Add property for registering default Servlet

This commit adds a new configuration property for configuring the
registration of the default Servlet in Servlet containers.

`"server.servlet.default-servlet.registered=false"`

The default of this property is still `true`, as it's been the case in
previous releases.

Closes gh-21214

Author: bclozel

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ServerProperties.java

@@ -38,6 +38,7 @@
 import org.springframework.boot.web.server.Http2;
 import org.springframework.boot.web.server.Shutdown;
 import org.springframework.boot.web.server.Ssl;
+import org.springframework.boot.web.servlet.server.DefaultServlet;
 import org.springframework.boot.web.servlet.server.Encoding;
 import org.springframework.boot.web.servlet.server.Jsp;
 import org.springframework.boot.web.servlet.server.Session;
@@ -226,6 +227,9 @@ public static class Servlet {
 		 */
 		private String applicationDisplayName = "application";
 
+		@NestedConfigurationProperty
+		private final DefaultServlet defaultServlet = new DefaultServlet();
+
 		@NestedConfigurationProperty
 		private final Encoding encoding = new Encoding();
 
@@ -263,6 +267,10 @@ public Map<String, String> getContextParameters() {
 			return this.contextParameters;
 		}
 
+		public DefaultServlet getDefaultServlet() {
+			return this.defaultServlet;
+		}
+
 		public Encoding getEncoding() {
 			return this.encoding;
 		}

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

@@ -53,6 +53,8 @@ public void customize(ConfigurableServletWebServerFactory factory) {
 		map.from(this.serverProperties::getAddress).to(factory::setAddress);
 		map.from(this.serverProperties.getServlet()::getContextPath).to(factory::setContextPath);
 		map.from(this.serverProperties.getServlet()::getApplicationDisplayName).to(factory::setDisplayName);
+		map.from(this.serverProperties.getServlet().getDefaultServlet()::isRegistered)
+				.to(factory::setRegisterDefaultServlet);
 		map.from(this.serverProperties.getServlet()::getSession).to(factory::setSession);
 		map.from(this.serverProperties::getSsl).to(factory::setSsl);
 		map.from(this.serverProperties.getServlet()::getJsp).to(factory::setJsp);

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/ServletWebServerFactoryCustomizerTests.java

@@ -74,6 +74,14 @@ void testCustomizeDisplayName() {
 		verify(factory).setDisplayName("TestName");
 	}
 
+	@Test
+	void testCustomizeDefaultServlet() {
+		ConfigurableServletWebServerFactory factory = mock(ConfigurableServletWebServerFactory.class);
+		this.properties.getServlet().getDefaultServlet().setRegistered(true);
+		this.customizer.customize(factory);
+		verify(factory).setRegisterDefaultServlet(true);
+	}
+
 	@Test
 	void testCustomizeSsl() {
 		ConfigurableServletWebServerFactory factory = mock(ConfigurableServletWebServerFactory.class);

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/server/DefaultServlet.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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.servlet.server;
+
+/**
+ * Default Servlet properties.
+ *
+ * @author Brian Clozel
+ * @since 2.3.0
+ */
+public class DefaultServlet {
+
+	/**
+	 * Whether to enable the registration of the default Servlet with the container.
+	 */
+	private boolean registered = true;
+
+	public boolean isRegistered() {
+		return this.registered;
+	}
+
+	public void setRegistered(boolean registered) {
+		this.registered = registered;
+	}
+
+}