Add convenient WebAppInitializer base classes

This commit introduces three abstract WebApplicationInitializers, to be
used in the typical setup of a Spring-based web application.

 - AbstractContextLoaderInitializer provides an abstract base class for
   registering a ContextLoaderListener.

 - AbstractDispatcherServletInitializer provides an abstract base class
   for registering a DispatcherServlet, with an optional root context.

 - AbstractAnnotationConfigDispatcherServletInitializer provides an
   abstract base class for registering a DispatcherServlet and optional
   ContextLoaderListener based on annotated (e.g. @Configuration)
   classes.

Issue: SPR-9300

spring-web/src/test/java/org/springframework/web/context/ContextLoaderInitializerTests.java

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2002-2012 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
+ *
+ * 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.web.context;
+
+import java.util.EventListener;
+
+import javax.servlet.ServletContextEvent;
+import javax.servlet.ServletException;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import org.springframework.mock.web.MockServletContext;
+import org.springframework.web.context.support.StaticWebApplicationContext;
+import org.springframework.web.context.support.WebApplicationContextUtils;
+
+import static org.junit.Assert.*;
+
+/**
+ * Test case for {@link AbstractContextLoaderInitializer}.
+ *
+ * @author Arjen Poutsma
+ */
+public class ContextLoaderInitializerTests {
+
+	private static final String BEAN_NAME = "myBean";
+
+	private AbstractContextLoaderInitializer initializer;
+
+	private MockServletContext servletContext;
+
+	private EventListener eventListener;
+
+	@Before
+	public void setUp() throws Exception {
+		servletContext = new MyMockServletContext();
+		initializer = new MyContextLoaderInitializer();
+		eventListener = null;
+	}
+
+	@Test
+	public void register() throws ServletException {
+		initializer.onStartup(servletContext);
+
+		assertTrue(eventListener instanceof ContextLoaderListener);
+		ContextLoaderListener cll = (ContextLoaderListener) eventListener;
+		cll.contextInitialized(new ServletContextEvent(servletContext));
+
+		WebApplicationContext applicationContext = WebApplicationContextUtils
+				.getRequiredWebApplicationContext(servletContext);
+
+		assertTrue(applicationContext.containsBean(BEAN_NAME));
+		assertTrue(applicationContext.getBean(BEAN_NAME) instanceof MyBean);
+	}
+
+	private class MyMockServletContext extends MockServletContext {
+
+		@Override
+		public <T extends EventListener> void addListener(T listener) {
+			eventListener = listener;
+		}
+
+	}
+
+	private static class MyContextLoaderInitializer
+			extends AbstractContextLoaderInitializer {
+
+		@Override
+		protected WebApplicationContext createRootApplicationContext() {
+			StaticWebApplicationContext rootContext = new StaticWebApplicationContext();
+			rootContext.registerSingleton(BEAN_NAME, MyBean.class);
+			return rootContext;
+		}
+	}
+
+	private static class MyBean {
+
+	}
+}