Merge branch '2.1.x'

Closes gh-17928

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

@@ -71,8 +71,7 @@ class TomcatEmbeddedContext extends StandardContext {
 			Wrapper wrapper = (Wrapper) child;
 			int order = wrapper.getLoadOnStartup();
 			if (order >= 0) {
-				grouped.computeIfAbsent(order, ArrayList::new);
+				grouped.computeIfAbsent(order, (o) -> new ArrayList<>()).add(wrapper);
-				grouped.get(order).add(wrapper);
 			}
 		}
 		return grouped.values().stream().flatMap(List::stream);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactoryTests.java

@@ -540,6 +540,21 @@ class TomcatServletWebServerFactoryTests extends AbstractServletWebServerFactory
 				() -> factory.getWebServer((context) -> context.addListener(new FailingServletContextListener())));
 	}
 
+	@Test
+	public void registerJspServletWithDefaultLoadOnStartup() {
+		TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory(0);
+		factory.addInitializers(new ServletContextInitializer() {
+
+			@Override
+			public void onStartup(ServletContext servletContext) throws ServletException {
+				servletContext.addServlet("manually-registered-jsp-servlet", JspServlet.class);
+			}
+
+		});
+		this.webServer = factory.getWebServer();
+		this.webServer.start();
+	}
+
 	@Override
 	protected JspServlet getJspServlet() throws ServletException {
 		Tomcat tomcat = ((TomcatWebServer) this.webServer).getTomcat();