Defer WebApplicationContext initialization

Update SpringBootServletInitializer so that the WebApplicationContext is
not initialized until ServletContextListener.contextInitialized() is
called. This makes it easier to subclass SpringBootServletInitializer
to add additional listeners to be called in a specific order.

Fixes gh-2070

spring-boot/src/main/java/org/springframework/boot/context/web/SpringBootServletInitializer.java

@@ -20,6 +20,7 @@ import javax.servlet.Filter;
 import javax.servlet.Servlet;
 import javax.servlet.ServletContext;
 import javax.servlet.ServletContextEvent;
+import javax.servlet.ServletContextListener;
 import javax.servlet.ServletException;
 
 import org.apache.commons.logging.Log;
@@ -62,20 +63,7 @@ public abstract class SpringBootServletInitializer implements WebApplicationInit
 
 	@Override
 	public void onStartup(ServletContext servletContext) throws ServletException {
-		WebApplicationContext rootAppContext = createRootApplicationContext(servletContext);
-		if (rootAppContext != null) {
-			servletContext.addListener(new ContextLoaderListener(rootAppContext) {
-				@Override
-				public void contextInitialized(ServletContextEvent event) {
-					// no-op because the application context is already initialized
-				}
-			});
-		}
-		else {
-			this.logger.debug("No ContextLoaderListener registered, as "
-					+ "createRootApplicationContext() did not "
-					+ "return an application context");
-		}
+		servletContext.addListener(new Listener());
 	}
 
 	protected WebApplicationContext createRootApplicationContext(
@@ -136,4 +124,36 @@ public abstract class SpringBootServletInitializer implements WebApplicationInit
 		return application;
 	}
 
+	/**
+	 * {@link ServletContextListener} used to load the context
+	 */
+	private class Listener implements ServletContextListener {
+
+		private ContextLoaderListener contextLoaderListener;
+
+		@Override
+		public void contextInitialized(ServletContextEvent event) {
+			ServletContext servletContext = event.getServletContext();
+			WebApplicationContext applicationContext = createRootApplicationContext(servletContext);
+			if (applicationContext == null) {
+				SpringBootServletInitializer.this.logger
+						.debug("No ContextLoaderListener registered, as "
+								+ "createRootApplicationContext() did not "
+								+ "return an application context");
+			}
+			else {
+				this.contextLoaderListener = new ContextLoaderListener(applicationContext);
+				// Don't delegate contextInitialized event as we're already setup
+			}
+		}
+
+		@Override
+		public void contextDestroyed(ServletContextEvent event) {
+			if (this.contextLoaderListener != null) {
+				this.contextLoaderListener.contextDestroyed(event);
+			}
+		}
+
+	}
+
 }