partial Servlet 6.1 support is available on the `main` Undertow branch,
but there isn't any milestone version available for the undertow 2.4
generation. At this stage of our 4.0 schedule, we think it's safer to drop
Undertow support now.

Closes: gh-46917

Author: bclozel

README.adoc

@@ -116,7 +116,7 @@ The main library providing features that support the other parts of Spring Boot.
 
 * The `SpringApplication` class, providing static convenience methods that can be used to write a stand-alone Spring Application.
   Its sole job is to create and refresh an appropriate Spring `ApplicationContext`.
-* Embedded web applications with a choice of container (Tomcat, Jetty, or Undertow).
+* Embedded web applications with a choice of container (Tomcat, Jetty).
 * First-class externalized configuration support.
 * Convenience `ApplicationContext` initializers, including support for sensible logging defaults.
 

configuration-metadata/spring-boot-configuration-metadata-changelog-generator/build.gradle

@@ -128,7 +128,6 @@ def dependenciesOf(String version) {
 			"spring-boot-tomcat",
 			"spring-boot-tracing",
 			"spring-boot-tx",
-			"spring-boot-undertow",
 			"spring-boot-validation",
 			"spring-boot-web-server",
 			"spring-boot-webclient",

documentation/spring-boot-docs/build.gradle

@@ -111,7 +111,6 @@ dependencies {
 	implementation(project(path: ":module:spring-boot-security"))
 	implementation(project(path: ":module:spring-boot-test-autoconfigure"))
 	implementation(project(path: ":module:spring-boot-tomcat"))
-	implementation(project(path: ":module:spring-boot-undertow"))
 	implementation(project(path: ":module:spring-boot-web-server-test"))
 	implementation(project(path: ":module:spring-boot-webclient"))
 	implementation(project(path: ":module:spring-boot-webflux"))
@@ -126,7 +125,6 @@ dependencies {
 	implementation("io.micrometer:micrometer-registry-jmx")
 	implementation("io.opentelemetry.instrumentation:opentelemetry-logback-appender-1.0")
 	implementation("io.projectreactor.netty:reactor-netty-http")
-	implementation("io.undertow:undertow-core")
 	implementation("jakarta.annotation:jakarta.annotation-api")
 	implementation("jakarta.jms:jakarta.jms-api")
 	implementation("jakarta.persistence:jakarta.persistence-api")

documentation/spring-boot-docs/src/docs/antora/modules/ROOT/pages/redirect.adoc

@@ -684,8 +684,6 @@
 * xref:how-to:webserver.adoc#howto.webserver.configure-http2.netty[#howto.webserver.configure-http2.netty]
 * xref:how-to:webserver.adoc#howto.webserver.configure-http2.tomcat[#howto-configure-http2-tomcat]
 * xref:how-to:webserver.adoc#howto.webserver.configure-http2.tomcat[#howto.webserver.configure-http2.tomcat]
-* xref:how-to:webserver.adoc#howto.webserver.configure-http2.undertow[#howto-configure-http2-undertow]
-* xref:how-to:webserver.adoc#howto.webserver.configure-http2.undertow[#howto.webserver.configure-http2.undertow]
 * xref:how-to:webserver.adoc#howto.webserver.configure-http2[#howto-configure-http2]
 * xref:how-to:webserver.adoc#howto.webserver.configure-http2[#howto.webserver.configure-http2]
 * xref:how-to:webserver.adoc#howto.webserver.configure-ssl.pem-files[#howto.webserver.configure-ssl.pem-files]
@@ -701,8 +699,6 @@
 * xref:how-to:webserver.adoc#howto.webserver.discover-port[#howto.webserver.discover-port]
 * xref:how-to:webserver.adoc#howto.webserver.enable-multiple-connectors-in-tomcat[#howto-enable-multiple-connectors-in-tomcat]
 * xref:how-to:webserver.adoc#howto.webserver.enable-multiple-connectors-in-tomcat[#howto.webserver.enable-multiple-connectors-in-tomcat]
-* xref:how-to:webserver.adoc#howto.webserver.enable-multiple-listeners-in-undertow[#howto-enable-multiple-listeners-in-undertow]
-* xref:how-to:webserver.adoc#howto.webserver.enable-multiple-listeners-in-undertow[#howto.webserver.enable-multiple-listeners-in-undertow]
 * xref:how-to:webserver.adoc#howto.webserver.enable-response-compression[#how-to-enable-http-response-compression]
 * xref:how-to:webserver.adoc#howto.webserver.enable-response-compression[#howto.webserver.enable-response-compression]
 * xref:how-to:webserver.adoc#howto.webserver.enable-tomcat-mbean-registry[#howto-enable-tomcat-mbean-registry]

documentation/spring-boot-docs/src/docs/antora/modules/ROOT/pages/system-requirements.adoc

