Add createDispatcherServlet hook point

Add an extra hook point in `AbstractDispatcherServletInitializer` to
customize the `DispatcherServlet`.

Issue: SPR-13222

spring-webmvc/src/test/java/org/springframework/web/servlet/support/DispatcherServletInitializerTests.java

@@ -1,11 +1,11 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2015 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
+ *      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,
@@ -70,6 +70,7 @@ public class DispatcherServletInitializerTests {
 		assertNotNull(servlets.get(SERVLET_NAME));
 
 		DispatcherServlet servlet = (DispatcherServlet) servlets.get(SERVLET_NAME);
+		assertEquals(MyDispatcherServlet.class, servlet.getClass());
 		WebApplicationContext servletContext = servlet.getWebApplicationContext();
 
 		assertTrue(servletContext.containsBean("bean"));
@@ -84,6 +85,7 @@ public class DispatcherServletInitializerTests {
 		assertEquals(ROLE_NAME, registration.getRunAsRole());
 	}
 
+
 	private class MyMockServletContext extends MockServletContext {
 
 		@Override
@@ -104,6 +106,11 @@ public class DispatcherServletInitializerTests {
 			return SERVLET_NAME;
 		}
 
+		@Override
+		protected DispatcherServlet createDispatcherServlet(WebApplicationContext servletAppContext) {
+			return new MyDispatcherServlet(servletAppContext);
+		}
+
 		@Override
 		protected WebApplicationContext createServletApplicationContext() {
 			StaticWebApplicationContext servletContext =
@@ -129,7 +136,12 @@ public class DispatcherServletInitializerTests {
 	}
 
 	private static class MyBean {
+	}
 
+	private static class MyDispatcherServlet extends DispatcherServlet {
+		public MyDispatcherServlet(WebApplicationContext webApplicationContext) {
+			super(webApplicationContext);
+		}
 	}
 
 }