Enable Undertow to use HTTP/2

Author: pvorb

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/DefaultServletWebServerFactoryCustomizer.java

@@ -47,6 +47,7 @@
 import org.springframework.boot.web.embedded.jetty.JettyServerCustomizer;
 import org.springframework.boot.web.embedded.jetty.JettyServletWebServerFactory;
 import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
+import org.springframework.boot.web.embedded.undertow.UndertowBuilderCustomizer;
 import org.springframework.boot.web.embedded.undertow.UndertowServletWebServerFactory;
 import org.springframework.boot.web.server.Http2;
 import org.springframework.boot.web.server.WebServerFactoryCustomizer;
@@ -415,6 +416,8 @@ private static class UndertowCustomizer {
 		protected static void customizeUndertow(final ServerProperties serverProperties,
 				Environment environment, UndertowServletWebServerFactory factory) {
 
+			customizeUndertowHttp2(factory, serverProperties);
+
 			ServerProperties.Undertow undertowProperties = serverProperties.getUndertow();
 			ServerProperties.Undertow.Accesslog accesslogProperties = undertowProperties
 					.getAccesslog();
@@ -456,6 +459,15 @@ protected static void customizeUndertow(final ServerProperties serverProperties,
 					.setEagerFilterInit(undertowProperties.isEagerFilterInit()));
 		}
 
+		private static void customizeUndertowHttp2(UndertowServletWebServerFactory factory,
+				ServerProperties serverProperties) {
+
+			Http2 http2Properties = serverProperties.getHttp2();
+			factory.addBuilderCustomizers((UndertowBuilderCustomizer) builder -> {
+				builder.setServerOption(UndertowOptions.ENABLE_HTTP2, http2Properties.isEnabled());
+			});
+		}
+
 		private static void customizeConnectionTimeout(
 				UndertowServletWebServerFactory factory, final int connectionTimeout) {
 			factory.addBuilderCustomizers((builder) -> builder.setSocketOption(