@@ -32,8 +32,6 @@ Spring Boot supports the following embedded servlet containers:
 | Jetty 12.0
 | 6.0
 
-| Undertow 2.3
-| 6.0
 |===
 
 You can also deploy Spring Boot applications to any servlet 5.0+ compatible container.

documentation/spring-boot-docs/src/docs/antora/modules/how-to/pages/deployment/cloud.adoc

@@ -179,7 +179,7 @@ web: java -Dserver.port=$PORT -jar target/demo-0.0.1-SNAPSHOT.jar
 ----
 
 Spring Boot makes `-D` arguments available as properties accessible from a Spring javadoc:org.springframework.core.env.Environment[] instance.
-The `server.port` configuration property is fed to the embedded Tomcat, Jetty, or Undertow instance, which then uses the port when it starts up.
+The `server.port` configuration property is fed to the embedded Tomcat or Jetty instance, which then uses the port when it starts up.
 The `$PORT` environment variable is assigned to us by the Heroku PaaS.
 
 This should be everything you need.

documentation/spring-boot-docs/src/docs/antora/modules/how-to/pages/webserver.adoc

@@ -12,8 +12,8 @@ This section answers those questions.
 
 Many Spring Boot starters include default embedded containers.
 
-* For servlet stack applications, the `spring-boot-starter-web` includes Tomcat by including `spring-boot-starter-tomcat`, but you can use `spring-boot-starter-jetty` or `spring-boot-starter-undertow` instead.
-* For reactive stack applications, the `spring-boot-starter-webflux` includes  Reactor Netty by including `spring-boot-starter-reactor-netty`, but you can use `spring-boot-starter-tomcat`, `spring-boot-starter-jetty`, or `spring-boot-starter-undertow` instead.
+* For servlet stack applications, the `spring-boot-starter-web` includes Tomcat by including `spring-boot-starter-tomcat`, but you can use `spring-boot-starter-jetty` instead.
+* For reactive stack applications, the `spring-boot-starter-webflux` includes  Reactor Netty by including `spring-boot-starter-reactor-netty`, but you can use `spring-boot-starter-tomcat` or `spring-boot-starter-jetty` instead.
 
 When switching to a different HTTP server, you need to swap the default dependencies for those that you need instead.
 To help with this process, Spring Boot provides a separate starter for each of the supported HTTP servers.
@@ -40,16 +40,16 @@ The following Maven example shows how to exclude Tomcat and include Jetty for Sp
 </dependency>
 ----
 
-The following Gradle example configures the necessary dependencies and a {url-gradle-docs}/resolution_rules.html#sec:module_replacement[module replacement] to use Undertow in place of Reactor Netty for Spring WebFlux:
+The following Gradle example configures the necessary dependencies and a {url-gradle-docs}/resolution_rules.html#sec:module_replacement[module replacement] to use Tomcat in place of Reactor Netty for Spring WebFlux:
 
 [source,gradle]
 ----
 dependencies {
-	implementation "org.springframework.boot:spring-boot-starter-undertow"
+	implementation "org.springframework.boot:spring-boot-starter-tomcat"
 	implementation "org.springframework.boot:spring-boot-starter-webflux"
 	modules {
 		module("org.springframework.boot:spring-boot-starter-reactor-netty") {
-			replacedBy("org.springframework.boot:spring-boot-starter-undertow", "Use Undertow instead of Reactor Netty")
+			replacedBy("org.springframework.boot:spring-boot-starter-tomcat", "Use Tomcat instead of Reactor Netty")
 		}
 	}
 }
@@ -116,7 +116,7 @@ Contrary to a test, application code callbacks are processed early (before the v
 [[howto.webserver.enable-response-compression]]
 == Enable HTTP Response Compression
 
