Disable Jetty's default Server header

Following the upgrade to Tomcat 8.5, Jetty became the only container
that sends a Server header by default. This commit updates the factory
for Jetty to disable the default Server header bringing it into line
with Tomcat and Undertow.

All three containers continue to support server.server-header which
can be used to configure a custom server header.

Closes gh-4730

spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyEmbeddedServletContainerFactory.java

@@ -891,6 +891,13 @@ public class JettyEmbeddedServletContainerFactory
 			ServerConnector connector = new ServerConnector(server, acceptors, selectors);
 			connector.setHost(address.getHostName());
 			connector.setPort(address.getPort());
+			for (ConnectionFactory connectionFactory : connector
+					.getConnectionFactories()) {
+				if (connectionFactory instanceof HttpConfiguration.ConnectionFactory) {
+					((HttpConfiguration.ConnectionFactory) connectionFactory)
+							.getHttpConfiguration().setSendServerVersion(false);
+				}
+			}
 			return connector;
 		}
 
@@ -912,8 +919,16 @@ public class JettyEmbeddedServletContainerFactory
 						.findMethod(Server.class, "setThreadPool", ThreadPool.class)
 						.invoke(server, threadPool);
 			}
-			catch (Exception e) {
+			catch (Exception ex) {
-				throw new RuntimeException("Failed to configure Jetty 8 ThreadPool", e);
+				throw new RuntimeException("Failed to configure Jetty 8 ThreadPool", ex);
+			}
+			try {
+				ReflectionUtils
+						.findMethod(Server.class, "setSendServerVersion", boolean.class)
+						.invoke(server, false);
+			}
+			catch (Exception ex) {
+				throw new RuntimeException("Failed to disable Server header", ex);
 			}
 			return server;
 		}

spring-boot/src/test/java/org/springframework/boot/context/embedded/AbstractEmbeddedServletContainerFactoryTests.java

@@ -813,6 +813,16 @@ public abstract class AbstractEmbeddedServletContainerFactoryTests {
 		assertThat(response.getHeaders().getFirst("server")).isEqualTo("MyServer");
 	}
 
+	@Test
+	public void serverHeaderIsDisabledByDefault() throws Exception {
+		AbstractEmbeddedServletContainerFactory factory = getFactory();
+		this.container = factory
+				.getEmbeddedServletContainer(exampleServletRegistration());
+		this.container.start();
+		ClientHttpResponse response = getClientResponse(getLocalUrl("/hello"));
+		assertThat(response.getHeaders().getFirst("server")).isNull();
+	}
+
 	@Test
 	public void portClashOfPrimaryConnectorResultsInPortInUseException()
 			throws IOException {