Enable HTTP/2 support for Tomcat and Undertow

This commit enables HTTP/2 support for Tomcat and Undertow, for both
Servlet-based and Reactive applications.

Enabling the `server.http2.enabled` configuration flag is enough with
Undertow.

Tomcat has a few prerequisites:

* Tomcat 8.5 requires JDK8+ and the proper libtcnative version installed
on the host
* Tomcat 9.0.x requires JDK9+

Closes gh-10043

spring-boot-project/spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -734,6 +734,54 @@ sample project for an example.
 
 
 
+[[howto-configure-http2]]
+=== Configure HTTP/2
+You can enable HTTP/2 support in your Spring Boot application with the
+`+server.http2.enabled+` configuration property. This support depends on the
+chosen web server and the application environment, since that protocol is not
+supported out-of-the-box by JDK8.
+
+[NOTE]
+====
+Spring Boot does not support `h2c`, the cleartext version of the HTTP/2
+protocol. So you must configure <<howto-configure-ssl, configure SSL first>>.
+====
+
+Currently, only Undertow and Tomcat are supported with this configuration key.
+
+
+[[howto-configure-http2-undertow]]
+==== HTTP/2 with Undertow
+As of Undertow 1.4.0+, HTTP/2 is supported without any additional requirement
+on JDK8.
+
+
+[[howto-configure-http2-tomcat]]
+==== HTTP/2 with Tomcat
+Spring Boot ships by default with Tomcat 8.5.x; with that version,
+HTTP/2 is only supported if the `libtcnative` library and its dependencies
+are installed on the host operating system.
+
+The library folder must be made available, if not already, to the JVM library
+path; this can be done with a JVM argument such as
+`-Djava.library.path=/usr/local/opt/tomcat-native/lib`. More on this in the
+http://tomcat.apache.org/tomcat-8.5-doc/apr.html[official Tomcat
+documentation].
+
+Starting Tomcat 8.5.x without that native support will log the following error:
+
+[indent=0,subs="attributes"]
+----
+ERROR 8787 --- [           main] o.a.coyote.http11.Http11NioProtocol      : The upgrade handler [org.apache.coyote.http2.Http2Protocol] for [h2] only supports upgrade via ALPN but has been configured for the ["https-jsse-nio-8443"] connector that does not support ALPN.
+----
+
+This error is not fatal, and the application starts with HTTP/1.1 SSL
+support still.
+
+Running your application with Tomcat 9.0.x and JDK9 doesn't require any native
+library installed. To use Tomcat 9, you can override the `tomcat.version`
+build property with the version of your choice.
+
 [[howto-configure-accesslogs]]
 === Configure Access Logging
 Access logs can be configured for Tomcat, Undertow, and Jetty through their respective

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/SslConnectorCustomizer.java

@@ -45,22 +45,21 @@ class SslConnectorCustomizer implements TomcatConnectorCustomizer {
 	private final SslStoreProvider sslStoreProvider;
 
 	SslConnectorCustomizer(Ssl ssl, SslStoreProvider sslStoreProvider) {
+		Assert.notNull(ssl, "Ssl configuration should not be null");
 		this.ssl = ssl;
 		this.sslStoreProvider = sslStoreProvider;
 	}
 
 	@Override
 	public void customize(Connector connector) {
-		if (this.ssl != null && this.ssl.isEnabled()) {
-			ProtocolHandler handler = connector.getProtocolHandler();
-			Assert.state(handler instanceof AbstractHttp11JsseProtocol,
-					"To use SSL, the connector's protocol handler must be an "
-							+ "AbstractHttp11JsseProtocol subclass");
-			configureSsl((AbstractHttp11JsseProtocol<?>) handler,
-					this.ssl, this.sslStoreProvider);
-			connector.setScheme("https");
-			connector.setSecure(true);
-		}
+		ProtocolHandler handler = connector.getProtocolHandler();
+		Assert.state(handler instanceof AbstractHttp11JsseProtocol,
+				"To use SSL, the connector's protocol handler must be an "
+						+ "AbstractHttp11JsseProtocol subclass");
+		configureSsl((AbstractHttp11JsseProtocol<?>) handler,
+				this.ssl, this.sslStoreProvider);
+		connector.setScheme("https");
+		connector.setSecure(true);
 	}
 
 	/**

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatReactiveWebServerFactory.java