-HTTP response compression is supported by Jetty, Tomcat, Reactor Netty, and Undertow.
+HTTP response compression is supported by Jetty, Tomcat and Reactor Netty.
 It can be enabled in `application.properties`, as follows:
 
 [configprops,yaml]
@@ -221,7 +221,7 @@ These properties should instead be defined using `spring.ssl.bundle.<type>.<name
 [[howto.webserver.configure-ssl.sni]]
 === Configure Server Name Indication
 
-Tomcat, Netty, and Undertow can be configured to use unique SSL trust material for individual host names to support Server Name Indication (SNI).
+Tomcat, and Netty can be configured to use unique SSL trust material for individual host names to support Server Name Indication (SNI).
 SNI configuration is not supported with Jetty, but Jetty can https://eclipse.dev/jetty/documentation/jetty-12/operations-guide/index.html#og-protocols-ssl-sni[automatically set up SNI] if multiple certificates are provided to it.
 
 Assuming xref:reference:features/ssl.adoc[SSL bundles] named `web`, `web-alt1`, and `web-alt2` have been configured, the following configuration can be used to assign each bundle to a host name served by the embedded web server:
@@ -292,13 +292,6 @@ Developers can choose to import only the required dependencies using a classifie
 
 
 
-[[howto.webserver.configure-http2.undertow]]
-=== HTTP/2 With Undertow
-
-Undertow supports `h2c` and `h2` out of the box.
-
-
-
 [[howto.webserver.configure]]
 == Configure the Web Server
 
@@ -309,7 +302,7 @@ See the list of xref:appendix:application-properties/index.adoc[].
 
 The previous sections covered already many common use cases, such as compression, SSL or HTTP/2.
 However, if a configuration key does not exist for your use case, you should then look at javadoc:org.springframework.boot.web.server.WebServerFactoryCustomizer[].
-You can declare such a component and get access to the server factory relevant to your choice: you should select the variant for the chosen Server (Tomcat, Jetty, Reactor Netty, Undertow) and the chosen web stack (servlet or reactive).
+You can declare such a component and get access to the server factory relevant to your choice: you should select the variant for the chosen Server (Tomcat, Jetty, Reactor Netty) and the chosen web stack (servlet or reactive).
 
 The example below is for Tomcat with the `spring-boot-starter-web` (servlet stack):
 
@@ -335,10 +328,6 @@ In addition Spring Boot provides:
 | javadoc:org.springframework.boot.web.embedded.jetty.JettyServletWebServerFactory[]
 | javadoc:org.springframework.boot.web.embedded.jetty.JettyReactiveWebServerFactory[]
 
-| Undertow
-| javadoc:org.springframework.boot.web.embedded.undertow.UndertowServletWebServerFactory[]
-| javadoc:org.springframework.boot.web.embedded.undertow.UndertowReactiveWebServerFactory[]
-
 | Reactor
 | N/A
 | javadoc:org.springframework.boot.web.embedded.netty.NettyReactiveWebServerFactory[]
@@ -402,7 +391,7 @@ By default, javadoc:org.springframework.boot.web.servlet.ServletComponentScan[fo
 [[howto.webserver.configure-access-logs]]
 == Configure Access Logging
 
-Access logs can be configured for Tomcat, Undertow, and Jetty through their respective namespaces.
+Access logs can be configured for Tomcat and Jetty through their respective namespaces.
 
 For instance, the following settings log access on Tomcat with a {url-tomcat-docs}/config/valve.html#Access_Logging[custom pattern].
 
@@ -420,24 +409,6 @@ NOTE: The default location for logs is a `logs` directory relative to the Tomcat
 By default, the `logs` directory is a temporary directory, so you may want to fix Tomcat's base directory or use an absolute path for the logs.
 In the preceding example, the logs are available in `my-tomcat/logs` relative to the working directory of the application.
 
-Access logging for Undertow can be configured in a similar fashion, as shown in the following example:
-
-[configprops,yaml]
-----
-server:
-  undertow:
-    accesslog:
-      enabled: true
-      pattern: "%t %a %r %s (%D milliseconds)"
-    options:
-      server:
-        record-request-start-time: true
-----
-
-Note that, in addition to enabling access logging and configuring its pattern, recording request start times has also been enabled.
-This is required when including the response time (`%D`) in the access log pattern.
-Logs are stored in a `logs` directory relative to the working directory of the application.
-You can customize this location by setting the configprop:server.undertow.accesslog.dir[] property.
 
