Allow ALPNServerConnectionFactory to negotiate use of HTTP/1.1

Closes gh-14444

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/SslServerCustomizer.java

@@ -124,7 +124,6 @@ class SslServerCustomizer implements JettyServerCustomizer {
 			HttpConfiguration config, SslContextFactory sslContextFactory) {
 		HTTP2ServerConnectionFactory h2 = new HTTP2ServerConnectionFactory(config);
 		ALPNServerConnectionFactory alpn = new ALPNServerConnectionFactory();
-		alpn.setDefaultProtocol("h2");
 		sslContextFactory.setCipherComparator(HTTP2Cipher.COMPARATOR);
 		sslContextFactory.setProvider("Conscrypt");
 		SslConnectionFactory ssl = new SslConnectionFactory(sslContextFactory,

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/jetty/SslServerCustomizerTests.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2012-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.web.embedded.jetty;
+
+import java.net.InetSocketAddress;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.eclipse.jetty.alpn.server.ALPNServerConnectionFactory;
+import org.eclipse.jetty.http2.server.HTTP2ServerConnectionFactory;
+import org.eclipse.jetty.server.ConnectionFactory;
+import org.eclipse.jetty.server.HttpConnectionFactory;
+import org.eclipse.jetty.server.Server;
+import org.eclipse.jetty.server.SslConnectionFactory;
+import org.junit.Test;
+
+import org.springframework.boot.web.server.Http2;
+import org.springframework.boot.web.server.Ssl;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link SslServerCustomizer}.
+ *
+ * @author Andy Wilkinson
+ */
+public class SslServerCustomizerTests {
+
+	@Test
+	@SuppressWarnings("rawtypes")
+	public void whenHttp2IsNotEnabledServerConnectorHasSslAndHttpConnectionFactories() {
+		Server server = createCustomizedServer();
+		assertThat(server.getConnectors()).hasSize(1);
+		List<ConnectionFactory> factories = new ArrayList<>(
+				server.getConnectors()[0].getConnectionFactories());
+		assertThat(factories).extracting((factory) -> (Class) factory.getClass())
+				.containsExactly(SslConnectionFactory.class, HttpConnectionFactory.class);
+	}
+
+	@Test
+	@SuppressWarnings("rawtypes")
+	public void whenHttp2IsEnabledServerConnectorsHasSslAlpnH2AndHttpConnectionFactories() {
+		Http2 http2 = new Http2();
+		http2.setEnabled(true);
+		Server server = createCustomizedServer(http2);
+		assertThat(server.getConnectors()).hasSize(1);
+		List<ConnectionFactory> factories = new ArrayList<>(
+				server.getConnectors()[0].getConnectionFactories());
+		assertThat(factories).extracting((factory) -> (Class) factory.getClass())
+				.containsExactly(SslConnectionFactory.class,
+						ALPNServerConnectionFactory.class,
+						HTTP2ServerConnectionFactory.class, HttpConnectionFactory.class);
+	}
+
+	@Test
+	public void alpnConnectionFactoryHasNullDefaultProtocolToAllowNegotiationToHttp11() {
+		Http2 http2 = new Http2();
+		http2.setEnabled(true);
+		Server server = createCustomizedServer(http2);
+		assertThat(server.getConnectors()).hasSize(1);
+		List<ConnectionFactory> factories = new ArrayList<>(
+				server.getConnectors()[0].getConnectionFactories());
+		assertThat(((ALPNServerConnectionFactory) factories.get(1)).getDefaultProtocol())
+				.isNull();
+	}
+
+	private Server createCustomizedServer() {
+		return createCustomizedServer(new Http2());
+	}
+
+	private Server createCustomizedServer(Http2 http2) {
+		Ssl ssl = new Ssl();
+		ssl.setKeyStore("classpath:test.jks");
+		return createCustomizedServer(ssl, http2);
+	}
+
+	private Server createCustomizedServer(Ssl ssl, Http2 http2) {
+		Server server = new Server();
+		new SslServerCustomizer(new InetSocketAddress(0), ssl, null, http2)
+				.customize(server);
+		return server;
+	}
+
+}