@@ -30,6 +30,7 @@ import org.apache.catalina.core.AprLifecycleListener;
 import org.apache.catalina.loader.WebappLoader;
 import org.apache.catalina.startup.Tomcat;
 import org.apache.coyote.AbstractProtocol;
+import org.apache.coyote.http2.Http2Protocol;
 
 import org.springframework.boot.web.reactive.server.AbstractReactiveWebServerFactory;
 import org.springframework.boot.web.reactive.server.ReactiveWebServerFactory;
@@ -141,8 +142,13 @@ public class TomcatReactiveWebServerFactory extends AbstractReactiveWebServerFac
 		// If ApplicationContext is slow to start we want Tomcat not to bind to the socket
 		// prematurely...
 		connector.setProperty("bindOnInit", "false");
-		TomcatConnectorCustomizer ssl = new SslConnectorCustomizer(getSsl(), getSslStoreProvider());
-		ssl.customize(connector);
+		if (getSsl() != null && getSsl().isEnabled()) {
+			TomcatConnectorCustomizer ssl = new SslConnectorCustomizer(getSsl(), getSslStoreProvider());
+			ssl.customize(connector);
+			if (getHttp2() != null && getHttp2().getEnabled()) {
+				connector.addUpgradeProtocol(new Http2Protocol());
+			}
+		}
 		TomcatConnectorCustomizer compression = new CompressionConnectorCustomizer(getCompression());
 		compression.customize(connector);
 		for (TomcatConnectorCustomizer customizer : this.tomcatConnectorCustomizers) {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactory.java

@@ -59,6 +59,7 @@ import org.apache.catalina.webresources.AbstractResourceSet;
 import org.apache.catalina.webresources.EmptyResource;
 import org.apache.catalina.webresources.StandardRoot;
 import org.apache.coyote.AbstractProtocol;
+import org.apache.coyote.http2.Http2Protocol;
 
 import org.springframework.boot.web.server.ErrorPage;
 import org.springframework.boot.web.server.MimeMappings;
@@ -297,8 +298,13 @@ public class TomcatServletWebServerFactory extends AbstractServletWebServerFacto
 		// prematurely...
 		connector.setProperty("bindOnInit", "false");
 
-		TomcatConnectorCustomizer ssl = new SslConnectorCustomizer(getSsl(), getSslStoreProvider());
-		ssl.customize(connector);
+		if (getSsl() != null && getSsl().isEnabled()) {
+			TomcatConnectorCustomizer ssl = new SslConnectorCustomizer(getSsl(), getSslStoreProvider());
+			ssl.customize(connector);
+			if (getHttp2() != null && getHttp2().getEnabled()) {
+				connector.addUpgradeProtocol(new Http2Protocol());
+			}
+		}
 		TomcatConnectorCustomizer compression = new CompressionConnectorCustomizer(getCompression());
 		compression.customize(connector);
 		for (TomcatConnectorCustomizer customizer : this.tomcatConnectorCustomizers) {

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/undertow/UndertowReactiveWebServerFactory.java

@@ -22,6 +22,7 @@ import java.util.Collection;
 import java.util.List;
 
 import io.undertow.Undertow;
+import io.undertow.UndertowOptions;
 
 import org.springframework.boot.web.reactive.server.AbstractReactiveWebServerFactory;
 import org.springframework.boot.web.reactive.server.ReactiveWebServerFactory;
@@ -89,6 +90,9 @@ public class UndertowReactiveWebServerFactory extends AbstractReactiveWebServerF
 			SslBuilderCustomizer sslBuilderCustomizer =
 					new SslBuilderCustomizer(getPort(), getAddress(), getSsl(), getSslStoreProvider());
 			sslBuilderCustomizer.customize(builder);
+			if (getHttp2() != null) {
+				builder.setServerOption(UndertowOptions.ENABLE_HTTP2, getHttp2().getEnabled());
+			}
 		}
 		else {
 			builder.addHttpListener(port, getListenAddress());

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/undertow/UndertowServletWebServerFactory.java

@@ -36,6 +36,7 @@ import javax.servlet.ServletException;
 
 import io.undertow.Undertow;
 import io.undertow.Undertow.Builder;
+import io.undertow.UndertowOptions;
 import io.undertow.server.HttpHandler;
 import io.undertow.server.handlers.accesslog.AccessLogHandler;
 import io.undertow.server.handlers.accesslog.AccessLogReceiver;
@@ -235,6 +236,9 @@ public class UndertowServletWebServerFactory extends AbstractServletWebServerFac
 			SslBuilderCustomizer sslBuilderCustomizer =
 					new SslBuilderCustomizer(getPort(), getAddress(), getSsl(), getSslStoreProvider());
 			sslBuilderCustomizer.customize(builder);
+			if (getHttp2() != null) {
+				builder.setServerOption(UndertowOptions.ENABLE_HTTP2, getHttp2().getEnabled());
+			}
 		}
 		else {
 			builder.addHttpListener(port, getListenAddress());