 Finally, access logging for Jetty can also be configured as follows:
 
@@ -537,14 +508,6 @@ server:
 
 
 
-[[howto.webserver.enable-multiple-listeners-in-undertow]]
-== Enable Multiple Listeners with Undertow
-
-Add an javadoc:org.springframework.boot.web.embedded.undertow.UndertowBuilderCustomizer[] to the javadoc:org.springframework.boot.web.embedded.undertow.UndertowServletWebServerFactory[] and add a listener to the `io.undertow.Undertow.Builder`, as shown in the following example:
-
-include-code::MyUndertowConfiguration[]
-
-
 
 [[howto.webserver.create-websocket-endpoints-using-serverendpoint]]
 == Create WebSocket Endpoints Using @ServerEndpoint

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/messaging/websockets.adoc

@@ -1,7 +1,7 @@
 [[messaging.websockets]]
 = WebSockets
 
-Spring Boot provides WebSockets auto-configuration for embedded Tomcat, Jetty, and Undertow.
+Spring Boot provides WebSockets auto-configuration for embedded Tomcat and Jetty.
 If you deploy a war file to a standalone container, Spring Boot assumes that the container is responsible for the configuration of its WebSocket support.
 
 Spring Framework provides {url-spring-framework-docs}/web/websocket.html[rich WebSocket support] for MVC web applications that can be easily accessed through the `spring-boot-starter-websocket` module.

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/web/graceful-shutdown.adoc

@@ -1,7 +1,7 @@
 [[web.graceful-shutdown]]
 = Graceful Shutdown
 
-Graceful shutdown is enabled by default with all four embedded web servers (Jetty, Reactor Netty, Tomcat, and Undertow) and with both reactive and servlet-based web applications.
+Graceful shutdown is enabled by default with all four embedded web servers (Jetty, Reactor Netty, and Tomcat) and with both reactive and servlet-based web applications.
 It occurs as part of closing the application context and is performed in the earliest phase of stopping javadoc:org.springframework.context.SmartLifecycle[] beans.
 This stop processing uses a timeout which provides a grace period during which existing requests will be allowed to complete but no new requests will be permitted.
 
@@ -26,10 +26,9 @@ The exact way in which new requests are not permitted varies depending on the we
 Implementations may stop accepting requests at the network layer, or they may return a response with a specific HTTP status code or HTTP header.
 The use of persistent connections can also change the way that requests stop being accepted.
 
-TIP: To learn more about the specific method used with your web server, see the `shutDownGracefully` API documentation for javadoc:org.springframework.boot.web.embedded.tomcat.TomcatWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[], javadoc:org.springframework.boot.web.embedded.netty.NettyWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[], javadoc:org.springframework.boot.web.embedded.jetty.JettyWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[] or javadoc:org.springframework.boot.web.embedded.undertow.UndertowWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[].
+TIP: To learn more about the specific method used with your web server, see the `shutDownGracefully` API documentation for javadoc:org.springframework.boot.web.embedded.tomcat.TomcatWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[], javadoc:org.springframework.boot.web.embedded.netty.NettyWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[], or javadoc:org.springframework.boot.web.embedded.jetty.JettyWebServer#shutDownGracefully(org.springframework.boot.web.server.GracefulShutdownCallback)[].
 
 Jetty, Reactor Netty, and Tomcat will stop accepting new requests at the network layer.
-Undertow will accept new connections but respond immediately with a service unavailable (503) response.
 
 
 

documentation/spring-boot-docs/src/docs/antora/modules/reference/pages/web/index.adoc

@@ -2,7 +2,7 @@
 = Web
 
 Spring Boot is well suited for web application development.
-You can create a self-contained HTTP server by using embedded Tomcat, Jetty, Undertow, or Netty.
+You can create a self-contained HTTP server by using embedded Tomcat, Jetty, or Netty.
 Most web applications use the `spring-boot-starter-web` module to get up and running quickly.
 You can also choose to build reactive web applications by using the `spring-boot-starter-webflux` module.